The AM Forum

This is the second mica cap that has went bad in 30 years of owning this rig. The cap is reading about 100 ohms. I am tempted to replace it with a disc ceramic or a door knob. I have old stock parts I could replace it with orignal equipment.  It just seems dumb to ask for the same problem down the road. It won't look orignal but it will be dependable.  I have all ready replaced the low voltage transformer. The old one was potted the new one not so. Any thoughts on this subject.?

Johnathan N8QPC

I've had good luck replacing mica capacitors with high-quality NP0/C0G ceramics.  YMMV, but in my opinion the ceramics perform just as well, if not better.  I usually buy them new from Mouser -- they will usually be much smaller and you might have to solder on lead extensions depending on your situation.

Steve, KW4H

A doorknob ceramic cap would be a good replacement but do NOT use the "ceramite" brand caps which tend to have very low RF current capability.

If I recall correctly, the 32V-2 doesn't have a safety choke on the output which would short the HV to ground and blow a fuse if the coupling/DC blocking cap shorts.  If not, I would add one while I was in there to protect the owner and antenna system from a failed coupling cap. 

Several years ago, I repaired the HT-31 amplifier out of the Hallicrafters SR-500 console for its prior owner and the output coupling cap on it was shorted causing the fuse to open because it was connected to a dummy load.  Had it been feeding a typical dipole without a balun, it would have happily put full plate voltage on one leg of the antenna. 

I now own the SR-500 and the repair experience reminds me to add a safety choke to the output of vintage transmitters while I am going through them.

Rodger WQ9E

I’m converting an RCA broadcast transmitter and I’ve got a dilemma regarding a safety choke. In stock form, the modulation monitor output is derived from a coil tap, an auto transformer actually, on the output. This step down transformer also provided a DC path across the output as well as being a static drain.

However, it’s crap as an inductor at HF.  It’s got many self resonances, some right in 80 and 40.  I need some other device for a safety choke.

What criteria should I use to choose one? The plate supply here can deliver 3kV at more than 1 amp continuously, so in a fault condition, the part has to be able to blow the overload without the risk of it failing instead.  Will an ordinary little pi choke like I see in ordinary ham lineages do the job, or do I need something else?

I am afraid the typical small pi type safety choke would act as a fuse in this case offering no protection.  It MIGHT be OK if a HV rated fuse was used in the HV output. 

In this case, for a safety choke you would need to use a plate feed style choke as the safety choke.

Rodger WQ9E

Answering K8DI, not the original OP.

I wound my own safety choke for my 813 rig. I have BC iron on this rig that can easily do 1.5A at 2kv. This one is wound with #20 on a 1inx6in thinwall PVC form. I think there are around 220 wraps. LC meter measured around 200uH if I recall correctly. Works great on 160M/80M 40M. Tom K1JJ suggested this approach. He said to install it, give it a try on all the bands one at a time and see if it gets warm. Stays nice and cool.

Ignore the sloppy soldering. That has been fixed.

John

John,

That was the style of choke I was thinking of for a high power safety choke.  It is built like those typically found in modern linear amplifiers and will easily handle the DC current to open the fuse/breaker of the plate supply if the blocking cap fails.

Rodger WQ9E

It's alive I put a HEC HH58 doorknob in rated for 5kv. It wont ever be a problem again. There isn't a lot of room to spare in there. The cap seems to work better that the original.  It doesn't look beautiful but it isn't going any where. The rig is back to it's old self keys 85 watts and swings 350 on my old Yeaseu Peak meter.  :) :) :).

Johnathan N8QPC