Last week I posted about an Emerson Research "home theatre" system that I
was given. Two 5A fuses on the CT transformer secondaries blown. This unit
has outputs for two rear speakers (L/R), a center channel (DIN connector)
and two front satellites (L/R). Now the inputs are analog RCA audio so this
box must be just simulating 5.1 using a delay or some other trickery.

What I'm looking for here is an understanding of how the various output
components interact... in particular how the NPN/PNP pairs function

Anyway, there are six output transistors, 3 x TIP41C (NPN) and 3 x TIP42C
(PNP). It _APPEARS_ by looking at the board traces that one set goes to
the rear output RCA connectors (can't really see how it's connected as the
connector block takes up a lot of real estate on the board), one set goes to
the satellite RCA outputs, and one set goes to the subwoofer single output
which is enclosed in the cabinet with the PCB board. The 41/42 pair
adjacent to the subwoofer output are the only two (of the six) output
transistors which are NOT blown.

Now, another poster suggested that I check the bias and driver transistors,
but I'm not sure which ones these are. here's what I've got, I wish I were
skilled in ascii art but I'm not so bear with me.

The output transistors are all in a row along a large aluminum heatsink
which runs the width of the PCB. They alternate --- In between each
PNP/NPN transistor is a smaller TO-92 NPN which is adjacent to the heatsink
as well, sort of stuck there with thermal grease. Adjacent to each TO-220
Tip transistor but on the side opposite the heatsink, each output transistor
is paired with another TO-92 Like so:

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | |
| |
c945 a733 c945 a733 2sc2245 2sa965
bad

I pulled ALL of these transistors and tested them. Of the TO-92's, all test
good except one of the a733's. My question is this: do I need to replace
BOTH a733's, i.e. to maintain matched set characteristics? I'd be looking
at an NTE290A as a replacement. Or are the c945/a733 some sort of matched
pair. I see that NTE offers either a) two matched 290A's or b) matched sets
of 289A/290A (289A is NOT listed as a replacement for 2sc945).

As I noted at the beginning, I'd like to understand which transistors drive
what if it's at all possible to determine from the information I've
provided. I can trace out the relationship between the transistors if
necessary but maybe the number, placement and type of units is enough.

Any replies greatly appreciated.

Dave

Dave wrote:

Last week I posted about an Emerson Research "home theatre" system that I
was given. Two 5A fuses on the CT transformer secondaries blown. This unit
has outputs for two rear speakers (L/R), a center channel (DIN connector)
and two front satellites (L/R). Now the inputs are analog RCA audio so this
box must be just simulating 5.1 using a delay or some other trickery.

What I'm looking for here is an understanding of how the various output
components interact... in particular how the NPN/PNP pairs function

I have little idea what you know and what you dont... npn/pnp pair is
your standard output pair, normally in more or less the same
configuration, but for any more details you'd need to either trace the
circuit yourself or get a cct diagram.


good except one of the a733's. My question is this: do I need to replace
BOTH a733's, i.e. to maintain matched set characteristics? I'd be looking
at an NTE290A as a replacement. Or are the c945/a733 some sort of matched
pair. I see that NTE offers either a) two matched 290A's or b) matched sets
of 289A/290A (289A is NOT listed as a replacement for 2sc945).

Little chance of needing matched pairs for anything. Transistors
distort so much they get lots of nfb applied to them, so it doesnt
matter whether theyre matched.


As I noted at the beginning, I'd like to understand which transistors drive
what if it's at all possible to determine from the information I've
provided.

not really.


NT

Well, typically these output transistors are usually arranged in a push pull
type circuit. One transistor amplifies the positive side of the waveform
and the other does the negative side.

But I have to ask why. Emerson is junk. Not worth spending time and money
on. Buy a new one.

DaveL


"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...
Last week I posted about an Emerson Research "home theatre" system that I
was given. Two 5A fuses on the CT transformer secondaries blown. This
unit has outputs for two rear speakers (L/R), a center channel (DIN
connector) and two front satellites (L/R). Now the inputs are analog RCA
audio so this box must be just simulating 5.1 using a delay or some other
trickery.

What I'm looking for here is an understanding of how the various output
components interact... in particular how the NPN/PNP pairs function

Anyway, there are six output transistors, 3 x TIP41C (NPN) and 3 x TIP42C
(PNP). It _APPEARS_ by looking at the board traces that one set goes to
the rear output RCA connectors (can't really see how it's connected as the
connector block takes up a lot of real estate on the board), one set goes
to the satellite RCA outputs, and one set goes to the subwoofer single
output which is enclosed in the cabinet with the PCB board. The 41/42
pair adjacent to the subwoofer output are the only two (of the six) output
transistors which are NOT blown.

