Problem w/Fender custom shop Vibrasonic

Dave Zirbel · Post by **Dave Zirbel** » 2 Aug 2002 9:49 pm

Hi folks! Something strange is happening with this amp. It's a 1990's Fender Vibrasonic Custom (not custom shop, oops)!. It doesn't have the point to point circuitry like the old Fenders, it has the circuit boards inside. The thing sits around quite a lot. I haven't played it much lately. The problem is that one of the tremelo controls(far right) has decided it wants to be a gain control, but it gets louder and more saturated as you dial it towards zero. Not only are the controls doing the wrong thing but it's backwards. Do any of you experienced electricians know where I can start? Could it be something to do with the footswitch. I never had the matching footswitch that came with the amp. I tried my Mesa Boogie pedal. Could that have confused something in the amp?

Thanks, Dave Z<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Dave Zirbel on 03 August 2002 at 09:02 AM.]</p></FONT>

Ken Fox · Post by **Ken Fox** » 3 Aug 2002 8:23 am

Sounds as though the photo-resistor in the "Tremelo Bug" has shorted on the output side. Sometimes the legs of these can be touching each other. The "bug" looks like a small black plastic package with two wires out of each end. The output (side you are concerned with) will have a leg to ground on that end. In the old handwired amps, that would be the end pointing towards the rear of the amp chassis.
The pedal merely applies ground and starts the tremelo oscillator, no problem caused there.
Be careful if you open it up. You will want to do the following to discharge the amp:

-unplug amp
-remove reverb cables and remember where they were plugged in
-unplug speaker
-remove power cord retaining screw and 4 chassis straps and screws
-remove chassis
-You might want to remove the power tubes, to keep them from being damaged. This allows the amp to sit on the tranformers, instead of the tubes
-Apply a small jumper wire from chassis to pin one of tube #1 and turn on the amp. This will allow the B+ to bleed off thru a 100K ohm load resistor. Should take about 30 seconds.
-Now it is safe to check that "bug"

Ken Fox · Post by **Ken Fox** » 3 Aug 2002 9:36 am

Another thing to check, pull tube #5, 12AX7, next to the last small tube before the power tubes. This is the tremelo oscillator tube. If problem is still there, check as in the above post. You can determine with an ohm meter to ground if the output of the bug is grounded. If the legs are not touching and causing the ground, replace it. Mojotone is a good source for the bug. I am not sure if they use the same bug as in the older amps, either. I assume it is the same. The current part # for it is 037836.

Dave Zirbel · Post by **Dave Zirbel** » 3 Aug 2002 10:00 am

Thanks Ken. I already got it working. It turned out to be a piece of the foil shielding was peeled back and touching the components in the chassis. The amp sounds better than ever with new power tubes and a JBL speaker. Thanks for the advice!

Dave Z