Red Knob Twin hum until V9 removed

[01redcobra]

Is there any measurable Vdc on the reverb pot side of C103? (V9B plate cap .0022uf)
If there is, C103 may be at fault.

Only about .004 vdc at the most. Hum increases when checking any of the leads from the reverb pot. If I check the grid on V9 at pin 7 with the voltmeter, it induces a very loud hum, buzz. Is there that much gain on that tube? Hang with me on this amp, it's got me going in circles.

[Jazz P Bass]

I am not all that sure what they are doing with that optocoupler (signal limiting?)
Lift it to see if that is the cause.

[01redcobra]

I am not all that sure what they are doing with that optocoupler (signal limiting?)
Lift it to see if that is the cause.

Lifted that vactrol and my noise is down to right around 8mv. I think it's to turn the reverb on and off. Showing on the schematic as off at 1.6vdc and on at .25vdc.

[Jazz P Bass]

Which begs the question, 'was the vactrol causing the noise or is the reverb in an Off state & the noise is still there but before the vactrol.'
More exploration is in order.

[01redcobra]

Which begs the question, 'was the vactrol causing the noise or is the reverb in an Off state & the noise is still there but before the vactrol.'
More exploration is in order.

The vactrol is definitely bad. Checking the diode side is showing me it is bad, voltage in both directions. There is no resistance between any of the leads on the other end, showing open either side. Obviously not switching at all.

[01redcobra]

So I most likely should replace that dead opto and go from there. Anyone know if a VTL5C3/2 will work?

[Jazz P Bass]

Try it.
Are there any markings on the opto?

It couldn't hurt to call Fender.
Maybe they can help in the identification.
Or they may very well have the replacement.

[g1]

Anyone know if a VTL5C3/2 will work?

That should be the exact part.
According to the parts list, the 4 opto's are all 016282. According to Darren Riley, that is a VTL5C3/2 :
Fender Optoisolator VTL5C3/2 0016282000

[g1]

Here's the schem with parts list and layout showing the unusual numbering order for the preamp tubes (top view), very useful.

[01redcobra]

That should be the exact part.
According to the parts list, the 4 opto's are all 016282. According to Darren Riley, that is a VTL5C3/2 :
Fender Optoisolator VTL5C3/2 0016282000

Thanks Jazz P and G-one ! I'll have to get some ordered in. Newark and Allied are out of them. Mouser didn't even list them on a search. I suspect they are a high failure part as no one has them in stock. I did find CE.Dist supposedly has some, so I guess I'll be ordering some tubes and stuff too.

[Enzo]

No, not a high failure part, just an obsolete part. Mouser doesn;t sell vacuum tubes or VCR parts either. This amp is 25 years old, designed with parts that were current 25 years ago.

[01redcobra]

Good to see you on the board again Enzo. Best wishes on your recovery!

[01redcobra]

So the saga continues... Got the new vactrol and installed it. Turned the amp on and really didn't hear the hum like before. Yay! But then the ugh. It seems as now I have massively reduced volume on the clean channel. Scoped at V7 and I have a good signal into and out of V7b, but almost no signal into V7a pin 2. This is with the input in the channel switching inputs. If I send it in thru the paralleled inputs, it gives me a nice strong signal and it's volume seems up to snuff. I see another vactrol there coming off the wiper on the volume pot for channel one and I'm wondering if that is giving me the problem. This amp is beating me up. ugh again.

[01redcobra]

I'm not understanding why the new vactrol for the reverb caused the diminished volume on channel 1. I've double checked all the wiring to be sure something didn't come off while the board was being moved around. The owner doesn't have the footswitch so I switch channels with the switched pot. Channel 2 volume is the same as before. Any ideas out there?

[Jazz P Bass]

Lift out LDR 1.
Either it is bad or there is a LDR control issue.