PCB oil cap advice needed

[W1ATR]

Hi everyone. I have a question about PCB type oil caps.

Now I've been reading on the net here for about an hour, looking though various sites regarding oil filled capactiors containing PCB's.

I have these two caps, and I'm thinking of using one in a project. They are tagged as follows:

General Electric Corp.
Pyranol Capacitor
No. C25684
Cat No. 14F13    MU-F 5.0
V10000   cy. DC
contains 1.9gal non-flammable liquid.

They are in like new condition, clean, and of-course non leaking. Size is 16" tall, 14" wide, and 6" deep. weight feels around 50-60lbs. If I'm not mistaken, these were the spare decoupling caps used in a heising modulator circuit in a continental BC txer. I could be wrong about that however, been a long time since they showed up at my door. They've been in my storage vortex for probably 10+ years now with a piece of wire between the terminals.

That about sums up the description, now for the question. Are these OK to put into service? I read a dozen sites that say PCB caps are fine to use as long as they aren't leaking, then there's a few paranoid sites that tell me to run from the house and call the EPA this second. (Imagining the robot from lost in space with rubber arms flailing about. "DANGER WILL ROBINSON, DANGER!!!")

One of them will work for my project, and being as large as they are, I would rather use it than spend the mucho buckstons to replace it with an equivalent.

Thanks in advance for any thoughts.

Jared W1ATR

[John K5PRO]

For home use, you are fine with the Pyranol cap. Actually, the PCB oils were less flammable, and safer in that respect. However, they did cause tumors in a wart-hog when immmersed in the stuff for much of its' life. Hi Hi.

If you are a commercial or gov't operation, then you gotta get 'em out when you can. I had a big resistor at work, size of a bathtub, that exploded 15 years ago, unbeknownst to me.  It used PCB oil cooling, and the stuff was splashed all over the walls, ceiling. We worked in there doing maintenance (it was in a large capacitor bank)over the years, smelling that oil, before someone finally measured and went ballistic. Danger Will Robinson all over. We had to bring in the guys in white suits, with special drums to haul their cleaning rags out. They took apart the place, light fixture, etc, and cleaned it up. Then we replaced that resistor with an air-cooled design.

The chances of getting ill from a single PCB capacitor are quite slim, but you don't want to eat and drink the stuff, if it every spills. Clean it up, wash up, take a shower, etc. Dealing with the waste product, if you are a homeowner, is your business. If you are commercial or Gov't its a nightmare.

For filter caps in PS, I would probably leave it, but (Disclaimer) that is only a free opinion. In coupling capacitor for a modulation transformer/reactor, where it might get more stress and heat up/leak, I would replace, as that is an inexpensive capacitor (few uF at ___ kV).

[k4kyv]

I would just go ahead and use it, but keep an eye on it for leaks.  Part of the paranoia is that when PCB burns it may give off dioxyn, which is supposed to be extremely toxic.  They are worried obout what happens if the building burns.

Just don't drive any nails into the capacitor case and you will be OK.  I have lost count of  how many PCB-filled caps I have here, but they all serve their purpose well.

In broadcast equipment the transformers and caps are supposed to be labelled with yellow warning decals.  When I picked up my BC1-T, someone had stuck those yellow decals on everything, including tar-filled transformers, electrolytic caps and even an enclosed relay.

I peeled off the ones I could, and for the hell of it, re-stuck one to the bumper of my 1984 rust-eaten pickup truck.

[W1ATR]

Thanks guys. Your advice pretty much goes along with what I had in mind, but I needed to hear it from someone else as well.

The transmitter plate voltage (2800) isn't anywhere near the voltage rating on these caps, that's why I figured one of them would work nicely to decouple the mod iron.

Naturally, 2 more questions just came up as I type, but first I'll describe the project a little so you have an idea what's I'm trying to build over here.

I'm putting together a modulator for a TMC GPT-750 using 833's in B to modulate a pair of 4-400 finals.(Overkill? yes definately, but wait.) The mod iron and reactor are both potted units made by Moloney that came out of a BC1-F. The mod tranny is too big, and flat out just too damn heavy to go into the center drawer on the TX, so it's all getting mounted in a 5 foot tall, 19" rack to stand beside the transmitter. (That left me with an empty center drawer that had a nice 10-1/2" high by 19" hole in the front that I stuffed an r-390a into...Does that make it a TMC transceiver now? )

Anyway, the idea is to move this whole thing over to a 4-1000's tx after I find some smaller gear to fit the drawer on the TMC.

