Remove epoxy from Stereo 410 PC-28 board?

GregDunn

Active Member
OK, I've been rebuilding and restoring Dynaco gear for (mumble) years now, and this is something I've never seen. I got this amp essentially for free, but it spent many years in a garage in Florida so I anticipated spending a lot of effort in cleaning it up. However, before stripping it I thought it was a good idea to run some electrical tests so I could see what I needed to order for repair parts - or if it was even worth cleaning up the chassis.

The good news is that the transformer, rectifier, and caps are in perfect shape. These old Sangamo caps that Dyna spec'ed on their power amps are something else: the 10k µF units measured about 11-13k with amazingly low leakage. I was able to re-form them and they are in impressive shape.

Measuring the nominal resistance for the output stage test points gave 3000 ohms pretty much on the nose; so there's nothing shorted or defective on the outputs.

However, the PC-28 boards are obviously in need of some repair; I got too much bias current on one channel, and the other one is OK but has a DC offset which increases as I bring up the power supply voltage. These can be dealt with, I'm sure. But when I started examining the boards for obvious damage, I saw this on one of them.

IMG_2267.jpg

It looks like someone epoxied ... something ... to the board which subsequently broke off. It resists chipping or scraping, and I'm going to have to remove the board from the amp before I do anything further. The rectangular piece looks like the carcass of a sandblock resistor, but the goop around it is definitely epoxy or something very similar.

Has anyone else had to remove a hunk of epoxy from electronics before? Fortunately, it's on the top side so it won't take any PC traces with it, and I think the grommets for Q213 are the only things I'll have to be careful of. What's the best approach? Grind it off carefully with a Dremel tool? Drill holes and try to break it off in chunks? This is a bizarre experience and I'm torn between amusement and horror. :)
 
Localized heat from a high watt iron will soften and degrade the bond. Gently chip or cut it away with a good blade after you heat it up to about 200* F. Are both boards like that?
 
Nope, just the one board. Do you suppose a paint stripper on low setting might work as well? I'll try my soldering gun with a heavy tip first, to see if it will even soften the goop...

Edit: I put the cutting tip on my big soldering gun and gingerly worked it into the epoxy. What happened was that it started turning into a powdery dry substance and almost literally fell off the board. I think I can get the vast majority of it off without damaging the resistors, let alone the board. The big sandblock carcass thing might be a little harder if any epoxy got under it, but I'm encouraged by the initial test!
 
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I am glad you had some success. I have worked with all kinds of epoxy in my past life. Good luck on your restoration.
 
Glad that worked out. If it was epoxy, remember that the PCB is also epoxy, so chemical strippers are a bad idea. Local heat or mechanical removal is the best option.
 
Oh yeah, I ruled out chemicals almost immediately; ages ago, I tried PC board cleaner on a unit to remove some flux and adhesive - it started to dissolve the components! That's why I asked here, figuring that others had prior experience with epoxy and could save me some hassle.... :thumbsup:
 
Done. A little of the surface of the PC-28 board came along with the epoxy, but not enough to ruin the mechanical integrity or impinge on the solder holes. I'll give it a scrub with some alcohol and start ordering replacement parts. Incidentally, the resistors all seem to measure OK - even the one which was totally submerged in the epoxy - but I'm replacing them all just to be sure.
IMG_2305.jpg
 
A quick update - I got all the parts for a complete refresh of the PC-28 boards and did a quick survey of the boards to see if I could get them working with minimal soldering. I plan to completely strip the chassis and have it re-done, but really wanted to be sure there wasn't something major wrong with the electronics first. Sometimes it's easier to work on an opened-up amp with some mechanical support than to have it spread all over the bench with the risk of wires crossing and boards touching. The first thing I did was replace the questionable resistors under and near the epoxy glob.

I think the PS caps are going to have to be replaced - they measured OK but seem to have a lot of leakage at the 80V levels required by the amp with any significant current draw. Also, one channel seems to have an offset problem and the other one has a bias problem. The values in question rise beyond safe limits as the variac voltage is increased. The output stage static resistance is well within Dynaco spec, so I'm pretty sure the issues are confined to the PC-28 boards. Whoever was in here before me substituted some bizarre choices for transistors, so perhaps I should just swap all the active components on PC-28 before testing again. And I'm leaning toward upgrading the driver bias resistors, as they are all old carbon comp units and probably under-rated. I've had these crack in half before due to overheating on my ST-400, so they're always a concern.
 
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