The AM Forum

While on 15 meters Saturday, I noticed power output dropped by about 50%, so I switched to a dummy load to investigate. Then there was a big "Pow", a smell of burned component, and a blown HV fuse.

Investigation revealed that C401 (final plate blocking capacitor) had blown.  It now reads 94 ohms.  It must have shorted, and when I moved to the dummy load, the current blew it up (its clearly cracked open).  Fortunately, it didn't appear to damage other equipment or living things.  However, it took out one of the diodes in one of the solid state 5U4 rectifier replacements.  I can't find any other damage.

C401 is a 1000uuf, 2500V mica.  It's bolted with another capacitor to the inside of the final tank compartment under the main tuning coil. 

Question is, how do I get to it?  Does anyone have any experience in getting to this component?  I understand this is a relatively common problem.  I'm thinking I can leave it in place and disconnect its leads and put a new smaller component around it.  I see a mica capacitor of the right specs, but smaller form factor for about $8 available at Mouser.  Any reason why these would not work? 

I don't want to have to pull the whole RF deck if I don't have to.  Any insight or experience would be appreciated.

Steve
NQ5V 

 

Steve,

The RF current carrying capacity is a key spec for this location so make sure the Mouser cap is up to the task.  The continuous duty nature of AM will quickly expose the problem of RF heating if the replacement cap isn't rated for sufficient current.

Does the 32V-3 not have a safety choke on the output network (provides a DC path to ground IF the DC blocking cap fails)?  If not I would definitely add one as part of your repair.  Otherwise a failed cap will allow an antenna which does not present a DC ground to rise to full HV which is definitely not good for safety.  In addition significant DC on the output can damage some types of antennas.

Thanks for the reply.  Your comment on the current capacity of the replacement is timely.

In looking around the web, I found new/old stock exact replacement parts available at a reasonable price.  These are by Sangamo, the same maker, and have the same specs and form factor.  They appear to be an exact replacement.  Now all I have to do is figure out how to get to it.

If you know anyone who has actually done this before, I'd like to talk to them.  To quote Apollo 13: "Houston, I really need a procedure!" In a rig this tight, pulling things at random could be a recipe for disaster.

Also, good point about the  safety choke.  There is not one in the rig or on the schematic.  They probably figured the cap would last long enough and no one would be using it in 60 years.  I do need to add a choke for safety.  If I hadn't switched to the dummy load, there would have been 700V on the tuner, T/R switch,  and antenna. 

Steve
NQ5V

Maybe some of the images here will help:

http://www.qsl.net/wa5bxo/32v3/

Thanks - I have the original owner's manual and have looked over it with a magnifying glass, and tried to figure out what can be unbolted and moved without dropping the gang tuned multipliers. 

"measure twice - cut once"  I think is the applicable adage.

Steve