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

[[email protected]]

I have an Onkyo TX-82 stereo receiver.

What happens is that, at random moments during use, the speaker
protection relay may open and cut the sound and then close.

I've tried reflowing all solder joints on the power amp board and
replaced various electrolytic capacitors, but to no avail. The
receiver keeps cutting out at random times.

The receiver seems to amplify normally, even up to the point where the
relay opens.

I'm guessing that the relay itself could be at fault, but how? I
physically tap the relay during use and it doesn't exhibit any faults.

Any suggestions on what could be going on?

 

[Arfa Daily]

I have an Onkyo TX-82 stereo receiver.

What happens is that, at random moments during use, the speaker
protection relay may open and cut the sound and then close.

I've tried reflowing all solder joints on the power amp board and
replaced various electrolytic capacitors, but to no avail. The
receiver keeps cutting out at random times.

The receiver seems to amplify normally, even up to the point where the
relay opens.

I'm guessing that the relay itself could be at fault, but how? I
physically tap the relay during use and it doesn't exhibit any faults.

Any suggestions on what could be going on?

Bad joints on the relay coil connections. Bad joints in the power supply
causing one rail to drop out momentarily. Bad joints in the relay control /
fault sense circuit. Resistor on relay delay timing going high / open.
Timing cap int o/c. Genuine problem such as defective speaker or wiring.
Those are the most common problems I come across.

Arfa

 

[[email protected]]

You can probably look elsewhere for the fault.  I would first suspect that the
circuitry that controls the relay, i.e., the protection circuitry, is
activating.

Some possible reasons that the protection circuitry is opening the relay....
(1) DC offset voltage is appearing at the speaker terminals.  Causes...Idle
current set wrong or component failure in the amplifier chain causing theoffset
to go haywire.
(2) Temperature compensation circuitry faulty.  Causes... defective temperature
sensing component (thermistor, diode, transistor).
(3) Protection circuitry faulty.  Causes... component failure in that area.
(4) Anything else that would upset the DC balance in the amplifier.

Things to look for:
Check for DC voltage present at the speaker terminals.  If it's more than a few
millivolts, you need to find the reason for it.
Is the DC offset only on one channel (more likely) or both (unlikely)?  Use the
good channel to compare voltage measurements along the amplifier chain and try
to isolate the bad stage.
Does the protection relay activate with no audio, or only with audio present?
Do you have a schematic of the unit?
--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the
address)

Experience: What you get when you don't get what you want

The relay trips whether or not there is any audio and whether or not
the power amp circuit has any load or not.

Checking for DC in the speaker terminals. Bank A left reads 8.3mV.
Bank A right reads 5.9mV. Bank B left reads 8.4mV. Bank B right reads
6.1mV.

Measures taken when receiver is idle with no source.

Unfortunately, I have no service data for this receiver.

 

[[email protected]]

Have you sat there watching the meter to see if the offset jumps up when the
relay opens? (I'm sure you have better things to do with your time!) Some
DMMs have a trending feature that keeps track of the highest voltage.

Arfa's suggestions are a good place to start.

Right now, I want to figure out what is going on and I have the time
at this moment to do that.

I measured the power and, on the left channel, the offset fluctuates
between a peak of 11.3 mV and 8.3 mV. Is this significant?

Unfortunately, I missed the meter reading right at the moment the
protection relay tripped.

 

[[email protected]]

You can probably look elsewhere for the fault.  I would first suspect that the
circuitry that controls the relay, i.e., the protection circuitry, is
activating.

Some possible reasons that the protection circuitry is opening the relay....
(1) DC offset voltage is appearing at the speaker terminals.  Causes...Idle
current set wrong or component failure in the amplifier chain causing theoffset
to go haywire.
(2) Temperature compensation circuitry faulty.  Causes... defective temperature
sensing component (thermistor, diode, transistor).
(3) Protection circuitry faulty.  Causes... component failure in that area.
(4) Anything else that would upset the DC balance in the amplifier.

Things to look for:
Check for DC voltage present at the speaker terminals.  If it's more than a few
millivolts, you need to find the reason for it.
Is the DC offset only on one channel (more likely) or both (unlikely)?  Use the
good channel to compare voltage measurements along the amplifier chain and try
to isolate the bad stage.
Does the protection relay activate with no audio, or only with audio present?
Do you have a schematic of the unit?
--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the
address)

Experience: What you get when you don't get what you want

I think I may have solved the problem.

I managed to find the service manual and performed a test and
adjustment of the idling current.

The service manual states that the idling current at open load must be
7.5 mV with a tolerance of 1.5 mV

Left channel was about spot-on at 7.0mV, but the right channel was WAY
off at over 30 mV! Seems the trim pot that controlled the idling
current has changed in value.

