SOLDANO designed amp/Yamaha

[Doc Stitches]

This SOLDANO designed amp powered up good for about 5 minutes and then the volume cut out almost completely unless the reverb is turned on. No reverb = no volume.

It is a T100C

Any help is appreciated.

Could old caps cause this? All but one look original.

Thanks
Jim

[Don Moose]

I recently acquired a T50 that had a similar problem - Yamaha, in their infinite wisdom moved the reverb return to after the masters, and apparently published the mod. Whoever did mine was a hack - it took a while to find all the trace cuts, moved plate resistors and other crap.

Soldano's solution is to put it back where HE had it in the first place. If you have the time and patience, I think you might try a trick from the StewMac pickup wiring book - instead of having the 'verb feed the 'tops' of their pots and pick off the signal from the wipers, try feeding TO the wipers and picking off the tops. Moving 4 wires around on the reverb pots has to be easier than what I did.

Schematics attached.

[Doc Stitches]

Hey Don....here are the photos

Doc

[Just Mike]

Yes, a necro thread, but relevant to my current situation. The reverb controls almost act like master volumes. So what exactly is the mod to fix this? (the reverb issue).

[g1]

The reverb controls almost act like master volumes. So what exactly is the mod to fix this? (the reverb issue).

Did you try the mod shown in post #2 schematics?

[Just Mike]

That's what I'm unsure about. The pic with the penciled in mod is correct, and the published mod in the Yamaha service manual is wrong? The pencil mod has an unknown cathode resistor value.

[g1]

That's what I'm unsure about. The pic with the penciled in mod is correct, and the published mod in the Yamaha service manual is wrong?

If you have the official Yamaha mod, please post it.

[Just Mike]

Here's the service manual. It has a pretty comprehensive explaination of the mod. But from what ive been reading, this mod is NOT the Mike Soldano mod.
It turns out that my amp already has this mod and the reverb is unuseable.

[g1]

The description of the yamaha mod seems unrelated to reverb controls acting as masters. They were trying to fix something else.
Post #1, he said the amp worked fine at first, then the dry signal cut out. To me this sounds like it could not be related to any mod, but rather that something broke. Like maybe the opto's handling the dry signal quit.
Post #2, the pencil drawing does not quite match the yamaha mod in the service manual you posted. I don't see why the mod would make the reverb controls kill the dry signal.

What exactly about your reverb is 'unuseable'? Best not to assume any cause and approach it from a fresh perspective.

[TomCarlos]

A year or so back, I had both a T50 and T100 that needed caps jobs, minor repairs, etc - things I am able to do. I spotted the notice for the MOD with the problem statement as shown by G1's post. The T50 did not have the mod. The T100 had the mod. Neither I or the amp owner could detect the problem reported. So avoid the mod if you can.

[Just Mike]

G1- Noted. But my point is that this mod was done before I got the amp and it has a problem with the reverb in that when the reverb is turned up, the sound thins out and on the High gain channel it actually introduces feedback. Somewhere somebody said Mike Soldano said it should be modded back to his original design. Where is that original design? Is it this? (from earlier in this thread?

[g1]

That picture has the yamaha factory mod pencilled in (but with errors).
The other drawing form post #2 (without the pencil stuff) is the original design.

[Just Mike]

Do you know what the errors are?

[g1]

Do you know what the errors are?

Yes. I doesn't quite match the mod described on pg. 34 of the manual you provided.

The unmarked resistor is 6K8, and what is drawn in after the .0047uF does not match the description step #6.
Should look like this:

[Just Mike]

OK, back at it. g1,You are correct. But the reverb returns are at the optoisolators POST MV's. And this is easily verifiable in that the masters don't turn down the reverb. I think there's a design flaw in the "fix". The original design had the returns going into V4b (looks like a mixer used to mix reverb & effects returns), the signal originally (pre mod) went to both triodes of V5 which drives the 2 tone stacks and on thru the masters. Now with the mod, the output of V4b (the reverb) is post master and the reverb signal now can be very overpowering and dominates the dry signal to the point of feeding back on the drive channel. Also, if one adjusts the master volume, you have to adjust the reverb because it's essentially in parallel with the masters.


V7 pin 2 is the input to the PI (post master). I believe the reverb return should be where it was at the input to V5.