Variac - Method Of Usage - UK Vintage Radio Repair and Restoration Discussion Forum

Rejectostat · 31st Oct 2004, 4:34 pm

Until now I have never bothered with a variac, but I am considering doing so for the occasional receiver. I would only be using it on receivers that have not seen power applied for decade(s) in the hope of a reduced stress start for components. How do other Forum members use these devices? Would a reasonable usage be 50v, 100v, 150v, 200v applied at 30 minute intervals, followed by full voltage? Would any "resting" time be required? I would only be using the variac on 1930's AC mains sets, its a basic Claude Lyons 0-240v "Rigavolt".

I've not used a variac as yet as I've heard the view that radios initally run up on variacs are never inherently reliable ("hit a capacitor with 240v straight away, if it survives it will probably be reliable etc, gradually run it up and you can never be sure" kind of thing). Any views?

BlowFish · 31st Oct 2004, 6:23 pm

Hello
When I use my variac I apply the voltage slowly upwards- starting at 0v then stopping to check at 50v and 100v, if everything appears ok (No smoke or bangs) I continue to 150v then to 200v and finally to full power (240v)

Quote:

hit a capacitor with 240v straight away, if it survives it will probably be reliable

Not recommended I always reform mine !(the electrolytic mains smoothing ones)

Quote:

I've heard the view that radios initally run up on variacs are never inherently reliable

I have ran 4 radios so far on a variac havent encountered any problems

wireless_paul · 31st Oct 2004, 6:35 pm

I always use a Variac when testing a "new" receiver. Gradually increase from zero to full voltage over an hour+. Always have an AVO in circuit to check the current.
Paul E

Sideband · 31st Oct 2004, 7:00 pm

This is one of those areas where people have their own ideas. Personaly I have never used a variac on a radio of any age. One problem with valve rectifiers is that the heater will not reach proper emitting temperature until you are pretty much at full mains anyway so the idea of using a variac seems somewhat pointless. Of course with selenium rectifiers they could perhaps be more effective in bringing the HT up slowly and hopefully reforming electrolytics at the same time.

Before I put mains near any unknown set, I make a point of replacing the mains filter cap and subbing the smoothing and then changing any HT decouplers and check for HT shorts. Then I check the mains transformer DC resistance and mains wiring, pull out all the valves and then apply mains via a lamp limiter. I then chack the outputs from the transformer and if all seems ok, I replace the valves except the rectifier. If all the valve light up OK and there are no nasty smells or smoke I then fit the rectifier (with the subbed electrolytics, it's unlikely there will be any major problems). So far in every set I have restored including some that have not been used for over 40 years, I have not had any problems. Once the set is basically running you can then decide on reforming the original electrolytics or (descretely) fitting new ones etc.

Like I said, we all have our own ideas so use whatever method you feel best with.

Rich.

jim_beacon · 31st Oct 2004, 8:24 pm

on AC only sets, there is usually HT available at low level, when the mains input voltage reaches 100V.

Jim.

1st Nov 2004, 1:09 am

Hi,

Yes, we all have our ideas. I agree with Rich as I follow the same process on old valve equipment.

A variac is good to check for certain failures that will only show themselves at full or close to full voltage and of course to reform old or to condition new caps. I would like to mention though that if you use a variac you should still consider using an isolation transformer on live chassy sets. Hopefully it will save your life if you happen to touch something live. You still will get a nasty shock though.

Andrew

mjizycky · 1st Nov 2004, 11:30 am

Never used a variac on an old radio either (plenty of colour TV sets though) - trying to draw current from underrun valve heaters isn't a good idea as the risk of poisoning the cathode is quite great. My personal preference is to either overhaul the set completely before applying power, then look for any other faults or alternatively run the set up on its own mains transformer (having performed an insulation test first) but remove the rectifier and supply HT current from a separate variable HT supply. This latter method of course precludes you from working on an AC/DC receiver or many post-war AC Bush sets with series heaters unless you put in a resistor matching the value of the rectifier's heater!

IMO I'd say that Richard's method is the best practical way of bringing a set back to life.

XTC · 1st Nov 2004, 6:03 pm

I usually use a variac for a soft start.

On a couple of occasions I've come across equipment with 110V mains transformers, and no external indication that this was the case. I was very happy that I'd checked to see what was happening at 100V. The transformers weren't tapped at 110V, they were 110V mains only.

I thought cathode poisoning was caused by running valves at the proper heater voltage , but no cathode current.. Some special quality valves were specifically designed to withstand this sort of service.

Pete.

