Yes, indeed. I had bought two used 425s many years ago, and while I
don't remember the exact symptoms, I wound up replacing the P/S
electrolytics in both of them. I started replacing one at a time as I
found faulty ones, but after that became a continual ritual, I decided
to just replace all of them. They're easy to replace because they're
simply mounted and electrically connected with screws.
73, Joe
K2XX
Jim Brown <mailto:jim@audiosystemsgroup.com>
Tuesday, December 12, 2017 2:00 AM
I'd start by checking the string of big 450V caps in the HV section of
the power supply. You want an old analog VOM, like a Simpson 260, on
a medium high resistance scale. First, disconnect from AC, make sure
the HV is completely discharged. Then use the VOM to go down the stack
(8 caps, I think) reading resistance, then reversing the leads, then
reversing again. For a healthy cap, you should see low resistance from
charging current, gradually dropping to zero each time you reverse the
leads. Go down the entire stack, remembering what the strongest caps
look like. Chances are you'll find only one that's weak, sometimes
two. While you could replace the entire string, it's rare that all are
bad. Buy replacements at the usual industrial electronics sources. The
last time I bought was ten years ago, think it was either Newark or
Allied.
I've got three of these amps. Schematics that make troubleshooting
easier are on my website. k9yc.com/publish.htm Scroll down.
73, Jim K9YC
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mike george via Amps <mailto:amps@contesting.com>
Tuesday, December 12, 2017 1:12 AM
hi new to list hope someone can help with this. fired up the 425 and
noticed the 1500wts was down 3 or 4 hundred wts. looked at the rf deck
and the lights were dimming out on peaks.the plate and grid current is
ok but the plate voltage drops from 2200 to 1500 on peaks.i tried on
dummy load same result. other amp on same 220 outlet works fine on
same ant. i dont see anything obvious inside the power supply. tubes
are not getting hot. not sure where to go. thanks mike george
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