The AM Forum

I know a few people here have used the Russian caps with success. Is it normal to see some spots on the rotor? They seem to have vacuum and test good with a cap meter, but I don't have a hipot tester. The 350pf has faint spots but the 500pf I attached a pic of has me a little worried.

I've had a few that looked like that.   Could it be breakdown arcing between the rotor and stator when the capacitor is meshed near max and operating under RF high-voltage conditions?  Maybe  the stator has the same opposite-side pitting if you took the cap apart and looked inside.

Arcing can occur when the vacuum/seals are failing, thus the voltage breakdown rating goes down. OR, the cap was run well beyond its voltage ratings.

If real arcing had occured, they will sometimes show a glowing blue look (gassy like a tube) before the break down occurs. They will still work, but at greatly reduced voltages.  A hi-pot test would probably show up any deterioration changes.

This is based on real experience I've had with bad vac caps. Hopefully some of the guys who have a deeper understanding of what is going on inside the cap will chime in...

T

That puppy looks to have been HOT! :o

73DG

Mine also looks like that, have not had any problems with it in my link-coupled tuner at 1kw though.

My first thought was arcing but it was sealed in plastic and appears to be unused. I looked at more on ebay and found a lot of these caps have similar defects but most sellers are taking pictures with the caps at full mesh. I bought it as a backup for the k1jj tuner so when the rain stops I'll give it a test and see what happens. I ordered another one from a different seller a couple of weeks ago to build a balanced L. I'm curious to see what it looks like.

Hmmm... then could that be part of the manufacturing process, like gettering a tube?  

No answer would surprise me at this point.  I have seen acring that looks like this, but IIRC, it also had some black/soot marks of real damage too.  I might take some pictures of my bad ones and post.

Maybe someone in the manufacturing area here would know.

eBay samples below:

Vac cap 1 and vac cap 2 show signs of that pattern

Vac cap 3 is as clean as a whistle.

Question:  Does your 500 pF cap (pictured in your first post) have pits and craters that have height and ridges (3-D) that extend upwards above the flat surface? If so I would think they could arc more easily. If they are perfectly flat (2 dimensional) it would just be a blemish and not hurt the voltage rating.

T

I have been using the Russian stuff for years. I have had conversation with people in Russia. These marks are in there from the factory. I don't know why but it is normal. I have used these caps to their full ratings plus some and not had any crap outs. I have had less than great success with Comet stuff.

John N8QPC

I'll second John's comments.

I've used the 5kv caps at 6kv before.

A few had markings like that.  Almost looks like water or or mineral discolorations.

I think the commie stuff is so under rated that any defects still pass muster.  I've seen the 3 to 30 pf caps used at 10 kv by the CB crowd. 

They are rated at 5kv.

--Shane
KD6VXI

John,

That's good to know, thanks. It will add more choices at the flea markets where I would have normally rejected.

Yes, the Russians make great RF stuff. I have used a lot of it. I have a few HV vacuum relays from Russia that have never failed and I trust enough to use as the T/R antenna relays on the big rigs. If that relay arcs over, it could destroy my receiving system.

T

You can see some light pitting if you look through the glass at the right angle. I don't see any high spots though. It looks like erosion from whatever the parts were contaminated with.

Thanks for all the input. It's good to know these things are holding up past their rating.