krellmk
Chief Journeyman
- Joined
- Nov 3, 2016
- Messages
- 736
- Location
- Brooklyn NY
- Tagline
- Are you satisfied with the life you're living
My friend bought an MC 225 recently from the local market and listened the good sound quality. Later found that it sounded a bit less punch at low frequency, and all four 7591 output tubes are not hot enough even after few hour playing.
I measured the plate current for each output tube were all about 2 ma but the normal value is about 22 ma.
The B+ supply was only 340V while it should be 400V. The filter capacitors C26 & C27 at voltage doubler dried out and lost most capacitance from 150 uf to 2 uf.
See picture below:
After replacing them with good capacitors and the B+ supply was ok back to 410V. But the output tube plate current only increased to 5 ma, still far away from normal value.
Carefully measured the negative bias of output tubes found -23.5V but it should be -21V.
It was because bias supply dummy load resistor R51 changed the value to 25K from 22K.
See picture below:
Everything is ok after replacing R51 with 22K correct value.
I measured the plate current for each output tube were all about 2 ma but the normal value is about 22 ma.
The B+ supply was only 340V while it should be 400V. The filter capacitors C26 & C27 at voltage doubler dried out and lost most capacitance from 150 uf to 2 uf.
See picture below:
After replacing them with good capacitors and the B+ supply was ok back to 410V. But the output tube plate current only increased to 5 ma, still far away from normal value.
Carefully measured the negative bias of output tubes found -23.5V but it should be -21V.
It was because bias supply dummy load resistor R51 changed the value to 25K from 22K.
See picture below:
Everything is ok after replacing R51 with 22K correct value.
