A neat workbench is the sign of a sick mind.

Even if the caps can't be re-formed, you can gut them and use the original cases over the replacement caps to give that original look to your equipment.

That's a clever and field expedient idea Sarge!

 

A neat workbench is the sign of a sick mind.

Good that it's working better. Don't toss out those caps that you removed. Even though they are ancient, you may be able to revive them (once you get the proper equipment) using a process known as "re-forming". This won't work if the caps are physically damaged or the electrolyte has dried out, but it's certainly worth a try.

Even if the caps can't be re-formed, you can gut them and use the original cases over the replacement caps to give that original look to your equipment.

Haha, I am a neat freak, that's for sure.

Hum, well most of the electrolytics that I removed were starting to hiss, bubble, and ooze out puss looking stuff. Does this mean that they can't be revived?

 

Hum, well most of the electrolytics that I removed were starting to hiss, bubble, and ooze out puss looking stuff. Does this mean that they can't be revived?

Yes. Once they start leaking, that's the end.

If you have more ancient equipment that you want to restore, it would be much better if you tried to re-form the capacitors prior to powering up the equipment.

The process of re-forming capacitors is to bring them up to their rated voltage very slowly with a current-limited supply. When they are at their rated voltage and the leakage is within acceptable tolerance, they are ready for return to service.

You would need a DC power supply capable of generating up to the capacitor's rated voltage. Current limiting can be effected by using a suitably-sized resistor. Current should be limited to 10mA or less during the re-forming process. Ohm's Law is your friend here.

 

That's a clever and field expedient idea Sarge!

Have to confess; wasn't my idea - someone else posted photos of the process on an antique radio restoration site somewhere; I don't remember where offhand.

It's great if you can use original/authentic components in a restoration; but if you can't, you can still make it LOOK original. Capacitor technology has advanced quite a bit in the last few decades.

 

Yes. Once they start leaking, that's the end.

The process of re-forming capacitors is to bring them up to their rated voltage very slowly with a current-limited supply. When they are at their rated voltage and the leakage is within acceptable tolerance, they are ready for return to service.

You would need a DC power supply capable of generating up to the capacitor's rated voltage. Current limiting can be effected by using a suitably-sized resistor. Current should be limited to 10mA or less during the re-forming process. Ohm's Law is your friend here.

I do have a 0 - 3kv 0-6mA power supply with precise voltage controls. I bet that would work well for reviving caps. How do I test for leakage though?


Now I just need to get that VTVM working!

 

Here's the basic idea:

V1 is your 3kV supply.
R1 limits maximum current, in case the cap is shorted.
C1 is the cap undergoing the re-forming process.
R2 shows you the capacitor leakage current. 1v = 1mA.
When V1 is increased or decreased, it also shows charge/discharge current.
Charge current is shown as negative (subtracts from leakage current); discharge current shows as positive.

It can take many hours to re-form a capacitor, perhaps even a couple of days. It is not a process to hurry or be careless with, particularly if you are dealing with high-voltage capacitors.

The idea is to slowly charge the electrolytic capacitor to it's rated voltage to allow the dielectric to re-form, without allowing sufficient current for it to short out and blow up. [eta] Of course, you should not exceed the capacitor's rated voltage. This can happen at the last stages of re-forming if you aren't monitoring the voltage across the capacitor pretty regularly.

The allowable leakage rate is:
0.1mA or (uF x voltage rating x 1.5)/10,000 - whichever is larger.
For example, if an 8uF cap rated for 150v had a leakage of 0.2mA when charged to 150v, it needs more re-forming time. If the current does not continue to drop after a few hours, then it won't likely re-form.
(8uF x 150v x 1.5)/10000 = 0.18mA; this particular capacitor's maximum leakage limit.

 

UPDATE: Well it is finished. The restoration of the HP-200C is now complete. I finished off by trying to fix up the appearance of the device. The final results are not too shabby. I want to thank everyone for the helpful pointers. (Hum the server won't let me upload attachments) so here are the pics below...

 

I think I can safely say that I'm not the only old dog here that is impressed with a young blood having appreciation for the classics.

NICE JOB!!

 

Very nice work.

congradulations.

 

So now you've finished what are you going to do with it, if I might ask?

 

So now you've finished what are you going to do with it, if I might ask?

I'll probably use it to test frequency responses on audio equipment, but for the mean time i'll put it on display somewhere and use it as a conversational piece.

 

Nice job!

Enjoy your "new" piece of test gear.