Now, another poster suggested that I check the bias and driver
transistors, but I'm not sure which ones these are. here's what I've got,
I wish I were skilled in ascii art but I'm not so bear with me.

The output transistors are all in a row along a large aluminum heatsink
which runs the width of the PCB. They alternate --- In between each
PNP/NPN transistor is a smaller TO-92 NPN which is adjacent to the
heatsink as well, sort of stuck there with thermal grease. Adjacent to
each TO-220 Tip transistor but on the side opposite the heatsink, each
output transistor is paired with another TO-92 Like so:

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |
|
c945 a733 c945 a733 2sc2245 2sa965
bad

I pulled ALL of these transistors and tested them. Of the TO-92's, all
test good except one of the a733's. My question is this: do I need to
replace BOTH a733's, i.e. to maintain matched set characteristics? I'd be
looking at an NTE290A as a replacement. Or are the c945/a733 some sort of
matched pair. I see that NTE offers either a) two matched 290A's or b)
matched sets of 289A/290A (289A is NOT listed as a replacement for
2sc945).

As I noted at the beginning, I'd like to understand which transistors
drive what if it's at all possible to determine from the information I've
provided. I can trace out the relationship between the transistors if
necessary but maybe the number, placement and type of units is enough.

Any replies greatly appreciated.

Dave

In article m1jbh.6192$dX4.81@clgrps13,
says...
Last week I posted about an Emerson Research "home theatre" system that I
was given. Two 5A fuses on the CT transformer secondaries blown. This unit
has outputs for two rear speakers (L/R), a center channel (DIN connector)
and two front satellites (L/R). Now the inputs are analog RCA audio so this
box must be just simulating 5.1 using a delay or some other trickery.

With only 3 amp channels, it probably just has a low pass filter to
drive the subwoofer.

What I'm looking for here is an understanding of how the various output
components interact... in particular how the NPN/PNP pairs function
snip

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | |
| |
c945 a733 c945 a733 2sc2245 2sa965

My guess, TIP41/42 are outputs, 2SC945/2SA733 are drivers connected
as a darlington configuration with the outputs, 2SC945 is bias
regulator connected between the bases of the drivers. 2SC2245/
2SA965 perform the same driver function but the subwoofer amp
runs at different supply voltages. No need for matching.

What you have drawn is just the current amplifying stages, there
are more transistors or IC's upstream that may or may not be
damaged. And there's a good chance you have some open emitter
resistors on the outputs.

"dave" nospam wrote in message
. ..
But I have to ask why. Emerson is junk. Not worth spending time and
money
on. Buy a new one.


Well I'd like to hook it up to my PC at work. I've got a pair of Polk Audio
satellites but they've got no bass, I mean NONE. I don't expect high
fidelity out of it, but it was entirely free and, given the fact that I'll
be listening at low volume, it's entirely adequate for my humble needs. So
far my investment is two 5A fuses, four power transistors and one driver
transistor, maybe $10 all together. Given that this "system" likely cost,
oh, maybe $80, if you'd like to chip in the extra $70 I'd be real happy to
"buy a new one".

That's why.

Dave

"Bill S." wrote in message
...
In article m1jbh.6192$dX4.81@clgrps13,
says...

With only 3 amp channels, it probably just has a low pass filter to
drive the subwoofer.


How can you output 2 x rear, 2 x front, 1 x ctr, 1 x subwoofer with ony 3
channels? I don't see how it could be done unless the two input channels
are amplified and sent to both front and rear outputs, PLUS are combined and
sent to center channel, leaving one amp channel left over to drive the
subwoofer on its' own...


My guess, TIP41/42 are outputs, 2SC945/2SA733 are drivers connected
as a darlington configuration with the outputs, 2SC945 is bias
regulator connected between the bases of the drivers. 2SC2245/
2SA965 perform the same driver function but the subwoofer amp
runs at different supply voltages. No need for matching.

What you have drawn is just the current amplifying stages, there
are more transistors or IC's upstream that may or may not be
damaged. And there's a good chance you have some open emitter
resistors on the outputs.

