Worth Saving this Relay Board?
 

I'm getting my car running again after an 8 year garage slumber
and noticed that my relay board is completely disintegrating.

I did have an electrical fire around 16+ years ago when
a light came loose out of the back of the tach and gounded
against the casing. Funny how such a small thing caused so much
damage. Anyway, I fixed/replaced quite a few wires in the dash
and things were working fine many years before the car
was "stored".

Now the entire bottom of the relay board, which looks like
dried tar, is flaking away.
Has anyone else ever seen this?
I guess I could repair it with some new polyester resin
but is it worth it?
How much does a 74 relay board usually run?
Anybody got one to sell?

--- Bill
http://forums.pelicanparts.com/uploa...1076633470.jpg

If the board works OK (you can get the diagram off this website and ring out every circuit) then you can dissolve the tar with solvent and re-pot with modern epoxy. I think the epoxy cost me about $20 a board. PITA but most of them are headed in the direction of yours with that old material and high underhood temps.

While you have the potting material off, solder all of the crossing connections.

Well, an update on the relay board.
I went through every connection on the board and there is an issue.
It was quite strange. I had to scrape off the tar to see
what was happening.
The trace directly below the 25A fuse that feeds most of
the relays is broken. (the thin line at the end of the arrow)
But strangely enough it still makes marginal contact.
It shows up as about 70-80 ohm resistance.
If you press on the fuse contact hard enough it will actually open.
The break is right where the rivet attaches the fuse holder
to the trace.

This fuse holder has some evidence of having been very hot.
(slight melting at the base)

[see photos]

Not sure if this was caused by my earlier electrical fire years
ago.

I would have never thought of checking for a marginal trace.

Ok, so now I understand the comment about soldering
over the cross connections.

So now the question is, would soldering a new wire over the
trace and up into the rivet hole carry the current necessary
to be a permanant solution? I'm assuming it would.

For anyone who has removed this tar coating.
what solvent did you use that will disolve the tar but not
damage the plastic board?

Also, once all the tar is gone, won't alot of the traces simply
fall out?
Sounds kind of like a tricky operation.

--- bill

http://forums.pelicanparts.com/uploa...1076650595.jpg
http://forums.pelicanparts.com/uploa...1076650645.jpg

The traces are held in by the rivets.
They won't fall out.

I used a lot of things to get the tar off. I never figured out the "best" way. Lots of scraping helps. Get the big chunks off first.

The bad connection at the rivets is how these boards usually flake out and die. That's why James suggested soldering all the rivet connections. That will assure a good connection assuming you can clean them up enough to get solder to stick!

You don't have to solder any wire in--just solder the rivet to the copper trace!! :) Do that to all the rivets and places where the traces connect to each other.

Then get yourself to your local electronics supply store and buy some "potting compound". Follow the directions on the container and apply it to the back of the board, let it set up, and you're done! You've got a relay board that should outlast the car, at least if your solder joints are any good. ;) (Mine ain't so great...)

--DD

You can get clear conformal coating that works too. Then you can SEE what is going on under the coating, and it is much easier to strip off if you ever need to.

Can anyone please share his experience how the original tar-like substance can be easiest removed ? I'm a bit hesitant to simply scratching it off - afraid to demage the connections more than necessary - thanks in advance, Harry

I 2nd the recommendation to use clear conformal coat that's what we use in the board industry.

I would be very concerned about the right hand fuse mount that's in your pics. It looks like it was subject to high heat and arcing If the plastic base of that prong is crumbling you should consider getting a replacement board. I don't think that you can adequately repair that base so that the fuse is held tightly.

I scraped the large pieces with out going too deep. Then I used carb cleaner and a stiff brush.