28-02-2023, 08:17 PM
Don't worry too much about the mains switch for now. Leave it shorted out and switch off at the mains socket. When working on a set it's always a good idea to unplug it before doing anything. Most of us have found out the hard way that the switch in the set isn't as good as it ought to be. I like to see an air gap
Ultimately you have several options with a duff mains switch. You can put up with it (I've certainly done that before), try to find a complete replacement pot or transplant a switch from another pot.
As for R18, if the original part isn't open circuit just put it back. Otherwise a pair of 470R 5W (or more) parts will be fine. The values aren't critical. If you have any wirewound resistors to hand in the 100R to 1K range just put them in for now. They will be adequate to get some life from the set.
C43 and C44 will definitely need replacing. Even if slightly leaky these will put a positive voltage on the grid of V3. The result is that V3 will overheat and possibly cook the audio output transformer too.
I've noticed that the volume control, V15, is also the grid leak resistor for V3. This is not a good arrangement. Any intermittent contact in the wiper will cause excess current in V3. Also if V3 is passing any grid current (it shouldn't but it happens) the volume control action will be horribly crackly. Not a worry yet, but something to be tackled later.
Ultimately you have several options with a duff mains switch. You can put up with it (I've certainly done that before), try to find a complete replacement pot or transplant a switch from another pot.
As for R18, if the original part isn't open circuit just put it back. Otherwise a pair of 470R 5W (or more) parts will be fine. The values aren't critical. If you have any wirewound resistors to hand in the 100R to 1K range just put them in for now. They will be adequate to get some life from the set.
C43 and C44 will definitely need replacing. Even if slightly leaky these will put a positive voltage on the grid of V3. The result is that V3 will overheat and possibly cook the audio output transformer too.
I've noticed that the volume control, V15, is also the grid leak resistor for V3. This is not a good arrangement. Any intermittent contact in the wiper will cause excess current in V3. Also if V3 is passing any grid current (it shouldn't but it happens) the volume control action will be horribly crackly. Not a worry yet, but something to be tackled later.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
