Vox AC30 vib-trem trouble shoot

[Quatro]

Hey,

i got a hold of what I believe is a '70s AC30/6. The vib-trem channel doesn't vib or trem. I have swapped out the tubes from that part of the circuit and tried some new caps in some spots suggested by another amp guy. I've also jumpered the footswitch to make sure it's not bad. The only time I hear vib-trem is when the amp is shutting down as the signal fades out. Right before it disappears, you can hear the effect working. Any ideas on how to debug this complicated circuit?

Thanks

[tubeswell]

Hey,

i got a hold of what I believe is a '70s AC30/6. The vib-trem channel doesn't vib or trem. I have swapped out the tubes from that part of the circuit and tried some new caps in some spots suggested by another amp guy. I've also jumpered the footswitch to make sure it's not bad. The only time I hear vib-trem is when the amp is shutting down as the signal fades out. Right before it disappears, you can hear the effect working. Any ideas on how to debug this complicated circuit?

Thanks

Sounds like the wiggle is kicking in as the tube current dies out, which possibly suggests that the wiggle voltage isn't strong enough to affect the tube current when its properly powered up.  You say you replaced the caps in the LFO circuit - did you replace all of them? And what about the resistors? (its not so common for resistors to lose their mojo - but old resistors can drift quite a bit)

[Quatro]

I friend who knows more about trem circuits suggested a weak point hat I should swap out caps. They are c32,c33,c34 on this schem:

http://www.voxamps.com/downloads/circuits/ac3093pr.jpg

I also replaced the filter cap for this part of the circuit. I haven't checked every resistor or tried swapping caps elsewhere because this part of the circuit has so many parts and the layout makes it hard to see what's what. That's why I was hoping someone might be able to suggest some common weak points to start looking at first.

Thanks

[Willabe]

Try C46 is an electrolytic cap, 22uF@25v, if you haven't yet.

These caps dry out over the years. 1970-ish amp, it's time to get replaced anyway.

[Quatro]

Ok -  we have some progress. Replaced suggested cap and thought I heard a hint of flutter. I checked the resistors next door, R66 and R67. Both had drifted in value. One by about 2x the original value. Also, according to the 2 layouts i found and tracing the circuit, these resistors were in the wrong positions on the board. Flip flopped -  from the factory. I replaced them both and put them were they should be and we have vib -trem! however, not completely fixed. The channel sounds like it has a "green ringer" in front of it. My next step is to trace out the rest of this part of the circuit on the board and check resistor values. Any other suggestions?

Thanks

[Quatro]

update -  regarding the switched resistors -  both layouts show the same resistor positions but I am noticing one says "two resistors switched!"

http://www.blueguitar.org/new/schem/vox/ac30_lay.jpg

Does this mean the layout is wrong that both copied from? i believe that would mean mine was right and i now made it wrong (although better,  as one value was way off)?

[HotBluePlates]

Have you checked that the internal trimpot is turned up?

This somewhat assumes your amp matches the schematic posted, or older AC-30 models. There is an internal trimpot that sets the intensity of the vib/trem, which cannot be adjusted from outside the amp (as with a Fender amp "Depth" control).

In the schematic you linked, it is "RV1" between the two halves of V11. This is also labeled "470k preset".

Being very careful, adjust that trimpot with the amp on, and see if your intensity comes up.

[Quatro]

Thanks.

I did adjust the trim pot.

I think i have a bad plate resistor somewhere in the trem stage food chain. A lot of popping and frying bacon on this channel and the circuit sounds like it has too much gain. I am going to pursue this avenue next and see if that helps.

[HotBluePlates]

If you're trying to fix the trem, other noises are probably not an issue.

If you meter the plate voltage of the oscillator tube, do you get an steady d.c. voltage with the footswitch in one setting, and a bouncing d.c. voltage in the other footswitch setting?

[Quatro]

Thanks for all the help - I think i got it.

A bad plate resistor. R61. Value was correct, but must have had physical damage from heat or vibration. Replaced it and all noise and freakyness is gone.