That's the quick and dirty, now for the question behind door number 1

1. On my drawing, I'm placing the cap between the mod iron sec and b+ after the reactor going to the final. Then tying the other side of the mod sec directly to ground. Somewhere I read something about putting the cap between the mod sec and ground and tying the other side of the sec to B+, but the explaination wasn't real clear.

Should I stick to plan A, and put the cap between mod sec and B+, or is there a good reason to wire it up the other way with the cap to ground?


2. I read something about putting the mod iron up on insulators. Now, I already built spark gaps on both the pri and sec of the mod iron.  Which is the better way to go, spark gaps, or float the core, or both?


Thanks, I definately appreciate your time spent answering my Q's. Without the  advice from guys that have "been there, done that", I'd have to shoot from the hip on a lot of this stuff, and hope the smoke didn't get out, as there aren't any other AMer's around here that I know of.

Thanks again,
Jared W1ATR

[k4kyv]

1. On my drawing, I'm placing the cap between the mod iron sec and b+ after the reactor going to the final. Then tying the other side of the mod sec directly to ground. Somewhere I read something about putting the cap between the mod sec and ground and tying the other side of the sec to B+, but the explaination wasn't real clear.

I would put the cap between the bottom side of the mod sec and ground, and tie the top side directly to the final.  That keeps the secondary winding at B+ voltage, so that there is less difference in DC voltage between the two windings, with less liklihood of the insulation between windings failing.  Grounding the mod sec directly would keep the secondary winding at DC ground potential, while the primary is at modulator B+ potential, so the insulation between windings must withstand the full B+ voltage over a 100% duty cycle.  This is an invitation to disaster, especially with decades-old transformers in which the insulation (usually nothing more than plain paper impregnated with varnish) may have deteriorated with age. 

An alternative would be to wire the transformer as you described, but instead of grounding the bottom end of the secondary, return it to the final amplifier B+.  The disadvantage of this is that any residiual a.c. hum from the HV power supply or other unwanted junk that might appear at the power supply output will be coupled in series with the output from the modulator and appear as additional hum and distortion on the modulated signal.  Also,  the audio from the modulator will be returned to ground through the power supply filter cap, effectively putting the power supply filter cap and coupling cap in series, thus reducing the amount of coupling capacitance.

2. I read something about putting the mod iron up on insulators. Now, I already built spark gaps on both the pri and sec of the mod iron.  Which is the better way to go, spark gaps, or float the core, or both?

I would do both.  The spark gaps protect the winding in case there is a voltage spike due to some transient, by breaking down and arcing across, before the transformer winding breaks down.  The idea is to set the spark gap as close as possible, without having occasional sparks during modulation peaks.  The spark gaps should go from either side of the mod B+ line to the plate connection of each modulator tube.  Don't just put one spark gap across the secondary side, since the arc-over could actually generate transient spikes that might couple back and break down the insulation on the primary side.  It should be ok to use 3 spark gaps, one across each half of the primary and one across the  secondary.

The purpose of mounting the entire transformer on insulation is to protect the insulation between the core and the windings.  The windings are at HV DC potential, and it serves no useful purpose to ground the core.  That just puts unnecessary stress on the insulation.  Letting the core float at DC potential removes that stress on the insulation.  One word of caution:  for safety's sake, always operate on the assumption that the core is wired directly to the +HV and treat it as such.  If you happened to touch the transformer case and the insulation failed, you would be in contact with the full +DC HV.  I wouldn't risk my life and well-being on the integrity of decades-old paper insulation.  It might not be a bad idea to attach some kind of warning notice near the transformer, in case a visitor or subsequent owner, unaware of how the circuit is wired,  might be probing around inside the transmitter with the high voltage turned on.

[W1ATR]

Thankyou very much Don, you answered some key questions that are helping me with my final design. I made up a set of teflon blocks to support to mod tranny in the bottom of the rack and bent up a nice strong shelf on the brake for the reactor and cap to sit on. This leaves me just enough room for the 833's and the small stuff.

This rack uses a door switch safety circuit for it's original application, so I'm going to take advantage of that and wire it into the TMC transmitters safety loop for it's drawers and panels. A couple well placed HIGH VOLTAGE stickers along with plexiglass guards in front of the "easy to get at" terminals ought to take care of the rest.

Is there any need to place the reactor up on blocks as this would be easy to do at this point?

For home use, you are fine with the Pyranol cap. Actually, the PCB oils were less flammable, and safer in that respect. However, they did cause tumors in a wart-hog when immmersed in the stuff for much of its' life. Hi Hi.

So what are you saying John? You mean that time I mopped up 10 gallons of the stuff with bath towels and threw them in the hamper wasn't a good idea?