When Is A Capacitor Not A Capacitor?

[W1UJR]

So when is a capacitor not a capacitor? When its a resistor!

Check out the reading on the Simpson meter, the leads right across the cap.
Notice anything......like 0 ohms of resistance, a dead short?

Yep, this cap is shot, shorting the full plate voltage to ground, yikes!
Makes for good fireworks in the 866s but not good for the big vintage Stancor plate xformer.
Of course the jumped out fuse holder might have given me an indication that something was wrong. 

From the W1KSC transmitter project -->> http://www.w1ujr.net/bruces_bench.htm

[k4kyv]

Shorted out oil capacitors are very rare, but I have seen a few of them. Perhaps it was abused sometime in the past with over-voltage. 

I have a 4 mfd 3 kv one in one of my homebrew rigs that must be over 70 years old and still works FB OM, and sails through the leakage test.  The National Recovery Administration logo stamped on it is a clue to its age.

[W1UJR]

First one that I've seen go bad as well Don, other than those darn "bathtub" caps on the Collins 32V and 75A lines.

Vivisection of the defective capacitor below.

If it was an oil cap, it is certainly dried out now.
I drilled two drain holes in the bottom, expecting to see oil stream out, nothing at all.

You can note, on one of the ends, that the oil seems to have settled out and formed a layer.
Makes sense as the cap was installed on its side, under the chassis, and having been built in the 1940s, 60 some years worth of time may have taken its toll.

Perhaps these are best installed in the vertical position with the leads either facing up or down?

[W1EUJ]

What value is the cap? I've got a bunch I'm 'dumping' for free at NEARFest.

[k4kyv]

Some "oil" capacitors are really oil-impregnated paper caps, without any liquid oil sloshing about inside.  You will find only an oily residue on the paper that insulates between the foil sheets if you disassemble the cap.  That's exactly what my c.1934 one is.  Its case isn't even hermetically sealed.

[W1UJR]

Some "oil" capacitors are really oil-impregnated paper caps, without any liquid oil sloshing about inside.  You will find only an oily residue on the paper that insulates between the foil sheets if you disassemble the cap.  That's exactly what my c.1934 one is.  Its case isn't even hermetically sealed.

May be that this one is just dried up Don?
The gold colored material may be what remains of the oil, its a rather brittle amber looking substance.

[W1UJR]

What value is the cap? I've got a bunch I'm 'dumping' for free at NEARFest.

Hi Dave,

No sticker on it, I imagine a 4-8MFD at 1,500+ volts should do the trick.
I've got a 8MFD at 1KV which I might use for testing, but the plate xformer is 600+ volts either side of center tap, so that's a marginal voltage rating.

Dimensionally mine is about 5" long, 4" wide and about 1 1/4" thick.

If you've got one, could you drop it with Larry NE1S during the fest?
I'm not sure that I'm going to be able to make the spring NEARFest this year.

Tnx OM,
Bruce

[WA1GFZ]

Bruce,
Be careful with that gold substance. It could be rich in PCBs and toxic.
Might want to get rid of the reamins. fc

[W1UJR]

Bruce,
Be careful with that gold substance. It could be rich in PCBs and toxic.
Might want to get rid of the reamins. fc

Good point Frank.
Think it may also cause global warming? 

[Bill, KD0HG]

I have never, ever seen a Sprague Vitamin Q impregnated capacitor fail, no matter how old, unless it had been abused.

[Carl WA1KPD]

I have never, ever seen a Sprague Vitamin Q impregnated capacitor fail, no matter how old, unless it had been abused.

Geez Bruce,
Maybe I should not have plugged the rig into the 230 volt socket. The plug did fit kind of funny.

BTW I like the AR QSL in the background...... Do you want a high res scan of it?

Glad to hear it is coming along. You have already done more to the old girl then I did in two years.

Great to see you guys last weekend and look forward to our next eyeball.

[W1RKW]

Had a discussion with a seasoned engineer today and about young engineers and training for new guys on the block.  The youngin's were trouble shooting a problem on a Trident boat this week and he suggested checking on the PS ripple on the PS's to ensure that it was were it should be. Because of the lack of experience they poo-poo'd it and figured that it shouldn't mattered despite the DC level on a meter. After many hours of troubleshooting they concluded to check the ripple of the PS's. There was enough ripple on the supply to wreak havoc with the trim and drain system computers on the ship that the computers wouldn't compute their data properly.  This was a result of power supply caps that were 25+ years old. The power appeared stable on a meter but on a scope it was not. They repaired the power supplies and all was well and the caps were half their rated value after all those years.

[N3DRB The Derb]

bruce - a black wrinkle safety guard around those bleeders made with cane metal would look sharp and be easy to make. I get a bit distressed seeing bleeders hanging out in the open air like that.

[W1UJR]

bruce - a black wrinkle safety guard around those bleeders made with cane metal would look sharp and be easy to make. I get a bit distressed seeing bleeders hanging out in the open air like that.

Yes, its not exactly the most safe of arrangements.
But I've only started on it Derb, so your suggestion may very well be added.
Good tip, thanks OM!

