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Onkyo TX-108 pops main fuse

jupitersspot

jupitersspot

Active Member
New-to-me TX-108 blows the 7 Amp fuse on the power input board. However, the secondary transformer remains powered and the front displays light up nicely (although switches do nothing). If I install my dim bulb tester it doesn't blow the fuse (I'm limiting current to maybe 750mA), but it oscillates -- relay closes, bulb gets bright, voltage drops, relay opens, voltage rises, relay closes...lather, rinse, repeat.

I'm off to go look at the output transistors. Interestingly, nothing on the board looks like it ever got hot.

Perhaps related: the front panel took physical damage at one point, and the left-most of the line of buttons is missing and some plastic is broken. Google images of other front panels lead me to believe I'm missing the "Alternate Audio" button.

If you have TX-108 front panel bit, give me a shout.
 
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https://www.hifiengine.com/manual_library/onkyo/tx-108.shtml
Looking at the schematic here, it looks like there is a small transformer that must run the display as well as the larger one, run off the fuse that's blowing, going off in all directions.

I'd check around for impact damage and broken traces then start ohming out the outputs.
The last Onkyo amp I worked on was similar but newer. The fuse was blown and so were the outputs (owner had shorted the speaker wires while it was playing pretty loudly). Strangely, nothing else on the board was visibly or electrically damaged aside from the outputs. I was expecting to see the wonderful cascade of fried components that you find in most direct coupled amps but was pleasantly surprised.
 
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Boredom is a great motivator in these times of pandemic: my 18 year old decided to come help diagnose. We did simple resistance checks across the power transistors while in circuit, and identified two that looked like shorts. We desoldered them and measured again, and they're shorted. They are Q636 (2SC2837) and Q637 (2SA1186) for those counting.
 
I have a feeling a lot of back burner projects are getting looked at due to the virus. At least some good can come out of it.

Sounds like you're on the right track. Check the emmiter resistors and the drivers. If they are okay you might be lucky and can just pop new outputs in there. I'd replace all of the outputs on the bad channel even if they test ok. They've taken a hit and are likely injured.
 
Success :-)
We replaced the two transistors and set the idle current, and we've been playing music happily for the last hour. It's a huge machine, and it has tons of power. I rather like it. Now all I need to do is figure out how to source the aluminum extruded button labeled "Alternate Audio" for the front panel, and I'll be off to the races.
 
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