Woe is me, conductive fiber board

[Helmholtz]

Another example/video with cleaning the board: https://www.youtube.com/watch?v=5OWSUYKZsag

Seems the problem here mainly was conductive flux residue.

[Leo_Gnardo]

This is a great idea. Though with all the plate resistor and coupling cap nodes (which I think constitute a high enough voltage to be potentially problematic?) the effort might be a wash.

I had that thought some decades back, when I started to apply the tie strip method. So I chose to move the audio circuits off the board instead, only those obviously affected by leakage from the HV nodes. So far it's worked fine, hundreds of occasions. I'll stick with what's worked for me & my crustomers.

Let's not forget those cloth insulated wires jammed under the board. It's always an extra challenge when I've done the usual fix above the board, then still find noises leaking in through those, particularly volume control wipers to preamp grids - which is another source for scratchy noises. Also wire from input jacks to first stage grid - when DC is picked up here it makes guitar volume control scratchy sounding.

[pdf64]

I think a AB763 board might be able to be used, but depending on the particular model it is, some features might be lost, eg pull boost.
It would be more difficult if the model is the 70W.

https://el34world.com/charts/Schemat...str_rev_2_.pdf

https://el34world.com/charts/Schemat..._bmstr_rev.pdf

[Leo_Gnardo]

In other words, is a CBS Master Volume Super Reverb still AB763?

AFAIK Fender stopped using those ABxxx & similar designations by the time they started making amps with master volume controls. Aside from the MV, any differences might be some component values especially in the invert/drive circuit. Also any of their "overdrive" schemes, like volume/master labeled "pull for boost." That is, if you & customer decide they are worthy of keeping. I usually get "nah, I never use that, it sounds awful! Get rid of it."

If you can identify the amp you're dealing with in Mr. Hoffman's collection of Fender schemos, or you can send him the correct schemo from any other source, I'm sure you'll be a big step closer to circuit board success, and a terrific sounding Super Reverb.

[Randall]

Well, I put in the driveway for a full day today, and half day yesterday. It has been sunny and slightly breezy at 80 degrees and low humidity. It is now down to under 2 volts, and under 1 volt depending where I measure. The reverb is now not lowering the volume like it was. I am going to try this a couple more days if the favorable weather holds out. Maybe get away with not having to deal with a board rebuild. I'm getting too old for this sh+t.

[Helmholtz]

If hygroscopic effects are involved the results will not be permanent.
I'd at least do some scratching around eyelets.

[Leo_Gnardo]

Well, I put in the driveway for a full day today, and half day yesterday. It has been sunny and slightly breezy at 80 degrees and low humidity. It is now down to under 2 volts, and under 1 volt depending where I measure. The reverb is now not lowering the volume like it was. I am going to try this a couple more days if the favorable weather holds out. Maybe get away with not having to deal with a board rebuild. I'm getting too old for this sh+t.

Hooray for solar power! The last guy who needed such an amp dryout I encountered in May 2009. His Mesa MkII's had gotten soaked in Florida downpours before & after that years' StuporBowl halftime show, and they were randomly blowing fuses ever since. Very disconcerting in live performance, to be sure! I passed along advice to his tech, he should give the unboxed chassis a month of sunbaths on the New Jersey boardwalk not far from his house. No time for that! Instead, he got two freshly made Electrodyne Mesas, and of course that solution worked a treat. About time he got fresh new amps anyway.

I'm still wondering how the reverb control might diminish overall volume in the way you describe Randall. All I can think of, the unplanned conductivity of the board may have turned the reverb return circuit into a strong ultrasonic oscillator, and that's what killed off overall volume as it was added to the dry signal. Would have been something to see on the 'scope.

I hope the sunbath solution works out for your crustomer's deflicted amp Randall. Sure would beat a total boardectomy. Keep us posted!

[DrGonz78]

Then one day they all started installing dehumidifiers in their amps. Living in Arizona this sort of thing just doesn’t happen as often. My whole gripe with the industry is that they keep making the same mistakes. You see it with Marshall’s and Boogies where the circuit board becomes conductive. So an old Fender I with a conductive board is easier to forgive.

[Leo_Gnardo]

My whole gripe with the industry is that they keep making the same mistakes.

"Make it cheap as you can!" Companies run by bean-counters, owned by vulture capitalists.

[Chuck H]

"Make it cheap as you can!" Companies run by bean-counters, owned by vulture capitalists.

Or, more like give 'em what they want.?. The black fiber boards are for sale all over the place. Enough so to prove that people must be buying them? The "mojo" attached to the original vintage materials is more than offset by the inconsistency from current makers of these products that surely haven't taken the trouble to understand there's a problem. Paper absorbs moisture. Carbon (still used as the only true black pigment) is conductive. But vintage spec boards continue to be more readily available than boards offering a solution to the conductivity problem. I think the problem with new boards of this construction may be exacerbated by materials inconsistencies as well. More or less black pigment. The amount of resin used to stabilize the paper fiber, etc. Sorry to be on the clearly racist end of the issue but east Asian manufacturing of this design isn't going to make it better. A couple of members here have already used reproduction boards for these amps with almost immediate results of conductivity. Face it!!! You can't make a paper fiber product soaked in "lamp black" (actual carbon and still the major product used in most manufacture of black pigmented products) and not expect that you don't have a semiconductor. Board material should ideally be made from an insulative material. Not a semi conductive one. I fear that it's not only the people buying these that are ignorant, but also the manufacturers at this point. The currently manufactured boards are done that way for strictly cosmetic marketing reasons. And they may be even worse than what Fender was using seventy years ago.

[Leo_Gnardo]

There's not really more of a PITA than this and it seems quite unnecessary and stupid. Couldn't someone do a little more research before choosing a board material? It's like building a multimillion dollar house on a loose sand foundation. I feel for ya, Randall.

Fender wasn't expecting people would still be using those amps 50, 60, more years after manufacture. At the time the construction method was standard for the big exciting item of the day, televisions. Also tabletop radios & other common electrical equipment. If it lasts 5 years, that's a victory.

It IS annoying today's kit suppliers haven't substituted an improved version, but if you want to build "authentic" copies that's what you get, warts and all.

[Randall]

The reason increasing the reverb diminished the volume was because somehow it was adding more neg voltage to the bias. I really dont understand how, but I definitely could measure it as such.

I have had it out in my sunny driveway for 5.5 days and have gotten it down to 1 volt or less. More humid weather is on the way, so I guess I'm done with that. I'm going to advise the customer to not take it on outdoor summer gigs, or next to water gigs. Maybe even sell to someone in AZ. Get away with a $125 bill rather than 3 -4 times that.

[Chuck H]

Get away with a $125 bill rather than 3 -4 times that.

$425 to rebuild a Fender circuit board is pretty reasonable since you are essentially rebuilding easily half an amp to do it. Plus salvaging and reusing components where possible so that adds to the labor. But I get ya. If drying it out makes it work all you can do is apprise the customer of the circumstances and let them choose.

[tedmich]

I believe all fiberboard is harvested from the tree of Woe ...