I picked up a vintage 1992ish Onkyo TX900 receiver from a neighbor recently. So far I have determined that the unit powers up, and has audio on both channels and through the headphone jack. But unfortunately the display isn't working.
I found the service manual online, but it's fairly high level, and doesn't show individual components from what I can see.
Does anybody have any suggestions on where to start troubleshooting? I'm a mechanical engineer and trying to learn more about diagnosing electronics, and have had some minor success so far.
My plan is to remove the board that controls the display and look for any obvious issues, and start measuring resistors. I was able to repair a much newer Onkyo AV receiver that lost it's display because of a bad resistor, but that's because someone else figured it out before me.
Thanks!
Ian
I found the service manual online, but it's fairly high level, and doesn't show individual components from what I can see.
Does anybody have any suggestions on where to start troubleshooting? I'm a mechanical engineer and trying to learn more about diagnosing electronics, and have had some minor success so far.
My plan is to remove the board that controls the display and look for any obvious issues, and start measuring resistors. I was able to repair a much newer Onkyo AV receiver that lost it's display because of a bad resistor, but that's because someone else figured it out before me.
Thanks!
Ian
Ian,
I assume you have the service manual?
Measure AC voltage between pins 1,2 and 45,46 of the display. Should be 4.9VAC
Alternatively you should see the same voltage at the power transformer pins marked AC3.
There are no resistors involved.
Hugo
I assume you have the service manual?
Measure AC voltage between pins 1,2 and 45,46 of the display. Should be 4.9VAC
Alternatively you should see the same voltage at the power transformer pins marked AC3.
There are no resistors involved.
Hugo
Thanks Hugo, I have the service manual but I can't find the two photos you are showing. Do you have a better manual?
This is the one I found online. I didn't see any voltage references on it which is part of my confusion.
This is the one I found online. I didn't see any voltage references on it which is part of my confusion.
I have the same manual.
Does this help?
The display is actually a tube and the 4.9V is supplied to the filament.
Hugo
Does this help?
The display is actually a tube and the 4.9V is supplied to the filament.
Hugo
Aha! User error. I got to the packing section and assumed anything after that was a different language. Oops. Now I see it.
Thanks!
I'll take the "Dumb-***" award for today! LOL.
Thanks!
I'll take the "Dumb-***" award for today! LOL.
OK I got the display board off and was able to measure the voltage across 1/2 to 45/46 and it's zero.
There is also this spot of the display which has me worried. That looks like a spot that blew to me, but I'm far from an expert.
Is the display shorted and dead?
Thanks,
Ian
There is also this spot of the display which has me worried. That looks like a spot that blew to me, but I'm far from an expert.
Is the display shorted and dead?
Thanks,
Ian
I think the spot is normal. What do you measure at the transformer pins AC-3?
It's hard to tell from the picture but these look like bad solderings:
Hugo
It's hard to tell from the picture but these look like bad solderings:
Hugo
That side of the transformer is most likely the primary.
The other side will have the 4.9V, or better, should have.
Hugo
The other side will have the 4.9V, or better, should have.
Hugo
I have 123.7V AC at those two pins.
And I was switching my DMM to DC on the other side. I have been playing with car stereos recently. Oops.
So if I measure AC Volts I get 4.99 V across 1/2 45/46 and 5.04V across the top 2 pins on the transformer, which I assume is AC-3.
Now what?
Danke! I'm learning!
Ian
And I was switching my DMM to DC on the other side. I have been playing with car stereos recently. Oops.
So if I measure AC Volts I get 4.99 V across 1/2 45/46 and 5.04V across the top 2 pins on the transformer, which I assume is AC-3.
Now what?
Danke! I'm learning!
Ian
Well, either the display or the microprocessor is toasted I guess. That's page 14 and 15 of the manual.
You could measure if there is +5V DC between ground (pin 32) and pin 64 of the microprocessor.
Hugo
You could measure if there is +5V DC between ground (pin 32) and pin 64 of the microprocessor.
Hugo
I will keep poking around. Thanks for your help Hugo, I'm learning a ton. If I can't get the display working this will become my workshop stereo since I only use it for one source.
I remember an Akai video recorder with no display and we needed to change a small cap on the +5V.
In your case that would be C701 just underneath pin 64 on the schematic. (5.5V 0.1µF)
A 10 or 20V cap would be fine as long as it's 0.1µF.
Hugo
In your case that would be C701 just underneath pin 64 on the schematic. (5.5V 0.1µF)
A 10 or 20V cap would be fine as long as it's 0.1µF.
Hugo
You're getting good advice from Hugo!
I'm concerned about the ~120VAC you mentioned. Would you provide details? It may be harmless leakage currents, but be careful!
Do you see any other indications processor activity?
I'm concerned about the ~120VAC you mentioned. Would you provide details? It may be harmless leakage currents, but be careful!
Do you see any other indications processor activity?
Yes, I have 4.92V between pins 32 and 64.
I had to pop the display loose to get to it.
I also see the capacitor under the display. It doesn’t look bad but I will remove it and test it.
I have a 50V 0.1 microfarad capacitor. Will it work? It looks like a different kind or at least a different form factor.
The rest of the receiver seems to be working but I don’t see any signs of life on the display. I just measured and I do have 5V across it.
The 123V is the line in voltage from the power cord and at the transformer.
The filaments of the display are the end pins at each end of the display. Pins 1 and 2 are electrically the same as are pins 45 and 46. The filament voltage is measured between pin1/2 and pins 45/46
In the dark you might just make out the filaments glowing a very dull red and the display should feel slightly warm after a few minutes.
All the other pins should have a negative voltage on them, perhaps as much as -25 volts. If you get a resistor (say 4k7) and connect one end to ground and brush the other end over all the pins of the display (apart from the filaments) then different segments should illuminate really brightly. If that happens the display is OK.
If you suspect the uP is faulty then try removing the backup cap and leave it all for an hour or two with no power applied, then with the cap still removed apply power and see it the fault has changed.
This transistor is the -26 volt regulator for the display. Check this voltage is OK.
The backup cap.
In the dark you might just make out the filaments glowing a very dull red and the display should feel slightly warm after a few minutes.
All the other pins should have a negative voltage on them, perhaps as much as -25 volts. If you get a resistor (say 4k7) and connect one end to ground and brush the other end over all the pins of the display (apart from the filaments) then different segments should illuminate really brightly. If that happens the display is OK.
If you suspect the uP is faulty then try removing the backup cap and leave it all for an hour or two with no power applied, then with the cap still removed apply power and see it the fault has changed.
This transistor is the -26 volt regulator for the display. Check this voltage is OK.
The backup cap.
Oh yes, that's a totally different thing. It's a memory cap and you can't replace it with the cap you have.
With mains power off, you could measure if it still holds 5.5V.
Not 100% sure but that most likely won't be the display's problem.
It is there to keep the processor powered with 5V when the receiver is off.
Hugo