Original Electrolytics - UK Vintage Radio Repair and Restoration Discussion Forum

oldticktock · 29th Jan 2010, 4:57 pm

Hi, Looking for some opinions on a couple of things.

Is it possible to reform old electrolytics via a variac?

Scenario is I have a couple of DAC90/A's which are totally original and untouched. The main C20/21 (trader 1161) on both look ok, and i'm sure I read somewhere on here that the Bush smoothers were very good.

Once work is underway and having changed all the waxies, I'm not sure of the process for the smoothers. I have some modern replacements to hand but would rather use the originals. I have a Heathkit capacitor checker (IT-28) would I check the can via this first or does this dump full power to the can with the potential to take my head off, or could I slowly wind it up on the variac over a few hours, if ok then check it.

OTT

kalee20 · 29th Jan 2010, 5:08 pm

What I would do is:

Get a cheap rectifier diode like 1N4007. Connect in series with a highish value resistor, like 47kΩ 3W (or use several resistors to make up this value).

Remove all the valves from the DAC90a, and connect this diode-resistor combination between any of the taps on the mains dropper, and the HT electrolytic (dioode cathode to electrolytic) so that it charges up the HT rail. Connect a voltmeter to measure the HT rail voltage.

Plug in via the variac, wind to 200V or so, and leave it, have a look at the voltmeter reading from time to time. If you achieve 250V on the capacitor sections, it'll be OK. If the capacitors are duff (unlikely) the voltage will fall, or perhaps never get up in the first place. The idea of the high-value resistor is to limit the current if the capacitors are naff, so you don't get bits of tin foil and wet blotting paper thrown everywhere.

ppppenguin · 29th Jan 2010, 5:10 pm

The problem with a variac is you won't get HT until the voltage is pretty high. This is assuming a valve HT rectifier as in the DAC90. Better use a lamp limiter. This will limit the current while allowing the heaters to warm up and give HT. Best of all you can tell at a glance if something is in trouble by looking at brightness.

PS: I cross-posted with kalee20. His advice is good.

kalee20 · 29th Jan 2010, 6:11 pm

Quote:

Originally Posted by ppppenguin

The problem with a variac is you won't get HT until the voltage is pretty high.

That's why I suggested use of semiconductor diode above - you can wind up the variac from zero and the diode will be working all the time. Perhaps I should have explained this, for the benefit of OTT! The connections I give above don't involve any soldering, either.

The idea of using a Variac to very slowly bring up a valve set isn't one that appeals to me, as it's bad practice to run valve heaters significantly below normal voltage. Taking 10 seconds to wind up to 80% of nominal voltage, and then increasing to nominal over the next couple of minutes - OK. Taking to 70% of nominal (which might just get the valves hot enough for some emission), and leaving for a couple of hours to exercise the electrolytics - no!!

Thanks for the compliment, Jeff.

oldticktock · 29th Jan 2010, 6:25 pm

OK thanks for the explanation, I've not used the variac to reform, but have slowly wound up the voltage to check for problems normally 20volts/10 mins increasing watching for issues. I've found a bubbling capacitor block on one set this way.

What about the capacitor checker will that tell me if the cap is ok or will it go pop when connected? I've used it on non electros and it works.

Boom · 29th Jan 2010, 6:28 pm

I'm probably up in the 50s now for the amount of DAC90As that I have restored over the last 3 years. I have had a duff smoothing can in just two or three.

Maybe I've been lucky but I don't see the need to reform them. The UY41 being indirectly heated doesn't come straight on so there is no surge to worry about.
The volts across the smoother will build up over a minute or so and in my humble opinion the cap' will then reform itself if it needs to during normal use.

As I say just MHO.

Dave

cmjones01 · 29th Jan 2010, 9:38 pm

While I know that under-running valve heaters is bad news in the long term, I did use a variac to reform the electrolytics in the power supply for my R1155.

It was disconnected from the set, so just a transformer, valve rectifier (6X5), smoothing choke and a couple of electrolytics.

I connected a resistor (10k, I think) in series with each capacitor in turn, with a voltmeter connected across it.

