Modded Vox Cambridge 30 Reverb Problem

[efdi]

Hello everybody!

I've got a problem that i hope someone here may help me with. I have a 12 year old Vox amp, a hybrid solid state with a 12AX7 tube Vox Cambridge 30 Reverb which i have modded (i replaced the problematic optocoupling from a LT9914 to a VTC5L3 and i changed the original TDA2050 chip with a much better LM3875). The modding done about a year ago fixed the "cambridgeitis" and made the clean channel sound superb.

Lately, though, i've been having a problem. Sometimes, in the middle of my playing or right when i kick start the amp early in the morning, the reverb is gone! Couriosly, it always comes back when i press the "boost gain" button from channel 2, even when channel 2 isn't activated! (i usually only use channel one with reverb)

I tried changing the 12AX7 tube with a new one, checked the wires leading from the spring reverb to the board and even resoldered some the reverb related connectors from the back of the board (from the reverb knob and from where the reverb wires meet the board + plus any other solder that looked "dry").

I don't think it's a heating problem since the problem appears even when i start the amp fresh, i talked with an electronic engineer friend which suggested it might be some sort of circuit problem since the reverb is always restarted when i press the "boost gain" button from channel 2, even when channel 2 is not activated.

Does anyone have any idea what might be causing this or what i might do to fix it? I really love the reverb on this amp and i would be very sad to have to buy a new one, but i can't go on any gig knowing the reverb might shut down randomly.

If anyone can help, it would be much appreciated! My engineer friend said he would help me, but he doesn't know much about amps so any tips as to where he should look for problems would help a lot. Also, i have attached the service manual with the circuit schematics to anyone that might be interested.

Thank you very much!

Vox Cambridge30R.pdf

[Jazz P Bass]

So you must concentrate on the reverb circuit.
When the circuit is in a failed condition, start looking at the opamps.
IC5B should have a signal at pin 7 at all times.
IC5A pin 1 should have signal if the reverb switch is "On".
I cannot seem to locate point "B" on the schematic.
I would assume it is the on/off for the reverb.
Look at these points when it is working & when it is not.
Something must be different.

[efdi]

Thank you very much for the reply!

I will check these out and see what is different when the reverb is working or not and post my findings here.

[efdi]

Also, there is no on/off switch for the reverb, the reverb knob controls everything. When at minimum the reverb is off.

[Enzo]

Oh, but there IS a reverb switch, it is the footswitch jack. Look lower left on the drawing, JK6. See the center contact trails down to point B? Now look at the reverb, and point B controls Q7 which kills the return to IC5. Are you using a footswitch in that jack? A miswired footswitch could do this.


Monitor the voltage at teh base of Q7 while turning that boost switch off and on, is the voltage there affected? If so, the reverb uis being turned off and on there, and we just need to find what is causing that control voltage.

[Jazz P Bass]

Thankyou Eagle Eyes Enzo.
I looked for that "B" node for ten minutes.
On my monitor it appears as a smudge.

[Enzo]

I was looking at the first one, there are three, only difference is mains voltage as far as I know. Perhaps one is clearer than the others for you?

Of course it could be something else entirely, but imagine a FS with an open ground connection. The reverb and channel switches on at the same time would ground the reverb kill as long as the panel switch for channel is on. Turn off that panel switch and the unwanted path to ground is broken and reverb no longer killed. It's just a hypothesis.

[efdi]

No, i don't use a footswitch jack. I just got a tip that the "ext foot switch" jack at the back of the amp might be corroded and cause these symptoms. This fits perfectly with your hypothesis as well!

I'm going to get a contact spray and clean that jack, see if the problem comes back. If it does i'll go in there and disable the jack and the switch completely. I'll post my results here.

[efdi]

Ok, i got some contact spray and cleaned all the jacks. So far everything works, i'll have to wait and see if the problem resurfaces.

[efdi]

Hello!

Just posted to say that the contact spray did the job! It's been more than a week and no sign of the problem. Big thanks to everyone who helped out!