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In Reply to: Re: Mcintosh 240 dysfunction posted by corerosin on February 25, 2007 at 21:57:16:
I would suspect that there has been a failure of a resistor somewhere in the power supply. Corerosin is on the right track as the high voltage doubler is part of that circuitry.I've seen this with older tube gear after its been worked on - the failure of some associated components after the amp has been run in a bit. I tend to want to keep someone's amp for about a month for use in regular operation to ensure that there's no drift in spec from the original resistors that are left in the amp.
I've seen these resistors 1) develop cracks which cause noise or arcing, 2) drift out of spec and/or 3) flame out from heating. Lastly, I've also had new, replacement metal-film resistors of the proper wattage fail due to high voltage arcing across the surface of the resistor when used in high voltage circuits. In this case, the fix is to use a larger wattage resistor of the same value in order to increase the spacing between the two ends of the resistor.
I'd suggest you pull the cover and look for part that looks 'heated' in some way. Post some pics, if you'd like, for others to see, too.
As always, know that there's a lot of high voltage in that amp so I do not recommend you attempt to work on it unless you're comfortable you know how to get around in there.
Hope that helps.
Cheers,
Follow Ups:
Thanks to both of you. I'll open it up and see if anything looks burnt, etc.Isn't it odd that the tubes still light up? Does that narrow it down at all?
but it does indicate that the main fuse if ok and there's current being passed through the power transformer to the filament windings.It doesn't rule out the power supply.
Cheers,
...It's located on the small rectifier/bias supply terminal board. This fuse may have blown. It rarely blows so one needs to find the root cause of failure before replacing it and repowering.
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