Bringing up the mains slowly on the variac, I watched the current flowing into the capacitor and stopped increasing the mains voltage when the current reached 20mA or so. Then I waited until it dropped below 1mA, then increased the variac some more. Rinse and repeat until we get to full mains voltage.

The whole process only took about 15 minutes.

The 8uF capacitor dated 1953 reformed absolutely fine. The 4uF dated 1944 started out at 2uF and finished as an open circuit. Oh well - you win some, you lose some...

Chris

29th Jan 2010, 4:57 pm	#1
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Original Electrolytics Hi, Looking for some opinions on a couple of things. Is it possible to reform old electrolytics via a variac? Scenario is I have a couple of DAC90/A's which are totally original and untouched. The main C20/21 (trader 1161) on both look ok, and i'm sure I read somewhere on here that the Bush smoothers were very good. Once work is underway and having changed all the waxies, I'm not sure of the process for the smoothers. I have some modern replacements to hand but would rather use the originals. I have a Heathkit capacitor checker (IT-28) would I check the can via this first or does this dump full power to the can with the potential to take my head off, or could I slowly wind it up on the variac over a few hours, if ok then check it. OTT

29th Jan 2010, 5:08 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Original Electrolytics What I would do is: Get a cheap rectifier diode like 1N4007. Connect in series with a highish value resistor, like 47kΩ 3W (or use several resistors to make up this value). Remove all the valves from the DAC90a, and connect this diode-resistor combination between any of the taps on the mains dropper, and the HT electrolytic (dioode cathode to electrolytic) so that it charges up the HT rail. Connect a voltmeter to measure the HT rail voltage. Plug in via the variac, wind to 200V or so, and leave it, have a look at the voltmeter reading from time to time. If you achieve 250V on the capacitor sections, it'll be OK. If the capacitors are duff (unlikely) the voltage will fall, or perhaps never get up in the first place. The idea of the high-value resistor is to limit the current if the capacitors are naff, so you don't get bits of tin foil and wet blotting paper thrown everywhere.

29th Jan 2010, 5:10 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Original Electrolytics The problem with a variac is you won't get HT until the voltage is pretty high. This is assuming a valve HT rectifier as in the DAC90. Better use a lamp limiter. This will limit the current while allowing the heaters to warm up and give HT. Best of all you can tell at a glance if something is in trouble by looking at brightness. PS: I cross-posted with kalee20. His advice is good.

29th Jan 2010, 6:25 pm	#5
oldticktock Retired Dormant Member Join Date: Nov 2009 Location: Weardale, UK. Posts: 1,981	Re: Original Electrolytics OK thanks for the explanation, I've not used the variac to reform, but have slowly wound up the voltage to check for problems normally 20volts/10 mins increasing watching for issues. I've found a bubbling capacitor block on one set this way. What about the capacitor checker will that tell me if the cap is ok or will it go pop when connected? I've used it on non electros and it works.

29th Jan 2010, 6:28 pm	#6
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Original Electrolytics I'm probably up in the 50s now for the amount of DAC90As that I have restored over the last 3 years. I have had a duff smoothing can in just two or three. Maybe I've been lucky but I don't see the need to reform them. The UY41 being indirectly heated doesn't come straight on so there is no surge to worry about. The volts across the smoother will build up over a minute or so and in my humble opinion the cap' will then reform itself if it needs to during normal use. As I say just MHO. Dave

29th Jan 2010, 9:38 pm	#7
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,677	Re: Original Electrolytics While I know that under-running valve heaters is bad news in the long term, I did use a variac to reform the electrolytics in the power supply for my R1155. It was disconnected from the set, so just a transformer, valve rectifier (6X5), smoothing choke and a couple of electrolytics. I connected a resistor (10k, I think) in series with each capacitor in turn, with a voltmeter connected across it. Bringing up the mains slowly on the variac, I watched the current flowing into the capacitor and stopped increasing the mains voltage when the current reached 20mA or so. Then I waited until it dropped below 1mA, then increased the variac some more. Rinse and repeat until we get to full mains voltage. The whole process only took about 15 minutes. The 8uF capacitor dated 1953 reformed absolutely fine. The 4uF dated 1944 started out at 2uF and finished as an open circuit. Oh well - you win some, you lose some... Chris