Murphy A26

[Radio Fixer]

All the spray paints and primers that I have tested measured as non-conductive as is Araldite Steel.

Its surprising how low resistance the original valve coatings were leastwise the Mullard gold ones. The lowest resistance reading I got for one NOS Mullard VP4A, that I had in stores, was 6 Ohms but this was variable up to 10. This was measuring from the metalising and cathode pin to near the top of the valve. Making a reliable contact isn’t easy and the best way was to use the meter probe in a pad of fine wire wool lightly rubbed on the surface.

To replace it I used on one set Electrolube Conductive Nickel Spray which had an Oms / Sq of 1 from DC to blue light. Later I used it for another radio but the formula had changed and even with the help of Electrolube there was no way I could make it stick to the glass. Apparently the formula was changed 'cos of 'elf and safety. Annoying.

The Graphit33 at 1000 to 2000 Ohms / Sq may not be good enough, certainly not for a high gain TRF such as a Philips SI (Yes! been there). For a Supie it might be adequate.

Be interesting to see if it works for you Gary

[AdrianPH]

When I used the Graphit33 as a replacement Aquadag on a CRT I got it down to under 1K Ohms per inch. What I can do is try the cold zinc galv on a glass jar in the garage, let it dry to see how it performs. If sticking to the glass is an issue, then possibly a coat of Graphit33 and then when dry overpaint with the zinc may be worth a try.

It will be a try it and see job, there is no rush.

Adrian

[Radio Fixer]

Here is a pic of a rare valve re-metalised with the Original Electrolube Nickel Spray.  Finish was a dark grey but sprayed over with gold paint.  Still good.

Dont bother with the new stuff:  I tried many primers, even one sent by Electrolube, and none of them made the nickel stay on the valve.

Bet the Zinc Primer wont be conductive, none of the Weld Thru Zinc Primers are. Yes! to a welding gun maybe but not to tiny voltages and RF.

   

Gary

[AdrianPH]

Well I have sprayed one side of a coffee jar with Graphit33 and the other side I have plastered on some Galva zinc. I will leave them for 24 Hours to cure and then may even give them a bit of heat and see what the results are like.

Adrian

[Mike Watterson]

And silver PCB repair paint is more expensive than it sounds! I painted it on once with a pin from an IC to a SMT miniature connector.

I've occasionally wrapped Aluminium foil earthed via a spring to a neighbouring IFT. There was conductive nickel spray 30 years ago for EMC shielding of plastic cabinets. I bought it once only because it was so expensive and after that used glued on aluminium foil on cases, sometimes with a paper clip for the earth wire if a screw and tag wasn't possible.

[AdrianPH]

A few days later and the tests on the glass are.

The Graphit33 is down to just under 3 Kohm, so more coats would get it a bit lower, but the Galva Zinc is in the Megs like 30 plus. So unless I got for tis off Ebay "Car Screen Window Electrical Heated Rear Screen demister Repair Paint" at £14.49 for 3 gram. I think I will be using Graphit33.

Adrian

[Mike Watterson]

Much what I'd expect. The 3 K might be good enough with four fine vertical wires in it soldered to an earth ring at the bottom and taped at the top till a coat of Graphit33 sets/dries. It can always be over-sprayed later to silver, light grey, red or gold to match an actual model.

[AdrianPH]

I have spent the past few days working on the A26.  I finally got around to de-packing the capacitor block and using modern caps within it.  Replaced a few wires were the insulation was crumbling away and also managed to replace the wax paper capacitors, every one I tested was showing a very high ESR and % loss as well as being way over value on my simple TC1.
many of the waxes have been re-packed with modern ones, awaiting a trip to Golborne so get a few values I am out of such as a 0.047uF so I can replace the last one in it's tube, for now it has a 68nF mustard cap in it's place.

You can see very easily one wire I replaced, but the other 5 which are black, not so much.  I did wonder why much of the wiring is done in what I guess is double cotton covered and some was in rubber?
   
The two valves have also been sprayed with Graphit 33 and tomorrow will be fitted.  I have done a few tests with an insulation tester with reasonable results on the mains transformer.  New mains lead fitted and have taken the chassis to mains earth in the lead.

Still do not have a mains variac, so will see if I can blow the fuse or not tomorrow!

Adrian

[AdrianPH]

I changed a few of the resistors on the radio that have gone well out of any tolerances some had doubled in value.  I used some modern ones, but in reality they do not look correct as opposed to the dog-bone style that is fitted.  There are more to replace but I will have to have a think about what types I will use.

When I first powered it up I could receive LW but no MW stations.  This was down to the band-switch having oxidized contacts, so a bit of wet and dry/emery paper and switch cleaner soon sorted that.

It needs a good wire antenna as there is nothing built in, so at some point I may build a frame aerial for it, or just stick another wire in the loft?

The AC/VP1 valve at some point was replaced with a VMP4G it seems to be slightly taller as the anode lead is at it's limit, but the set is working.  It could do with more resistors changing, but that will wait for a while.
   
If anyone has a spare locking nut of the correct type for the knobs I would be grateful, reasonable costs covered of course.

Adrian

[Radio Fixer]

" band-switch having oxidized contacts, so a bit of wet and dry/emery paper and switch cleaner soon sorted that."

I wouldnt have used the emery paper. The contacts are usually brass with a little plating often silver. Go thru that and you have brass which oxidises much quicker. Unless the set has a removeable bottom then its another chassis out job to squirt some more switch cleaner in the not too distant future.

Switch cleaner and working the switch to and fro is all thats usually needed.

But GL with your success

Garu