Pioneer M-790 amp adjustment

[James Sweet]

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.

 

[Mark D. Zacharias]

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.

 

[Asimov]

"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" <[email protected]>
JS> From: "James Sweet" <[email protected]>

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]

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.

 

[James Sweet]

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!

 

[Mark D. Zacharias]

Comments inserted.

it


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.

 

[sofie]

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.

 

[James Sweet]

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
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Well I'll give this ridiculous contraption a shot, and if it blows up again
it's off to the recycler.

I'm gonna bypass that "protection" circuit for good and add some fuses
instead, no way it's gonna burn the house down.

 

[Mark D. Zacharias]

I still make good money on these - I just educate the customers WITHOUT FAIL
regarding speaker wires etc. Re-do's haven't been a problem for this reason.

The "prop" I use is a AA battery and a small speaker with wire. I
demonstrate how the condition of the wire should be, then "POP" the speaker
with the battery. Have the customer check theirs likewise, and if they hear
no pop, don't hook up that speaker / wire. You can almost see the light come
on above their heads. They really seem to get it for a change.

Mark Z.

 

[Mark D. Zacharias]

James,

You can send the module to me for repair if you like - I have an M-4000
"test bed" which uses the same module.

Mark Z.

 

[Mark D. Zacharias]

James,

I really recommend NOT leaving the blow function disabled. You're just
asking for trouble. Lots of customers, when the 8A fuse blows, just keep
replacing it as if that will fix it, and when that doesn't work, even try
bigger fuses. I know it's tempting but don't do it. Bad idea.

Mark Z.

 

[Asimov]

"sofie" bravely wrote to "All" (31 May 04 19:41:45)
--- on the heady topic of "Re: Pioneer M-790 amp adjustment"

Power amps often require a lot of patience in testing and therefor
take "a lot of time" to do right. Otherwise one ends up with a pile of
expensive burnt replacements and have to start all over again. As far
as I'm concerned if a consumer amp can't play into any kind of load,
including a short circuit, without damage then its design is flawed.
Modules are very bad for this. Fusing their output might help.

so> From: "sofie" <[email protected]>

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

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

 

[Sofie]

Asimov:
Only very, very, very, very few amplifier designs can play into a
short-circuit without damage. Even expensive and well designed power amps
can have catastrophic failures when loaded into a short-circuit.
As you know, fusing the speaker outputs will help protect the speakers from
the dc rails if the output stages short out, but the amp will be already
fried.
It is my frequent observation that the Pioneer design that we are discussing
is not tolerant of much at all and is prone to failure when other amps keep
on working. It is my opinion that putting expensive repair money into a
flawed design is a waste and would be better put to use in purchasing a
decent Yamaha or similar receiver.

 

[James Sweet]

James,

I really recommend NOT leaving the blow function disabled. You're just
asking for trouble. Lots of customers, when the 8A fuse blows, just keep
replacing it as if that will fix it, and when that doesn't work, even try
bigger fuses. I know it's tempting but don't do it. Bad idea.

Mark Z.

Well the owner of this amp is one of my friends and he knows better, if he
fries something that's his problem as this is the first and last time I'm
repairing this particular unit.

 

[Asimov]

"Sofie" bravely wrote to "All" (01 Jun 04 15:23:29)
--- on the heady topic of "Re: Pioneer M-790 amp adjustment"

So> From: "Sofie" <[email protected]>

So> Asimov:
So> Only very, very, very, very few amplifier designs can play into a
So> short-circuit without damage. Even expensive and well designed power
So> amps can have catastrophic failures when loaded into a short-circuit.
So> As you know, fusing the speaker outputs will help protect the speakers
So> from the dc rails if the output stages short out, but the amp will be
So> already fried.

I don't disagree but I was just making a case for Utopia. The most
common mistake to be found in amp designs is that there should always be
some sort of limiting resistor on the base drive to limit runaway Icbo
when the collector to base junction breaks down. If currents are limited
when a transistor is driven into thermal breakdown it can often recover
after cooling down with perhaps only a small reduction in Hfe. I have an
early 60's amplifier with Ge transistor outputs that when overheated
will do this. The first time it happened I thought I had fried it good
but it had only blown the line fuse. It didn't even kill the rectifiers.


So> It is my frequent observation that the Pioneer design that we are
So> discussing is not tolerant of much at all and is prone to failure when
So> other amps keep on working.

If it was for myself I would modify it to correct the problem for
example as was suggested or perhaps further mods but only if it was
worth it performance wise. Many consumer amps are too noisy even with
moderately sensitive speakers. I try for 20db spl or less noise floor.
Basically I like near silence when I stick my ear to the speakers.


So> It is my opinion that putting expensive
So> repair money into a flawed design is a waste and would be better put to
So> use in purchasing a decent Yamaha or similar receiver.

Yamaha equipment is better indeed but not perfect as they often have
minor irritating problems. Their customer relations is great though.

"sofie" bravely wrote to "All" (31 May 04 19:41:45)
--- on the heady topic of "Re: Pioneer M-790 amp adjustment"

Power amps often require a lot of patience in testing and therefor
take "a lot of time" to do right. Otherwise one ends up with a pile of
expensive burnt replacements and have to start all over again. As far
as I'm concerned if a consumer amp can't play into any kind of load,
including a short circuit, without damage then its design is flawed.
Modules are very bad for this. Fusing their output might help.

.... Batteries not included.

 

[jakdedert]

Asimov:
Only very, very, very, very few amplifier designs can play into a
short-circuit without damage. Even expensive and well designed power
amps can have catastrophic failures when loaded into a short-circuit.
As you know, fusing the speaker outputs will help protect the
speakers from the dc rails if the output stages short out, but the
amp will be already fried.
It is my frequent observation that the Pioneer design that we are
discussing is not tolerant of much at all and is prone to failure
when other amps keep on working. It is my opinion that putting
expensive repair money into a flawed design is a waste and would be
better put to use in purchasing a decent Yamaha or similar receiver.

<snip>

Granted the above, but I remember a review of the Crown DC-300 in the
70's...quote as nearly as I recall was "...a screwdriver across the output
binding posts at full power elicited some pretty impressive sparks, but no
damage..."

They don't make 'em like the used to.

jak

 

[Leonard Caillouet]

Sofie is 100% correct. With respect to the DC300, it was reputed to be able
to "drive a nail", and was a very bulletproof, or should I say idiotproof
amp. Having used many of them under many varying load conditions, its
reputation was well deserved, but even the DC300 could not tolerate a dead
short for more than an instant, if that. And if you did blow it up it could
be a bear to fix.

The pioneer design discussed here leaves much to be desired in terms of
reliability, repairability, stability, and durability. We see units
frequently that have been blown by users shorting the output leads and also
that seem to fail under normal use. I tend to agree that repairing them is
often not a good choice.

Leonard

 

[Hubert Littau]

"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" <[email protected]>
JS> From: "James Sweet" <[email protected]>

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.

 

[Wild Bill]

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]

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.