Customer comes in with a Marshall Origin 50 that once played, but then one day the sound just "turned to sh*t" as he put it.
I found out that the amplifier was stuck in low power mode (3 power settings via toggle, controls screen grid voltages of power tubes).
I discover that the power is being limited by the circuitry related to TR11/12. If I remove TR11, the amplifier can be set to all three power levels and works normally.
If you look at the 4R 8R 16R jacks (speaker outs), you see there are three diodes. Their job is to pull base of TR11 low and enable full power operation.
The way it's supposed to work is that when the barrel of 1/4" plug is inserted to any jack, the cathode of one of the 1N4148 diodes gets pulled to ground.
However, that is not happening. It's the opposite. When the plug is inserted, the contacts open up, preventing the diodes from being grounded. The other side (terminal NR) seems to be the one that SHOULD be connected to the diode, but it's floating on this schematic.
I tested all three jacks with a plug into it and they all behave the same. Plug inserted, D201/202/203 lose ground. Yet the amp supposedly worked, according to customer.
I have removed TR11 to test my hunch and the amp can be set to all three power levels now. The way this is wired, I don't see how this ever could have worked.
I found out that the amplifier was stuck in low power mode (3 power settings via toggle, controls screen grid voltages of power tubes).
I discover that the power is being limited by the circuitry related to TR11/12. If I remove TR11, the amplifier can be set to all three power levels and works normally.
If you look at the 4R 8R 16R jacks (speaker outs), you see there are three diodes. Their job is to pull base of TR11 low and enable full power operation.
The way it's supposed to work is that when the barrel of 1/4" plug is inserted to any jack, the cathode of one of the 1N4148 diodes gets pulled to ground.
However, that is not happening. It's the opposite. When the plug is inserted, the contacts open up, preventing the diodes from being grounded. The other side (terminal NR) seems to be the one that SHOULD be connected to the diode, but it's floating on this schematic.
I tested all three jacks with a plug into it and they all behave the same. Plug inserted, D201/202/203 lose ground. Yet the amp supposedly worked, according to customer.
I have removed TR11 to test my hunch and the amp can be set to all three power levels now. The way this is wired, I don't see how this ever could have worked.
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Help ampexperts out.
Hi Fi fans, continue listening to your music playback systems, and continue reading the Tubes / Valves Threads on diyAudio.
Help ampexperts out.
Hi Fi fans, continue listening to your music playback systems, and continue reading the Tubes / Valves Threads on diyAudio.
Mods, please delete this thread. Posted to wrong area. Thanks. I found the correct one. Thanks 6A3sUMMER!