The AM Forum

Up until this evening the 3B28s in the power supply/modulator for the Johnson 500 have been nice quiet toooobs, not so much as even glow from the filaments.

However now when keying the mic in AM or turning on the HV for CW, one of the 3B38s flashes, nice arc!! And I can hear the arc, maybe a little more hum from the PS/mod.

I think this is a sign from the heavens something is amiss. Could I be on the verge of a crapout of a 3B28? Don't have any spares at this time for plan B.

Opinions and comments welcome from voices of experience.

wd8kdg
Craig,

Craig,

Sounds like you have a tube going south on you.  I just updated my Johnson Valiant that I purchased and put S/S rectifiers (p/n 1N2637) in it vise going with the 3B28's.  I hope you are able to get the problem fixed and get it back up on the air, I just love the sound of an old Johnson.  If you want to go the route of S/S on the HV, I have 6 extra rectifiers that are NOS just sitting around here, just let me know if you would like a couple.

Richard

Don't forget the HV jacks in back of the RF deck and also on the power supply.  Those banana jacks are notorious for going south on the Johnson 500s.

73 de jay/w5jay..

I'll do a double take on the tip plugs/jacks on the rf deck, ps, and cables. All of the above were replaced and new cables made during restoration. Guess it is something to keep an eye on. I've got good cable clamps on the HV bundle to make sure the tip plugs don't back out of the jacks.

Tnx,
Hoopie

Try swapping positions of the 3B28's.  If the same tube still arcs in the new position, the problem is most likely in the tube.  That would be the easiest problem to correct.

If the arcing stays with the same tube socket, make sure the tube is getting proper filament voltage (with the tube in the socket), and that the HV a.c. voltage to the plate cap of each tube is normal.  You could have shorted turns in the plate transformer primary that is causing abnormally high secondary voltage, although this usually causes a fuse to blow.

I have had rectifier tubes to act squirrelly because the tube pins and contacts in the socket were not making good contact due to oxidation.  Sometimes, the contact pin loses it springiness once it overheats from poor contact.  If the problem appears it be in the socket and simple cleaning fails to resolve it, the best solution is to replace the entire socket if one is available.

Hi Don,

Tried the tooob swap minutes ago. Same tooob arcing in different socket, only worse. In the original socket there was just the one initial arc as HV was applied, either in the CW mode or on AM when keying the mic.

In the other (different) socket when I turn on the HV, switch to transmit in CW, the tooob has a steady arc. Nice color, but I don't think this is what I'd rather watch. Time to invest in NOS 3B28s.

Another subject, grins and giggles, I got out the old logs. Our last QSO on 3.880 was June 1, 1982 while I was living in West by God Virginia. Wondering if Russ, WA4JFQ, from London, KY is still around?? He always had a gud signal to the Northern part of the Union.

Don, thanks  for the advice!


Hoopie

Russ went SK quite a while ago.  He was inactive his last couple of years.  It was probably at least 10 years ago last time I talked to him.