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pcmacd
09-24-2012, 08:40 PM
I've been reading about replacing caps in Soundcraftsmen vintage amps.
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I've a ~22 year old PM860 that, after a modest warm up period, simultaneously goes up and down in power (volume) level on both channels simultaneously.
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DC offsets are modest @ output. In the mv range.
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I've not tested the amp at non-idle conditions (need a non-speaker load; thinking about some light bulbs?)

some 50-100 watt bulbs seem to be about four ohms at thermally stable conditions
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Thoughts? If you please?
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They are not inductive or capacitive, so they seem to be a good load?



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At no load, p/s rails are stable at ~70VDC.
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I presume that this is either a problem with the regulation circuit or, perhaps, the output filter caps?

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I've posted a schematic at http://www.mediafire.com/?fw5oikud3d1az54.
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The original is here: http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=49614&d=1296216160
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I presume the regulation circuit is the upper left hand corner of the schematic?
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If not, please advise. .



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I'm an engineer, an ME, and am gasping for air here.


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And, when I read of replacing caps, does this include ALL caps, or just the electrolytics?

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AND, btw, I have ALL manner of test gear at my disposal. LOTS AND LOTS OF STUFF, including:
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dmms -- some pretty awesome ones
o'scopes
power supplies; all manner of such
low frequency network analyzers -> 50 KHz dual channel, 100 KHz single
transient capture devices, lots of 'em
counters

etc etc etc.



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Thanks for you help. WE DO appreciate your inputs...
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mac :banghead:

lgvenable
09-25-2012, 10:55 PM
I replaced the two electrolytic caps in each of the amp modules in my 13 BGW 250D or 250E amps, and got better bass response . that meant 4 electrolytic caps per amp>>there were two caps in channel 1 and two caps in channel 2. I have not done this yet to my 750B's; but wouldn't be squeamish after my first experience.

They were all in the signal path; and really wasn't too expensive (3.00 /channel). Just remember there is a + and - on electrolytic caps.

I didn't replace the two much larger caps in the center of the amps...as an FYI. Pretty easy to do, just use a good electronics grade silver-tin eutectic solder, and while you're at it reflow any suspicious solder joints while you're in there.

Oh and invest in a good soldering station, I got an industrial station for ~ 75.00 in great shape off eBay.

rickedd46
11-16-2012, 03:40 PM
It is better to replace them all.

lgvenable
11-16-2012, 09:48 PM
It is better to replace them all.

Agreed...Just more expensive ;)