I've finished the filter capacitor replacement in my Dad's MLA-2500.
I also added a step-start, a new power cord (surprise!), and did some
serious cleaning of the band switch.
In doing the latter, I discovered why it had arced: the factory had
inserted too much wire into the solder lug. The wire extended well
past the lug and traversed some of the distance between that contact
and a ground contact. I'd say that it encroached at least 1/16" into
the free space that's supposed to be between the contacts, reducing
the available air gap. That was apparently enough to cause the arc
when Dad operated on 75 m. He had commented that sometimes on 75 he'd
heard the amp arc, but it was never anything he chose to pursue since
he didn't operate on 75 much. This was years ago, when I lived in
Colorado and we maintained a weekly sked, usually 40 m daytime.
Sometimes we'd move it to the evening and use 75 m.
The step start makes a huge difference: I'm no longer tempted to step
back as I turn it on.
The amp tunes as I remember: it runs out of plate C at the bottom of
80 m and simply doesn't do very well at all on 160 m: it runs out of
load-C and won't cover the bottom of the band. It's also very
inefficient on 160 m. It does very well on 75 m and (based on my
metering which may not be very accurate) makes about 1000 W out on 75
m. That's also the key-down output on 40, 20, and 15 m. B+ sags to
about 2000 V with 0.8 A of plate current
It's merely OK on 10 m, where it acts like it has too much plate C,
likely because it has too much L in the tank. If Dad ever uses it
again, I'm sure it will be on either 75 or 40 m, perhaps 20 m. I
think 15 and 10 m is unlikely and he has no antennas for 160 m. Even
the antenna he has for 75 right now is pretty marginal.
So, the beast lives again, complete with the original 8875 tubes.
Thanks to everyone for their help and suggestions. I especially want
to thank to Ron, W2CQM/3 who serendipitously showed up on the list
with a set of replacement filter capacitors!
73,
Kim N5OP
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