MC240 HELP PLEASE

There's a good possibility that the thermistor used to slow the current inrush during power up has left this earth. They work by heating up as their resistance drops.

If they're original, the heating/cooling cycles will cook them over a 40 year lifetime. I've had them fall apart just by touching them. The good news is that they're still available from Mac! I believe they're a CL-90 thermistor, if you've got a supplier close by, though.

To confirm this is your problem pull the bottom cover and you'll see a component about the size of a US 25 cent piece with 2 wires welded to its sides. If this is still intact, then look at the power switch and see if you can find some burned spots. Arcing as you saw would generally leave some hints.

Just be careful if you plug it in with the bottom off and with it plugged in. The part of the circuit I'm talking about will have the full 120vAC available to bite you.

Hope that helps.

Cheers,

David

If the tubes light up, it isn't the thermistor. Everything will be off if the thermistor is dead.

There are many capacitors in there and I bet many people wouldn't replace all of them like I did. Look at the caps and see if they look like originals. It should be pretty clear. There will be two tall silver ones on the left of the choke inductor (looks like a mini power transformer, half the size of your fist). If they are labeled by engraved print, they are probably original - new ones have sticker labels in general. Then look at the two caps on the right of the choke. If they have paper outer coverings, they too are original. New ones have plastic wrap. If any of these 4 caps have a problem, your amp is in trouble and will need a specialist to assess the damage.

If all these caps seem ok, replace the fuse. Sometimes they can look fine but not be. If you replace the fuse and nothing happens, you need to have your amp checked out.

Good luck and keep us posted.

Arthur

Check if there are loose soldering joints.

If your tubes light, the fuse and thermistor are fine. If the tubes light and do not self destruct, you either are missing the high voltage supply, or something else common to both channels (other than the bias supply) is wrong. Check the mono/twin amp/stereo switch (not that it's the only possible cause) if you have both 435 volts and 140 volts from the HV supply.