messa mkiv and capacitor problems

[TimmyP1955]

Alcohol isn't always what it takes to clean away residue, especially if it's from a drink. Gunk from a water-based drink will often require water to dissolve it.

[Chuck H]

Alcohol isn't always what it takes to clean away residue, especially if it's from a drink. Gunk from a water-based drink will often require water to dissolve it.

Alcohol usually works for me. It is water compatible and fairly hot as solvents go. I could see where it could be too fast to do enough work before it evaporates though. In which case plain old soap and water might be better.

[trobbins]

The use of a modern insulation resistance (generic megger) tester can be a very simple, elegant check of coupling caps. My tester is just a handheld 9V battery powered device, with DC 100V, 250V, 500V, 1kV ranges, and happily charges up a typical coupling cap in a few seconds. Definitely worthwhile paralleling a DVM to check the actual applied voltage, and when the test is finished it can act as a discharge path and verification that you aren't about to handle a charged cap. Perhaps some caution to initially check that the tester is applying DCV, with no AC.

I guess only having a few preset ranges may be seen as a little restrictive, but for first up QC of checking a new cap, or that old mustard - it neatly utilises one instrument for multiple uses - and may well pick up signs of voltage dependant leakage.

[cyclone]

One other thing, always check those flyback diodes Mesa uses on the power tube plates, doubt its the source of your frustration with this amp but its been the source of mine a couple of times.

[gearhead63]

^^^^^^What he said. And I'll add: It may not be necessary to dremel through the board. Often times you can remove only the burnt area. I've often simply carved a notch with an exacto knife.

I had a Marshall AVT50 head that the TDA7293 was toasted in. When I replaced it I had to take a razor knife and scrape the top layer of the board
until it looked clean. Didn't really have any carbon that you could see, was just a little darker looking. But was definitely conducting.