pcmacd
09-24-2012, 07:40 PM
I've been reading about replacing caps in Soundcraftsmen vintage amps.
.
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.
.
DC offsets are modest @ output. In the mv range.
.
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
.
Thoughts? If you please?
.
They are not inductive or capacitive, so they seem to be a good load?
.
At no load, p/s rails are stable at ~70VDC.
.
I presume that this is either a problem with the regulation circuit or, perhaps, the output filter caps?
.
I've posted a schematic at http://www.mediafire.com/?fw5oikud3d1az54.
.
The original is here: http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=49614&d=1296216160
.
I presume the regulation circuit is the upper left hand corner of the schematic?
.
If not, please advise. .
.
I'm an engineer, an ME, and am gasping for air here.
.
And, when I read of replacing caps, does this include ALL caps, or just the electrolytics?
.
AND, btw, I have ALL manner of test gear at my disposal. LOTS AND LOTS OF STUFF, including:
.
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.
.
Thanks for you help. WE DO appreciate your inputs...
.
mac :banghead:
.
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.
.
DC offsets are modest @ output. In the mv range.
.
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
.
Thoughts? If you please?
.
They are not inductive or capacitive, so they seem to be a good load?
.
At no load, p/s rails are stable at ~70VDC.
.
I presume that this is either a problem with the regulation circuit or, perhaps, the output filter caps?
.
I've posted a schematic at http://www.mediafire.com/?fw5oikud3d1az54.
.
The original is here: http://www.audioheritage.org/vbulletin/attachment.php?attachmentid=49614&d=1296216160
.
I presume the regulation circuit is the upper left hand corner of the schematic?
.
If not, please advise. .
.
I'm an engineer, an ME, and am gasping for air here.
.
And, when I read of replacing caps, does this include ALL caps, or just the electrolytics?
.
AND, btw, I have ALL manner of test gear at my disposal. LOTS AND LOTS OF STUFF, including:
.
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.
.
Thanks for you help. WE DO appreciate your inputs...
.
mac :banghead: