135W Twin w/Leaky Eyelet Board

[Ted]

I have a 135W Twin here, which has been stored in a damp place for some years.

DC levels are all over the place - lifting components from their eyelets and making direct connections to them brings the voltages to more sensible levels.
(Also - completely disconnected eyelets have DC voltages on them)

This appears to have been a long term problem as a number of tube connections have been made direct to resistors - this must have been done before the damp storage.

Two questions:

i) Will I be wasting my time trying to dry the board out?
ii) Will an AA769 type eyelet board fit? - preamp schematic seems identical to the 135W

[Leo_Gnardo]

i) Will I be wasting my time trying to dry the board out?
ii) Will an AA769 type eyelet board fit? - preamp schematic seems identical to the 135W

i) yes deafinitely. That board has been "leaky" since day one.

ii) some similarities yes, but another carbon-black loaded circuit board ? ? ? ? ? Don't do that. Also missing the 5-band EQ.

What I do in cases like this is rebuild the audio circuits on tie strips. Particularly the preamp/eqs, and reverb mix/recovery circuit. Some amps need more help than others. It's time-consuming but can bring excellent results.

If you have lots of time and pounds you can redo the whole kit on fibreglas epoxy turret board or eyelet board.

[J M Fahey]

Hey !!!!!
You are in LONDON !!!!!!!!
That explains it

That said, i'd make/buy a replacement eyeletted board made out of fiberglass or at least Pertinax and transfer parts.

[Chuck H]

I'm usually the FIRST to say "Dump that carbon impregnated board!", but...

In many cases it is only moisture that is causing the problem. Get the amp into a low humidity environment. Heat the board with a blow drier repeatedly. After a couple of days of this you can re test the amp. If the problem was only moisture reacting with the carbon it will go away. If the problem is that the carbon content of the board has aligned to form multiple resistors the failures will remain and the board should be replaced.

NOTE: It's been my observation that on many SF amps I've dissected the black board is impregnated with wax! I can only guess that this was an attempt to minimize the moisture sensitivity of those paper boards saturated with carbon. Ya think?!? If your board is already wax impregnated you almost certainly need to replace it If not, and drying it out helps, it may help to impregnate the board with something non conductive and heat stable. Though I'm not sure what. Silicone mixed with solvent maybe.

[überfuzz]

Listen to Chuck and... your willingness to tinker with amps. In addition to this, how does that old beast sound? I've seen amps with funky readings that sounded fiercely good. If you like the sound you could check if any component sees voltages/currents that isn't healthy. Me, well I love the sound of a 135W twin so I'd go for a new board without messing about like Chuck suggested.

[Mick Bailey]

Silicone mixed with solvent maybe.

Silicone can give good results on a dried out board, but there's the big drawback of silicone migration over the inside of the amp. It gets everywhere and makes any future work on the amp problematic; soldering is a real pain and the silicone is very difficult to remove. I've used Servisol Silicone grease mixed with aerosol-can silicone oil to thin it out. Brush it over the board, warm it with a hair dryer and keep reapplying until it gets soaked in. If you do end up replacing the board after using silicone you'll have a big challenge in re-using any components or wiring.

You could try re-waxing with potting wax (1 part beeswax + 4 parts paraffin wax) while the board is heated enough to melt the wax. It needs lots of patience to keep going until the board stops absorbing the wax. Results are variable. I think it comes down to how much the composition of the board has been affected as much as how much moisture it's absorbed. You also need to remove as much of the original contaminated surface wax as possible beforehand.

Either way, the board needs to be initially dried out through prolonged gentle heating. It can take a long time to get them thoroughly dry to the point where you can see the DC leakage dropping off.

[Enzo]

For conductive eyelet boards I have had fine results just clearing the surface with a heat gun. The waxy residue absorbs moisture. I apply the stream of very HOT air from the heat gun and just go down the board. I can easily see the "melt" line as I work each area. Kinda looks like a frosty windshield in my car as it thaws out. The heat evaporates the waxy film.

That doesn;t take a long time, and there is no labor time to build a new board.