Yamaha T100C -Another Bleeder!

[drewl]

Lovely amp designed by Soldano, looks better than it sounds.

Anyway, clean channel is bleeding into the Overdrive channel:
With clean preamp up, Unit switched to OD channel, clean master turned down and OD master turned up.

turn down Clean preamp pot, bleed goes away.

Amp uses Vactrol 5C1 opto-couplers for switching

When I ground the signal at V2b pin 7 the bleed stops, but I still have OD signal- that shouldn't be according to the schematic.
Opto #51 looks like it's supposed to ground the OD signal.

Unless I was confused, it was getting late.

[drewl]

In the 43 page service manual, there is a mod to separate the plates and cathodes of V4: In OD mode turning up Clean preamp effects the reverb sound, but right now I don't have the reverb tank connected and both reverb pots are turned down.

It sure looks as though the clean signal is not turned off or shunted when OD channel is selected.
It would be easy enough to add another opto, or relay to accomplish this, if in fact that is the way it's designed,
And I'm not just losing my mind here.

[Helmholtz]

Ideally opto 51 should be shorting and opto 52 should be open.
But optos are not ideal switches, so I guess a little bleed-over is normal.

LDR resistance of opto 51 can be measured in circuit. LDR of opto 52 needs to be disconnected for R measurement, in circuit R can't measure higher than 1.5M+330k.

[Jon Snell]

Have you checked the 470u smoother for the low voltage rail feeding the Vactrols? also check the 150R feed resistors.

[drewl]

I'll check all that, but correct me if I'm wrong: There are no Opto's shunting the clean signal when switched to OD channel, right?

[Jon Snell]

You are correct, 5 1 is the only shunt because the OD adds to the overal sound, it doesn't remove the Clean sound. Removal is down to personal preference and pot settings.

[Enzo]

And don't overlook the contributions of crosstalk.

[drewl]

Sure, but in this amp the clean signal is right there along with the OD signal in OD mode.
It may be getting swamped by the larger OD signal, but it is still there.

[drewl]

Well I did replace the 470uf filter for the switching supply, lowered ripple nicely.

I installed another opto to shunt the clean signal to ground after V1b.
It removed the bleed and amp sounds much better on the OD channel.

Worthwhile mod in my opinion if you have one on the bench.