31st Oct 2004, 4:34 pm	#1
Rejectostat Hexode Join Date: May 2003 Location: Hertfordshire, UK. Posts: 308	Variac - Method Of Usage Until now I have never bothered with a variac, but I am considering doing so for the occasional receiver. I would only be using it on receivers that have not seen power applied for decade(s) in the hope of a reduced stress start for components. How do other Forum members use these devices? Would a reasonable usage be 50v, 100v, 150v, 200v applied at 30 minute intervals, followed by full voltage? Would any "resting" time be required? I would only be using the variac on 1930's AC mains sets, its a basic Claude Lyons 0-240v "Rigavolt". I've not used a variac as yet as I've heard the view that radios initally run up on variacs are never inherently reliable ("hit a capacitor with 240v straight away, if it survives it will probably be reliable etc, gradually run it up and you can never be sure" kind of thing). Any views? Last edited by Paul Stenning; 26th Dec 2004 at 7:47 pm. Reason: Fix link or code for vBulletin

31st Oct 2004, 6:35 pm	#3
wireless_paul Hexode Join Date: Mar 2003 Location: World Heritage Village of Saltaire, Shipley, West Riding of Yorkshire. Posts: 324	Re: Variac - Method Of Usage I always use a Variac when testing a "new" receiver. Gradually increase from zero to full voltage over an hour+. Always have an AVO in circuit to check the current. Paul E Last edited by Paul Stenning; 26th Dec 2004 at 7:48 pm. Reason: Fix link or code for vBulletin

31st Oct 2004, 7:00 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,566	Re: Variac - Method Of Usage This is one of those areas where people have their own ideas. Personaly I have never used a variac on a radio of any age. One problem with valve rectifiers is that the heater will not reach proper emitting temperature until you are pretty much at full mains anyway so the idea of using a variac seems somewhat pointless. Of course with selenium rectifiers they could perhaps be more effective in bringing the HT up slowly and hopefully reforming electrolytics at the same time. Before I put mains near any unknown set, I make a point of replacing the mains filter cap and subbing the smoothing and then changing any HT decouplers and check for HT shorts. Then I check the mains transformer DC resistance and mains wiring, pull out all the valves and then apply mains via a lamp limiter. I then chack the outputs from the transformer and if all seems ok, I replace the valves except the rectifier. If all the valve light up OK and there are no nasty smells or smoke I then fit the rectifier (with the subbed electrolytics, it's unlikely there will be any major problems). So far in every set I have restored including some that have not been used for over 40 years, I have not had any problems. Once the set is basically running you can then decide on reforming the original electrolytics or (descretely) fitting new ones etc. Like I said, we all have our own ideas so use whatever method you feel best with. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

31st Oct 2004, 8:24 pm	#5
jim_beacon Retired Dormant Member Join Date: Mar 2003 Location: Bishop's Waltham, Hants, UK. Posts: 939	Re: Variac - Method Of Usage on AC only sets, there is usually HT available at low level, when the mains input voltage reaches 100V. Jim.

1st Nov 2004, 1:09 am	#6
Andrew_-_XVK6ZEA Guest Posts: n/a	Re: Variac - Method Of Usage Hi, Yes, we all have our ideas. I agree with Rich as I follow the same process on old valve equipment. A variac is good to check for certain failures that will only show themselves at full or close to full voltage and of course to reform old or to condition new caps. I would like to mention though that if you use a variac you should still consider using an isolation transformer on live chassy sets. Hopefully it will save your life if you happen to touch something live. You still will get a nasty shock though. Andrew

1st Nov 2004, 11:30 am	#7
mjizycky Retired Dormant Member Join Date: Dec 2002 Location: London, UK. Posts: 69	Re: Variac - Method Of Usage Never used a variac on an old radio either (plenty of colour TV sets though) - trying to draw current from underrun valve heaters isn't a good idea as the risk of poisoning the cathode is quite great. My personal preference is to either overhaul the set completely before applying power, then look for any other faults or alternatively run the set up on its own mains transformer (having performed an insulation test first) but remove the rectifier and supply HT current from a separate variable HT supply. This latter method of course precludes you from working on an AC/DC receiver or many post-war AC Bush sets with series heaters unless you put in a resistor matching the value of the rectifier's heater! IMO I'd say that Richard's method is the best practical way of bringing a set back to life.

1st Nov 2004, 6:03 pm	#8
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: Variac - Method Of Usage I usually use a variac for a soft start. On a couple of occasions I've come across equipment with 110V mains transformers, and no external indication that this was the case. I was very happy that I'd checked to see what was happening at 100V. The transformers weren't tapped at 110V, they were 110V mains only. I thought cathode poisoning was caused by running valves at the proper heater voltage , but no cathode current.. Some special quality valves were specifically designed to withstand this sort of service. Pete.