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MC240 rebuild, my latest project.

Pio1980

Pio1980

AK Member
For an elderly a'phile in Atlanta, I'm working my way thru his wall-O'- Mac.
I've already done a C24 and now this MC240, next up a pair of MC75, then a MR-71 and lastly a MX-110. This is his bedroom system BTW, he built the speakers in the 1970s when he assembled the system. I've set up a work station there to do the work while we converse and he observes the process. I have some background in x-ray technology (his field of employment) and share his interests in fine vintage cars and technology in general incl the odd stuff.

The original problem was no B+, a blown fuse in that line as it turned out undoc'ed on my skiz indicating the need for a thorough PS rebuild. I concurrently replaced all the striped BBODs on the signal and rectifier boards as a matter of course and the rectifier diodes with fast-recovery types. I also later added a couple film bypasses (not shown) to the voltage divider caps as per good modern practice, and that bumblebee snubber cap on the doubler is of course now gone. All resistors were in tolerance, unstressed, and therefore stayed in. Terry DeWick sez in a related posting that the Sprague 160P BBs are utterly reliable and should stay to preserve the signature Mac sound, and so they did.
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Here the amp is recapped and ready for bench check.
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Couple more pix

Some before pix, the blue cans for the doubler bridge were another tech's prior work.
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What many don't realize (you apparently do) is that the black Sprague capacitors with red lettering are usually (always?) Mylar and should be still good. On the other hand, any of those striped bumble bee capacitors are to be replaced. How do you know that the bumble bee capacitor was bad? Well, if it makes a "clunk" sound when it hits the bottom of the trash can, it was bad...
 
I left the two input blocking BBBBDs in as any leakage there would be inconsequential and I had ran out of replacements on hand after bypassing the lower B+.
 
battradio said:
Very Nice work Steve ,

Want to hear about how it all sounds when your finished .:music:

Mark
Click to expand...

Thnx Mark! I'd prefer to hear it on my stuff but that isn't going to happen with this amp. I'll be listening to it on the owners unfamiliar-to-me system which hasn't worked right in quite some time, what with the '240 for the surround/rear dead and one of the 75's in the primary channels very ill. The C24 pre was absolutely miserable and the external switching system for the tape system was non-functional. I'm bringing it back one piece at a time and will sub the '240 for the pair of 75's in the primary/front until they are done and ready to go back into service. Then the MR71 gets a look-see before I go downstairs to deal with the other system in the Great Room with the sick Altec 1590s and the malfunctioning MX110. I subbed that pending-project Adcom GFA-535 for the Altecs and put a passive control in front of it from the CD [player to sub for the Mac to get good sounds back for him down there for the time being.
 
Inrush protector replacements?

Well, the '240 went back together and on the shelf where it decided to flash one of the 7027 outputs in most spectacular manner and blow a fuse, this time thankfully the one you can replace without disassembly. Odd since it behaved during the dummy load symmetrical clipping test. A quartet of 6L6GBs, hook up inputs and then to find the stuff that didn't show up during the bench check. An intermittent input switch and RCA socket became evident which we will live with while this amp stands in for the primary amps now on the table belly-up, a pair of MC75 with consecutive sn, purchased new at the factory by this original owner. Most of the small caps and all the e'lytics go and other stuff to be sorted out along the way.

I do need to know what to replace the damaged inrush surge protectors.
Would CL-120 be OK?
 
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I've used CL-90s for inrush. One hint, rebias the output tubes if the selenium rectifier for the bias voltage was replaced. See the Hints and Kinks stickey in the Mac Forum for the procedure.

Cheers,

David
 
dshoaf said:
I've used CL-90s for inrush. One hint, rebias the output tubes if the selenium rectifier for the bias voltage was replaced. See the Hints and Kinks sticky in the Mac Forum for the procedure.

Cheers,

David
Click to expand...
Thnx much, David!
I think Terry recommends a 470^ resistor be added in series with the replacement silicon diode, which is what I did with the '240.
Found it! http://www.audiokarma.org/forums/showpost.php?p=275837&postcount=34
Now I just need for the MC75.
 
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