AA764 VibroChamp build started acting up... need help with fix.

[eurekaiv]

Hello! Hoping this is a good place to get some help fixing my VibroChamp head that has started having a strange issue. I troubleshoot and repair electronics by trade (pinball machines) and know how to use a meter and can solder just fine. I'm not an electrical engineer however and when things aren't obvious in a tube amp, I get pretty dizzy pretty fast. Still, my troubleshooting background usually lets me sort things out but I'm not having much luck with this so I figure it's time I ask for help.

A few notes on the amp... I didn't build it (bought it used as a kit build from someone on eBay) but I did mod it slightly. It has a negative feedback lift switch and it's set up to be able to take a big bottle tube. I did these mods and these mods only, otherwise it's stock to the schematic best I can tell. It worked great for a couple years but it sat for awhile and last time I took it out it was misbehaving. Now that I'm stuck at home on semi-quarantine, I'm trying to clear up some projects that have been sitting underneath pinball machines waiting to be looked at.

Problems:
The treble control isn't doing anything until you get to about 6.5 at which point all sound cuts out completely until around 9. At that point the sound comes back and the tone is very bright.

The vibrato speed control doesn't change the speed. If I push/pull on the left lug of the speed pot (connected to the two ceramic caps, see VC3 pic attached) with a chopstick the speed will sometimes start working again until I let go.

What I've done:
Replaced the treble pot, no change. Pulled and tested the caps in the tone circuit with an LCR meter for ESR failure. Touched up solder joints, chopsticked around to see if poking anything caused the treble failure to go away. Tested all voltages (everything looks perfect, no DC on the tone controls), tried a few different tubes. Tested for continuity between the various portions of the circuit that are acting up.

Any ideas?

[shooter]

how close does yours match this;

https://el34world.com/charts/Schematics/files/Fender/Fender_vibrochamp_aa764_schematic.pdf

[eurekaiv]

Pretty sure the layout is the same other then no 330pf cap at the output tube and a screen grid resistor added. I can't find anything different anyway...

[shooter]

it was misbehaving.

so if you leave the knobs alone it rocks?, tremolos?

it's worth it to "baseline" the VDC against the schematic

doe the taper of the pot you took out match the schematic

[eurekaiv]

The amp worked fine for a couple years so I don't think this is a pot taper issue. I checked voltages, everything matches the schematic within a few volts except for the preamp cathodes which are 1.9 vs. 1.6.

[eurekaiv]

Thought of something else to add... the bass and volume controls do not effect the treble issue. The problem persists no matter how loud or how much bass is in or out of the circuit.

[DummyLoad]

see attached.


--pete

[eurekaiv]

Connects back to the .1uf cap for the bass control as per the aa764 diagram & schematic. I probably buzzed this out but I’ll double check tomorrow.

[DummyLoad]

Connects back to the .1uf cap for the bass control as per the aa764 diagram & schematic. I probably buzzed this out but I’ll double check tomorrow.

in the pic - it looks like a poor solder joint.


--pete

[eurekaiv]

It’s solid... going through with a scope now. 

[shooter]

going through with a scope now

be aware scoping "inside a TS" will get you wonky results most of the time.  I look going in, I look coming out, I just replace everything in-between if it don't look right.

[eurekaiv]

Well, sorted it out... bad treble pot. I was 99% sure I replaced it straight away but I guess not. Unf no spare 250k lying around so it's making do with a 500 which bumps the mids up slightly.