There are only 1 or 2 other transistors on the board but several IC's, a
couple of 14- or 16-pin and one bad-lookin' dog on the underside which looks
to be SMT, many pins. If the outputs were shorted, would this not smoke the
outputs, then the drivers, and so on? I tested the resistors to the bases
of the output transistors and all were ok, will look at the
emitter/collectors next.

Thanks a lot for your help, your reply as to the operation is exactly what I
was hoping for.

Dave

wrote in message
ups.com...
Dave wrote:


Little chance of needing matched pairs for anything. Transistors
distort so much they get lots of nfb applied to them, so it doesnt
matter whether theyre matched.


The closest thing I can find locally as a replacement is a 2SA817.

The original 2SA733 has I(c) of 150mA, power dissipation 0.25W and hfe of
between 90 and 600 (200 typical).

The 2SA817 has an I(c) of 500mA, power dissipation of 650mW and hfe of 70 to
240.

Is this close enough?

"Dave" wrote in message
news:5Snbh.7889$dX4.4025@clgrps13...


The 2SA817 has an I(c) of 500mA, power dissipation of 650mW and hfe of 70
to 240.

Sorry, the 2SA817 has an I(c) of 300mA, power dissipation of 600mW

"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |
|
c945 a733 c945 a733 2sc2245 2sa965
bad


Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put them in
I figured I'd power up the unit with no output devices installed. Checked
the bias voltage on the base of each output transistor position.

On the two pairs that are driven with C945/A733 pairs, there is 300-400mV DC
at the base. At the 2SC2245/2SA965 driven pair the I read a whopping
15.5VDC at the NPN and -15.5VDC at the PNP output base socket. This can't
be good. As the pair uses a common bias transistor, I replaced the bias
transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp. I've
made a schematic from the outputs back, but I don't have anything to compare
my values to because it's a completely different amp channel than the two
other channels which are identical.

Going backwards from outputs, I see outputs - drivers - dual op-amp IC.
Behind the op-amps are more op-amps (quad op-amp IC TL074CN), another
half-dozen NPN's (all c945's), two diode half-wave bridges, all kinds of
good stuff. Is there any hope at all, given that I don't have an identical
working channel to reference, of finding the source of this dc voltage? Or
should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning
exercise if there is a potential positive outcome (i.e. light at the end of
the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

Dave wrote:
"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |
|
c945 a733 c945 a733 2sc2245 2sa965
bad

Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put them in
I figured I'd power up the unit with no output devices installed. Checked
the bias voltage on the base of each output transistor position.

On the two pairs that are driven with C945/A733 pairs, there is 300-400mV DC
at the base. At the 2SC2245/2SA965 driven pair the I read a whopping
15.5VDC at the NPN and -15.5VDC at the PNP output base socket. This can't
be good. As the pair uses a common bias transistor, I replaced the bias
transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp. I've
made a schematic from the outputs back, but I don't have anything to compare
my values to because it's a completely different amp channel than the two
other channels which are identical.

Going backwards from outputs, I see outputs - drivers - dual op-amp IC.
Behind the op-amps are more op-amps (quad op-amp IC TL074CN), another
half-dozen NPN's (all c945's), two diode half-wave bridges, all kinds of
good stuff. Is there any hope at all, given that I don't have an identical
working channel to reference, of finding the source of this dc voltage? Or
should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning
exercise if there is a potential positive outcome (i.e. light at the end of
the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

If its all dc coupled youre most definitely asking for punishment. I'd
dump it and get some other chuck out to work one, one youve got a
decent chance of fixing.


NT

Dave wrote:
"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

blown pair blown pair good pair

----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |

c945 a733 c945 a733 2sc2245 2sa965
bad


Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put
them in I figured I'd power up the unit with no output devices
installed. Checked the bias voltage on the base of each output
transistor position.
On the two pairs that are driven with C945/A733 pairs, there is
300-400mV DC at the base. At the 2SC2245/2SA965 driven pair the I
read a whopping 15.5VDC at the NPN and -15.5VDC at the PNP output base
socket. This
can't be good. As the pair uses a common bias transistor, I replaced
the bias transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp. I've
made a schematic from the outputs back, but I don't have
anything to compare my values to because it's a completely different
amp channel than the two other channels which are identical.

