I am not familiar with the 374 -- but I am very familiar with the 78 --
so I will assume that they are the same (except for the tube count).
I've experienced two problems that could result in this symptom.
First: Bad ceramic resistor
There are three -- one in each leg of the input voltage line
(these get isolated by the step start relay after a delay) -- and (the
more likely one to be a problem) a 10Kohm that is used as a voltage
divider and is mounted in the same place -- adjacent the transformer at
the base of the unit -- vertically. When they get old, they develop
little microcracks -- you can measure them with a VOM and they look
marvelous -- you put any current through them and they open right up.
Best way to test it is to swap a known good one (i.e., new one) and see
if the problem goes away.
Second: High Voltage short to ground
This can happen in lots of ways -- most are rather obvious to
your eyes and nose. The toughest one I ever saw was a too-tightly bent
piece of red HV wire from the cover switch -- it was routed right along
the chassis and tightly bent AND tightly held by a cable clamp -- result
is after several years the insulation cracked on the side you couldn't
see and as soon as you would get HV -- it would cycle off -- since you
don't seem to get this far in the start up sequence, I'd bet on the 10K
resistor.
Dave N7UE
(own 2 78s and get to fix 5 others)
On Sun, 9 Mar 1997, K7BG Matt Trott wrote:
> The old 374A seems to be acting up a bit. Every time I turn it on it trips
> off. It did stay on one time long enough for the amber meter lights to turn
> to green, but something is definitly haywire. Most of the time it only stays
> on for 10 seconds or less and never gets to the "green light" stage.
>
> The HV is okay and it doesn't appear to be a tube problem. Anyone ever have
> this problem? Bad relay? No blown fuses, but I'm not gonna try to turn it on
> again until I figger out what's going on.
>
> Thanks, Matt--K7BG
>
> P.S. Wouldn't you know I've been hearing Europeans on 160 the last couple of
> nights which is a first for me and I can't put any fire in the wire to work
> them!!
>
>
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David B. Ritchie, Esq.
D'Alessandro & Ritchie, P.C. -- Intellectual Property Lawyers
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