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Hi all,
My "new" vintage amp ( 1980 Mk IIA 60w combo with reverb, no graphic) needed a new power cord - done.
While inside I noticed a badly burnt and somewhat cracked 100ohm filament resistor off 6L6 pin 2. Replaced.
Since then the channel switching won't work and the relay is buzzing when activated.
I've traced an injected signal to the last coupling cap before the relay in the lead channel (.047uf off V3B pin6 - V3A and V3B are reversed on my diagram) - and after the cap the CRO-measured signal gets shorted to ground, as per circuit design, but even in lead channel the signal continues to get shorted to ground instead of going to the lead channel master vol.
Incidentally I've noticed my first new 100ohm resistor was an old old carbon compound type from an old radio - was trying to maintain the "vintage" status, and caused more buzzing in the relay - a new 100ohm give less buzzing but still no switching..
When first switched, the buzzing is not so bad, but increases in volume over the next 10 - 15 seconds and then stays there.
I can see some movement in the relay contacts when switched on/off, but difficult to tell if the contact is complete, I suppose I could remove the components after the relay and then check continuity when switched, but it's crammed pretty tightly in there..
I get good control voltages on the coil side of relay (8V - 0v footswitch side, and 8v - 5v transformer side as indicated on c/diagram), and it seems like it's trying to switch..
I'm about to check if there's and DC after the relay which migh be holding the contact in place, and if so would point to a dodgy de-coupling cap, esp the .003uf off V3A pin2.
What's the collective wisdom?????
try another 100ohm?? though I can't really see how it can make all that much difference, as the coil voltages suggest the distant side of the coil is dropping to 0v when switched..
Can the clear cover come off the relay to clean contacts?
What else comes to mind ??
thanks for any thoughts,...
Regs, Dave
My "new" vintage amp ( 1980 Mk IIA 60w combo with reverb, no graphic) needed a new power cord - done.
While inside I noticed a badly burnt and somewhat cracked 100ohm filament resistor off 6L6 pin 2. Replaced.
Since then the channel switching won't work and the relay is buzzing when activated.
I've traced an injected signal to the last coupling cap before the relay in the lead channel (.047uf off V3B pin6 - V3A and V3B are reversed on my diagram) - and after the cap the CRO-measured signal gets shorted to ground, as per circuit design, but even in lead channel the signal continues to get shorted to ground instead of going to the lead channel master vol.
Incidentally I've noticed my first new 100ohm resistor was an old old carbon compound type from an old radio - was trying to maintain the "vintage" status, and caused more buzzing in the relay - a new 100ohm give less buzzing but still no switching..
When first switched, the buzzing is not so bad, but increases in volume over the next 10 - 15 seconds and then stays there.
I can see some movement in the relay contacts when switched on/off, but difficult to tell if the contact is complete, I suppose I could remove the components after the relay and then check continuity when switched, but it's crammed pretty tightly in there..
I get good control voltages on the coil side of relay (8V - 0v footswitch side, and 8v - 5v transformer side as indicated on c/diagram), and it seems like it's trying to switch..
I'm about to check if there's and DC after the relay which migh be holding the contact in place, and if so would point to a dodgy de-coupling cap, esp the .003uf off V3A pin2.
What's the collective wisdom?????
try another 100ohm?? though I can't really see how it can make all that much difference, as the coil voltages suggest the distant side of the coil is dropping to 0v when switched..
Can the clear cover come off the relay to clean contacts?
What else comes to mind ??
thanks for any thoughts,...
Regs, Dave
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