>Hi there. I have a problem with an Heathkit SB-200 that I'd like to
>throw at the list. I did a cursory search of the archives and did not
>find anything specific to my problem.
>
>Background:
>
>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).
>
>I'm thinking that it might be that the antenna relay is not very happy
>with life right now. I'm a bit of a novice here and wanted to get some
>insight from the group.
? Newer 572Bs reportedly have more gain than those produced in days of
yore, More gain means more ability to regenerate/oscillate. My
un-expert guess is that your new tubes had an intermittent VHF parasitic.
Lower (than original) VHF-Q suppressors might be in order. The tradeoff
is c. 2% less power at 29MHz. If you have a dipmeter, measuring the VHF
resonance at the blocking cap. will tell you what the likely freq. of the
parasite is. If you want to try a low VHF-Q parasitic suppressor
retrofit-kit, let me know and we will send one. They are guaranteed to
make the amplifier less squirrely or you don't owe us the $13.
>
cheers, Chuck
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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