Hello!
I have a Mark IV B combo. Worked well for about 5 month. Recently I've noticed several problems.
First, hum. It' not very loud but noticable. None of the controls affect it except Lead and R2 channels louder than R1.
Second, really loud popping and cracking when switching from standby to ON and loud pop when switching to standby. Triode/pentode affects loudness. In pentode there is almost no popping. In triode popping is really loud. When I use simul-class with triode occasionally after I switch from standby to ON there is continuous loud cracking for about 1-2 seconds.
That's all bad but bearable. Main problem with tubes. Two of them are overheating - they are really hot and plates inside glowing orange (not red) after working for about 30 seconds. They are both located on the right side of the combo. On the Mark IV tube chart they are labeled as D and C.
The tubes are all new (same thing with old power and pre-amp tubes) and tone is not affected (at least no major differences).
Clearly it's an amp problem. First thing that I've checked is bias supply DC voltages. Here is power amp schematic where I labeled with red my voltages http://img209.imageshack.us/img209/2467/mesaboogiemark44.png (188 kB). Overheating tubes are V7 and V9. I've checked all resistors along the way from bias supply to tube grids after desoldering one leg - they are allright. I couldn't properly check capacitors so may be problem with one of them.
Please help, any advice will be helpfull. I really don't want to go to tech guy if it's just a shortened capacitor or some other simple problem.
I have a Mark IV B combo. Worked well for about 5 month. Recently I've noticed several problems.
First, hum. It' not very loud but noticable. None of the controls affect it except Lead and R2 channels louder than R1.
Second, really loud popping and cracking when switching from standby to ON and loud pop when switching to standby. Triode/pentode affects loudness. In pentode there is almost no popping. In triode popping is really loud. When I use simul-class with triode occasionally after I switch from standby to ON there is continuous loud cracking for about 1-2 seconds.
That's all bad but bearable. Main problem with tubes. Two of them are overheating - they are really hot and plates inside glowing orange (not red) after working for about 30 seconds. They are both located on the right side of the combo. On the Mark IV tube chart they are labeled as D and C.
The tubes are all new (same thing with old power and pre-amp tubes) and tone is not affected (at least no major differences).
Clearly it's an amp problem. First thing that I've checked is bias supply DC voltages. Here is power amp schematic where I labeled with red my voltages http://img209.imageshack.us/img209/2467/mesaboogiemark44.png (188 kB). Overheating tubes are V7 and V9. I've checked all resistors along the way from bias supply to tube grids after desoldering one leg - they are allright. I couldn't properly check capacitors so may be problem with one of them.
Please help, any advice will be helpfull. I really don't want to go to tech guy if it's just a shortened capacitor or some other simple problem.
