October 29th, 2008, 09:36 PM
What would cause the Vibrato's intensity control to control the actual volume of the amp. As I go from 1 to 10, the volume drops along with it.
I'm fixing to get in there and troubleshoot it.
I've been restoring the amp and have done lots. Re-capped, found faulty carbon comp, now this vibrato issue.
I noticed that the pin on the driver tube, pin 8 plate, is around 2.1volts, when I turn the vibrato from 1 to 10, it jumps to about 6volts. If I remove the 12ax7 vibrato tube, this doesn't happen. I thought that the pot was the culprit, but it measures correctly when I check it with my meter, open from any connections.
When I have the rca jumper removed and the vibrato circuit activated the issue is no longer there.
October 29th, 2008, 10:09 PM
I figured out what's wrong but, will have to think about how to fix it, unless someone else knows the answer.
The vibrato is stuck in High mode when deactivated.
In other words the vibrato signal pulsates from 0-5 volts on the driver plate. 0 is low, full guitar signal, 5 is High, no signal.
When deactivated it's at 5 V. When I turn the intensity of the vibrato from 1 to 10, the 5V is applied to the Plate. Driving the driver into harder conduction.
How do I reverse this?
October 31st, 2008, 04:25 AM
Anything on why the Intensity pot controls the volume when the vibrato is dis-engaged?
October 31st, 2008, 08:38 PM
The trem's output modulates the bias of a preamp stage. On or off is only for the AC it sends out. It sends out DC even when off and this is what reduces the bias of the stage it modulates.
Turn it down when off or leave it on so you remember to turn it down when not wanted.
No pedal needed on this amp to use trem.
November 2nd, 2008, 05:19 PM
Cool, I was so over analyzing the circuit. I'm just used to the other vibrato amps that utilize a pedal or you can short the RCA jack. Fender's own little design flaw....haha!
I've had a Vibro Champ before, but never used a switch on the vibrato.
Now if I UPS can hurry on my new 5y3gt so I can have this amp pumping away.
I believe the rectifier is failing.......when driven hard the volumes drops out and stays out till the pwr section recovers. At low volumes, it's fine. All new caps and re-tensioned sockets.
But, thanx again for point me to the obvious......I tend to think out of the box.
November 6th, 2008, 12:41 AM
Ok, I've narrowed down all the culprits that are obvious with the problem that I am having. Here's a list of all the things that I've done.
64' Fender Vibro Champ - Cuts out when driven hard, not when vibrato is activated though?
1. All electrolytic caps replaced
2. 1k and 10k pwr supply and 6v6 cathode resistor changed to 1k metal oxide. To run cooler
3. All pots checked and cleaned, one replaced(bass)
4. All tubes swapped with new and other NOS(5y3, as well)
5. Solder Joints re-flowed
6. Checked for DC leakage in Modulator and coupling caps, <45mv
So what I've noticed happening today......
When I play normal volumes the amp sounds fine, but lacks some sustain after awhile.
When I turn it to 10 and crank on it, immediately, the volume cuts out and sputters. I looked at the 6v6 and it's arching when this happens. If I increase the intensity to barely on for the vibrato, it stops the problem. I might change out that pot after all for the intensity.
Tonight, I'm gonna clean the tube sockets and check continuity through them. Any other idea's?
November 7th, 2008, 11:00 AM
Messing with the amp today and I found a broken ground lead.
The 220K on the grid of the 6V6 has broken off on the copper plate. I never would
have noticed it unless I used my poker stick. Re-flowed the solder joint, wala! It's