Going backwards from outputs, I see outputs - drivers - dual op-amp
IC. Behind the op-amps are more op-amps (quad op-amp IC TL074CN),
another half-dozen NPN's (all c945's), two diode half-wave bridges,
all kinds of good stuff. Is there any hope at all, given that I
don't have an identical working channel to reference, of finding the
source of this dc voltage? Or should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning
exercise if there is a potential positive outcome (i.e. light at the
end of the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

Unfortunately, testing with no output transistors in place will give
erroneous readings. I even had an HK 570i receiver flame out on me when I
tried this back in the '80s.
The problem is that the base-emitter junction of each output transistor is
effectively a diode to the diving circuit, clamping at around .6 volt,
unless the outputs are Darlintons, of course...


Mark Z.

wrote:
Dave wrote:
"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put them in
I figured I'd power up the unit with no output devices installed. Checked
the bias voltage on the base of each output transistor position.

On the two pairs that are driven with C945/A733 pairs, there is 300-400mV DC
at the base. At the 2SC2245/2SA965 driven pair the I read a whopping
15.5VDC at the NPN and -15.5VDC at the PNP output base socket. This can't
be good. As the pair uses a common bias transistor, I replaced the bias
transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp. I've
made a schematic from the outputs back, but I don't have anything to compare
my values to because it's a completely different amp channel than the two
other channels which are identical.

Going backwards from outputs, I see outputs - drivers - dual op-amp IC.
Behind the op-amps are more op-amps (quad op-amp IC TL074CN), another
half-dozen NPN's (all c945's), two diode half-wave bridges, all kinds of
good stuff. Is there any hope at all, given that I don't have an identical
working channel to reference, of finding the source of this dc voltage? Or
should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning
exercise if there is a potential positive outcome (i.e. light at the end of
the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

If its all dc coupled youre most definitely asking for punishment. I'd
dump it and get some other chuck out to work one, one youve got a
decent chance of fixing.

then youve still got as set of output trannies yu can use in a project.
Not often I'd say forget it, but with dc amps I wouldnt bother.


NT

wrote in message
oups.com...
wrote:

then youve still got as set of output trannies yu can use in a project.
Not often I'd say forget it, but with dc amps I wouldnt bother.


NT

Yeah, that's about where I'm at with it. Given it was a cheap unit to start
with, it's not worth it... I just have a hard time letting go once I've sunk
my teeth into a project. However the board does have LOTS of useful
regulators and diodes on it, plus I recover a nice 5A CT transformer.

Thanks for the pragmatic advice.

Dave

Dave wrote:
wrote in message
oups.com...
wrote:

then youve still got as set of output trannies yu can use in a project.
Not often I'd say forget it, but with dc amps I wouldnt bother.

Yeah, that's about where I'm at with it. Given it was a cheap unit to start
with, it's not worth it... I just have a hard time letting go once I've sunk
my teeth into a project. However the board does have LOTS of useful
regulators and diodes on it, plus I recover a nice 5A CT transformer.

Thanks for the pragmatic advice.

Dave

Well, the good news is the sooner you get onto another piece of kit the
sooner you're going to have a result.
Plus you can use your TIPs to make a nice big power amp.
Coincidentally, you've even got the psu, case & preamp all already
there.


Mark Z wrote:

The problem is that the base-emitter junction of each output transistor is
effectively a diode to the diving circuit, clamping at around .6 volt,
unless the outputs are Darlintons, of course...

I reckon this one has horrors instead of darlings for the output


NT

wrote in message
ps.com...
Well, the good news is the sooner you get onto another piece of kit the
sooner you're going to have a result.
Plus you can use your TIPs to make a nice big power amp.
Coincidentally, you've even got the psu, case & preamp all already
there.

The pre-amp/amp are *heavily* integrated, same PSU. I'd have to look pretty
hard to find a point to break into the circuit at pre-out. I think I'll
just build myself a 20-150Hz variable low-pass filter and 50W push/pull amp
for the subwoofer with the TIPs. I can cut the board off (the PSU is well
isolated off by itself) and use the relatively large empty space for my new
amp. Conveniently, the 4-channel audio processor chip which provides input
switching btween three sets of input jacks (even a high-gain set!) are all
on the PSU end of the board.

I'm disappointed I can't get it working fully as it *potentially* does
exactly what I want.

Oh well, success is knowing when to cut your losses.

Dave

Mark Z wrote:

The problem is that the base-emitter junction of each output transistor is
effectively a diode to the diving circuit, clamping at around .6 volt,
unless the outputs are Darlintons, of course...

I reckon this one has horrors instead of darlings for the output


NT

Dave wrote:
wrote in message
ps.com...

