Onkyo TX-82 stereo receiver fault. Suggest possible solutions...

[[email protected]]

The manual has been uploaded to you.

However, it has been sent using a different e-mail address than
"[email protected]."

 

[[email protected]]

Forgot to follow the directions on your sig regarding sending a
message to your e-mail.

The PDF has been re-sent.

 

[Franc Zabkar]

Go ahead with whatever you'd like to do with the file.

Here it is:
http://www.eserviceinfo.com/download.php?fileid=37140

Page 25 appears to be the relevant section.

- Franc Zabkar

 

[[email protected]]

Here it is:http://www.eserviceinfo.com/download.php?fileid=37140

Page 25 appears to be the relevant section.

- Franc Zabkar

Here's where it gets hard for me (and shows how much of a noob I am, I
guess). I can't comprehend the schematic diagram.

I mean, I know what some of the symbols mean, but I can't totally
fathom their interaction in the circuit.

 

[Franc Zabkar]

Here it is:
http://www.eserviceinfo.com/download.php?fileid=37140

Page 25 appears to be the relevant section.

It appears that the protection circuitry is in two parts. Q551 and
Q651 sense the current in the emitter resistors, whereas Q552 and Q553
sense the DC offset at the amplifier outputs. Both protection circuits
trigger the Protect signal at pin 24 of the LM6405G microprocessor via
Q554, and the uP then trips the relay via pin 30.

Disconnecting R556 will disable the DC offset protection, and
disconnecting R553 and R653 will disable the current overload
protection on the left and right channels, respectively. Disconnecting
each of the three resistors one at a time may help narrow down the
source of the problem.

By my reckoning, you would need a DC offset of 2.7V before Q552 would
turn on, and you would need 1.2A in the emitter circuit of Q552 and
Q553 in order for Q551 to turn on. The latter is much higher than your
out-of-spec 60mA idling current.

- Franc Zabkar

 

[Franc Zabkar]

By my reckoning, you would need a DC offset of 2.7V before Q552 would
turn on, and you would need 1.2A in the emitter circuit of Q552 and
Q553 in order for Q551 to turn on.

Sorry, that should be Q508 and Q507, not Q552 and Q553.

- Franc Zabkar

 

[[email protected]]

It appears that the protection circuitry is in two parts. Q551 and
Q651 sense the current in the emitter resistors, whereas Q552 and Q553
sense the DC offset at the amplifier outputs. Both protection circuits
trigger the Protect signal at pin 24 of the LM6405G microprocessor via
Q554, and the uP then trips the relay via pin 30.

Disconnecting R556 will disable the DC offset protection, and
disconnecting R553 and R653 will disable the current overload
protection on the left and right channels, respectively. Disconnecting
each of the three resistors one at a time may help narrow down the
source of the problem.

By my reckoning, you would need a DC offset of 2.7V before Q552 would
turn on, and you would need 1.2A in the emitter circuit of Q552 and
Q553 in order for Q551 to turn on. The latter is much higher than your
out-of-spec 60mA idling current.

- Franc Zabkar

I guess the offset is fixed instead of adjustable. And, I'm guessing
that I'm most likely wrong but, does the idling current adjustment
have any relation to and significant effect with the DC offset?

I don't know why but, after setting the idling current to factory
specs, the amp appears to be stable; the heatsink and transistors
don't run hot and the protection never trips while playing music
through it for 4 hours straight with a pair of Yamaha S55 monitors.
It never worked that well until after the adjustment was done.

And, when setting R607 to where the test lead shows 7.5mV on R ch. and
doing the same for R507 for L ch., the resistance reading were, more
or less, ended up being the same on both pots.

But, when I set R607 to a setting that ends up reading 100mV at the
test point, the heatsink gets kind of toasty and the protection kicks
in within a few minutes.

 

[Arfa Daily]

It stands to reason that rotating the pot fully CCW or CW should not
damage the amp, otherwise not many would have left the factory. If the
trimmer is wired as below, then why not measure the idling
voltage/current at both extremities and verify whether this trips the
relay?

|---|
| |
| V
--|-/\/\/\/\/\---

If neither setting causes the relay to trip, then the problem must lie
elsewhere.

