Reforming old electrolytics - UK Vintage Radio Repair and Restoration Discussion Forum

Daimlernut · 23rd Jun 2015, 2:37 pm

Hi all

I know this has been covered a lot on this forum but Im in the process of rebuilding a Cossor 339 and have a bit of a dilemma, after many hours of reforming the 8uf caps the result was disappointing, out of the 5 only one showed a leakage below 1ma one was 2.5ma all dated Dec 46 whereas an 8uf dated Feb 69 after the reforming process shows a leakage of 23 micro amps. Has anyone had better results reforming caps as old as 1946?

Alan

Bazz4CQJ · 23rd Jun 2015, 9:37 pm

The idea of re-forming an old cap depends on the question of whether or not other changes have happened besides depolarisation. I recall removing electrolytics from my ~1940 HRO and being astonished at how little they seemed to weigh and concluded that the old rubber seals had allowed the electrolyte to dry out. If changes like that happen, they are probably beyond re-forming. Have you checked what value of capacitance they are showing?

B

joebog1 · 25th Jun 2015, 7:14 am

If you are rebuilding, REBUILD it! I have tried reforming caps that were brand new and high quality, but had been stored for years.
It's a dead set waste of time, UNLESS the unit must be original.

If that's the case then I pity you as caps made over 70 years ago were, by today's standards, beyond use when brand new! Sorry for being so heavy footed

You can always pull out the rubber seal, remove the old "guts" ( wear rubber gloves) and drill a small hole just large enough for the negative lead in the case. Push the negative lead out through the said hole and solder it to the original wire.
Drill another hole in the centre of the rivet used to couple the positive lead to the rubber "gasket seal plug", solder the positive lead to the old positive lead wire.
Then carefully "jam" the rubber plug back into the case.

The time you spend doing this will buy 50 caps the same value and voltage rating, WITH today's leakage specs.

Either that or learn to rewind old power transformers with original wire and paper etc etc etc .

I wish you luck

best regards
Joe

Daimlernut · 27th Jun 2015, 12:38 pm

Yes, the problem of replacing the electrolytics with new is that 8uf 750v are now impossible to find. I have cut the originals open where the clamp fits, removed the inside, and replaced with 8uf 800v Polypropylene caps.
The original cut is then hidden by the clamp.

Joe, I appreciate what you are saying, but this is a vintage oscilloscope and I want to preserve the look, which I thought was the essence of this site. There are just no capacitors of 8uf 750v available new, not that I can find anyway, so rebuild seems the only option. When you power these scopes up before everything is up and running the HT voltage reaches 700v so 750vw at least is advisable in my opinion.

Alan

Sideband · 27th Jun 2015, 4:32 pm

You can always use two 16uF 450V in series with 470K resistors across each. That will give you 8uF at 900V. You only need the resistors to balance out the voltage across each.

23rd Jun 2015, 2:37 pm	#1
Daimlernut Triode Join Date: Aug 2011 Location: Bagshot, Surrey, UK. Posts: 38	Reforming old electrolytics Hi all I know this has been covered a lot on this forum but Im in the process of rebuilding a Cossor 339 and have a bit of a dilemma, after many hours of reforming the 8uf caps the result was disappointing, out of the 5 only one showed a leakage below 1ma one was 2.5ma all dated Dec 46 whereas an 8uf dated Feb 69 after the reforming process shows a leakage of 23 micro amps. Has anyone had better results reforming caps as old as 1946? Alan

23rd Jun 2015, 9:37 pm	#2
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Re: Reforming old electrolytics The idea of re-forming an old cap depends on the question of whether or not other changes have happened besides depolarisation. I recall removing electrolytics from my ~1940 HRO and being astonished at how little they seemed to weigh and concluded that the old rubber seals had allowed the electrolyte to dry out. If changes like that happen, they are probably beyond re-forming. Have you checked what value of capacitance they are showing? B

25th Jun 2015, 7:14 am	#3
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: Reforming old electrolytics If you are rebuilding, REBUILD it! I have tried reforming caps that were brand new and high quality, but had been stored for years. It's a dead set waste of time, UNLESS the unit must be original. If that's the case then I pity you as caps made over 70 years ago were, by today's standards, beyond use when brand new! Sorry for being so heavy footed You can always pull out the rubber seal, remove the old "guts" ( wear rubber gloves) and drill a small hole just large enough for the negative lead in the case. Push the negative lead out through the said hole and solder it to the original wire. Drill another hole in the centre of the rivet used to couple the positive lead to the rubber "gasket seal plug", solder the positive lead to the old positive lead wire. Then carefully "jam" the rubber plug back into the case. The time you spend doing this will buy 50 caps the same value and voltage rating, WITH today's leakage specs. Either that or learn to rewind old power transformers with original wire and paper etc etc etc . I wish you luck best regards Joe Last edited by Mike Phelan; 25th Jun 2015 at 9:02 am. Reason: Typos.

27th Jun 2015, 12:38 pm	#4
Daimlernut Triode Join Date: Aug 2011 Location: Bagshot, Surrey, UK. Posts: 38	Re: Reforming old electrolytics Yes, the problem of replacing the electrolytics with new is that 8uf 750v are now impossible to find. I have cut the originals open where the clamp fits, removed the inside, and replaced with 8uf 800v Polypropylene caps. The original cut is then hidden by the clamp. Joe, I appreciate what you are saying, but this is a vintage oscilloscope and I want to preserve the look, which I thought was the essence of this site. There are just no capacitors of 8uf 750v available new, not that I can find anyway, so rebuild seems the only option. When you power these scopes up before everything is up and running the HT voltage reaches 700v so 750vw at least is advisable in my opinion. Alan

27th Jun 2015, 4:32 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Reforming old electrolytics You can always use two 16uF 450V in series with 470K resistors across each. That will give you 8uF at 900V. You only need the resistors to balance out the voltage across each. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....