Well, the good news is the sooner you get onto another piece of kit the
sooner you're going to have a result.
Plus you can use your TIPs to make a nice big power amp.
Coincidentally, you've even got the psu, case & preamp all already
there.

The pre-amp/amp are *heavily* integrated, same PSU. I'd have to look pretty
hard to find a point to break into the circuit at pre-out. I think I'll
just build myself a 20-150Hz variable low-pass filter and 50W push/pull amp
for the subwoofer with the TIPs. I can cut the board off (the PSU is well
isolated off by itself) and use the relatively large empty space for my new
amp. Conveniently, the 4-channel audio processor chip which provides input
switching btween three sets of input jacks (even a high-gain set!) are all
on the PSU end of the board.

I'm disappointed I can't get it working fully as it *potentially* does
exactly what I want.

IIUC all you've lost is your power amps, the rest works. I'm not clear
what you mean by pre and main being integrated, I've not seen anything
like that since the valve days. Same pcb sure, same psu of course, but
circuit intertwined? I cant think how it would be.

Simple no frills power amp modules are quite easy to make. Use a fast
opamp and you can do away with output bias circuits altogether. Forget
about all the nonessential points and you can get some very simple
power amps done. If you cant get them into the case any way, they can
always sit outside it on their own, using the main units psu.

Anyway, It ounds like you'll get something out of it one way or
another.


NT

wrote in message
ps.com...
Dave wrote:
wrote in message
ps.com...


IIUC all you've lost is your power amps, the rest works. I'm not clear
what you mean by pre and main being integrated, I've not seen anything
like that since the valve days. Same pcb sure, same psu of course, but
circuit intertwined? I cant think how it would be.

I'm not that clear on where the amp begins and where the pre-amp stops in
this case. It's not like an amp with discrete components where everything
after the tone/balance pots is amp and everything before is pre. I'd expect
at least four of the 12 op-amps on the board (2 x dual & 2 x quad) to be a
buffer/integrator for the subwoofer signal... I guess that's the equivilant
of a "pre-amp" for the subwoofer. Maybe four more (or less) to combine the
signal for the center channel output? I really know very little about
multi-channel (I mean more than two) systems and this one is a real deviant
because I think it's basically just some sort of delay circuit to simulate
surround sound from using only two analog R & L inputs.

If the amp is just the drivers and output transistors, and I've replaced the
outputs and a good portion of the drivers, and I've still got DC bias
problems, that would lead me to think that the source of the problem may
well be in the preamp UNLESS there is a coupling cap between the pre-amp and
amp stages. Hmm, if I can figure out where to make the split, maybe I can
add such a cap which would at least tell me on which side of the fence (pre
or main amp) the problem lies... ah, but I was going to dismantle it and use
the components... silly me.

Dave wrote:
wrote in message
ps.com...
Dave wrote:
wrote in message
ps.com...

IIUC all you've lost is your power amps, the rest works. I'm not clear
what you mean by pre and main being integrated, I've not seen anything
like that since the valve days. Same pcb sure, same psu of course, but
circuit intertwined? I cant think how it would be.

I'm not that clear on where the amp begins and where the pre-amp stops in
this case. It's not like an amp with discrete components where everything
after the tone/balance pots is amp and everything before is pre. I'd expect
at least four of the 12 op-amps on the board (2 x dual & 2 x quad) to be a
buffer/integrator for the subwoofer signal... I guess that's the equivilant
of a "pre-amp" for the subwoofer. Maybe four more (or less) to combine the
signal for the center channel output? I really know very little about
multi-channel (I mean more than two) systems and this one is a real deviant
because I think it's basically just some sort of delay circuit to simulate
surround sound from using only two analog R & L inputs.

If the amp is just the drivers and output transistors, and I've replaced the
outputs and a good portion of the drivers, and I've still got DC bias
problems, that would lead me to think that the source of the problem may
well be in the preamp UNLESS there is a coupling cap between the pre-amp and
amp stages. Hmm, if I can figure out where to make the split, maybe I can
add such a cap which would at least tell me on which side of the fence (pre
or main amp) the problem lies... ah, but I was going to dismantle it and use
the components... silly me.

right, it sounds like the problem is one of circuit tracing then. When
you use a capacitor coupled power amp(s), the dc conditions on its
input dont matter. So you can follow the signal back from the (now
blown) output tr pair using a probe thats a single opamp amplifier and
a 2" speaker until you get a nice clean souding signal, and take it
from there. Or, if it uses a pot for volume, you could tap 2 channel
putput straight off the volume pot.

