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

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

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

"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

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.

"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

"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

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:
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:
"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.