Fender Custom Vibrolux Reverb Cutting In and Out. Help?

[teddycaster]

Hey all, Ted from Philly here. First post. So Hi! Just wanted to see if I can get some insight from yous guys first in hopes some solution comes about before I have to pay a tech and get the same information that I already figured. Anyways here's what's up:

I've had this Fender Custom Vibrolux Reverb amp for a whilllle and have toured with it and got a lot of use out of it. I know these are mostly hated but I'm one of the few that really makes use of its tone and it works perfect for me. This one always seems to be a little buggy, but after warming up it always seemed to get into a good spot. But typically it would sound like it was scratchy, cutting in and out of volume, the reverb cutting in and out and just sounding like it was clearing its throat if that makes sense. Again, I would typically just crank a low E and strum real hard and it would like get cool. Now it just is realllly bad and doesn't quite shake it off. Also the pilot light is intermittent and is mostly off but flashes on and off when the amp is having ups and downs. It's scrambly, static-y, in and out of volume, will be at like 1/4 volume for a while then spurt loud randomly, just sounds awful. I swapped tubes immediately and it seemed cool then instantly went back after a half hour.

Where it gets weird: I've done a few changes over the years so in my troubleshooting I tried a few things outside of the chassis first. When it was acting up with the 2 10" speakers in the back (Celestion g10 and a Jensen ceramic, both 8 ohms together at 4 ohms), I would try bypassing those and plugging into a 4x12 cab of unknown ohms, and the problem ceased. Wasnt sure if it was an output transformer issue and maybe it couldnt handle 4 ohms and this cab was more digestible??? not even sure if that makes sense. To test this I tried it with a 4ohm 2x12 ampeg cab and again sounded good then immediately acted up. I even swapped out the celestion speaker to have matching jensens and it actually just got worse. I tried it again with the 4x12 that was previously good and it seemed great still but then started acting up, to which I think maybe it could be an ohm thing but it doesnt officially fix the problem. Maybe that's all a red herring, I'm not sure but there was a clear diff between the cab and the orig speakers at first.

The pilot light also is mostly off and occasionally flashes on or may even stay on, and seems like it freaks out when the amp freaks out. Through my research it's sounding like the pilot light is tied to the PT and I'm wondering if these issues sound like a classic PT or OT problem and if there are other things I could check out to diagnose. I haven't opened the hood to test the taps yet and am a little unsure how to get it hot and cranked and simulate the problem while looking at values/if that's something I'm comfortable with. (I've done a lot of amp fixes over the years, nothing this in depth tho).

SO.

Before I take it to a tech, I just wanted to see if I should grab a new PT or check out some other issue that I'm not aware of. Just don't want to have to spend another $100 to be told I need to get a $200 PT that I already figured. Any advice would be appreciated!


MODS:
Spring reverb upgraded to 3-spring, same impedance etc.

Replaced reverb driver transformer w new one

Was using a Dr. Z air Brake for a while, not sure if this could hurt anything?

Tubes replaced, was the first thing I swapped out to diagnose

Re Capped a few years back

- Not sure if any of that matters just wanted to put that out there. Thanks! - Ted
 

[Latole]

Your problem seems to come from a bad contacts or solders in the power supply circuits.

Nothing serious in my opinion, but you have to take a close look at the printed circuits and solder at each tubes socket and redo the solder or the contacts (clean or fix them ?)

Do you talk about the "Blackface Custom Vibrolux or the 68 Custom Vibrolux ?

I have both schematics and service manual for the Blackface

First picture ; the Blackface Custom Vibrolux
Second:  '68 Custom Vibrolux

[SILVERGUN]

I had a similar symptom with a different amp. The pilot light was telling me that there was an intermittent open in my heater supply. I was able to see that the trace had heated up and lifted off of the board. When the pilot light was working ok the amp was working ok. When it started to flicker I would experience the same type of cutting out.
I was able to repair mine by jumping across the burnt section of trace with some wire.
First you have to find it. I would not just throw a PT at it.

[Bieworm]

Just throwing it in here, not to put you on the wrong track.. but I had the cutting out once with the choke gone bad.  New choke and the thing ran like new. But the intermitting pilot light makes me suspect a bad connection too...

[teddycaster]

Thanks team! I'll crack her open and look for broken traces. To answer the previous question regarding which kind, it's the Fender "Custom" Vibrolux Reverb, not the new 68' reissue or anything like that. It's the 90s-2000s one with the white knobs, Normal and Bright Channel. here's the Schematic. https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Custom-Vibrolux-Reverb-Reissue-Schematic.pdf

Now, one question, the pilot light looks like it's coming straight off the PT, could a break somewhere else down the line affect that? I
'm just wondering if there's a shot winding somewhere in the PT that is jostling in and out of operation. Anyways, thanks for the help I'll take a look under the hood soon and see what's up.

[Bieworm]

I think some jangly winding in a PT would not result in some simple crackling... there would be bigger problems.
Good luck on finding a cracked trace on that pcb.. it's a hell to remove it from the chassis. Take lots of pictures of the lead dress. My guess is if there's something loose or cracked it should be induceable by moving stuff and prodding with yer chopstick...

[SILVERGUN]

the pilot light looks like it's coming straight off the PT

I'm assuming you are assuming based off of the placement on the schematic (?)

The layout is incomplete because it doesn't show how the pilot light is wired, but I'm going to assume that CP13 and 14 are where the secondary ties in....not directly to the pilot light

My theory should be easy to test for. Get some test leads and find a way to clip your meter onto either CP13 and 14 OR R62 and 63 and monitor the voltage.
My issue wasn't happening immediately and I'd have to let it warm up a bit before it showed up.

[sluckey]

Since the pilot lamp cuts out at the same time the amp cuts out I suspect the filament voltage is cutting out. Likely suspects are CP13 and CP14. Maybe just a loose connection. Maybe a bad solder joint.

Looks like the layout shows twisted green wires going to the tubes and twisted black/white wires going to the pilot.