Preamps routinely have dc bias on them, its not an issue.

If you probe around with something very low power, you wont scare your
ears or damage anything, and you should find where its all good. The
signal quality may not be good within some parts of the old dead power
amp, because the dead output trs will have made the feedback loop open
there. Just keep going back till its good and clean.


NT

wrote in message
ps.com...

If you probe around with something very low power, you wont scare your
ears or damage anything, and you should find where its all good. The
signal quality may not be good within some parts of the old dead power
amp, because the dead output trs will have made the feedback loop open
there. Just keep going back till its good and clean.


NT

Well, turns out it IS as simple as discrete components: the pre-amp ends
just past the tone/balance *IC*, and the power amp starts there. I made a
probe out of the inputs to a disposable powered computer speaker.

Thanks for your insight.

Dave

wrote:
wrote:
Dave wrote:
"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I
put them in I figured I'd power up the unit with no output devices
installed. Checked the bias voltage on the base of each output
transistor position.

On the two pairs that are driven with C945/A733 pairs, there is
300-400mV DC at the base. At the 2SC2245/2SA965 driven pair the I
read a whopping
15.5VDC at the NPN and -15.5VDC at the PNP output base socket.
This can't be good. As the pair uses a common bias transistor, I
replaced the bias transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp.
I've made a schematic from the outputs back, but I don't have
anything to compare my values to because it's a completely
different amp channel than the two other channels which are
identical.

Going backwards from outputs, I see outputs - drivers - dual
op-amp IC. Behind the op-amps are more op-amps (quad op-amp IC
TL074CN), another half-dozen NPN's (all c945's), two diode
half-wave bridges, all kinds of good stuff. Is there any hope at
all, given that I don't have an identical working channel to
reference, of finding the source of this dc voltage? Or should I
just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a
learning exercise if there is a potential positive outcome (i.e.
light at the end of the tunnel). But, maybe I'm just asking for
punishment...

Any and all responses appreciated

Dave

If its all dc coupled youre most definitely asking for punishment.
I'd dump it and get some other chuck out to work one, one youve got a
decent chance of fixing.

then youve still got as set of output trannies yu can use in a
project. Not often I'd say forget it, but with dc amps I wouldnt
bother.


NT

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so are
DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all right,
but I'm not at all sure I'd classify it as such a difficult repair simply
because it may be a "DC" amp. As I say, I fix such amps all the time. No
problem.

Mark Z.

Mark D. Zacharias wrote:
wrote:

then youve still got as set of output trannies yu can use in a
project. Not often I'd say forget it, but with dc amps I wouldnt
bother.

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so are
DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all right,
but I'm not at all sure I'd classify it as such a difficult repair simply
because it may be a "DC" amp. As I say, I fix such amps all the time. No
problem.

Mark Z.

which puts you in a very different position to the OP.


NT

"Mark D. Zacharias" wrote in message
t...
wrote:
wrote:
Dave wrote:

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so
are DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all
right, but I'm not at all sure I'd classify it as such a difficult repair
simply because it may be a "DC" amp. As I say, I fix such amps all the
time. No problem.

Mark Z.

As I noted in my post, I'm willing to spend some time on this and would love
to learn something.

I've checked all of the driver transistors inasmuch as is possible using the
diode check function of a DMM. The ones that didn't pass were replaced.
I've checked all of the diodes and resistors in-circuit, everything passed,
resistors all "in the zone", i.e. within 40% of their stated values. My
multimeter also has a transistor check but it's pretty flaky especially with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

I can power up the amp with output transistors removed, as soon as they go
back in the two in-line 5A fuses on the transformer secondaries blow. there
are no dead shorts across the outputs. I get proper voltages at the inputs
to my IC's (really the only values I can check as I have no service manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

The new output transistors are NOT fried, they're rated at 10A peak and the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a few
times. ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

I'm going to do something I should have done a few posts ago... install the
outputs one pair at a time which will at least localize my problem to one of
the three channels.

I know there's no substitute for experience, but does anyone know if any
books or online publications that cover DC coupled (audio in particular)
amplifiers and the troubleshooting thereof? I know you're all busy and
probably tend to lose interest when you see posts and think "this guy ain't
never gonna' fix this" but I'm a relatively smart guy with a couple of years
of Electrical Engineering schooling (although that was more than 20 years
ago we were still in the transistor age).

