Thanks, Tom.
Yes, there are carbon comp resistors across the caps and I plan to replace
those with metal film; I'll look for carbon comp resistors across the
rectifiers and pull them.
The story I read (someplace) on carbon comp resistors is mixed: in any service
where they might dissipate anything more than about 50% orftheir rating (maybe
less), they will drift down. I've seen this happen a few times always with
disastrous (or nearly so) results. The other end is if they never get very
warn, or are simply stored. In such cases, what I read claimed that they drift
*up* because the clay in the composition material swells slightly, reducing the
overall resistance. This I haven't seen, but I've never run across an old stash
of carbon comp resistors, either.
The amp never saw much service and the 8875 tubes are still in good shape. I
need to add grid protection circuitry to them since they'll probbaly last
forever if and only if the grids are treated with kid gloves. My Dad *may* have
a spare ste of them, too, but given they they now consist of unobtainium, I
want to insure the integrity of what's there.
Thanks again!
Kim, N5OP
----- Original Message ----
From: Tom W8JI <w8ji@w8ji.com>
To: amps <amps@contesting.com>; Kim Elmore <cw_de_n5op@sbcglobal.net>
Sent: Monday, February 19, 2007 4:32:04 AM
Subject: Re: [Amps] ML!-2500 Filter Cap Replacement
>I have my Dad's MLA-2500 in my shack for what I suspect is
>a filter
> capacitor replacement job. It hadn't been operated in a
> long time and
> Dad wanted me to check everything out (he's had health
> problems). I
> was tuning it up and all seemed well until I heard a soft
> pfft,
> smelled a Bad Smell, and heard the PS hum loudly, clearly
> drawing
> lots of current.
There are a few component selection errors in Dentrons that
should always be taken care of immediately.
The major error is they used carbon resistors across the
electrolytics. Carbon resistors should never be used for
critical voltage equalizing or voltage dropping
applications, because carbon resistors age down in value
with heat and time. That causes a destructive cycle where
the resistor decreases in value until something fails,
usually an electrolytic or the resistor itself.
Get rid of ANY carbon resistor you see across any
electrolytic in any amp, and replace them with metal based
resistors.
Also they used low voltage rated resistors across diodes.
That's another component selection issue. You can just
remove them and leave only the caps.
Many Dentron amps using sensitive control grid tubes also
don't have grid protection. Some, very stupidly, don't even
have a grid current metering system. You might want to be
sure the MLA has a grid current overload system installed.
73 Tom
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