Mesa Mark III blowing caps

[AtomicMassUnit]

Hey guys. I bought a broken Mark III and I'm trying to fix it. It had several blown caps, including one of the main 220uf filters, and the whole bias section with the diodes and resistors were burnt. I replaced all the parts that were burnt, checked my connections, and fired it up (literally). I didn't even turn it off of standby, just turned the mains power on, and the 470uf 16v cap that goes to ground across the 1n5352 diode blew up after about 20 seconds. I didn't use a 5352, I used a 4007. Could that be the reason this blew up? Or should I be looking elsewhere? If you look on the schematic ( http://www.schematicheaven.com/boogi...ogie_mkiii.pdf ) on the top left hand side of page 2, there is the cap/diode in question. I'm really hoping to get it working. Any ideas?

[Enzo]

Yes, that COULD be the reason. If you look at the schematic, that diode is drawn as a zener. Even says right next to it 15V 5W. The 1N5352 is in fact a 5w 15v zener diode. ANy voltage over 15 volts is shunted to ground. The 1N4007 won't conduct in reverse until you get past 1000 volts, and even then is will be an unwilling zener.

What happens is the supply can get as high as it wants there without the zener. I would guess the poor 16v cap saw as much as 70VDC. I don't doubt it gave up.

I hope the 6v zener above it survived, and for that matter all those LDRs and the 6v zener upper right by the FS jack.

If the bias supply blew up, make sure the new caps are wired correct - positive to ground.

Once you correct that and replace the cap, fire it up in standby and see if there is in fact 15vDC across that zener. WHile at it check for proper bias voltage at the power tube pin 5s. MAke sure ALL FOUR power tube sockets get bias voltage. Watch the schematic as the sockets are not all the same. But we want something like 45-60 volts negative at each.

The main B+ filters should be hot too, so are they sitting at 500v or so?

If all that is OK, hazard flipping the standby switch. Probably best without power tubes the first time.

[AtomicMassUnit]

Thanks Enzo. You know so much about electronics it blows my mind! I replaced the part, fired it up and it didn't blow up the cap. I measured across the diodes, 15v, I'm getting 500v at the main filters. I checked for bias voltage and only two of the tubes are getting the right voltage. The other two aren't getting anything, and I noticed that the wire coming off the board feeding the resistors that go to pin 5 is missing. I'm replacing that and should get my bias voltage, then I guess I'm going to try turning it on and praying I don't see smoke or fireworks.

By the way, this is an amp that had been sitting at a tech's house for years unfixed, so it'll be a really exciting event if I can get this thing going.

[AtomicMassUnit]

Ok... fired it up. It works! Played through it a few minutes, Im gonna clean it up some more and make sure it stays ok.

By the way, this is the 2nd Mark III I've seen that has the shielding inside the cap peeling off and dropping little bits of aluminum into the chassis.

[Enzo]

Glad you got there.

Thanks, but there are plenty guys here who know more electronics than I do. I just know how to troubleshoot.

[Rick Erickson]

Ahh, experience...

Thanks, but there are plenty guys here who know more electronics than I do. I just know how to troubleshoot.

I know what you mean Enzo, but there are lots of people who have electronics degrees yet can't troubleshoot worth a darn. There's a lot to be said for experience. There's nothing like turning an electrolytic into a party favor to give one some practical experience.

RE

[Enzo]

Back in high school electronics shop, it was a regular prank to leave charged caps sitting around, in particular so the following class period would encounter them. This would have been well over 40 years ago, so it was still all tubes we learned on. We had B+ supplies built into our benches. Not only would an electrolytic make a nice weapon of surprise, but a .1/600v oil filled cap will hold a healthy charge for a long damn time. Talk about a party favor.

It is pretty impressive when a little radial lytic launches with a loud bang and whizzes right past your ear.

It is also sobering when a springy wire soldered to something comes free as you heat the joint and sproings molten solder at your face, and there on your glasses lens is a big splash of lead where it solidified a half inch from the eyeball it just saved. That is an experience that really talks to you.

[AtomicMassUnit]

Except for guys like me who don't wear glasses.

[Enzo]

I think that was my point - you REALLY should wear protective eyewear when soldering. You can be blind in a fraction of a second. Were it not for my glasses, I would be now.