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Mc240
Mc240
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Old 12th June 2010, 03:06 PM   #1
david62 is offline david62  United States
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Default Mc240

I have a Mc240 that has had the power supply electrolytics replaced,but still needs the remaining electrolytic caps and all of the bumblebee caps replaced.I was told to leave the 160P black beauty caps alone.I was also told that I don't need to replace the bumble caps with fancy caps and to go with Nichicon Qx. I am wondering if anybody has gone over this amp.and if they would advise one type of cap over another.I want to preserve the vintage sound,but also want a great sounding amp.for my main system.Thanks,
David
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Old 12th June 2010, 05:36 PM   #2
llwhtt is offline llwhtt  United States
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I used Sprague 716 Orange Drops and Atoms in the MC30s I have and for a couple of others for friends, they were happy with the results. The Spragues are not the ultimate in technology but neither are the parts coming out. They are cheap (inexpensive) to boot, and don't forget to have fun getting the old parts out, I mean have lots of fun! One more thing if the selenium rectifier is still in place, that has to go too, but measure the voltage of the negative supply before you tear it down. With a new silicon diode the voltage will be higher (more negative, is that higher or lower?) and you'll have to adjust one of the resistor values to compensate. The schematic might have the correct voltage on it also.

Craig
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Old 12th June 2010, 05:41 PM   #3
Eli Duttman is online now Eli Duttman  United States
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Do you want a valuable museum piece or do you want a good music maker? The "bumblebee" caps. are part of what interests collectors. OTOH, there is little to recommend them for music reproduction purposes.

FWIW, my view is that fine vintage amps, like the MC240, exist to make music, not collect dust on a shelf. With that in mind, I advise that film and foil caps. be used inside NFB loops and that Soviet surplus PIO parts be used outside said loops. ALL electrolytic caps. need replacement, as they literally dry out over time.

Resistors too need attention. Carbon composition parts drift out of tolerance and go noisy over time. Except for grid stopper positions, replace CC resistors with other types. Metal film parts are fine for grid to ground positions. Carbon film parts are OK elsewhere. OEM wirewound resistors require replacement if, and only if, physical deterioration has occurred.
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Old 12th June 2010, 08:45 PM   #4
david62 is offline david62  United States
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The selenium rectifier is going to be replaced with a silicon diode and a 470 ohm resistor in series with it.All remaining electrolytics will go.Which film and foil caps did you go with?Which caps would be most critical if I did want to use some Mundorfs or better grade caps?Thanks,
Dave
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Old 12th June 2010, 10:02 PM   #5
Eli Duttman is online now Eli Duttman  United States
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The 716P series Orange Drops, with their Copper leads, are good performers at modest expense. OTOH, 715P series Orange Drops have steel leads, which sound bad.

A look at the MC240 Schematic shows all signal caps., except C1 and C11, as being inside the NFB loop. Those parts can be "boutique". C4 and C14 are the AC grounds for the non-inverting triodes of the phase splitters. Those parts are important and should be of good quality. 400 WVDC MultiCap RTX would be good. You might increase the value to 330 nF.

BTW, any Selenium rectifiers MUST be replaced by modern Silicon parts.
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Old 12th June 2010, 10:43 PM   #6
david62 is offline david62  United States
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Eli,

Thanks for your advice.Terry Dewitt said that he uses Nichicon QXP caps which would probably be pretty close to the Sprague 716P orange drops.The C4 and C14 I believe are Sprague .22 160P Black Beauties which Terry said could stay and unlike the bumble bees are good caps and usually are still up to spec.The selenium rectifier will be going for sure.How about the two silicone diodes in the power supply?They are working well now,would it give me any sonic advantage to change them out?
Dave
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Old 13th June 2010, 12:49 AM   #7
Eli Duttman is online now Eli Duttman  United States
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Replacing the OEM Silicon rectifiers in the B+ supply makes sense. Those parts are "slow" and NOISY. UF5408s (1000 PIV/3 A.) are inexpensive and quite quiet.

According to Nichicon's site, the XP caps. are metalized film. OTOH, 716P series ODs are discrete film and foil, which is more transparent. Black Beauty 160P caps. are not junk, but they can't measure up to RTX MultiCap parts. Highly transparent caps. inside NFB loops are (IMO) the way to go. The idea is to remove contributions to the total error correction signal. The dielectric in 716Ps is polypropylene, while the dielectric in RTXs is polystyrene. Mixing up the film types contributes to hearing the amp, instead of hearing the plastic.
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Old 13th June 2010, 02:20 PM   #8
david62 is offline david62  United States
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Eli,

I really appreciate your help on this.I will go with the 716P caps on your advice.I will change the diodes in the main power supply as well.I will look up the RTX caps.It was mentioned that de-soldering the old caps from the binding posts is tough.Any advice on the extractions?
Dave
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Old 13th June 2010, 03:09 PM   #9
Eli Duttman is online now Eli Duttman  United States
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A hot iron and desoldering braid are essential, when removing old parts. A "solder sucker" can be useful too. You should be able to get by with several widths of desoldering braid and a good iron.
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Old 13th June 2010, 04:45 PM   #10
llwhtt is offline llwhtt  United States
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Good tools and lots of patience are necessary. The problem with old MCs is there are parts on both sides of the tag board. Part leads from one side go thru holes and then wrap around the tag and sometimes the lead you want to remove is under one or more other leads and those too need to be removed first. That's where the patience comes in and you'll probably use your four- letter word vocabulary quite often. I got to the point where I just made a diagram of everything and stripped the board. Not a fan of carbon comp resistors anyway. The MC30s I have were complete toast when I got them so there was no harm done, I wasn't trying to preserve a museum piece. Like I said before have fun it's worth it in the end.

Craig
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