[N3DRB The Derb]

yeah, wrinkled up, nobody would be able to tell it wasn't made that way in the first place. Of course, I dunno what your resto goal is for the rig. In this case I think the safety factor would outweigh others. Done right you would have ur cake and eat it too. No one would ever tell it was a add on if done correctly. it would actually look better than now.

Of course the jumped out fuse holder

 WHY OH SWEET BABY JESUS do they ALWAYS DO THAT??!?!?!?!??!one   

[Carl WA1KPD]

Of course the jumped out fuse holder might have given me an indication that something was wrong. 

Yikes!
I swear, "I knew nothing"

[ka3zlr]

I am Very Glad Bruce did not get hurt from the malfunction.

The NRA. Now there's a Piece of History. Sure could use it today...

[W1UJR]

I am Very Glad Bruce did not get hurt from the malfunction.

The NRA. Now there's a Piece of History. Sure could use it today...

Yes, glad here as well! 

I don't know when it did crap out, but suspect it was some time ago as it came to me with the fuse holder jumped out.
You know the theory, the fuse popped, don't have a spare, so just jumper her out, right?

Discovered another problem tonight, when testing after replacing the cap...the large toggle switch that switches the primary of the plate transformer has a tendency to stick on.
Yikes, but I thought the plate switch was turned off...
 
Goes to show, never assume, guess one could tell by the glowing plate pilot light, but...just in case, installing a new switch tomorrow.
Suspect the old one bit the dust when the primary was sucking down lots of current due to the short on the secondary filter cap.
It is 10:30PM here, and my rule #1...make that rule #2 (Rule #1 is never "beer goggle") is don't work on HV when tired.

I am really excited, thinking I might be able to get the beast making RF tomorrow.
Already checked the modulator stages, they work.
Next I've got to figure out the numerous switches and control, true old buzzard rig, nothing is labeled.

Night all.

[WA1GFZ]

I use a variac and series AC amp meter to check old power supplies. The switch may stick from turning it on into a short. It may have welded the contacts. Sometimes you can save the switch if you toggle it a bunch of times without power to wipe the contacts.
jumpers across fuse holders saves gas by avoiding trips to the store.

[N3DRB The Derb]

go green by eliminating all breakers and fuses! Some people will die, but hey! 

[W1UJR]

go green by eliminating all breakers and fuses! Some people will die, but hey! 

Actually, population control, or "culling", has been frequently voiced by the "Greens" as one method of saving the earth. 

I only see one fuse in this entire rig, and that's on the primary of the HV supply.
That, coupled with open/exposed bleeder resistors makes you wonder.
Apparently safety was not a key driver "back in the day".
Then again, its like the argument about labeling the controls; if you built the rig, you knew what switch or pot did what, just like you knew where the "hot spots" were.

Nice construction otherwise, and appears to have been built without a great number of power tools; most of the chassis holes are hand filed rather than punched or drill out large, wiring harnesses neatly laced, and that old classic of yesteryear, using tube socket bases as plugs, it the method of choice on chassis interconnect harnesses. The builder even thoughtfully installed a 120 VAC plug on the rear apron of the modulator deck so one could have a light source during those late night crap-out repairs.

The rig uses a Millen 90800 exciter, which makes me wonder about the vintage, think that might be too new for 1940s.
Of course the exciter could always have been installed later.

Right now I'm working on the meter for that exciter, seems the paper face which was glued onto the metal backing plate has come adrift. Surprised at this, but looks like a WWII surplus meter from the stampings, Millen just crafted a new face and glued over it.

[Carl WA1KPD]

The rig uses a Millen 90800 exciter, which makes me wonder about the vintage, think that might be too new for 1940s.
Of course the exciter could always have been installed later.

According to the Moore book that exciter was available from 46 to 49.

I found the Millen manual and will get it in the mail to you. The schematic is dated 10-2-45. If you need any particular information let me know and I can scan it

[W1UJR]

Hi Carl, thanks, you're a good guy, appreciate that.

I was hoping to have the old girl fired up today, but the needle on the exciter sticks, working on the meter now.
Nevertheless I've got the HV on, and was able to key the rig with my CW key, the sticking meter needle held me up.
I found found the set of leads which go to the RF output deck, they were tucked up under the chassis, and just clip on the output coil of the exciter deck.

So you and Larry were correct, he had the rig wired up to bypass the RF deck for low power operation, or one could use the clips leads to take the output of the Millen rig into the 812s. Clever builder, I'm appreciating his handiwork more and more. Thanks for chance to care for the old gal.

I'm still trying to figure out the mod deck lash up, seems he has some "extra" switches which I don't understand the purpose. I think one may be used to change the mic input impedance, the other I am clueless on. I believe he may have changed the design over time, there are 2 or 3 hole caps on the front panel which might have held switches.

I've been meaning to call you this weekend about your boats, would like to pick ur brain.

-Bruce

[N3DRB The Derb]

I am a joke to talk to anyone about safety - I ran a rig with full B+ on the tank coils for 4 years and it was a open relay rack at that. Miracle I didnt get zapped.

[W1RKW]

A light bulb in series works to but it's more subjective.

I use a variac and series AC amp meter to check old power supplies. The switch may stick from turning it on into a short. It may have welded the contacts. Sometimes you can save the switch if you toggle it a bunch of times without power to wipe the contacts.
jumpers across fuse holders saves gas by avoiding trips to the store.