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.

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

Ive found a fuse inside the amp, I will try to discribe it. On one end it has a spring type set up, off that spring there is very very thin copper fusewire it is now absent without leave,and I dont expect its going to return to active duty.

This looks like it may well have been an original fuse, the serial numuber is ,


313 3AG 3-2/10A 125V (this fuse is on the inside of the amp accessed by removeing the bottom plate.

Is there a substitute,I can use because I dont like my chances of finding this exact type here.

Could it really be that simple ? I hope so.

Thanks everyone Bye Paulie.

That's right - now that you mention it, there is a fuse in there. I think it is for the bias stage in case a tube goes bad.

If a fuse blows, it is because there is a problem. You will have to find the problem before you're back to normal. It sounds like you have a tube that has gone bad. But which one? That is a tough one without a tube tester... Look at each tube very carefully and see if you see anything strange. I have seen some develop black spots and/or sometimes you will see a whitish film where the vacuum was lost (near the base typically).

Arthur

Yup spot on there Athur, one tube in the very centre of the top plate glows a lot hotter than the rest Im useing nos tungsol kt66 .Im waiting on a replacement for it as I write.
Ive only been useing the kt66 for a couple of weeks now,before those I was useing svetlana c wing 6l6gc.

The kt66 sound good but they sound a bit lazy,compaired to c wing 6l6 the draw back seems to be that.
I have to drive the c wing harder for example. If Ive got the c wing 6l6 in and I ve got the volume a say 5 oclock. I would only need the volume a 3.00 with kt66 .The kt66 seem to have great mids and highs, but to me the bottom end is a bit lazy(I call them my wife).
While the c wing 6l6 seem to be a better allrounder great bottom end good mids and highs. (I call these my girlfriend).

My plan was to get my other mc240 back (from my brother no easy task) have the kt66 in one mc240, to run the mids/highs. And the other to with the c wing 6l6 in the other mc240 to run the drivers.
That way I can have the best of both worlds my wife and girl friend under the one roof all living in prefect harmony.

Tryed new fuses internal and external,it worked for about 20 minutes,until I pushed the gain up to far, the same thing happened again,clicking the silence.

My next idea is to get a new of tubes.

The problem may also be that, Im useing a 240 volt power stepdown to 117/125 volt,and that may just be high lighting a fault in amp.

Im going to have the amp fully checked out,and the power supply changed over to 240 volt power supply,while I'm at it have the amp rebuilt and a new chassis fitted at the same time .

This project will not happen to the new year, and on that note,I would like to thank ever one who responded and wish them a Merry Christmas and a happy new year.

Will let you know how it goes in the new year, after the rebuild.Bye Paulie

Try vintage 7027a tubes. They sound great on the MC30 and MC240.