Hi All,
As a happy 9+year owner of one of Bob's terrific ST-120 amps ... I've got one that I can't solve: A missing left channel.
It started as an intermittent fault and I thought that I'd seen the three left-side tubes lose their warm glow which I'd mentally noted as a potential dry joint to be resolved. But more recently when it stopped 100% the tubes have all been glowing nicely...
This morining, I've don'e some pre-cautionary re-soldering of the heater line for all three tubes and they all measure ~6Volts as expected.
Next, I did some rolling. Starting from the centre 12AT7 outwards including the power KT-120s - no change.
As part of this I re-biased and whilst the "front" pot was OK levelling out quickly after warming up, regardless of the tube changed, the back was still quite floaty between 0 and 0.5v - which I can't explain.
I also checked the rails and noted that outputz of the outer-left 12AT7 was a bit lower at about 200V than the right-hand(working) side at 230V (when the doc says 300V) - I will note that I'm in Australia and I'm using a stepdown tranny to drop from 240@50Hz to 110(still at 50Hz) - so there could be some differences there.
I did have an oscilliscope but it's in storage right now and haven't been able to have a look at signal path but, when testing the outputs of the outer 12AT7s, when I test the working right-side, I get a crackle in the speakers with my analogue multimeter, but no such crackle on the left.
Just out of interest, I took off the cover and checked that there's some continuity across the speaker tranny for the primary windings ... all good there.
But now I've run out of ideas and looking for some help...
Anthony
As a happy 9+year owner of one of Bob's terrific ST-120 amps ... I've got one that I can't solve: A missing left channel.
It started as an intermittent fault and I thought that I'd seen the three left-side tubes lose their warm glow which I'd mentally noted as a potential dry joint to be resolved. But more recently when it stopped 100% the tubes have all been glowing nicely...
This morining, I've don'e some pre-cautionary re-soldering of the heater line for all three tubes and they all measure ~6Volts as expected.
Next, I did some rolling. Starting from the centre 12AT7 outwards including the power KT-120s - no change.
As part of this I re-biased and whilst the "front" pot was OK levelling out quickly after warming up, regardless of the tube changed, the back was still quite floaty between 0 and 0.5v - which I can't explain.
I also checked the rails and noted that outputz of the outer-left 12AT7 was a bit lower at about 200V than the right-hand(working) side at 230V (when the doc says 300V) - I will note that I'm in Australia and I'm using a stepdown tranny to drop from 240@50Hz to 110(still at 50Hz) - so there could be some differences there.
I did have an oscilliscope but it's in storage right now and haven't been able to have a look at signal path but, when testing the outputs of the outer 12AT7s, when I test the working right-side, I get a crackle in the speakers with my analogue multimeter, but no such crackle on the left.
Just out of interest, I took off the cover and checked that there's some continuity across the speaker tranny for the primary windings ... all good there.
But now I've run out of ideas and looking for some help...
Anthony
