Marshall MB450H Schematic
The usual Marshall schematic sites appear not to have this particular model's schematic available. Anyone know where I can download a copy?
Amp had one shorted (C-E) device on each half of output stage. Replaced all outputs and did a low voltage mains test with about 10 vac at the line input, but noticed something appears to be loading down the positive power supply, which is producing tons of ripple and falls to .5 volts immediately after power is cut off. Resistance measurements across the supply rails are in the megohms. Circuit is convoluted and everything related to power and output are on one PCB, which is becoming a mess to troubleshoot without a schematic.
It appears that someone replaced the bridge rectifier before this hit my shop. At the moment, I've done all that I can without a schematic, chasing down the obvious areas.
It took a week and a half, but Marshall finally came through and sent me schematics. Once in hand, I found the last of multiple problems in about five minutes, had the part changed out and the amp working and undergoing torture testing/burnin within 30 minutes.
So what was it?
Crash GordonDrive It Like You Stole It
Curious minds want to know!
I hate the "I've fixed it/solved the problem" without WHAT fixed/solved it posts. Please enlighten us. Inquiring minds want to know.
Had someone before you had a similar/same issue AND posted the fix for it/details, it might've made it easier for you (and anyone else afterwards)
Although with a brand like Marshall (amps) , they want their gear working, so direct contact works well, I know Mesa Boogie/GK/Trace Elliot does.
Starting with the obvious:
one output transistor on each bank of the output stage had a C-E short.
Less obvious was a current steering transistor earlier in the circuit which was pulling the + bank toward the pos rail.
Finally, a quad op-amp near the input stage. That was it.