“De-CBSing” a ‘76 Fender Vibrolux Reverb, got some questions…

[Raoul Duke]

Here’s the differences I’ve found so far vs the schematic for an AA270 FVR (left to right on the board). I may have the function of the sub-circuit wrong, so please correct me if I do. Still learning how to read schematics correctly.

LTP:
Schem - 470R, 1M, 470R
Actual - 330k, 680R, 330k

Reverb recovery:
2k7 has a cap in the schem, not on board

Reverb inject:
470k replaced with 560k

Reverb driver:
2k2 with cap on schematic; board has a 680R with no cap

All in all, probably not as drastic as I first thought - but still different. I’ve attached a DIYLC of the board as I found it for context. Any observations or lessons are welcome and appreciated.

Thanks!

76 Vibrolux Reverb.png

[Raoul Duke]

Just about done with everything and planning to light it up tomorrow. Ended up with about 14” of extra wire just with “the easy to get at” stuff. Probably could have reached 20-22” if I had the motivation to pull everything out of the front of the chassis and do all the board grounds and pot leads. Everything looked pretty well tucked-in and “green goo-less” so I left it be with the exception of all the pot jumpers (goo end to end) and the vibrato channel bass pot lead (took about 2.25 inches to lose the goo).

Also came to the realization that I have an AA1270 circuit; not the AA270 I bought it as. Very similar but the output tube stabilization caps should have clued me in sooner - but live and learn. Definitely learned a lot my first time going through an old-ish Fender.

Hopefully everything works like it should tomorrow.

[Raoul Duke]

Got everything up and running with only one goof-up. Mistakenly put the 220k on the wrong side of the reverb pedal jack. Easy fix. All the DC is off the board as well as a ton of wax, wiring cleaned up and organized a little neater, tube sockets cleaned up and neater, heater wiring redone with correct polarity, lead dress fine-tuned, sounds great biased right at 60%.

The only mods I added were a bleeder resistor on the first filter cap, 1R/1%/2w bias sensing resistors in place of the stabilizer caps, cap across the roach to de-tick the tremolo (not in the pic), removed all the ground tabs from the PT and made real chassis grounds, re-did the 3 prong cord eliminating the ground switch and death cap but keeping the outlet. Also changed the first dropping resistor to 3k3 from the AA1270 2k2 to get the preamp tubes back down to the correct voltages - they were all running at ~25% above the schematic voltage range.

All in all, real happy with how it turned out. It’s quiet and sounds good. Learned quite a bit on my first Fender overhaul. And “no” - I didn’t write the reverb transformer wire colors in the chassis. Did the best I could to get rid of it, but didn’t get it all, lol.

IMG_0420.jpeg

[martin manning]

A very satisfying result, no doubt! Seems like the engineers missed an opportunity there, they could have told the accountants that the assemblers were wasting at least 10 cents worth of wire in every amp.

So, you measured DC on the board previously and now after cleaning it is gone? What process did you use?

Do you know what the "green goo" on the wire leads is? Corrosion?

BTW the safety ground should be bolted, not soldered, and it looks like there is already a nearby hole in the chassis for such.

[Raoul Duke]

There was at least 14 cents in there I think, lol.

I didn’t completely eliminate the DC, but got it under 3 mV consistently across the board including spots where it was close to a volt (around the V2 plate load resistors for instance). I scraped wax, lifted the components on one end, heated the board, wiped off with isopropyl, repeated until very little would leach out. Was able to get to 90+% of the board working in small increments.

The goo looks like verdigris but is slimy vs waxy. Seems common in ‘70s Fender amp wire stock. Supposedly becomes conductive and I’ve seen it eat the wire insulation. Research seems to indicate it becomes the root of many problems if not halted before it propagates.

Thanks for the tip on the safety ground. I was just trying to make it like older Fenders. I’ll fix that.

I have to admit that all the fixes I’ve applied are to the credit of a few YouTube channels I watch that go pretty deep into old Fenders. I got a lot of pointers from those videos.

The only thing I’m still thinking about is that the amp came with Sovtek 5881WXT power tubes in it. They sound good and bias fine, but the data sheets say they are only rated for 250v on the plates (they run at about 415v in this amp). As stated, the amp came with them and I’ve never thought about them until now. Maybe a set of real 6L6GCs would be a good idea?

[jelle]

I have used the Sovtek 5881WXT in Fenders at 450V plate voltage as well as in Music man power amps at 700V plate voltage without issues. This tube can handle the 415V just fine.

jelle

[Raoul Duke]

Thanks very much Jelle! I appreciate the benefit of your experience and advice.

While I’m talking tubes, anyone ever experienced premature failure of recent manufacture EH 6L6s?