Anyways, I readjusted both trim pots until I got the voltage spot-on
to 7.5 mV. Hopefully, that will fix it.

However, I am concerned about the fact that the pot on the right
output had drifted so drastically so I am entertaining the notion of
finding a replacement set of pots and redoing the idling adjustment.

What do you guys think?

 

[Franc Zabkar]

I have an Onkyo TX-82 stereo receiver.

What happens is that, at random moments during use, the speaker
protection relay may open and cut the sound and then close.

I've tried reflowing all solder joints on the power amp board and
replaced various electrolytic capacitors, but to no avail. The
receiver keeps cutting out at random times.

The receiver seems to amplify normally, even up to the point where the
relay opens.

I'm guessing that the relay itself could be at fault, but how? I
physically tap the relay during use and it doesn't exhibit any faults.

Any suggestions on what could be going on?

Assuming that the protection involves some current sensing or DC
voltage sensing components in each channel, then why not disconnect
the relevant components one channel at a time? I would think that you
would not risk any damage by doing this, since you are able to
reproduce the fault with the speakers disconnected.

Other replies have focused on looking for a DC voltage on the outputs,
but I've just been looking at the protection circuit for a current
model Onkyo receiver (TX-8211) that appears to monitor the load
current in the 0.22 ohm emitter resistors of the output transistor
pair. If either resistor were to intermittently go open, then this
would trigger the protection. I have seen this problem in a car amp.
In that instance, one of the pins of a ceramic emitter resistor
fractured due to vibration.

- Franc Zabkar

 

[[email protected]]

... I've just been looking at the protection circuit for a current

model Onkyo receiver (TX-8211) that appears to monitor the load
current in the 0.22 ohm emitter resistors of the output transistor
pair. If either resistor were to intermittently go open, then this
would trigger the protection. I have seen this problem in a car amp.
In that instance, one of the pins of a ceramic emitter resistor
fractured due to vibration.

- Franc Zabkar

Interesting thought.

According to the schematics and parts list, if I'm reading it right,
Q508 and Q509 (transistor pairs for the left) work with R512 and R513,
which are a matched pair of metal plate resistors rated at 2W and .47
ohms. The right channel transistor pair are Q608 and Q609 and their
resistors are R612 and R613. The resistors on the right channel are
identical with those that are on the left, of course.

So, how would I go about testing the resistors? Would I lightly tap
on each one with a plastic probe and see whether or not the protection
relay trips?

 

[Arfa Daily]

Have you sat there watching the meter to see if the offset jumps up when
the
relay opens? (I'm sure you have better things to do with your time!) Some
DMMs have a trending feature that keeps track of the highest voltage.

Arfa's suggestions are a good place to start.

Right now, I want to figure out what is going on and I have the time
at this moment to do that.

I measured the power and, on the left channel, the offset fluctuates
between a peak of 11.3 mV and 8.3 mV. Is this significant?

Unfortunately, I missed the meter reading right at the moment the
protection relay tripped.



It's hard to say whether or not that is significant. In itself, in terms of
absolute voltage, it is not significant. Imbalances up to about 20mV are not
uncommon, and are nothing for concern. However, the fact that it is
fluctuating may indicate that some component is 'hovering' around its
breaking down point, or that one of the rails is not stable. It is not
really usual for the voltage, whatever its actual value, to fluctuate. Does
your DMM have a peak hold facility as suggested elsewhere? This will tell
you if the midpoint shifts substantially at the time of failure. However,
note that if the relay circuit is 'quick', it might be *too* quick for the
DMM to see the voltage increase, before the relay disconnects it, so really,
you need to be taking any measurements actually at the amplifier output,
prior to the relay.

Does this amp have a set of discrete component output stages, or are they
STK hybrids ?

Monitor the supply rails as well - particularly those to the output stages -
and see if they are moving by a similar amount as the output offset. In DC
coupled amps, a very tiny shift in conditions back in a preamp stage, can
upset the DC conditions so much in the output stages, as to cause a relay
trip-out, which can make them very frustrating to troubleshoot. Use the butt
end of a reasonable sized screwdriver to have a bit of a bash around, to see
if you can provoke the problem at will. If you can, then swap to a biro pen
to pin down the actual area, and possibly, component.

You can also try a hairdryer, and a can of freezer, both sides of the board.
Bad joints which don't readily show up with disturbance testing, are often
surprisingly sensitive to heat and cold. Does the problem seem to get worse
or better the longer the amp is on ? Is it worse when the amp is 'doing some
work' ? All possibly significant factors in arriving at a diagnosis.

Arfa

 

[[email protected]]

Monitor the main +/- DC rails for changes/imbalance. Monitor the voltage
over any zener diodes associated with the protection circuitry.

Could the idling current being horridly off be a possible cause?

