McMichael 471 PSU and reforming... - UK Vintage Radio Repair and Restoration Discussion Forum

tamemin · 22nd Aug 2018, 12:52 pm

Hi chaps,

I've just started on a McMichael 471. This will be my first mains powered vintage set.

I'm looking at the PSU and so far the transformer looks ok (tested with a multimeter) There was a small voltage switcher contact problem that I fixed. The rectifier filaments are ok, so should hopefully work.

My worry was the 35 uf 350v twin reservoir capacitor and it's leakage, so I did a bit of testing at 20v and noticed that it doesn't hold a charge for long (4 secs)

So, after a bit of googling, I read about this cap reforming process ....hmm interesting so, I had a go at giving it 80v and monitoring current using a variable DC-DC converter...current initially high then fell back over a few minutes.

I basically stepped the voltage up, in increments, repeating the process until I got to about 300v. Did the same for both sides of the cap.

At the end of the process, the cap would consume about 1.2 watts at 300v, in leakage, per side.

What do you guys thing, will it be ok? or blow the little lamp fuse...

I've attached some pics.

Tam

G8HQP Dave · 22nd Aug 2018, 12:59 pm

4mA leakage is a bit higher than I would be comfortable with.

Boater Sam · 22nd Aug 2018, 1:50 pm

4mA is high, you should be able to get it down to below 1mA but it may take a long time, like a day!

If it refuses to reform or more likely goes off again after a short while, restuff the can with a couple of good 105 degree caps with a reasonable ripple rating especially for the reservoir one.

The are many threads on here about cutting and re stuffing cans so that they look original.

Nuvistor · 22nd Aug 2018, 2:12 pm

Or fit a new one if it will fit.
https://www.cricklewoodelectronics.c...Capacitor.html

cathoderay57 · 22nd Aug 2018, 2:50 pm

I totally agree with previous posts. Even if the fuse lamp doesn't rupture if you use the existing caps you will likely experience hum and instability problems, plus you are putting an unnecessary extra load on the HT winding of the transformer and the rectifier. Both of these would be much more expensive to replace than an electrolytic capacitor.

tamemin · 22nd Aug 2018, 3:53 pm

Thanks guys,

I like the look of that new cap on Cricklewood, it might even be the right physical dimension. I also like the keeping the thing looking original, as this set is in amazing condition and honestly I can't see that it has been tampered with at all since manufacture. In fact, I'll attach a pic now...

Tam

Nanozeugma · 22nd Aug 2018, 5:00 pm

By the time you add carriage, add the VAT to the price and the carriage a 32 + 32 from Cricklewood comes in just shy of £12 a pop.
Or... you could choose the same brand from China for £4.61 post free.
It might take a wee while to arrive, but I know which one I'd choose...

HamishBoxer · 22nd Aug 2018, 6:29 pm

Cheaper than a u/s rectifier valve or worse a mains transformer.

Boater Sam · 22nd Aug 2018, 6:32 pm

Or re stuff with new caps off ebay for about £1.80.......................

Nuvistor · 22nd Aug 2018, 7:01 pm

Lots of choices, all will do the job, better fitted than damage other parts.

22nd Aug 2018, 12:52 pm	#1
tamemin Triode Join Date: Jul 2018 Location: Oxfordshire, UK. Posts: 34	McMichael 471 PSU and reforming... Hi chaps, I've just started on a McMichael 471. This will be my first mains powered vintage set. I'm looking at the PSU and so far the transformer looks ok (tested with a multimeter) There was a small voltage switcher contact problem that I fixed. The rectifier filaments are ok, so should hopefully work. My worry was the 35 uf 350v twin reservoir capacitor and it's leakage, so I did a bit of testing at 20v and noticed that it doesn't hold a charge for long (4 secs) So, after a bit of googling, I read about this cap reforming process ....hmm interesting so, I had a go at giving it 80v and monitoring current using a variable DC-DC converter...current initially high then fell back over a few minutes. I basically stepped the voltage up, in increments, repeating the process until I got to about 300v. Did the same for both sides of the cap. At the end of the process, the cap would consume about 1.2 watts at 300v, in leakage, per side. What do you guys thing, will it be ok? or blow the little lamp fuse... I've attached some pics. Tam Attached Thumbnails

22nd Aug 2018, 12:59 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: McMichael 471 PSU and reforming... 4mA leakage is a bit higher than I would be comfortable with.

22nd Aug 2018, 1:50 pm	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: McMichael 471 PSU and reforming... 4mA is high, you should be able to get it down to below 1mA but it may take a long time, like a day! If it refuses to reform or more likely goes off again after a short while, restuff the can with a couple of good 105 degree caps with a reasonable ripple rating especially for the reservoir one. The are many threads on here about cutting and re stuffing cans so that they look original.

22nd Aug 2018, 2:12 pm	#4
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: McMichael 471 PSU and reforming... Or fit a new one if it will fit. https://www.cricklewoodelectronics.c...Capacitor.html __________________ Frank

22nd Aug 2018, 2:50 pm	#5
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 471 PSU and reforming... I totally agree with previous posts. Even if the fuse lamp doesn't rupture if you use the existing caps you will likely experience hum and instability problems, plus you are putting an unnecessary extra load on the HT winding of the transformer and the rectifier. Both of these would be much more expensive to replace than an electrolytic capacitor.

22nd Aug 2018, 3:53 pm	#6
tamemin Triode Join Date: Jul 2018 Location: Oxfordshire, UK. Posts: 34	Re: McMichael 471 PSU and reforming... Thanks guys, I like the look of that new cap on Cricklewood, it might even be the right physical dimension. I also like the keeping the thing looking original, as this set is in amazing condition and honestly I can't see that it has been tampered with at all since manufacture. In fact, I'll attach a pic now... Tam Attached Thumbnails

22nd Aug 2018, 5:00 pm	#7
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Re: McMichael 471 PSU and reforming... By the time you add carriage, add the VAT to the price and the carriage a 32 + 32 from Cricklewood comes in just shy of £12 a pop. Or... you could choose the same brand from China for £4.61 post free. It might take a wee while to arrive, but I know which one I'd choose... __________________ Thermionic Emission, warms the cockles of your tubes. Last edited by Nanozeugma; 22nd Aug 2018 at 5:01 pm. Reason: Typo.

22nd Aug 2018, 6:29 pm	#8
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,932	Re: McMichael 471 PSU and reforming... Cheaper than a u/s rectifier valve or worse a mains transformer. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

22nd Aug 2018, 6:32 pm	#9
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: McMichael 471 PSU and reforming... Or re stuff with new caps off ebay for about £1.80.......................

22nd Aug 2018, 7:01 pm	#10
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,433	Re: McMichael 471 PSU and reforming... Lots of choices, all will do the job, better fitted than damage other parts. __________________ Frank