- Franc Zabkar
--

Not disputing the "should" Franc, I was just thinking that if the pot was a
bit touchy, then whacking it from one end to the other with it farting
about, might just result in the magic smoke being released from the outputs.
Just being careful, which you, more than most on here, should appreciate is
a sensible thing to do with a DC coupled amp ... d;~}

Arfa

 

[Arfa Daily]

It appears that the protection circuitry is in two parts. Q551 and
Q651 sense the current in the emitter resistors, whereas Q552 and Q553
sense the DC offset at the amplifier outputs. Both protection circuits
trigger the Protect signal at pin 24 of the LM6405G microprocessor via
Q554, and the uP then trips the relay via pin 30.

Disconnecting R556 will disable the DC offset protection, and
disconnecting R553 and R653 will disable the current overload
protection on the left and right channels, respectively. Disconnecting
each of the three resistors one at a time may help narrow down the
source of the problem.

By my reckoning, you would need a DC offset of 2.7V before Q552 would
turn on, and you would need 1.2A in the emitter circuit of Q552 and
Q553 in order for Q551 to turn on. The latter is much higher than your
out-of-spec 60mA idling current.

- Franc Zabkar
--

OK then, how about this for a scenario. As both pots are now set in about
the same place to get the same idle current, and one of them wasn't before,
it follows that the one which was wrong, had almost certainly been set
wrongly for a long time. This means that the transistors and heatsink on
that channel have been running much hotter than they should have. If it uses
mica washers with heatsink paste, rather than silicone rubber insulators,
the excess heat may have dried the paste out over the years, such that it is
now doing a poor job, and the transistors can get hot enough to start
running away, which creates a large emitter current, which is then sensed,
and causes the shutdown. Even if the insulators are rubber, a close
inspection might reveal that the excess heat has done them no good, and they
have gone hard. Just a thought, as I agree with your thoughts regarding
offdet and current trip points.

Arfa

 

[[email protected]]

OK then, how about this for a scenario. As both pots are now set in about
the same place to get the same idle current, and one of them wasn't before,
it follows that the one which was wrong, had almost certainly been set
wrongly for a long time. This means that the transistors and heatsink on
that channel have been running much hotter than they should have. If it uses
mica washers with heatsink paste, rather than silicone rubber insulators,
the excess heat may have dried the paste out over the years, such that itis
now doing a poor job, and the transistors can get hot enough to start
running away, which creates a large emitter current, which is then sensed,
and causes the shutdown. Even if the insulators are rubber, a close
inspection might reveal that the excess heat has done them no good, and they
have gone hard. Just a thought, as I agree with your thoughts regarding
offdet and current trip points.

Arfa

The transistors do, indeed, use mica insulators with heatsink paste.

Having replaced capacitors and redoing solder on the board, I took it
upon myself to clean out the old thermal paste off the transistors,
mica, and heatsink and used fresh paste.

 

[[email protected]]

There are two ways to go about servicing...

1. Find out exactly what's wrong.

2. Get the device working again.

Most people smart enough to service electronic equipment tend to lean toward
the former. But it's inefficient, and should be reserved for times when it
seems to be really needed.

In your case, it appears that #2 worked (for whatever reason). I would
suppress my curiosity -- your "I don't know why, but..." -- for the time
being and simply enjoy the amplifier.

I also tend to lean on the latter.

As the saying goes, if it works then it's not broken. If it's not
broken, don't fix it.

 

[[email protected]]

OK then, how about this for a scenario. As both pots are now set in about
the same place to get the same idle current, and one of them wasn't before,
it follows that the one which was wrong, had almost certainly been set
wrongly for a long time.
Arfa

As for how R607 had been set wrong before, I have a very wild guess
about that.

I bought the amp used as a present to a friend, but in the midst of
testing, I found that the receiver had problems with intermittent
power loss. Turned out to have been cold joints on the secondary
power supply block. I think what could've happened was that whoever
owned the thing before I bought it might have tried to "fix" it by
tweaking the potentiometer. What doesn't support this, however, was
that it seems only R607 had been altered. The other pots, such as
R507 and the pots on the preamp circuit do not appear to be a altered
as the tuner and other parts of the preamp section appear to work
properly. Usually, a really bad weekend tech may just tweak anything
indiscriminately until he thinks he fixed it.

I think it makes more sense that R607 has drifted in value somehow.

