I've got a Pioneer M-790 here with all 8 power output transistors blown. I
found a number of cracked solder joints, is there anything else I should
check on this particular amp before I install new transistors and try it
out? Also how do I adjust the bias? I only see one pot in the whole amp.

There's a long history on these "blow line" models. Most likely only the
outputs and some 220 ohm emitter-base resistors are bad. You are aware, I
hope, of the so-called "protection" circuit on these, which INTENTIONALLY
blows the outputs if any fault is detected?

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"James Sweet" wrote in message
newsQAuc.22391$4A6.20874@attbi_s52...
I've got a Pioneer M-790 here with all 8 power output transistors blown. I
found a number of cracked solder joints, is there anything else I should
check on this particular amp before I install new transistors and try it
out? Also how do I adjust the bias? I only see one pot in the whole amp.

"James Sweet" bravely wrote to "All" (31 May 04 07:07:00)
--- on the heady topic of "Pioneer M-790 amp adjustment"

JS Reply-To: "James Sweet"
JS From: "James Sweet"

JS I've got a Pioneer M-790 here with all 8 power output transistors
JS blown. I found a number of cracked solder joints, is there anything
JS else I should check on this particular amp before I install new
JS transistors and try it out? Also how do I adjust the bias? I only see
JS one pot in the whole amp.

Yes, check the bias circuit "very" carefully and use resistors to
limit current in the power transistors during testing. The value of
these protection resistors depends on the supply and current but for
say 40 volts with 100mA, something like 100 to 330 ohms at 7W+.
Determine what is a safe value for this amp but it is very advisable
to use them for initial testing just in case.

Bias is often permanently set in some models and thus one must
use oem matched transistors for these or they won't track thermally.
The single pot is likely to adjust a regulated supply voltage: don't
touch it. Do some tracing starting from the wiper and determine its
function.

Be very careful to check the driver circuits as well because when the
output transistors go they tend to inject a heavy current back there.
It's like a chain reaction which propagates backwards from the output.

Well, this is probably not enough to help but I do wish you good luck!

A*s*i*m*o*v

.... A fail-safe circuit will destroy others.

The bias is set by soldering together or unsoldering specific adjacent foil
pads near the bias transistors. No practical way of telling you which to use
via these postings. For each bias transistor there is one pad pair which is
soldered to increase bias, and one to decrease bias. If both were soldered,
there would effectively be no change in bias.

There are surface-mount transistors which can be shorted emitter-base to
defeat the "blow" function. Assuming there is no other problems, the amp
would fire up OK after replacing the outputs and 220 ohm resistors.
The Blow line must measure less than 100mV under all conditions before it is
safe to unsolder the surface-mount transistors mentioned above.

ALWAYS bleed off the power supplies - all 4 of them - before any soldering
etc. These supplies do not go down or bleed off when the power is removed,
not even when unplugged.

Post back if your still into doing this one, and I can get more info - I've
done lots of these.

Mark Z.

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Asimov" wrote in message
...
"James Sweet" bravely wrote to "All" (31 May 04 07:07:00)
--- on the heady topic of "Pioneer M-790 amp adjustment"

JS Reply-To: "James Sweet"
JS From: "James Sweet"

JS I've got a Pioneer M-790 here with all 8 power output transistors
JS blown. I found a number of cracked solder joints, is there anything
JS else I should check on this particular amp before I install new
JS transistors and try it out? Also how do I adjust the bias? I only see
JS one pot in the whole amp.

Yes, check the bias circuit "very" carefully and use resistors to
limit current in the power transistors during testing. The value of
these protection resistors depends on the supply and current but for
say 40 volts with 100mA, something like 100 to 330 ohms at 7W+.
Determine what is a safe value for this amp but it is very advisable
to use them for initial testing just in case.

Bias is often permanently set in some models and thus one must
use oem matched transistors for these or they won't track thermally.
The single pot is likely to adjust a regulated supply voltage: don't
touch it. Do some tracing starting from the wiper and determine its
function.

