Gotta get the Fender Chassis out of the Vibrolux cabinet first.....

[nevetslab]

While still waiting for parts to arrive for the Ampeg SVT4-Pro, I set that aside and moved a new client's Fender Vibrolux Reverb combo up to the check-out bench. Noisy and intermittent fuse holder are a couple of the symptoms, so removed the upper rear panel to carefully extract the chassis from the cabinet. Famous last words with so many of the Fender Combo amps I receive.

I got the chassis two thirds of the way out before it locked up tight. The chassis width and the cabinet inside width are just tight enough for the sides of this folded chassis to grab hold of the inside Tolex flaps of the cabinet, where over time the adhesive bond has failed, and the chassis side details just sharp enough to peel the Tolex flaps back to lock up the chassis before you could fully extract it. OK, I'll push it back into place and see what I can do with my thin steel rules from the two sides. Nope. Can't put it back in either, as the beveled front edge of the chassis front panel functions perfectly as a Chisel and did a great job peeling off the front edge of the roof shield foil and stops the chassis from getting put back in. So, Adhesive bond failure on the top and side inside surfaces over time has left me with the challenge of somehow getting that POS chassis removed, and then take the cabinet apart in order to effect repairs on the Tolex flaps and the roof foil before getting to the electronics. Sigh.........

It's not that I haven't been down this road before. It is irritating, and sad that the owner of the amp will have to bare the cost of the labor to make his amp serviceable for future repairs.





You can also see on the bottom of the cabinet, what time has done with the adhesive bond of the Tolex Flaps at both ends of the bottom surface, where the adhesive has failed and curling is already taking place. I should have looked at the bottom first before attempting to extract that chassis. But, usually one is lucky and can get the chassis out of the cabinet without the cabinet telling you no way in hell are you getting this from me!!! A day in the life in the shop.

[The Dude]

That shi+ happens to me lots with certain Fender amps! It drives me bonkers that they can't just make the chassis a few mm smaller or the cab a few larger. It's just a lot of unnecessary work. The designers should be buried in coffins that are 2 inches shorter than they are tall.

[Helmholtz]

Never had that problem with 60s or 70s Fenders.

[Stratfordade]

The worst chassis removal experience I’ve had was just recently with an Ampeg BA115HP bass cube. Took an age of rocking and inserting thin plastic shims around chassis to finally remove it. It’s made harder by lack of good places to get a hold.

No adhesive failure but just a little excess tolex around the captive nuts clipped into the chassis. I trimmed off some of this and slightly sanded down tolex at these points to hopefully make reinstall easier (it’s still on bench). If you get rough with it there’s risk of bending chassis and/or damage to captive nut cut-outs.

[nevetslab]

After restoring the cabinet's roof foil and side Tolex Flaps using contact cement followed by T50-5/16" staples, everything is back to normal after putting the cabinet back together. The amp chassis slips in and out with the greatest of ease now. Still, what a PITA.

Now, I have very high HUM level in this amp. It's a Custom Vibrolux Reverb amp. 60Hz hum is dominant, so now chasing that.

Custom_Vibrolux_Reverb_0A4.pdf

[stokes]

Have had that tolex problem with just about every vintage Fender amp I opened. As for the hum, was it there before the chassis was removed and replaced? If not check that the foil shielding is making contact with the chassis ground.

[Pedro Vecino]

Another similar problem is what happens to the aluminum plate (of a certain thickness) when it deforms. It can happen with some Silverface amps and yesterday it happened to me with a JCM900 combo. I had to remove the side handles to access the screws on the front panel and tilt it back enough to have enough room.

[nevetslab]

Have had that tolex problem with just about every vintage Fender amp I opened. As for the hum, was it there before the chassis was removed and replaced? If not check that the foil shielding is making contact with the chassis ground.

I'm just getting to that problem, now having the cabinet restored. I had the hum problem from the first time I powered the amp up before even taking it apart. The pair of 100 ohm resistors to ground on the heater supply are present and do get to ground, but I'm planning on punching a mounting hole to the left of the PCB-mounted fuse on the rear of the chassis for a 100 ohm 5W WW hum balance pot. Also need to see what the plate current of the matched J/J 6L6GC power tubes are pulling. Initial visual and electrical inspection shows all looks and measures nominal, apart from knowing what the power tubes are pulling.

The foil shield was in contact with the chassis, and on the service cradle, I had an aluminum plate sitting on top of the chassis just to see there wasn't a problem from that. So, the pursuit begins.

[nevetslab]

Now having worked out the mechanics of installing the 100 ohm 5W WW Hum Balance Pot into the chassis, I removed the power switch, first having to remove the condom from the switch handle in order to get my 15mm nut driver over the mounting nut and remove the AC Mains switch, so could then unplug the AC mains Power Cord, needing that cord out of the way in order to drive the Whitney 5/16" punch thru the rear panel to the left of the AC Mains Fuse PCB. The shaft of the pot was too long which had to be cut down and a new screwdriver slot cut into it. So, took care of all those details and got a decent retrofit onto the shortened shaft. Then installed and wired up the pot. While Fender at least used twisted GRN leads from the PCB to the pilot light, they instead used same Blk & Wht AC Mains chassis wiring to/from the power switch for also wiring up the Heater Wires between all of the tubes from the PCB. I followed suit. After I got the pot wired up, I powered up the amp to see if this bought me what I needed.

It did, and it didn't. As we all have found in dialing up a hum balance pot, ideal setting fully depends on the channel(s) volume settings as well as the tone settings. Get it dead quiet, and then turn the channel volume up or down from where it was, changes the degree of hum, as does adding the second channel's volume & tone pots. So, I did my best to dial it in for what seemed reasonable. Final setting will have to be done with the owner's guitar plugged in and see how he likes it. At least it's now quiet and not buckets of hum as it was when I first heard it. And the Fuse Holder isn't a factor any more of the amp turning on or off. Clipped out R62 & R63 from the PCB before powering up the amp, of course. WHAT didn't go so well was putting the 'condom' back onto the power switch. I wasn't able to push the large portion of the mold back over the switch's mounting nut without it failing and tearing open. So, I guess these condoms are a one-use item like the real ones.





Chassis now fits in and out of the cabinet without any issues from the roof foil or the inside left and right Tolex Flaps, so it's a working amp again. There was a second Black Accutronics/Belton Model 34063 Reverb Tank sitting on top of the one inside the reverb pouch screwed to the floor. I had measured that tank's coils, and it measured just like a normal 4AB3C1D tank's DCR readings. Pulled the tank out of the pouch earlier and didn't find any broken springs. I found that reverb tank sounded nominal, so I left it in. Now I can call the owner to come have a listen.