 

[Franc Zabkar]

OK then, how about this for a scenario. As both pots are now set in about
the same place to get the same idle current, and one of them wasn't before,
it follows that the one which was wrong, had almost certainly been set
wrongly for a long time. This means that the transistors and heatsink on
that channel have been running much hotter than they should have. If it uses
mica washers with heatsink paste, rather than silicone rubber insulators,
the excess heat may have dried the paste out over the years, such that it is
now doing a poor job, and the transistors can get hot enough to start
running away, which creates a large emitter current, which is then sensed,
and causes the shutdown. Even if the insulators are rubber, a close
inspection might reveal that the excess heat has done them no good, and they
have gone hard. Just a thought, as I agree with your thoughts regarding
offdet and current trip points.

Arfa

The OP has stated that the relay trips and closes again, presumably
straight away. Is this consistent with a thermal runaway scenario?

It seems to me that the uP is sensing a fault condition and opening
the relay to disconnect the speakers. But opening the relay achieves
nothing in the case where the amp is unloaded. So if there is a
thermal runaway condition, then surely it will persist even after the
relay is deactivated, in which case the uP should not reactivate it.

BTW, there is an additional 1 ohm resistor in parallel with the two
0.47 ohm emitter resistors, so the misadjusted idling current appears
to have been 120mA, not 60mA as I previously calculated. This current
would result in 10W of dissipation in the output transistor pair
instead of only 2.5W when correctly adjusted. But surely even this
high figure is much less than that generated by the amp when running
under load?

- Franc Zabkar

 

[Franc Zabkar]

Not disputing the "should" Franc, I was just thinking that if the pot was a
bit touchy, then whacking it from one end to the other with it farting
about, might just result in the magic smoke being released from the outputs.
Just being careful, which you, more than most on here, should appreciate is
a sensible thing to do with a DC coupled amp ... d;~}

Arfa

I was following up your suggestion that rotating a dicky pot may have
cured it, in which case returning it to its original condition would
not recreate the original fault. Furthermore, if the pot in the above
case had an intermittently open wiper, then you should be able to
reproduce the same fault by driving the wiper to the far left.

Based on the OP's followup, it appears that the pot itself may not
have been the problem.

- Franc Zabkar

 

[Franc Zabkar]

I guess the offset is fixed instead of adjustable. And, I'm guessing
that I'm most likely wrong but, does the idling current adjustment
have any relation to and significant effect with the DC offset?

AIUI, the two are unrelated. The offset is close to zero due to the
negative feedback of R514 which ensures that the voltages on the bases
of the differential pair (Q501) are equal.

I don't know why but, after setting the idling current to factory
specs, the amp appears to be stable; the heatsink and transistors
don't run hot and the protection never trips while playing music
through it for 4 hours straight with a pair of Yamaha S55 monitors.
It never worked that well until after the adjustment was done.

And, when setting R607 to where the test lead shows 7.5mV on R ch. and
doing the same for R507 for L ch., the resistance reading were, more
or less, ended up being the same on both pots.

But, when I set R607 to a setting that ends up reading 100mV at the
test point, the heatsink gets kind of toasty and the protection kicks
in within a few minutes.

By my reckoning, 100mV would result in an idle current of 0.4 amp
which would in turn result in a dissipation of 32W.

Anyway, congratulations on getting your amp working.

- Franc Zabkar

 

[Arfa Daily]

I was following up your suggestion that rotating a dicky pot may have
cured it, in which case returning it to its original condition would
not recreate the original fault. Furthermore, if the pot in the above
case had an intermittently open wiper, then you should be able to
reproduce the same fault by driving the wiper to the far left.

Based on the OP's followup, it appears that the pot itself may not
have been the problem.

- Franc Zabkar
--

I think we're talking at cross purposes here Franc. I know what I meant, and
you've known me long enough on here that I think you probably do as well ...

Arfa

 

[[email protected]]

Anyway, congratulations on getting your amp working.

- Franc Zabkar

Thanks, but it concerns me that, since I can't understand why, then I
may be missing something, such as a condition where this could be a
temporary fix in the end.

 

[[email protected]]

I understand exactly what you're talking about, but it simply isn't
productive. There are better ways to use your mental energy.

Pots (or the circuits they regulate) can and do drift. I've bought several
of the original Sony FM Walkman, and all have required the stereo sync pot
to be readjusted (by ear is sufficient) before the tuner would switch to
stereo.

Thanks for the reassurance.