>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.
>
? no RF input is needed when it makes its own.
cheers, 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
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