I managed to obtain the service manual and was able to measure voltage
for the idling current.

According to the service manual, voltage should be at 7.5mV with a
tolerance of +/- 1.5mV. Left channel measured in at around 7.0 mV,
but the right channel was measured at over 30.0 mV.

 

[Franc Zabkar]

Interesting thought.

According to the schematics and parts list, if I'm reading it right,
Q508 and Q509 (transistor pairs for the left) work with R512 and R513,
which are a matched pair of metal plate resistors rated at 2W and .47
ohms. The right channel transistor pair are Q608 and Q609 and their
resistors are R612 and R613. The resistors on the right channel are
identical with those that are on the left, of course.

So, how would I go about testing the resistors? Would I lightly tap
on each one with a plastic probe and see whether or not the protection
relay trips?

I hate inducing faults in power amps because one faulty component can
take out many others. Why not upload your manual to eserviceinfo.com,
or link to it if you've found it on the Net, or scan and upload the
relevant section of the diagram to an image hosting site or to your
own web space?

- Franc Zabkar

 

[[email protected]]

If I may ask first.

Would too high a voltage for the idling current on one of the channels
be a possible cause?

 

[Arfa Daily]

If I may ask first.

Would too high a voltage for the idling current on one of the channels
be a possible cause?

There must be a good reason why the idle current is four times what it
should be. You have suggested elsewhere, that adjusting the pot has
corrected the situation, and appears to have stopped the fault from
occuring. In itself, the idle current being that high, is unlikely to cause
the protect circuit to drop the relay out intermittently. Usually, the only
effect will be that the output transistors on the affected channel, will run
rather hotter than would be considered 'comfortable'. That said, if you had
a 'dicky' pot, it may well be going intermittently effectively o/c, as the
wiper makes and then doesn't. The effect of this could be to shoot the idle
current up way high, which might be enough to trip the protect circuit,
depending on exactly what parameters of the output stages, it is monitoring.

If you have been able to correct the bad idle current to the specified
value, the next thing that you should do is to compare the pot's set
position, or its resistance setting, to the pot on the good channel. If it
is near enough the same, then the most reasonable conclusion is that you did
have a dicky pot, and rotating it has cleared the condition. A further
confirmation of this, would be if the pot had been returned virtually to its
original set point, and the idle current value was now correct. However, if
the pot has had to be rotated a significant distance from where it was
originally set, and is now set at a completely different point / resistance
to the pot on the other channel, then you have a genuine fault, which needs
locating and correcting.

If a preset pot goes intermittent, usually a drop of switch cleaner /
lubricant on the track, and a good scrub from end to end a few times (with
the amp turned off, of course ... ) is enough to produce a long term cure.

Arfa

 

[[email protected]]

The right channel pot is around 4.27 kilo-ohms in its current
position. The left channel pot is around 4.28 kilo-ohms.

Putting the right channel pot back to approximately where it was when
the problem existed yields a rating of 5.76 kilo-ohms.

 

[Franc Zabkar]

If you have been able to correct the bad idle current to the specified
value, the next thing that you should do is to compare the pot's set
position, or its resistance setting, to the pot on the good channel. If it
is near enough the same, then the most reasonable conclusion is that you did
have a dicky pot, and rotating it has cleared the condition.

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

 

[[email protected]]

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

Running it at a higher voltage for the idling current does get it to
run hotter. Could the protection be a thermal issue?

 

[[email protected]]

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

Increasing the idling current voltage has started the tripping again.

 

[Franc Zabkar]

Running it at a higher voltage for the idling current does get it to
run hotter. Could the protection be a thermal issue?

If I'm visualising the circuit correctly, then at 30mV the idling
current would be 30mV/0.47R = 60mA. If the supply rails are +/-40V,
then the dissipation in the output transistors would be 80 x 0.06 =
4.8W. At 7.5mV the dissipation would be only 1.2W. I can't imagine
either of these figures would give rise to thermal problems.

- Franc Zabkar

 

[[email protected]]

If I'm visualising the circuit correctly, then at 30mV the idling
current would be 30mV/0.47R = 60mA. If the supply rails are +/-40V,
then the dissipation in the output transistors would be 80 x 0.06 =
4.8W. At 7.5mV the dissipation would be only 1.2W. I can't imagine
either of these figures would give rise to thermal problems.

- Franc Zabkar

Would it be okay for me to e-mail you a PDF of the service manual?

 

[Franc Zabkar]

Would it be okay for me to e-mail you a PDF of the service manual?

Yes. I'll upload it to eserviceinfo for the group, if that's OK,
otherwise I'll put it on my web site.

- Franc Zabkar

 

[[email protected]]

Yes. I'll upload it to eserviceinfo for the group, if that's OK,
otherwise I'll put it on my web site.

- Franc Zabkar

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