Retro Tech & Electronics | Marshall JCM800 Lead Series Repair - Major Power Supply Issues 2 of 2 @retrotechandelectronics | Uploaded February 2019 | Updated October 2024, 5 hours ago.
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.
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.
