Yamaha A-700, blown channel (bad voltages)

Hi thanks for the reply!
- The lights turn on normally when powered up.
- There is a single protection relay for both channels, so it doesn't engage (I would imagine because of large DC offset on the bad channel due to the bad voltages). The relay only engages if both channels are good.
- I don't actually know for sure that the right channel is good either, but the voltages seem more normal on that one? It didn't blow output devices, that's all I know.

If it helps at all, I have an identical working amp that I've repaired a blown channel on before, and the relay will click with the 470Ω resistors in place of the outputs (while on the DBT) when all is well - so that much I know is OK. I can also take static measurements from it if needed, but I'm a bit reluctant to take live ones (it's used daily and I don't trust myself to not slip with the probe).

Welcome Ziabatsu !! Your in good hands with Mac.

What you can do to help prevent slippage with the probes is cover up most of the probe with tape or that liquid rubber and only leave the very tip exposed.

Nashou

Well I spent some time going over the schematics and looking over the voltages you posted. It looks like you have a problem with both channels. I was also taking into account all the parts you said you have already changed. After all this I really do not have any suggestions. These circuits are difficult to troubleshoot as Nash and I found out previously. You almost have to remove and test every component to find the problem. I don't even have any thoughts as to where to start. These circuits are not like newer amps and receivers that you can pinpoint better. If 1 part is bad is throws off voltages everywhere. Of even with just a bad connection. So I hope you can find the problem but I am sure it will take some effort. Sorry.

Thanks anyway, I'll keep trying to find something.
So far all I have is TR113 getting super hot. I've left its legs long so it doesn't burn up.

1.
I haven't checked TR147 - TR152 voltages, should I compose a list for those? Would B/E be enough for you or is C needed as well? I just find the collector the hardest to measure without shorting things, but I'll wrap up the leads anyway.

2.
I just checked the voltages going into the board at the blue/red wires on the L/R channel.
Both seem to match with +/- 50.75V. I think that's good. Are there any other points I should check power?

3.
I had another idea - do you think it's worth chocking the relay closed and measuring DC offset at the speaker terminals to see if the "good" channel is ~zero or not? Is it possible to check DC offset without outputs in place? I feel like it'd be useful to know for sure if one channel is okay.

4.
Do you guys think having an oscilloscope might help narrow this down? I watched a few youtube videos on connecting a tone generator to the input and then checking the output at various stages to try and narrow down where the signal stops showing. Would something about this amp design stop that being useful, or would that work (and can it be done while on DBT)? I could try and borrow one if it's useful (even for the sake of learning).

I'll get those voltages first and then think about what to pull. Are there any rules of thumb on what can be pulled without causing other things to blow up?

Hey Nashou -
Are you sure? This is measured while on DBT, would that not change the voltages?
And are you taking into account that the schematic voltages are supposedly incorrect too?

I can probably carefully double check this by putting my good amp on the DBT taking the same measurement (if you think 51 is wrong). Would that suffice?

Totally agreed on making sure the power supply is all good first.
Transistors are all from avionic's recommendation so I don't think there are any incorrect pinouts (and I've used a number of them to successfully fix other amps) - but there are definitely a handful of the smaller early-stage ones that I haven't used before / checked. So I'll do that.

Edit:
Been poking around with meter when bored, power off, found an anomaly in B-E resistance
TR118 (right ch) is the equivalent of TR117 on the left ch.

TR117 7.28 MΩ (bad channel)
TR118 1.58 MΩ ("good" channel)

TR117 0.87 MΩ (good amp)
TR118 0.90 MΩ (good amp)

Does that give you any clues?



Edit 2:
I took some more static B-E resistance measurements, clearly there's 3 transistors that aren't matching the "good" channel, the others all seem to match
I feel like there may be component/s parallel with those transistors' B-E legs that are bad (TR113, TR115, TR117).
The even numbers are the corresponding "good" channel transistors.



I might take the same static measurements from the working amp to check, but I suspect not having the outputs in place will cause a global anomaly. I might temporarily put the new outputs into the dead amp so that I can make valid comparisons, presumably the DBT should protect those guys anyway even if I turn it on.

Let us know if it works.