Be very careful to check the driver circuits as well because when the
output transistors go they tend to inject a heavy current back there.
It's like a chain reaction which propagates backwards from the output.

Well, this is probably not enough to help but I do wish you good luck!

A*s*i*m*o*v

... A fail-safe circuit will destroy others.

"Mark D. Zacharias" wrote in message
...
The bias is set by soldering together or unsoldering specific adjacent
foil
pads near the bias transistors. No practical way of telling you which to
use
via these postings. For each bias transistor there is one pad pair which
is
soldered to increase bias, and one to decrease bias. If both were
soldered,
there would effectively be no change in bias.

There are surface-mount transistors which can be shorted emitter-base to
defeat the "blow" function. Assuming there is no other problems, the amp
would fire up OK after replacing the outputs and 220 ohm resistors.
The Blow line must measure less than 100mV under all conditions before it
is
safe to unsolder the surface-mount transistors mentioned above.

ALWAYS bleed off the power supplies - all 4 of them - before any soldering
etc. These supplies do not go down or bleed off when the power is removed,
not even when unplugged.

Post back if your still into doing this one, and I can get more info -
I've
done lots of these.

Mark Z.



Oh joy, if I'd known how much hassle this would be I wouldn't have ordered
parts, oh well. Now that I'm into it I may as well see if I can finish. So
let me get this straight, the "protection" circuit *intentionally* blows the
output transistors?! What the hell were they thinking??

Where are the 220 ohm resistors? I was looking casually for the usual
largish resistors but I didn't see them. What's the general procedure for
setting the bias? The only amps I've ever done it on I had a listing of
exactly what current to adjust for and a pot which made it easy. This is a
rather unusual amp, please fill me in with any info you can. Thanks!

Comments inserted.

"James Sweet" wrote in message
news:R_Nuc.33672$n_6.12557@attbi_s53...

"Mark D. Zacharias" wrote in message
...
The bias is set by soldering together or unsoldering specific adjacent
foil
pads near the bias transistors. No practical way of telling you which to
use
via these postings. For each bias transistor there is one pad pair which
is
soldered to increase bias, and one to decrease bias. If both were
soldered,
there would effectively be no change in bias.

There are surface-mount transistors which can be shorted emitter-base to
defeat the "blow" function. Assuming there is no other problems, the amp
would fire up OK after replacing the outputs and 220 ohm resistors.
The Blow line must measure less than 100mV under all conditions before
it
is
safe to unsolder the surface-mount transistors mentioned above.

ALWAYS bleed off the power supplies - all 4 of them - before any
soldering
etc. These supplies do not go down or bleed off when the power is
removed,
not even when unplugged.

Post back if your still into doing this one, and I can get more info -
I've
done lots of these.

Mark Z.



Oh joy, if I'd known how much hassle this would be I wouldn't have ordered
parts, oh well. Now that I'm into it I may as well see if I can finish. So
let me get this straight, the "protection" circuit *intentionally* blows
the
output transistors?! What the hell were they thinking??

Yeah - their engineers should have gone to jail for this one. The
"protection" circuit monitors the output of a voltage divider, the output of
which is zero under normal circumstances. If the trigger line goes over
about 100 mV, a destructive bias voltage is applied. Just to save the cost
of a relay. In their all-in-one units, like the RX-570 and RX 590, etc, even
a bad cassette motor can blow the outputs.




Where are the 220 ohm resistors? I was looking casually for the usual
largish resistors but I didn't see them.

They are surface-mount types near each output transistor. You can measure
across the E-B connection after the shorted transistors are removed. One or
two will probably be good, a couple probably bad. If the resistor is bad
when the amp's power is applied, the amp will blow again.

The transistors involved in defeating the blow line are Q7401 and Q7406.
They are surface-mount types, and they are pretty hard to find, but once you
do, you can solder across the E-B junction of each one to defeat the BLOW
function. After the amp is repaired you must remove the short and restore
normal operation of this circuit. Otherwise someone could come along later
and sue your ass if the thing burns their house down, which is quite
conceivable on one of these if they fail and there's no "protection".

