A tale that may amuse, and may even evoke sympathy........
Last year, I changed antennas. The 205BA at 62 feet and interlaced 4 ele beams
for 10 and 15 at 68 feet went, replaced by a 4 ele Steppir at 62 feet. The
tower, fed as folded monopole, moved its resonance from 2.6 to 3.72MHz. No
longer could it be fed with an L network with a variable C - it needed variable
L as well. Built a remote controlled tuner, using a vacuum variable that I'd
been given. Hi Pot? never needed one.........beautiful purple glow in the
variable. Anybody know where I can get a new vacuum for it? SWR went through
the roof, of course. Tried a smaller vac variable, and tried making a padder
capacitor out of sheet glass. More fireworks, and SWR again through the roof.
Tried some HV ceramic capcitors and they couldn't cut it either - more
fireworks. So I bought a cap from Max Gain systems, and eventually got the
mechanics sorted out for the motor drive.
I then found that the periods of arcing in the tuner appeared to have screwed
the amp. It would work on 3.5 MHz, but even into dummy load, went mad at 3.7.
I changed the passive grid resistor to a Tin-Oxide, and still it went mad.
Plate current runs away, and I think the bias from the regulated bias supply
collapses, because the interlock drops out. OK on 20. The RF choke was
definitely small, and I played with that, with two chokes in series. One of
them was from the NCL2000, and being held off by about 85 microhenries from the
full RF at the plate, I was surprised when that had the smallest 'pi' start
burning up on 40 - from the outside. That took the series HV fuse. Stripped
burnt wire off, intennding to have another go. KH8SI appears, and I've no amp.
This morning, he's actually gotten around to working EU 30 minutes after the
band opened on 20, but I can't cut it without an amp - I did try.... Try firing
up the amp after the work on teh RC choke. Before the amp was put
into tx, there was a sparking noise. Open everything up, and check. HV fuse
OK, remember Tom Rauch's comments on equalising resistors and Rich Measures
comments on wirewounds. Measure them. There's 8 off 22k and 2 off15k in series
across the elctrolytic string, and they measure about 900k ohm. Figure this
needs looking into. Go to one capacitor with 22k across it. Put meter across,
and watch the resistance climb as the cap charges. gets up to about 30 K. OK.
Disconnect capacitor from resistor, check resistor again. 22.1K. Add capacitor
and resistance goes UP!!
Repeat a few more times, wondering 'what the hell?' Why should 800 mFd across a
22k resistor make the DVM indicate a higher than 22k resistance. Get out the
AVO - old fashioned VOM - and that gives sensible readings. Then note DVM is
intermittently flashing to say the battery is low. Change battery, and sensible
readings obtained. But that is a new one on me that the DVM can be that wrong
before it indicates low battery. Oh well, I got it for nothing
anyway...Probably something to do with spikes from the sampling integrating.
Go back to chasing the sparks. Of course, this is where you need a Hi Pot.
Havn't got one. there's a HV xfmr under the ebnch supposed to be for a scop.
Check it, get 1700 volts. two K2AW rectifiers, a couple of caps, drive it from
a Variac and we have a hiPot going up to 4.4kV. Now I can use that 40kV probe
for the DVM.....apply to amp, sparking. Turns out that there's an open on the
RF choke that was sparking. Sort that out.
Try firing up on 160, efficiency poor, and suddenly the mains fuse for the HV
goes.. No spares.....drill ends of fuse and fit 30 gauge copper wire. NOT best
practice with the inside of the glass copper coated. Decide to try 20meters in
case KH8SI is there again tomorrow morning - they've probably gone home, or
conditions will be bad or something. Still, it seems to fire up on 20 tonight.
Leave it alone....
Conclusions. This nearly 50 year old amp, the Rf deck of which was designed for
the military as a lower power tx and uprated with bigger valves and more volts
by the OEM, has probably got some components (for which I don't have, and can't
readily get, spares) breaking down under RF load as a result of the SWR when
the arcing took place They are hard to pinpoint because when they let go, all
hell lets loose and the thing blows fuses, and at lower power, theyr'e OK..
I've even got so depressed I've been looking up the prices of
Ameritrons......about the same number of pounds here as they are dollars in the
US. That is even more depressing....
The DVM thing is something totally new to me. Anyone else seen anything like
it? Hopefully, someone may learn soemthing from this tale of woe....maybe that
buying an Ameritron is a better idea than trying to build your own or rework
surplus!
73
Peter G3RZP
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