M-500 Refurb.

Got into my M-500 today. It had smoked a cap in the power supply and I decided to re-cap the entire unit. I found that someone had been into the power supply previously (one taped up cap and a bad solder job on the cap that smoked). Anyway, when I de-soldered the cap, there was a residue left that solder won't adhere to. Anybody know of something only moderately dangerous that I can use to remove that stuff. Otherwise, I can't get a good solder joint. I was thinking of using muriatic (hydorchloric) acid, but figured I might eat up the entire circuit board, or at least dissolve the traces.

Got into my M-500 today. It had smoked a cap in the power supply and I decided to re-cap the entire unit. I found that someone had been into the power supply previously (one taped up cap and a bad solder job on the cap that smoked). Anyway, when I de-soldered the cap, there was a residue left that solder won't adhere to. Anybody know of something only moderately dangerous that I can use to remove that stuff. Otherwise, I can't get a good solder joint. I was thinking of using muriatic (hydorchloric) acid, but figured I might eat up the entire circuit board, or at least dissolve the traces.

You should be able to flush it away with one of those cheap stiff brushes (used to be called an acid brush) and some isopropyl alcohol....it's probably electrolyte from the leaky cap.

Got into my M-500 today. It had smoked a cap in the power supply and I decided to re-cap the entire unit. I found that someone had been into the power supply previously (one taped up cap and a bad solder job on the cap that smoked). Anyway, when I de-soldered the cap, there was a residue left that solder won't adhere to. Anybody know of something only moderately dangerous that I can use to remove that stuff. Otherwise, I can't get a good solder joint. I was thinking of using muriatic (hydorchloric) acid, but figured I might eat up the entire circuit board, or at least dissolve the traces.

After cleaning, you might have to use a little flux on it to get the solder to flow properly. Be sure to use flux intended for electronic (solder) connections - not pipe fittings - they're not the same.

to clean up solder flux i use a can of PCB cleaning spray (i forget the name but you can probably buy it at radio shack) and a toothbrush. you can also get the cleaning fluid in those little pen sized cylindrical metal tubes that have a fibrous tip -- you have to push down on them to get the fluid to come out and then you can use the tip to scrub the stuff clean. now the make the stuff a lot less stinky. the last can i bought smells like oranges.

for electrolyte goo, i just use some water and a toothbrush.

Well, I'm in a little deeper. The M-500 (non-t) has a couple of "daughter" PCBs attached to the "mother" PCB power supply board. One of them has, I found out, 4 electrolytic caps on it along with a couple of transistors and resistors, and it's about 1" square! I think in the M-500t, they put the traces right on the mother board. Anyway, what a PITA to get off. The other daughter board doesn't have anything of interest to me in the refurb. area (right now). I popped the cover off to look at it - just a bunch of diodes, I think.

I was wondering where those caps were, as they are on the schematic, but didn't show up on the foil side image. The M-500t service manual had most of them. That's when I went looking at that funny black box on the mother board.

i wish i could help bill, but i've never been inside of a non-t M-500. i'd be happy to look at pics if you want to post some.

Well, so far so good. I got that daughter board off. I got a vice out to hold the 1" board so I'd have two hands to work with (one for soldering iron, and one to alternatively use the solder sucker or pull the cap off the board). Just have to do the work now. I'll keep y'all posted. BTW, below is a picture of that little sucker.

M-500 daughter board.JPG (159.52 KiB) Viewed 1554 times

Well, so far so good. I got that daughter board off. ....

That's the voltage regulator for the rails. IIRC, the ratio of R511 to R509 determines the difference between the rails voltages. It's on the main board in the t model. The other black module is part of the triac control network. It's done more simply in the t model. Only one cap in there, so as long as it starts up and doesn't make any chunking noises you're fine.

Don't see any electrolytics on the other daughter board.

Here's the daughter board after re-capping. Notice the Panasonic caps are smaller.

M-500 Daughter Board After.JPG (120.16 KiB) Viewed 1536 times

Half way done (actually, I think the hard part is over). The power supply is re-capped. Powered it up and the lights came on and nothing smoked. Powered it off and discharged the caps through a 100 Watt light bulb. Definitely lit the bulb. Anyway, here's a picture of the re-capped power supply.

M-500 Power Supply.JPG (239.8 KiB) Viewed 1500 times

Half way done (actually, I think the hard part is over). The power supply is re-capped. Powered it up and the lights came on and nothing smoked. Powered it off and discharged the caps through a 100 Watt light bulb. Definitely lit the bulb. Anyway, here's a picture of the re-capped power supply.

Looks very clean in there BillD. One thing you might try, since it looks like you like to keep them looking good inside as well as out (as do I).

The heatsink compound doesn't prevent the aluminum from oxidizing (I thought it would, but evidently not). If you unbolt the big rectifier down by the transformer and clean off underneath it, as you rub, the paper towel will get black with aluminum oxidation. Then recoat with heatsink compound and it should be a more efficient thermal sink. I did this underneath the output transistors too.

It's very dry out here and nothing much corrodes. I might try removing the rectifier, but I'm a little wary of prying up the output transistors.. BTW, for anybody watching and wanting to do this project, it's pretty easy. That little circuit board a few posts ago has four caps on it (tedious but easy). You can see the six large caps in the last picture. There are only two other electrolytic caps on the power supply board. The electrolytic caps on the amplifier board should be replaced "sometime", but have a much longer life in that they operate no where near their ripple current. So, re-capping the power supply and adjusting the bias goes a long way to making your amp like new again.

It's very dry out here and nothing much corrodes.

But there's air out there, so things oxidize. Probably not a big deal, but I'm kind of anal about such things.

I might try removing the rectifier, but I'm a little wary of prying up the output transistors.. ......

I was replacing the transistors as well, so it was a no-brainer for me. The M-500 mounting method is different from the M-1.0. Bending the M-1.0 output leads to clean underneath might put a lifting pressure on the pads, but then again it's an anodized heatsink with no heatsink compound anyway. The M-500 mounting method would leave only a lateral force on the legs, which shouldn't weaken the solder joint integrity. BTW, I have a 1 lb spool of 7% silver solder, and probably won't go through it in a lifetime if anybody needs some.

Untitled.jpg (9.42 KiB) Viewed 1482 times

Thanks for clearing up the concept of oxydation for us RichP. Most people assume that its the humidity (or water) that causes oxydation, but it is actually the oxygen in the air and in the humidity that does. Water causes accelerated rust on anything with iron in it unless it has been stabilized by alloying it with other metals (stainless steel anyone?)