Hello Peter:
So, what is the problem?
Colin K7FM
-----Original Message-----
>From: Peter Chadwick <g3rzp@g3rzp.wanadoo.co.uk>
>Sent: Aug 1, 2006 1:26 PM
>To: amps@contesting.com
>Subject: [Amps] Amplifier frustrations
>
>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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