Weird VOX problem

[Valvehead]

Working on a VOX Beatle head , Model V1143 . solid state

One of the problems- On the normal channel, plugging in a guitar i have no signal. Other 2 channels work fine.

the weird part - i can just have the cable plugged in and touch the tip and i get the normal loud buzz noise. Plug into a guitar and nothing !

its not the guitar-i tried 2 that work perfect.

tried a different cable .

i touched my noise gen throughout the channel circuit--full noise. also produces full noise when touching the tip of cable !

How is this possible ?? Q101 , Q102 show normal volts

[R.G.]

Touching the end of a cable makes for a relatively huge signal, much larger than a guitar produces. It is possible (in fact likely) that you have a coupling cap going high resistance.

[Valvehead]

ok, thanks -- any particular caps i should start checking/ swapping --that is , assuming you have the scem ?

[Valvehead]

so far i have swapped out C101 , 104 , 106

[Valvehead]

No luck so far --- could it possibly be one of the transistors doing this ? I think there are 4 or 5 in the normal ch. before its tone stack

[R.G.]

It is getting stranger. On the 1143, the front end transistor is an N-channel JFET, type 2N4303 if I remember right. That one dead might do it. The JFET input stages are a little finicky about transistors and biasing. C102 2uF NP is an exposure. So are all of the pot wipers and switch contacts. In the normal channel, the MRB switch being open could cause a similar problem.

It would make a great deal of sense to put a signal generator on the front end, then use an audio probe to listen at each successive transistor output along the way into the amp. Thomas Vox amps don't make a lot of voltage gain up until the mixer/limiter, so the signal level will be about guitar level up to there.

[Valvehead]

sorry guys-- i never did get it fixed -- the owner was in a hurry to sell it so its gone.
Thanks for your time though !

[R.G.]

PM me when the new owner wants to sell.

[Valvehead]

ha--- its long gone buddy .

thanks for your help RG .

[drewl]

Got to tune up a Beatle a couple months ago, I gotta' get one of these someday, I love that fuzz!