Marshall JCM800 Lead Series Repair - Power supply Issues

[jcrubin]

This JCM 800 landed on my bench had cutout issues as reported and was apparently blowing out fuses.  This sounded strange, but since it was blowing out fuses, this is going to have to be put into service position and troubleshoot with the power-off.  This will start with a visual inspection. The visual inspection immediately reveals the first issue, which was expanding electrolytic capacitors.  Also oxidation is exhibited in the tube sockets.  The underside reveals capacitor leakage. During a fuse inspection it was found that the owner was attempting to put higher than rated fuses in the HT side in an attempt to keep the amp running.  Hopefully this did not cause any internal damage.  The tube sockets were all hit with deoxit after being individually inspected and each tube tested, one finals tube was found to be shorted, very interesting.  A strategy is assembled to deflect the circuit board out of the way. This is required to reach one capacitor.  With that, all of the caps are swapped out one by one.


Part 1 of 2

[jcrubin]

In part 2 we continue testing the caps that were pulled from the amplifier and come to find that while the capacitance was good (with the exception of one that exploded) what it does in term of leak-down and its quickly shown that these had an event that appears to be a condition caused by the amp being turned on after a long time being dormant.

     Continuing on fuses are re-added and the test speaker is employed, Variac is setup for a test run.   With a good smoke test and voltage test, the bias of -37.4 is enough to get going.  This gives 32.5ma to get started with 418V on the plate.   With noise coming out of the speaker, cracking and cutout, I could quickly identify an issue in the pre-amp stage which turned out to be a failed tube.  After this the hum pot is setup for minimum.

    Testing all of the control with the guitar shows everything to be in working order.  Brings us to a first biasing event. I got actual matched sets which tube depot did a meh.... job on so i ended up having to mix and match all of the tubes to come up with something.    With the bias completed everything is re-assembled. Everything completed.

Part 2 of 2