Vox Cambridge 15 Cambridgitis

[Storing!]

Hi All

For small gigs I use 2 VOX Cambridge 15 amps. Great sounding little amps.
Now one of them has developed a case of Camebridgitis. This is caused by the use of a bad optocoupler in the tremolo circuit.
Actually the tremolo is build reversed, so for the sound to come through, the coupler has to be "on" all the time.

Since I build a very nice Tremface a couple of years ago, and the Ring Thing in my board has a very luscious tremolo, so I don't use the tremolo function on the amp. I Thought, lets short the coupler as an easy fix to this problem.
This gave a massive boost in volume. So much actually that I'm concerned if this is a save mod. Maybe I should install a tri(e)mpot?

Can anyone shed some light on this item,

Thanks, E

[thermionix]

Can you post a schematic?  And describe where you shorted it?

[Storing!]

Hmm don't have a schematic
The idea is to short theo pto coupler on the R side, so that it looks that the light is always full on.
What bothered me is that the amp produced more than twice the volume when i did that.

gr, E

[R.G.]

Hmm don't have a schematic

Bummer. Chances of success drop dramatically if you don't know what you're tinkering with. 'Cambridge 15" is an awfully inclusive abstraction.

Without knowing what's being modified, we're all as much in the dark as you are.

[thermionix]

Google brought up a Cambridge 30 schematic.  All solid state except for one 12AX7.  Bunch of 4558DDs.  Does this sound like what you have?  I didn't see an optocoupler but I didn't spend much time looking.  The pots aren't labeled as to what they control.

http://music-electronics-forum.com/attachments/5429d1242749475-cambr-schema.jpg

[PRR]

> I didn't see an optocoupler

Found the little bugger. (Unmarked cap off-left is 47uFd)

[thermionix]

So assuming he's shorting out the LDR portion of this, if it's even the same setup in his amp.  Who knows how that value swings (min/max) under normal operation, but if he thinks it's too loud shorted, maybe replace it with a trim pot (100k?) and fine tune it.  Or R42 if that's easier for whatever reason.

[Storing!]

Thanks this helps.

I'll go with a trimmer. Easy repair

Thanks, E

[slacker]

You could measure the resistance of the opto in the working amp with the tremolo turned off, and then replace the broken one with the same resistor value.