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Yamaha newbie A-700 resto thread

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I think I may have finally found the culprit of the original missing/low channel problem... This is the muting board. 2 of the 3 circled solder joints were loose and able to wiggle freely - likely happened over the years. It's been fixed now. Can't believe I missed this all along.

Will report back soon, about to put the heatsinks in...
 
Fired up on DBT with dummy resistors in place...

Bright to immediately dim...

holding breath....

Relay click! Voltages seem good on the dummy resistors. I guess the service manual is very wrong. But I'm getting +/- 0, 52, and 0.6 on E/C/B. About see if bias will adjust....
 
Should I be able to adjust bias with the dummy resistors in place? I think I have the DMM in the correct place, but it's reading 0mv and does not change with the adjustment pot, on both channels.
 
Well, I couldn't leave well enough alone. After quadruple-checking everything, I decided that I would remove the 470 resistors and put the output transistors in place...

All is still good on the DBT! Same behavior. Auto Class A button makes it brighter, as expected.

I know you shouldn't actually set bias on the DBT, but it does adjust. I have lowered it on both channels.

On the left channel, it sits perfectly steady.

On the right channel, though, even though it can be adjusted, it is constantly jumping around by a few (2-3) mV. Any suggestions? Note: I haven't replaced the bias pots, the ones I ordered were way too big.

I await further instructions before taking it off the DBT.
 
Soundscape said:
My 1st thought is TR124. The old bias pots are usual suspects too.
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I think I tested TR124 (good) but I'll double check in the morning. I need a damn oscilloscope already...

Soundscape said:
Be patient stud...There's unobtaniums in there. Wait for the pro's to chime in.
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I know, I felt pretty confident I was good to at least put the new devices in place and wait for further instructions. I need to stop being like Icarus, especially since I already fell from the sky once...

Before I put them in, I tested voltage at every single spot listed in the SM on both channels... I even pulled quite a few silicon devices and resistors to test values... Seemed too easy so I was paranoid and wanted to make sure there wasn't a secretly blown part somewhere down the line of the previous incident.
 
Bratwurst7s said:
Either you fellows are up late or I am up early.

Good morning.
James
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Good morning James & Analog Wisdom. it's all the same to me regarding time. Barking at the moon. :beerchug:

I'm supposed to be ordering caps but got distracted by music on the C-80 / M-80 combo & my Kona.
 
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Soundscape said:
Good morning James & Analog Wisdom. it's all the same to me regarding time. Barking at the moon. :beerchug:

I'm supposed to be ordering caps but got distracted by music on the C-80 / M-80 combo & my Kona.
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I am 67 today.

Wife, Kids and Friends are all at work so I got nobody to share it with except for my AK Pals.

I will celebrate by taking apart a troublesome amp.

Cheers,
James
 
Bratwurst7s said:
I am 67 today.

Wife, Kids and Friends are all at work so I got nobody to share it with except for my AK Pals.

I will celebrate by taking apart a troublesome amp.

Cheers,
James
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Well Happy birthday young man! I've learned that a guy is officially old when one dribbles an extra 5-10 seconds after ya think you're done wizzing. For real.
 
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