Dave

Dave wrote:

As I noted in my post, I'm willing to spend some time on this and would love
to learn something.

I've checked all of the driver transistors inasmuch as is possible using the
diode check function of a DMM. The ones that didn't pass were replaced.
I've checked all of the diodes and resistors in-circuit, everything passed,
resistors all "in the zone", i.e. within 40% of their stated values.

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.

My
multimeter also has a transistor check but it's pretty flaky especially with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

then why do you say those trs are bad?

I can power up the amp with output transistors removed, as soon as they go
back in the two in-line 5A fuses on the transformer secondaries blow. there
are no dead shorts across the outputs. I get proper voltages at the inputs
to my IC's (really the only values I can check as I have no service manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

a fault of any kind anywhere in the power amp that results in the
output trs being turned on.


The new output transistors are NOT fried, they're rated at 10A peak and the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a few
times.

very unlikely. If they handle it once you've been lucky.

ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

so its your output trs that short. Or the speaker wiring.

I'm going to do something I should have done a few posts ago... install the
outputs one pair at a time which will at least localize my problem to one of
the three channels.

I know there's no substitute for experience, but does anyone know if any
books or online publications that cover DC coupled (audio in particular)
amplifiers and the troubleshooting thereof? I know you're all busy and
probably tend to lose interest when you see posts and think "this guy ain't
never gonna' fix this" but I'm a relatively smart guy with a couple of years
of Electrical Engineering schooling

OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.

Base R values will depend on hfe of the TIPs.

Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.


NT


(although that was more than 20 years
ago we were still in the transistor age).

Dave

wrote in message
ps.com...

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.


I mean that when I test them in circuit I never get the exact correct value.
I figured that if I was looking for a gross over-current-causing resistor
fault, it'd likely be open or shorted. or close.

My
multimeter also has a transistor check but it's pretty flaky especially
with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

then why do you say those trs are bad?

Because they don't work in circuit. Case in point I replaced a PS
transistor in a tuner awhile back which was putting out completely
out-to-lunch voltages (I had a circuit diagram for this one). Problem
solved. Stuck transistor into my DMM transistor tester and it gave it an
hfe which was in the range of what the data sheet might lead one to expect.


I can power up the amp with output transistors removed, as soon as they
go
back in the two in-line 5A fuses on the transformer secondaries blow.
there
are no dead shorts across the outputs. I get proper voltages at the
inputs
to my IC's (really the only values I can check as I have no service
manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

a fault of any kind anywhere in the power amp that results in the
output trs being turned on.


The new output transistors are NOT fried, they're rated at 10A peak and
the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a
few
times.

very unlikely. If they handle it once you've been lucky.

These are 6A continuous rated. Why shouldn't they be protected behind a 5A
fast-blow fuse? Or isn't "fast-blow" fast enough?

Damn, probably used up a bunch of luck on those trannies, then. I have a
feeling I still am going to be in need of some down the road on this one...

ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the
fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

so its your output trs that short. Or the speaker wiring.


The amp was powered up with no load (no speakers connected). So unless I
got some bad brand-new TIP's I don't know how they could be shorting.


OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.


Can I cheat and use a 25W light bulb?

Base R values will depend on hfe of the TIPs.

hfe at 3A is

min=15
max=75.

Do I use avg=45? 1.7A / 45 = 38mA, 15V / 0.038A = 400 ohm 1/2W.


Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.


I've got it mostly drawn out... another hour or two and I should be there
with a schematic.

Dave wrote:
wrote in message
ps.com...

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.

I mean that when I test them in circuit I never get the exact correct value.
I figured that if I was looking for a gross over-current-causing resistor
fault, it'd likely be open or shorted. or close.

measuring them in circuit is going to give you all sorts of out there
readings. And resistors dont normally short, so I'm not sure if you'll
find much that way.


My
multimeter also has a transistor check but it's pretty flaky especially
with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

then why do you say those trs are bad?

Because they don't work in circuit. Case in point I replaced a PS
transistor in a tuner awhile back which was putting out completely
out-to-lunch voltages (I had a circuit diagram for this one). Problem
solved. Stuck transistor into my DMM transistor tester and it gave it an
hfe which was in the range of what the data sheet might lead one to expect.

perhaps the meter just reads dc collector current, and on faulty goods
its reading leakage? Try measuring a resistor from C to E connections
on the meter, see if it gives an hfe reading.


