Vox Buckingham Tremolo Problem

[fuzzman72]

Hello!

Months ago I've totally rebuilt a Buckingham for one of my customer.
Following R.G.'s suggestions (thank you once again Mr.Keen!) We ended up with a great sounding and more reliable amp.
Now the amp is in my shop again for a minor issue.
Though the Tremolo is working great the depth pot act as
a secondary volume even with the tremolo effect disingaged through the footswitch.
So if you turn the depth pot in the CW direction you have a considerably drop in volume; at the estreme CW position the signal appear distorted at the mixer stage input.

[Jazz P Bass]

What have you checked so far?
There are voltage markings galore on the schematic.

If I had to guess, I would shoot for C224 being bad.
(Between Depth pot wiper connection to Q207 base)

Vox Buckingham V1121 Service Manual.zip.

[R.G.]

I don't remember exactly what I advised you, but I'm pretty sure that somewhere in there I said "replace 100% of the electrolytic caps". Did you do that? If you did, you've sidestepped the issue of one more old electro cap failing every few weeks or months.

JPB's advice is sound, and C224 is one place to look. But first some understanding is in order.

The tremolo works by using a "see-saw" depth control. The tremo depth pot has buffered dry signal on the CCW side, the recovered tremolo signal on the CW side, and turning the pot lets in signal current from both in a mix determined by its position. This current is fed through C224 to one of the mixer channels.

And that's the first question - does the tremo control act like a secondary volume to all channels? Or just the Normal channel. The Bass and Brilliant channels would be unaffected if there was no signal on the tremolo output side of the tremo depth pot. If this affects all channels, what's happening is that a DC leak through the tremo control is dragging down the whole mixer, in which case a short in or around C224 is a good first guess.

[fuzzman72]

Thank You for your replies.
Yes, I've changed all the electrolytics, from the power supply to the last coupling cap.
However I've re-checked them but hey tested good, even C224.

The volume drop appear only in the NORMAL channel and doesn't affect the bright and bass one.

I took some voltages around Q204 and Q 205 (referring to schematic v1121)

The power supply is at 4,5V

Q204
C 4,35
B 0,67
E 0,05

Changing Q204 with a brand new BC238C (Hfe 400 c.a.) doesn't help.

Q205
C 3,92
B 0,85
E 0,23

I've replaced all the resistors in the tremolo circuit with new metal film with no results.

Feeding the amp with a 1KHz sine wave and monitoring Q204, when there's 5mV on the base I have 3mV on the emitter (and the sine wave look chopped on the lower peaks) and 30mV c.a. on the collector.
I was expecting the same amount of signal on the emitter and probably that's where the problem is.

[52 Bill]

Based on the description of the problem the circuit around Q204 is working properly. The circuit around Q205 is where the problem lies. Check cap C212 for leakage maybe.

[fuzzman72]

Bill, C212 is ok, but you're right, the problem is Q205 since it doesn't amplify the signal, modulated or not.