Pioneer says NOT to use a variac, use a 100W light bulb across the main fuse
instead. If the lamp glows bright for more than a split-second, cut power
immediately.
The way it should work if the amp is OK - the lamp glows for an instant as
the main power supply caps charge, then goes out, then may glow dimly
beginning a few seconds after that. I believe the blow line is monitored at
pin 6 of the connector at the end of the longer board. Should read less than
100 mV with respect to ground. In a borderline situation, say 90 mV, a 15K
resistor may be added from BLOW to ground to reduce the level a bit without
defeating the circuit.

What's the general procedure for
setting the bias?

This usually isn't necessary if the amp is OK, but there is NO emitter
resistors to measure across, so you theoretically would have to break open a
B+ line to monitor current. I haven't done this, just looked at the schem
and eventually decided which pads to solder together to reduce bias and make
it run cooler. As I said, this isn't usually necessary if the other stuff is
OK.


Mark Z.

Mark, James, & Asimov:
Quite frankly when I get these models in my shop with blown outputs I
usually recommend that the unit is not economically repairable.
Proper component level repairs are not only very time consuming and are
prone to "blowing" again for a variety of malfunctions .... also replacement
power output modules are becoming quite scarce and again can be prone to
blowing because of the very poor design..
In my opinion this is obviously not a shining example of Pioneer's
engineering and manufacturing prowess...... gone are the days of the very
well designed grand Pioneer "super" receivers of the 70's and 80's like the
SX 1250 and other fine pieces.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


"Mark D. Zacharias" wrote in message
...
Comments inserted.

"James Sweet" wrote in message
news:R_Nuc.33672$n_6.12557@attbi_s53...

"Mark D. Zacharias" wrote in message
...
The bias is set by soldering together or unsoldering specific adjacent
foil
pads near the bias transistors. No practical way of telling you which
to
use
via these postings. For each bias transistor there is one pad pair
which
is
soldered to increase bias, and one to decrease bias. If both were
soldered,
there would effectively be no change in bias.

There are surface-mount transistors which can be shorted emitter-base
to
defeat the "blow" function. Assuming there is no other problems, the
amp
would fire up OK after replacing the outputs and 220 ohm resistors.
The Blow line must measure less than 100mV under all conditions before
it
is
safe to unsolder the surface-mount transistors mentioned above.

ALWAYS bleed off the power supplies - all 4 of them - before any
soldering
etc. These supplies do not go down or bleed off when the power is
removed,
not even when unplugged.

Post back if your still into doing this one, and I can get more info -
I've
done lots of these.

Mark Z.



Oh joy, if I'd known how much hassle this would be I wouldn't have
ordered
parts, oh well. Now that I'm into it I may as well see if I can finish.
So
let me get this straight, the "protection" circuit *intentionally* blows
the
output transistors?! What the hell were they thinking??

Yeah - their engineers should have gone to jail for this one. The
"protection" circuit monitors the output of a voltage divider, the output
of
which is zero under normal circumstances. If the trigger line goes over
about 100 mV, a destructive bias voltage is applied. Just to save the cost
of a relay. In their all-in-one units, like the RX-570 and RX 590, etc,
even
a bad cassette motor can blow the outputs.




Where are the 220 ohm resistors? I was looking casually for the usual
largish resistors but I didn't see them.

They are surface-mount types near each output transistor. You can measure
across the E-B connection after the shorted transistors are removed. One
or
two will probably be good, a couple probably bad. If the resistor is bad
when the amp's power is applied, the amp will blow again.

The transistors involved in defeating the blow line are Q7401 and Q7406.
They are surface-mount types, and they are pretty hard to find, but once
you
do, you can solder across the E-B junction of each one to defeat the BLOW
function. After the amp is repaired you must remove the short and restore
normal operation of this circuit. Otherwise someone could come along later
and sue your ass if the thing burns their house down, which is quite
conceivable on one of these if they fail and there's no "protection".