These are 6A continuous rated. Why shouldn't they be protected behind a 5A
fast-blow fuse? Or isn't "fast-blow" fast enough?

Everyone asks this one.
1. Fuses dont limit current, so way above 5A flows when a 5A fuse blows
to clear a short
2. Fast fuses are very slow compared to what a tr can survive. Fuses
can not protect trs to a significant extent.
3. Fuses dont blow at 5A, 5A is the current they'll pass for decades.
Blowing one takes much more.

Look up fuse curves to see what it takes to pop one.


Almost makes you think there's a short before the amp section if the
fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

so its your output trs that short. Or the speaker wiring.

The amp was powered up with no load (no speakers connected). So unless I
got some bad brand-new TIP's I don't know how they could be shorting.

Yabbut theres wiring from the pcb to the external connectors, plus
maybe a headphone jack etc. Remove the audio output connection at the
pcb, if convenient, to isolate all that crp.


OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.

Can I cheat and use a 25W light bulb?

Filament bulbs have a power on surge of 8-10x run current, their cold
resistance is about a tenth of hot resistance. If you take this into
account you can use a bulb with the right cold resistance. 10 ohms cold
is 100 ohm hot. If you're on 110v thats a 100watt bulb.


Base R values will depend on hfe of the TIPs.

hfe at 3A is

min=15
max=75.

Do I use avg=45? 1.7A / 45 = 38mA, 15V / 0.038A = 400 ohm 1/2W.

If you use average hfe, half your trs wont get enough base drive to
deliver the required collector i. So use min hfe of 15. If you want to
run 1.5A thru collector, you need at least 1.5/15 = 0.1A into the base.


Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.

I've got it mostly drawn out... another hour or two and I should be there
with a schematic.

cool


NT

Dave wrote:
"Mark D. Zacharias" wrote in message
t...
wrote:
wrote:
Dave wrote:

Practically all the amps I work on are dc coupled. Not a problem in
most cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970
or so are DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all
right, but I'm not at all sure I'd classify it as such a difficult
repair simply because it may be a "DC" amp. As I say, I fix such
amps all the time. No problem.

Mark Z.

As I noted in my post, I'm willing to spend some time on this and
would love to learn something.

I've checked all of the driver transistors inasmuch as is possible
using the diode check function of a DMM. The ones that didn't pass
were replaced. I've checked all of the diodes and resistors
in-circuit, everything passed, resistors all "in the zone", i.e.
within 40% of their stated values. My multimeter also has a
transistor check but it's pretty flaky especially with small
transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.
I can power up the amp with output transistors removed, as soon as
they go back in the two in-line 5A fuses on the transformer
secondaries blow. there are no dead shorts across the outputs. I
get proper voltages at the inputs to my IC's (really the only values
I can check as I have no service manual or schematic), voltage
regulators are working at +15VDC/-15VDC/-6VDC.
what could cause this overcurrent situation?

The new output transistors are NOT fried, they're rated at 10A peak
and the new fuses are fast-blo at 5A. I'm sure they don't LIKE a big
pulse of instantaneous current but they're brand new and likely can
handle it a few times. ALL but two (drivers) of the transistors in
the amp circuits are 2SC945/2A733's which should vaporize with
anywhere near 5A of current.. theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the
fuses blow but not the drivers/outputs... but the thing runs fine
with outputs transistors removed.

I'm going to do something I should have done a few posts ago...
install the outputs one pair at a time which will at least localize
my problem to one of the three channels.

I know there's no substitute for experience, but does anyone know if
any books or online publications that cover DC coupled (audio in
particular) amplifiers and the troubleshooting thereof? I know
you're all busy and probably tend to lose interest when you see posts
and think "this guy ain't never gonna' fix this" but I'm a relatively
smart guy with a couple of years of Electrical Engineering schooling
(although that was more than 20 years ago we were still in the
transistor age).
Dave

This is basic and may have already been covered, but DO check the DC
resistance between the collector of the outputs and the heat sink. Should be
open-circuit, an insulator could be damaged or missing.
Also check resistance between each emitter resistor and both the + and -
supplies. Should not be a short - should show open or cap charging. If you
show a short here, then an output transistor is probably bad, despite
earlier testing.

The hFe showing on a leaky transistor is common with these checkers. Lift
the base lead. If the transistor still conducts with only the E-C connected,
it's bad.

Mark Z.