MC240 HELP PLEASE


Ive had a mc 240 for years, its never been a problem child, loud but never a problem.

I recently brought a 2nd one to drive my mids and tweeters.

I was useing the amp the other night,and it made some clicking/on/off type noise's near the 117/125 volt switch and then near or in the the tranformers,then it just went silent.No sparks, no smoke, no shock,just silence.And thats the way it stayed ever since just bloody silent.(now this one has become a problem child.Was useing cd player into c28 into mc240 in evo 40 speakers at the time

Ive checked the cd player the preamp the speakers with a second amp they are all fine.Tubes still light up, fuse seems to ok.

The amp had just been serviced before I brought it had new caps and more,the guy I brought it of on audiogon is 100% reliable and if he says it was serviced and caps replaced then it was .

Anyone got any ideas on what the problem might be.Bye Paulie.
Thank you to all that reply .
souterncross
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.
Thank you for all replys so far,no luck with it yet caps are new, amp was recapped by the original owner just a few weeks ago and was given a clean bill of health.

The only checking I can do is visual I removed the base to check for arcing ,and flick the switches I dont see any evidence of arcing and when I flick the switches it doesnt even give a static sound. thanks again Bye Paulie