Heathkit IO-4205 Power Supply Caps

[eryjus]

Hello,

First, I am a complete noob with high voltage stuff. I'm learning, but I need help by someone looking over my shoulder.

I recently came into posession of a Heathkit IO-4205 5MHz Dual Trace Oscilloscope. The documentation is copyright 1978. I'm told it works.

I opened it up to check the caps before I applied power, and found the following black caps and wanted to know what they were. They are on the power supply board. I was able to read the name and model and came up with, "Nytronics 162J-1, 0.1uF, 20% tolerance, 2000VDC."



I spent some time with Google and was not able to find much about these caps and only a very little bit about Nytronics in general. Nothing like a datasheet. Frustrated, I finally had a RTFM moment and checked the Heathkit Manual. These caps are located with diagrams and identified as paper caps.

Paper. Ok, I read questionable: Don't apply power, Adam, until you know the condition of the caps. But, I do not have any way to test ESR and I am unwilling to try any "wet finger test." I'm far more comfortable with replacing them outright.

I also have the complete schematic. I located the caps in question. The relevant part is here.



These caps (C301, C302, C303) appear to be smoothing and filter caps (please correct me if I am using the wrong terms here). But it also appears that the test voltage on one lead of one cap is -2000VDC -- right at the limit for the cap with absolutely no headroom. This does not seem smart to me.

So, I am proposing to replace these 3 caps with the yellow KEMET 0.1uF 5% film cap (polypropylene) rated for 3000VDC (KEMET C4CAYUD3100AA1J). I am reasonably confident they will do the job adequately.

This, then, brings me to my specific questions:
1. Am I considering the proper type (dielectric? what is the proper term?) of capacitor for these replacements?
2. Is my thinking related to using a 3000VDC rated cap correct, or is there still enough "room" available in the 2000VDC rated cap (also, if there is enough room in the 2K cap please explain how this is possible)?
3. The paper cap appears to be polarized (from the Heathkit Manual) and my proposed replacement appears not to be polarized. I recognize that it may be bad form to replace a polarized cap with a non-polarized one. Should I consider the polarized-to-polarized-replacement rule to be more of a hard-rule than a mild-suggestion? If hard rule, please help me find an appropriate replacement cap.
4. There are larger (1200uF) electrolytic caps for this power supply as well. They show no physical signs of leaking. Since I have no way of testing ESR, should I consider replacing them at the same time?
5. What am I not considering? I know this is a loaded question, but I honestly don't know what I don't know. So, can you think of anything I should be looking at before I put power to this device?


Thanks in advance!

[PeteS in CA]

Re: Heathkit IO-4205 Power Supply Caps

Impregnated paper has been used as a capacitor dielectric for more decades than I've been around. Polypropylene parts should be OK. That oscilloscope is probably pretty old, 1980s or 1970s vintage. I'd be concerned about the health of every electrolytic capacitor in it. They may have dried out. Also, while it is a triggered scope, its bandwidth is just 5MHz. Unless you are trying to restore it as a collector, I'm not sure its usefulness would justify the effort to recondition it.

[redwire]

Re: Heathkit IO-4205 Power Supply Caps

You can kind of gauge what parts do not last. It is best to test each electrolytic capacitor, 1978 was a while ago. Maybe consider buying an ESR meter like the Blue ESR.
HV paper caps I would replace. The circuit is a voltage-doubler so each cap sees around 1000Vpk. I think your replacement Kemet part looks fine. Sometimes I use a dab of silicone to stick them to the PC board.

In the old days, wound foil capacitors (paper/wax/plastic dielectric) have no polarity but they do have a stripe on one end, marking which lead goes to the outside foil. This is because the outside foil can cause a lot of electrical noise to nearby parts, in amplifiers. So you paid attention to orientation of the cap in certain circuits. Mr. Carlson made a youtube video about his circuit to ID the outside foil electrode. Heathkit has the stripe marking on the cap and PC board.

I would check the multi-section big can electrolytic C3, I've never found these to last. It has 100uF, 300uF, 100uF sections for the 160V and 130V rails. Nowadays they are replaced using separate caps.

Always be careful and discharge these caps through a say 1k-10k 5W resistor before going in.

The 1,200uF parts I would leave alone, they look OK. Unless there is a problem with the +/-15V rails.
Make sure the right mains fuse is there, so it pops if anything goes awry. It seems to be 0.5A slo-blow which is a bit sluggish.

#57-52 diode original 5D20 2000PIV 5mA and I would use a BY203 or BYT62, GP02-20 etc. as a substitute.
#57-27 diode original 1N2071 600PIV 0.75A and I would use a 1N4005 or 1N4007 as a substitute.

5MHz scope I used for many years, it was fine troubleshooting audio, TV etc circuits.

[eryjus]

Re: Heathkit IO-4205 Power Supply Caps

I would check the multi-section big can electrolytic C3

Good call. Forest & Trees. I would have missed that.

And without calling everything out line by line, thank you both for your responses.