SB-220, arcing, what do you guys think?

[WB2RJR]

Some years back my SB-220 started arcing in the Pi network caps. It started on 10 meters, but wouldn't do it if I only ran 1kw input instead of 2kw. So I only ran 1kw on 10 meters but this got worse until it started doing it on 20 at 1kw.

So I decided to fix it and there was a lot of stuff about changing out and redesigning the parasitic suppressors, HK did a bad job etc.,etc. Well I did all that and it didn't fix.

Basically I was thinking if this was so badly designed how come it worked fine for over 30 years. Something had to have changed. Well I'm not working on it right now, and doing a vertical for 80m and rebuilding my SB200 power supply but it got me to thinking about the SB-220.

Here is the print and pictorial. Looking at the B+ line to the 3-500Z, I was thinking what the hell is that 2nd RF Choke (RFC2) doing there. I've never seen that in an amp, it's not in my SB-200 (only the 50uh RFC and normal .001 ceramic disc bypass to ground). Not only that but looking at the pictorial it has a .001 at 6Kv doorknob as a bypass and then the normal .001 bypass on the B+ line. A doorknob? That means lots of possible current if I'm not wrong.

If that .001 doorknob opens or its reactance goes up it could be the source of the parasitic oscillation. What do you think. I'll probably pull it out this weekend to work on it as I should have the other stuff done by then.

Seems to me the SB-220 might have oscillated for Heathkit and they put in RFC2 and the doorknob to fix it. Has anyone else ever seen 2 RFC's in the B+ line?

73
Marty

[KD6VXI]

The suppressors didn't cause Ctune to arc.

Mine did the same thing.

During the production lifetime of the sb220, the Ctune cap was changed, to a part with a higher V rating.  Bands witches where the next thing to go.

KM1H advises a small cap from 40 to 10 on the band switch.  Indo not recall the value now, but just a couple pf.   There is also a washer to reduce corona you can solder to the back of your bandswitch.

But, palstar andbrfbparts both sell a cap with an increased voltage rating.  Rf parts version may be the palstar, I don't know.

It got more and more common with use because of carbon tracking.  Change CTune, and check the archives for Carl's fix on the bandswitch arcing, and your back in business.

--Shane
KD6VXI