Pioneer says NOT to use a variac, use a 100W light bulb across the main
fuse
instead. If the lamp glows bright for more than a split-second, cut power
immediately.
The way it should work if the amp is OK - the lamp glows for an instant as
the main power supply caps charge, then goes out, then may glow dimly
beginning a few seconds after that. I believe the blow line is monitored
at
pin 6 of the connector at the end of the longer board. Should read less
than
100 mV with respect to ground. In a borderline situation, say 90 mV, a 15K
resistor may be added from BLOW to ground to reduce the level a bit
without
defeating the circuit.

What's the general procedure for
setting the bias?

This usually isn't necessary if the amp is OK, but there is NO emitter
resistors to measure across, so you theoretically would have to break open
a
B+ line to monitor current. I haven't done this, just looked at the schem
and eventually decided which pads to solder together to reduce bias and
make
it run cooler. As I said, this isn't usually necessary if the other stuff
is
OK.


Mark Z.

On Monday, 31 May 2004 10:32:42 -500, "Asimov"
wrote:

"James Sweet" bravely wrote to "All" (31 May 04 07:07:00)
--- on the heady topic of "Pioneer M-790 amp adjustment"

JS Reply-To: "James Sweet"
JS From: "James Sweet"

JS I've got a Pioneer M-790 here with all 8 power output transistors
JS blown. I found a number of cracked solder joints, is there anything
JS else I should check on this particular amp before I install new
JS transistors and try it out? Also how do I adjust the bias? I only see
JS one pot in the whole amp.

Yes, check the bias circuit "very" carefully and use resistors to
limit current in the power transistors during testing. The value of
these protection resistors depends on the supply and current but for
say 40 volts with 100mA, something like 100 to 330 ohms at 7W+.
Determine what is a safe value for this amp but it is very advisable
to use them for initial testing just in case.

Bias is often permanently set in some models and thus one must
use oem matched transistors for these or they won't track thermally.
The single pot is likely to adjust a regulated supply voltage: don't
touch it. Do some tracing starting from the wiper and determine its
function.

Be very careful to check the driver circuits as well because when the
output transistors go they tend to inject a heavy current back there.
It's like a chain reaction which propagates backwards from the output.

Well, this is probably not enough to help but I do wish you good luck!

A*s*i*m*o*v

... A fail-safe circuit will destroy others.

When these came onto the market the service rep reccomended strongly
that you use a 60 watt lightbulb in line with the ac for testing, and
definatelt NOT a variac.
The light bulb will burn bright at turn on then settle to about half
at idle or low volume. A constant bright lite after repair usually
means you just saved a bunch of money. Go back and fix the cause.

Hi Jim, how are you doing with that amp?

I have a similar model sitting on the shelf that I was going to TRY to fix a
couple of years ago, and haven't gotten back to it.. yet, heheh.
I did get a complete new power section for it.

The service manual suggests putting some power resistors in the circuits
temporarily to check the bias. If you'd want more info, I'll look it up. I'm
sure Mark's steering you in the right direction, since he's repaired quite a
few of 'em.

Cheers
WB
...............

"James Sweet" wrote in message
newsQAuc.22391$4A6.20874@attbi_s52...
I've got a Pioneer M-790 here with all 8 power output transistors blown. I
found a number of cracked solder joints, is there anything else I should
check on this particular amp before I install new transistors and try it
out? Also how do I adjust the bias? I only see one pot in the whole amp.

"Wild Bill" wrote in message
...
Hi Jim, how are you doing with that amp?

I have a similar model sitting on the shelf that I was going to TRY to fix
a
couple of years ago, and haven't gotten back to it.. yet, heheh.
I did get a complete new power section for it.

The service manual suggests putting some power resistors in the circuits
temporarily to check the bias. If you'd want more info, I'll look it up.
I'm
sure Mark's steering you in the right direction, since he's repaired quite
a
few of 'em.

Cheers
WB
..............


I just got the parts the other day, haven't had a chance to look at it yet.