Blackface Super Reverb Vibrato only works when chassis is removed

[jvm]

Vibrato works great until chassis is installed into cab. Then vibrato becomes intermittent and also very faint. Chassis just slid in, no screws installed yet. Anybody seen this before?

[Jazz P Bass]

Ghost Busters

It's haunted.
Is this an absolutely repeatable issue?
If you take the chassis out it works.
If you install the chassis it does not work.
How many times was the process repeated.
Do these statements prove true, every time?
See where I am going with this.
Really, there is only a few things it can be.
Something is getting 'bumped' when you install the chassis.
Look in as the chassis goes into the cabinet.
Some component or wire is being moved.
Or the footswitch circuit and it's attendant wire/ connector/ footswitch is to blame.

[g1]

the footswitch circuit and it's attendant wire/ connector/ footswitch is to blame.

This gets my vote. Try it with a jumper plug instead of the footswitch. Also, in case it's some real weird fault with the neon/ldr/roach, try covering the chassis with a piece of cardboard to block the light while the chassis is out of the cab.

[jvm]

Sorry for the slow reply. I have been waiting on biopsy results for my daughter. All tests came back clean! Now that I no longer have that distraction, I can try to figure out this amp. With the chassis on my bench, I can lay a metal enclosure across the top of the chassis with the result being the same as turning Intensity control to "0". I can still hear the "ticking" but all intensity is gone. Same result when sliding chassis into and out of cabinet. The chassis is not being flexed, nor is the tag board or wiring being touched at all.

[jvm]

Thanks, g-one. I read your reply and tried covering the roach. Lo and behold, same problem. I removed the roach and saw that the top of the lamp was just slightly protruding out the LDR end of the heat shrink. new roach, problem solved. Thanks for suggesting something so simple. I have been very distracted these past few days and just needed a reminder.

[Jazz P Bass]

Good call, G-One.
I am glad that the problem was figured out.

[g1]

Funny thing was I almost didn't mention it because it seemed illogical/backwards to me . I would expect a fault where the roach works in the dark but not the light, this was the opposite. But there is nothing to say any fault modes need to be logical so illogic prevailed!

[52 Bill]

Funny thing was I almost didn't mention it because it seemed illogical/backwards to me . I would expect a fault where the roach works in the dark but not the light, this was the opposite. But there is nothing to say any fault modes need to be logical so illogic prevailed!

There is fault mode with neon lamps where they will not fire unless they are stimulated by an outside light source. I had read about it once in a CML catalog and I didn't believe it until I noticed that the neon light in one on my power strips wasn't lighting up when the room lights were turned off, but would light up when the overhead lights were on. So it may be that the trem lamp would only fire when the chassis was out of the case or uncovered.