HiWatt Leaky DC

[Buddha's Guitar Tech]

Rice is in the cupboard. That's science enough for me.

I stir-fried the amp with noodles and broccoli.

Actually, the amp is in a fairly dry room at the moment and it's awaiting some chianti and fava beans.

[Firestorm]

Just my opinion, but I am skeptical of whether boards that become conductive can reliably be salvaged. When they absorb moisture, the resistance drops: current flows; heat is generated; so the board should dry out. Except that the heat may carbonize the board microscopically so you wind up with a situation that can only get worse, not better. Depending on what is connected and how closely, some sections of the compromised board may become phantom capacitors and even phantom diodes in addition the phantom resistors. This kind of process must affect materials used in modern manufacturing, so perhaps a materials science lab will explore it properly someday.

[Buddha's Guitar Tech]

Yes, I would assume it would take a long time to dry out, as it probably took quite some time to get 'wet'.

Most sensible option would be to get the HT resistors off the board. Stands to reason that's where the rogue voltages are coming from.

Or just add a few extra coupling capacitors to block any unwanted DC getting to grids. Not sure what an extra .1UF cap would do to the tone. Can't imagine much.

[Firestorm]

Not sure what an extra .1UF cap would do to the tone. Can't imagine much.

It will change things if it's in series with another cap. You may have to swap out the original cap for a higher value so you wind up with the same capacitance you started with.

[Buddha's Guitar Tech]

You may have to swap out the original cap for a higher value so you wind up with the same capacitance you started with.

Good call.

Looking good so far. I did some PTP avoiding direct contact between the board and pot wipers using existing caps. (No new components yet.)

No more crackle with pot rotations in the tone controls. Just the master volume to go.

[gldtp99]

If you get the orig board dried out so that it works properly, what's to prevent it from soaking up moisture again in the future ? You do live in an area with generally damp comditions---- so i'd go with replacing the board.

i've used Hiwatt-type boards from Watts: http://www.turretboards.com/

i like the black (G10) with double turrets but any color G10 would probably work well.........................gldtp99

[Jana]

I remember reading that some of the different colored boards have some conductivity to them. I wish I could remember where I read it or which colors were worse. I vaguely recall a post saying that the black color caused some problems. I could be mistaken on that though. So, maybe it's not moisture but a "feature" of the board itself. I have a hard time understanding how epoxy could absorb moisture. Maybe it has to do with the dye or colorant that is mixed in with the epoxy? Well, that's all the vague, unsubstantiated rumors I have.

[gingertube]

If you can get the board dried out then you can prevent re-occurrance by spraying with PCB Conformal Coating. These come in 2 types (generally), a standard lacqueur and an Anti Corona type for EHT. Either should do.

When I worked in a hospital we had problems with pH meters (in blood gas analysers) with input impedance of 100 MOhms. Every 12 months we had to bake the board for 24 hours to drive out moisture and then apply a new coating.

I'm as little "suss" about this being a moisture problem (not saying it is'nt but in a tube amp environment I would not expect it to absorb much moisture).

I would give it a good scrub with iso-propyl alchohol first to make sure its not just a dirt/grease pick up problem.

Cheers,
Ian

[Buddha's Guitar Tech]

Well here's what I did and it sounds just fine.

Amp belongs to someone else, so it's not my money to spend.

May not look very pretty, but it sounds good and it's fairly solid.

Many thanks to all who gave suggestions, much appreciated.

[img:800:261]https://lh6.googleusercontent.com/-7ZdR ... G_1699.JPG[/img]