Stuck With a Vintage Blackface Fender Pro Reverb Repair

[cryptozoo]

I've hit a wall with a vintage (1965) Fender Pro Reverb amp, and am hoping some of the more experienced vintage Fender gurus here can validate my findings or point out anything I may have missed.

The amp came to me with bad power tubes / blown fuse, and after opening it up I discovered the plate resistors were shot. Most puzzling was that 99% of the components on the board had been replaced -- not a great job, but decent enough. Asian poly caps, carbon film resistors. The optocoupler was still original, as was the wire. Some mods had been done (splitting shared cathode caps / resistors on a couple of tubes) and some of the values used elsewhere deviated from the schematic. I replaced these values with correct values, removing mods as I went.

After I was finished, I fired it up and there was a lot of 'blow,' or noise, with the volume turned down. Sounded similar to bad plate resistors, but I'd actually replaced those during this process. In my troubleshooting, I isolated the noise to the Reverb channel / ch 2, noting that if it was disconnected the amp was dead quiet. After some preamp tube pulling, I then isolated the noise to V4, the reverb recovery / 2nd gain stage for ch 2. I then checked the Reverb transformer, grid / grounds and determined it was happening in the other triode, the pin 6-7-8 side. After replacing all parts associated with that triode (including the resistors and bypass cap) the problem was still there, and I figured it had to be the grid wire or perhaps the socket. I disconnected the wire and ran another wire and the noise remained, so I changed the socket. The noise remained. It's the occasional low-level electrical noise, followed by some scratchiness that was intermittent. Not insanely bad, but it's louder than it should be and fairly annoying at times. At this point I lifted and subbed components in for the stuff I'd replaced, checked all the pots again, checked grounds, etc. The only thing that killed the noise was disconnecting ch 2 directly at the 220k PI blend resistor or lifting a leg on the .1 cap that preceded it (I subbed in another cap to test, no change).

After getting some voltage readings on a batch of lifted capacitors that were still attached to the board, I started poking around, and here's where I'm at. I've heard the arguments about conductive boards being a myth, but I wanted to share my findings. I'm measuring almost half a volt DC at certain parts of the board -- I've confirmed this by lifting the B+ node that feeds the plates and confirming there's no voltage reading after that. The highest reading I'm getting is the area around where the plate B+ feeds in, and this happens to be very close to the reverb recovery resistors.

Now I'm starting to see why every component on this board had been changed.

The amp had previously had a full doghouse cap job as well (Sprague Atoms) and although I've tested them (all test fine) and the voltages are right, I have not replaced all the new caps. PT seems fine, voltages are steady and where they should be for 120vac at the wall. Choke checks out as well. So a complete fiberboard replacement might be the final frontier here, other than subbing in a new OT. I also don't think this is the issue because as I mentioned, ch 1 sounds great so long as channel 2 is disconnected.

Is there anything that sticks out here as something I may have missed? I'm really open to suggestions at this point. I had to leave it and get some distance for a bit, as I'm really scratching the ol' head here. Many thanks.

[Jazz P Bass]

Try the old hairdryer trick on the conductive board.

If that helps lower the measured voltage on the board, you may want to consider replacing the board.

[cryptozoo]

Jazz P, I'll give that a shot. The board is a bit wavy but I've seen much worse, and it doesn't seem to be damp. I may sandwich some desiccant packs in between the eyelet board and lower board and see if that helps at all after a couple of days. At this point, I've spent so much time on it that I wish I would've just replaced the board from the get-go, but symptoms like hiss and noise could really be any number of things.

[Leo_Gnardo]

In cases like this I rebuild the audio circuits on tie strip and "float" those over the main board. Done hundreds, maybe thousands by now this way successfully. For those who have the $$$$$.$$ yes a new board, made of something other than black cardboard, would be good.

With just about all the components replaced it looks like the previous tech was trying to find the noisy component, without realizing it's the BOARD that's noisy. Once, a long time ago, that was me.

[Enzo]

It isn't wet, it has absorbed moisture. Seriously, use a heat gun on it. I have yet to get one I could not bake into proper operation.


Ground your meter to chassis. Find an eyelet on the board that has several hundred volts on it. Now stick the meter red probe onto the black board right next to that eyelet but not touching it. Do you get any voltage there on the black board?

[cryptozoo]

Right on, that gives me some hope. I'll try the heatgun method and see if that'll do the trick. How many of these do you guys figure you'd run across?

Enzo, I'm typically getting between a tenth of a volt all the way up to a half-volt DC, depending on where I am on the board. The worst is near the plate resistors of V2 and the area where the preamp B+ connects to the middle of the board.

[Enzo]

If you measure any DC on the fiber board at all, it is conductive.


I aim my heat gun at the board and start "defrosting" it. The board itself starts out looking a little waxy, but you ever start the car on an icy day with a fogged up windshield, turn the defroster on and watch the windshield clear from a small area that grows until the whole thing is clear? I can see that waxy look evaporate away, leaving the board looking dry and clean and the contamination evaporates. I work from one corner across the board and down its length.


It happens enough to know about, but is a minority of repairs.