Yikes....lots of good info here. I replaced all of the blown components I could
find (RF Choke, the 33ohm from lug to ground) and threw the antenna switch ...
it still pegs and the 33 ohm resistor will blow again (or throw the breaker at
the panel...). This is on the bench with NO rf into it.
Chuck
"Gary J. Ferdinand" wrote:
> > Heathkit SB-200 Built circa 1977. Currently in need of a minor rebuild
> > for maximum life which I plan to undertake (Harbach power supply, new
> > antenna relay, replace some ancient caps, etc...). Currently has new
> > 572B's with < 5 hours operating time on them.
> >
> > Symptom:
> >
> > While transmitting a while back, I let the smoke out of a resistor that
> > is part of the RF choke on the anode clip (one of the 80 5w wirewound
> > guys with the coil wound around it). Obtained new resistor and soldered
> > it in.
> >
> > Proceeded to put amp back in service. Upon hooking it up, found a clear
> > frequency and got ready to transmit. As soon as the antenna relay
> > clicks shut, the plate and grid current peg and another resistor self
> > destructs. This one is off of lug 2 of V2 to ground (right most tube
> > socket when looking at the back of it).
>
> Chuck,
>
> Were you feeding power to the amp when it pegged or did you just throw it
> into transmit position but without the exciter putting any power into it?
>
> This sounds like a classic amp-taking-off parasitic problem. It would be
> interesting to note if it were happening by its lonesome or whether you fed
> 100W into it when it happened. It would also be interesting to know what
> band it was being tuned to.
>
> Others here are far more expert than I, but if I were you (and ONLY if the
> amp misbehaved when feeding power to it), I would feed a very small amount
> of power to it (barely cause the plate current to go above resting Ip) and
> determine if it is possible to find a point where it is resonant. Also
> determine whether it resonates on ANY band. One failure mode of the SB200
> is the bandswitch wafer. Another are tank circuit connections (solder joint
> failures). If the amp is not seeing a 50 ohm load (because of a screwed up
> tank circuit due to either of the above), all bets are off as to
> whether/when it remains stable.
>
> Post any new observations. I'm curious as to what it will turn out to be.
>
> Gary W2CS
>
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