Good Afternoon All,
Many thanks to everyone who took the time out of their own personal
schedules to respond back to me re. my issue here with the homebrewed 2x813
KW gradually losing output power on 160-meters...
I hope I've "C-C"ed everyone who contacted me for a direct response, but in
the event that I may have missed anyone, my apologies for getting back to
you impersonally via the Reflector like this.
Anyways, I'll cut to the chase!
I replaced the output coupling / blocking capacitor strictly on spec,
because it was a classic old door knob unit (I don't trust those): no
effect.
I then ascertained that the input circuitry was not the culprit, because the
SWR between the exciter & amplifier was a flat 1:1 in ALL instances of the
test, i.e. from maximum output, to declining output out of the amplifier.
I next elected to replace the bypass capacitor at the "cold" end of the B+
choke to the 813's, again on spec because it was an old, surplus door knob.
During this procedure, I noticed that the plate RF choke itself appeared to
be a tad "boiled" --- the dope coating on it gave the look of a part that
had undergone sustained internal heating (and possible shorting between
turns)...
Someone mentioned insufficient inductance as a potential step toward a cure
in my case, so I measured the "L" with my ARRL Lightning Calculator: it was
only 275-uh! What was I thinking (or drinking!) when I collected the parts
for this rig in 1989-90...?! The ARRL HANDBOOK circuits for RF chokes on
160-meter linears that I've seen all call for a minimum of 1-mh
inductance...
I procured a replacement from the junque box --- an old multi-pi National
affair (NOT an R-175!) that looks to have well in excess of 1-mh, and
installed it.
Subsequent testing here this afternoon indicates that the dropping RF output
condition is now gone, thankfully! I have no idea what such a large
inductance in the plate circuit might do to the efficiency of this amplifier
on higher bands, like 10- or 15-meters...but my main focus for now is 160,
and it seems to be happy "as is" on the band, for the time being.
If anyone wants to see what my "zorched" RF choke looks like removed from
the rig, go to:
http://www.flickr.com/photos/15888272@N05/
Now, I have one final favour /request to ask of the group: in the process of
getting my amplifier back on line, I experienced a momentary short in the
amplifier, which seems to have done consequent damage to my B+ voltage
meter. It reads the B+ voltage of 2.5-KV well enough, as before, but it will
NOT go back down to zero with the p.s. turned off. It "hangs" at around
2-KV, & will not budge, until such time as I switch on the B+, at which
point it goes back up to 2.5-KV...
Does anyone know of a fix for a meter movement like this, or is a
replacement in order at this time...? It's a standard-issue bakelite
"Simpson" brand, 2-1/4" x 2-1/4" in size, and reads 0 - 500 VDC (I use
multipliers to get it to read 0 - 5000 VDC).
If anyone has one in the junque box for sale, I'd love to hear from you!
Thanks again, everyone, for the excellent suggestions, & the moral support!
~73~ Eddy VE3CUI - VE3XZ
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