Onkyo TX-NR838 (smoking) trouble

[tibimakai]

I have picked this up from the bay, with a blown center channel.
Easy fix, right?
I have found all the parts, that are needed to be replaced.
All done, I have tested the mainboard twice and the pre-amp board twice.
All is good. I have started measuring the idle current and it was going up slowly(which is good). Around 5mV, suddenly it was dropping toward 0, and it reached 0.
I have took everything apart, and measured everything again. I did not find, anything wrong. I put it back together and this time, the idle current started going up way to fast to my liking and of course, it started smoking at the center channel pre-amp board. I have turned it right off, to don't cause bigger damage.
Found a 2.2 Ohm resistor(R6292) near the connector(emitter resistor for the TTA transistor-Q6102), with very high resistance. I have replaced it, could not find anything else wrong.
I have tried it again, after inspecting everything again. Still smokes.
I would need help, tracing down why it keeps smoking.
I have also replaced C6022/50V(capacitor between the two TTA/TTC transistors, just in case). I have to recognize, that I did not replace TTC, because it wasn't blown(initial repair), only the TTA.
In these amps, almost every time, blow the same components.
Capacitor has the right orientation, I have checked.
I would appreciate any help.

Tibor

[vinceroger69]

Re: Onkyo TX-NR838 (smoking) trouble

added service manual just in case you dont have it

[tibimakai]

Re: Onkyo TX-NR838 (smoking) trouble

I have it, thanks though.

[petehall347]

Re: Onkyo TX-NR838 (smoking) trouble

struggling with your id numbers .. transistor not on schem .. if i search 6102 it shows me r6102 emitter resistor 0.22 ohms ?
can you check your id numbers ?

[redwire]

Re: Onkyo TX-NR838 (smoking) trouble

I would say you have a damaged transistor. They are a bit harder to track down. You have to do the full test sometimes.

Compare the two diode-junction readings within a transistor, between transistors on other channels.
Look at VBE and VCB readings with a diode-test on a multimeter.
They should be close to the others, say 0.6V and if you get a lower or higher reading say 0.4 or 0.8, then the junction is suspect.
Another test is ohms between transistor pins should show many megs or nothing- but a partially burnt junction will show up leaky (out of circuit test).

I would say a driver or O/P transistor is damaged, and usually the vbe multiplier Q5461 2SA1036/Q5411 2SC2412 gets cooked.
If you short the vbe multiplier Q5461, bias should go to zero and it should run Class B unless the driver/output stage has an issue.

Temp sensor transistor Q6002 2SC2412 on the heatsink but it has R6022 2k2 to limit current so it is not likely damaged.

[tibimakai]

Re: Onkyo TX-NR838 (smoking) trouble

Sorry about having screed up the id numbers. Those are for a TX-RZ800. It should be R5372-2.2 Ohm, TTA- Q5252, C5442- 22uF.
I'm working on 4 Onkyos.

[tibimakai]

Re: Onkyo TX-NR838 (smoking) trouble

redwire, thanks for that. I will, check out the transistors again. What I have failed to do and maybe that is the problem, that when I have replaced TTA, I did not replace TTC, since it was showing OK.

[petehall347]

Re: Onkyo TX-NR838 (smoking) trouble

Q5252 plus more likely shorted

[tibimakai]

Re: Onkyo TX-NR838 (smoking) trouble

I have measured again everything and the only difference that I have noticed is the C5002(221J). I have used the Ohm meter. On every other channel it was starting to raise, from 126 kohm, but at the C5002, it was steady at 0.709 Mohm. I don't know, if it means anything.
I wanted to try again, to at least which part is smoking.
Turned it on and in like 15-20 seconds, nothing was smoking.
Than I hooked up the testing wires w/resistors and meter. As soon as I have turned it on, started to smoke, voltmeter was showing 2.9V(don't remember if it was + or -), Check Spk showed up on the display.
The only part that I have found, it was again the R5372 (2.2 Ohm resistor at the emitter of Q5252). Transistors check out good. This is a mystery.

[petehall347]

Re: Onkyo TX-NR838 (smoking) trouble

looks like too much current either from transistor being switched hard on. bias set too high maybe .. or something after it is too low resistance .

[tibimakai]

Re: Onkyo TX-NR838 (smoking) trouble

I have found a bad 82 Ohm resistor at the L channel(!), while comparing the channels. Most likely that was smoking. I don't understand how that went bad, since the issue it was with the C channel.
Just in case, I have replaced the TTC transistor at channel C and of course, the burned 2.2 Ohm, at the TTA emitter.
It is all good, working great.
Thank you, for your input.