Golborne Vintage Radio

On the topic of reforming electrolytic capacitors, I built the capacitor tester/reformer unit that featured in the winter 2008 issue of the BVWS Bulletin. The article had first appeared in Radio Bygones June/July 2008, which prompted me to build one. Several components wern’t easy to find, notably the LCD panel meter, analogue panel meter, IRFBE20PBF transistor, and the two 6V transformers. If anyone contemplated building that one, following info might help anyone else faced with the same problems:

At October 2013, the 500V LCD Digital Volt Panel Meter 3 1/2 DC is available for £4.69 with free delivery, and can be viewed and ordered online at the link below from:

http://cgi.ebay.co.uk/3-1-2-Blue-LCD-Dig...uipment_ET&hash=item230ffe6083

As with most of the digital meters of offer, the power circuit of the meter must not share ground of the circuit that is being measured, but that wasn't an issue for me - I used the spare winding on one of the mains transformers, though if anyone looks at the circuit in RB or the BVWS Bulletin, it shows a common ground. Anyone who builds this project will need to consult the actual meter instructions. I've attached the original RB circuit (same in the BVWS Bulletin) corrected by me with the common ground connection removed, and also a copy of the note and diagram which came with my meter showing how to correctly wire it.

As a cautionary tale, when they say “500V” that’s what they mean. I squirted 535V into mine and they don’t like it up ‘em! My fault entirely, through not having the voltage control backed off. The meter expired and I had to get a replacement, which arrived just as quickly as the first one, but I treated it with more respect.

As at October 2013, The IRFBE20PBF MOSFET is available and in stock from RS for 67p at this link: http://uk.rs-online.com/web/p/mosfet-tra...s/5429535/

Some constructors might wish to cull a transformer from a scrap set, and the article explains that option, but I used two new 6 Volt transformers coupled back to back (that is with the secondaries coupled together, so you put 240V into one transformer, and get 240V out of the other. A doubler circuit uplifts that to 500V (adjustable). I obtained the transformers from ESR Electronics –again, a most helpful firm I’ve used many times over the years: Side Tag Transformer 0 - 6, 0 - 6V (2 x 1A) Order Code 301-045 £8.88 plus VAT + P&P.

View the transformer here:

http://www.esr.co.uk/electronics/product...ormers.htm

ESR can also supply the analogue 0 - 10mA Panel Meter, Order Code 124-124 at a keen price of £4.26 each + VAT. View the meter here:

http://www.esr.co.uk/electronics/product...tequip.htm

ESR's website is here, from whence you can read or Download the latest version of their Catalogue, which features over 500 Semiconductors, Resistors, Capacitors and Diodes:

http://www.esr.co.uk/

I have no connection with any of these suppliers, other than as a satisfied customer. I hope this info might assist any forum members who might otherwise be discouraged from building this project due to problems sourcing these components.

I’ve attached a couple of pics showing the outside and inside of my reformer.

Hope that's of interest to someone.

(Updated October 29 2013)

David

Hi David, yet again another quality build, the unit seems to push out a fair voltage, to put the capacitors under working conditions, i have copied the usefull links, to my favorites. Nice proffesional unit, and good read thanks....regards.....Alan.

I agree with Alan - most useful links. Thanks for those. I will have to dig out my BVWS magazines and build one of those.

The reformer has proved very useful and effective, and the article is very informative on the topic of reforming electrolytic caps. For example, the author puts forward a formula for working out the time to reform a cap, as follows: T (in minutes) = 5 + M, where 'M' is the number of months that a capacitor is believed to have been in storage, either in an unpowered set, or N.O.S. So for example, if the cap has been stored for 40 years the total time to reform it is 5+ 40x12 = 485 minutes - about 8 hours.

In use, the voltage and current are set to the lowest settings and the reformer set to the 'Reform' oposition, then the current observed on the 10mA analogue meter, which should be much less that the maximum 10mA. The voltaage is slowly increased over time, keeping the current below 5mA. The author suggests that over a period of several hours, the voltage is increased in stages of a quarter, half, three-quarters and finally, the full working voltage of the cap, check at all times to ensure the the cap remains cool and the leakage current low.

That's just a brief precis of a seven-page article.

Just one important point to note:

In the circuit, the 6V winding of T1, which is rectified to provide 9V DC to power the digital panel meter, is shown as connected to ground. I've used several of these meters and the instructions that come with them make it clear that the power supply to the meter MUST NOT be connected to the equipment that the meter is measuring. Thus, the inut that powers the meter must be kept floating, and not share the ground that the rest of the equipment into which it is built. I've copied the circuit from the Winter 2008 Bulletin article (a sketch, as it appeared in the magazine), to show the error, and have attached an amended circuit showing the corrections.

I'm not sayng the author made an error - it may have been permissible to connect the digital penel meter that he used, in the manner depicted in the article. I'm simply saying that - to quote the instructions which came with the meters I've used they say 'power input must not share equipment ground or meter will damage and die'.

I've attached a diagram which came with the meters I've used which makes it very clear. These meters, which have a clear backlit display, are now quite cheap - as cheap if not cheaper than analogue meters, and can be used in all sorts of applications. I've used one in a homebrew zener diode tester for example, and am presently using a 9999 Hz frequency counter module (with a seven segment LED display rather than the more usuual LCD display), which cost about £12.00, to build a digital lathe speed indicator. (Of course one RPM is 60 times slower than 1 Hz, so for each rev of the lathe, 60 counts are called for to convert from Hz to RPM otherwise, if the lathe was running at 600RPM, the digital readout would only be 100. This conversion is made by fitting a disc on the lathe headstock with 60 slots in it, which interupts a beam in an opto interrupter).

That's another story, of which, more another time, but it illustrates just how versatile and useful these cheapo meter jobbies are, with a little imagination. The only downside, is that depending on the postal services from the PRC, they can take a few weeks to arrive, but hey - under the radar of VAT, and free post and pack - who's complaining!!?

Hope that's useful info, particularly to anyone contemplating building this cap reformer or using these meters in other applications. (They also sometimes have different power voltage requirements - my digital panel meter needs 12V for example).


David

Might I make one minor suggestion?
Put a resistor across the unused winding of T2.
Alan

(13-05-2011, 07:47 PM)AlanBeckett Wrote: [ -> ]Might I make one minor suggestion?
Put a resistor across the unused winding of T2.
Alan

Thanks for reading Alan, and for your observations.

I'd wondered about that myself at the time Alan. I'm not too well up on transformers, but I have heard that it's not good practice to have unused secondaries. What current do you think that the used secondary should draw to keep it happy? EG, a 220R resistor across that unused secondary would draw 27mA. (only 0.16W).

But that said, it was commonplace for mains transformers for valve power supplies to have several secondaries - for example, for different voltage rectifiers - 4v, 5V, 6.3V - sometimes tapped, sometimes separate windings, some of which were unused. I've sometimes used two secondaries in series to double up the voltage, or paralleled them in phase to double up the current carrying capacity.

For anyone contemplating building that reformer, the most satisfactory and cheapest option for the mains transformer arrangement would be to use one rather than two back to back. For example, a mains transformer culled from an old AC radio with 240V and 6.3V secondaries.

David.

David,
The reason I wasn't brave enough to quote a value was I didn't know what the voltage was. As you say, a few mA should be OK.
I don't know that I've ever seen windings left floating in commercial equipment. Anyway, they're usually too tight to have any spare windings.
Alan