McMichael Supervox Twin wiring.

[monomono]

Hi all, I recently was gifted a McMichael Twin Supervox, 933 AC mains set which had been stored away since 1955 at the back of a workshop, a bit of woodworm in the cabinet but otherwise in extremely good condition, all parts intact except the mains on/off sw was at one time found to be kaput and it was wired direct to the mains transformer (with tar tape).

The two wire cloth covered mains lead was in dangerous condition so I replaced it, also the wire from the transformer secondary to the metal rectifier was very burnt looking as were all the wires from the metal rectifier to the speakers so I disconnected the HT secondary and checked the mains transformer, it works, the pilot light came on and the valve heaters all work.

What I'd like to know before I proceed any further is, would 60 years of storage cause these wires to crinkle this badly or is it evidence of a short in the HT circuit?
Having never come across a radio this old with metal rectifiers I'm hopeful of some ideas from the forum.

Thanks
Seasons greetings to all.

[Herald1360]

That picture definitely looks like old perished rubber insulation- there's nothing specific to suggest overheating or other problems. It looks like a fair amount of replacement wiring is needed for safe operation- how you proceed depends on whether repair or restoration is your aim, though in either case you will need to replace old wax paper capacitors, out of spec resistors and any wiring insulation where there is any chance of unwanted contact between circuits. Valves will most likely be OK, or at least usable, but the metal rectifiers may well not be, though here replacements in the form of silicon diodes with some added series resistance are easy to hide, leaving the originals in place for appearance sake.

[PJL]

This is normal degredation of rubber wiring. See this https://www.vintage-radio.net/forum/...ad.php?t=72625. For solid wire it is possible to fit a new sleeve to the existing wire by unsoldering one end. For multi-stranded wire like yours, you will have to replace the cable. If you are as mad as I am you will sit in front of the TV and recover the cotten sleeving to cover the new wiring.

[mark pirate]

In most sets of that age, i would replace the smoothing caps before attempting to power it up, also check the condition of the wiring under the chassis.

These are a nice radio, well worth taking the time to restore properly, good luck with it.

Mark

[monomono]

Thanks.

It is a real nice piece of equipment and I'm looking forward to reviving it, the under chassis wiring looks very original, no repairs apparent and the speakers are OK. If I replace the metal rectifier would it be OK to use two diodes from a microwave or do i need a bridge? Not too sure how to go about that.
I'll rewire and replace old caps first and see how it goes, that will keep me busy over the season.

Padraig

[McMurdo]

Nice radios, redolent of the period, keep us posted and lots of pics please!

[monomono]

will do

[geofy]

Originally Posted by monomono

If I replace the metal rectifier would it be OK to use two diodes from a microwave or do i need a bridge?

This TRF set has a voltage doubler for its HT rectification. If the two microwave diodes are suitably rated they could be used, or two 1N4007 diodes would do, with the, usually, 100 Ohm resistor of around 5W in the DC output lead going to the smoothing choke/speaker field coil. The two capacitors in the voltage doubler as well as the two HT smoothing capacitors should be replaced.

The rusty chassis and speaker frame can be treated with a rust remover or rust proofing paint.

[monomono]

Thanks Geof, it will take a wee bit of rewiring before i get to sorting the HT, I have plenty spares from old junked radios, so i should get it operational. I will keep posting my progress(or lack of).
regards
padraig

[Station X]

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...ad.php?t=77757

[monomono]

Hello all,back on the mcmichael rv8 restoration again after a few months,got the power supply sorted,original metal rectifier still working,replaced the caps,a lot of the carbon resistors were gone high,ie C19 1.5k, was 3.4k so were also replaced ,so i powered it up with the lamp limiter,no problems,gave it full power, again no noticeable problems ,a very faint hum from the speakers ,so far so good. NO. The low freq coupling unit is open circuit on all but one winding (pic one) in the circuit diag this is a rc unit but in my unit its got two coils + 0.5uf cap.i have drawn a diagram of what i think it looks like internaly but it could be incorrect as i have never seen one of these before.
any help or advice from the forum greatly apreciated
regards
padraig

[PJL]

Your picture is too out of focus to see what is going on. What you have drawn would make sense with an AC coupled intervalve transformer. The primary 'coils' may just be resistance wire, do thay have a core? If not then I would replace with resistors.

The o/c intervalve transformer secondary is a bigger problem, you could replace the transformer with a 1M grid resistor but you will lose the gain provided by a transformer. It might be a good stop gap so you can get the set up & running whilst you have the transformer rewound though.

[monomono]

Hi pjl,heres a better picture,the top trans has an iron core it looks like a mini interstage ,two wires in two out,the bottom coil is a coreless centre tapped with very fine wire.
thanks
padraig

[monomono]

pjl you were right about the centre tapped coil,i opened it up and found its a wire wound centre tapped resistance ,so thats easily replaced with a 20k+30k.