What exactly have I just bought?! A 1930s Radiogram

[Techman]

A lot of old radios used heavy gauge solid wire like bus bar wiring for under chassis interconnections and this was either insulated or not insulated at all, so doesn't matter whether the insulation is brittle or falling off. If it's wiring that's stranded and floppy, or it's wound together and may touch, then it needs to be either replaced or re-covered - it all depends on what sort of restoration you're going to do.

I've seen that type of tuning drive before and I think I've got a couple of examples of it myself.

I think your set is probably one of the most unusual and interesting items of its type that has ever been documented on here - well done for saving it!

[EF80TVVALVE]

Yes there is some quite heavy gauge wire used throughout which I will keep but on the power supply it is relatively thin and easily moved. As these wires are laid so close together and overlapping I think it is for the best as condition wise they are just too poor considering the high voltages that will be passing through most of them.

I've tested the power supply for the speaker field coil today, the transformer checks out fine but the rectifier has failed. On close inspection it is indeed a copper oxide type, a shame really as this isn't something that is easily replaced with an identical looking part. I might look for a similar selenium type which may be a better bet as it looks like they are still in production and will look more 'correct' than a modern bridge, heatsink and series resistor.

As for the tuning drive it does look a generic part, I have wondered whether it's worth sourcing one as a lot of these old parts still turn up very often, I'm just not exactly sure who made it.

[Techman]

Originally Posted by EF80TVVALVE

I've tested the power supply for the speaker field coil today, the transformer checks out fine but the rectifier has failed. On close inspection it is indeed a copper oxide type,

Not so fast with the rectifier! How do you know it's faulty? You can't test them with an ordinary meter unless it's just short circuit and they always read o/c. You can only test it by putting proper volts on it, but take precautions with good current limiting or you might get fireworks if it is faulty and breaks down - stick a lamp in series with the supply.

[EF80TVVALVE]

I brought it up on a variac also observing the output voltage. It seemed to work more or less okay for a short amount of time before the DC output voltage began to drop and it began to draw a lot of current, the AC voltage on the output also began to rise as well as uneven heating across the fins, a few sections were running very hot compared to others which were relatively cool in comparison. DC resistance is only a few hundred ohms so it's definitely had its day.

[Techman]

Good test - sounds like it's had it!

Perhaps you could hide a replacement (diode - I guess it's half wave and there's probably a picture of it if I check back in the thread) near/behind/under it, leaving the original in place for looks.

[EF80TVVALVE]

Unfortunately so, it's a shame as it's quite decorative as well as it's functional purpose. It's actually a full wave bridge rectifier which is why I'm considering a selenium replacement. I can easily get hold of some modern square block bridge rectifiers but with the load on the output it'll likely need a heatsink as well as a dropper resistor as the modern replacement will be much more efficient than the original. With all of that in mind this is why I'm considering a selenium type as should this rectifier had been replaced in the 1940s-1950s then a selenium would most likely have replaced it. It's not quite original but it's a sympathetic repair due to its age, but this all depends on the cost of a new type as I'm sure they can't be cheap!

[Ed_Dinning]

Hi Bren, a selenium might have a higher forward drop and give you problems there, post a pic and details of its ratings and someone may have one

Ed

[Techman]

Originally Posted by Techman

there's probably a picture of it if I check back in the thread

There is, I just had a look back, in post #22 on the second page - I thought I remembered seeing one (or two). Shame it's duff.

[Lucien Nunes]

It's not quite original but it's a sympathetic repair due to its age,

Personally I would leave the existing copper oxide rectifier in situ and fit silicon. A selenium rectifier might look quaint and old-timey but it was not the original fitment (and only just becoming available then, whereas CuO was around from c. 1928). Nor would it be the conventional choice today, therefore to use selenium because it might have been used previously, implies a history that never took place. It also compels you to remove the CuO rec which sillcon does not. If you do need a dropper, and you might not, one or two surviving limbs of the CuO rec might work fine in that role, relieved of the reverse-voltage by the silicon. IME they do not go high-resistance in the way selenium does.

[EF80TVVALVE]

Yes I'd ideally like to leave the original in place but the biggest trouble going with the modern route is discreet fitment which I haven't thrown the idea out just yet. There isn't much space on the board which this supply is mounted upon and any modern rectifier will easily be seen and difficult to hide, hence why I thought of the possibilities of a selenium replacement.

I'm also considering if it is worth dismantling this copper oxide rectifier, removing the oxidised washers and heating them to produce a new oxidised coating, this looks to be how it was done originally but I don't know if its likely to just be a waste of time. If it repairs it then maybe its worth a shot. It's a very simple construction and would be easy to rebuild.

[Lucien Nunes]

How much current does the field coil take? You can get a 5A 40V schottky bridge in a SMDIP less than half an inch square that needs no heatsinking other than the leadouts due to its very low forward voltage.

[kalee20]

Originally Posted by EF80TVVALVE

I'm also considering if it is worth dismantling this copper oxide rectifier, removing the oxidised washers and heating them to produce a new oxidised coating

That was my thought too - there must be info 'out there' on how the process was done. Whether in air, or whether an oxygen environment was better, in principle, it's do-able!

[TonyDuell]

I think it's a bid harder than that. There are 2 oxides of copper. The common one, produced by heating copper in air is copper(II)oxide, CuO, also known as cupric oxide. It is black (it looks a bit like a soot coating when you make it) and is useless for recification. The other one, the one you need here, is copper(I)oxide, Cu_2O. It's brown.

I think there's a bit in 'Instruments of Amplification' about making it, in the chapter on making copper oxide transistors. But from what I recall it was not an easy or reliable process the way he did it in the book.

[TonyDuell]

Should have added that Copper(I)oxide, the one you want, is also called cuprous oxide.

[sparkymike]

Re. the ABC ARC in post 42, could that have been ABC, ie. ABC cinemas ?
Just a thought.
Mike.

[EF80TVVALVE]

Thanks for the information Tony, I presumed it was the former (cupric oxide) as the discs are black and exposed copper is very purple suggesting heat, although this could either be 90 years of age and use or even from methods of applying/forming Cu_2O.

Thinking about it, I suppose the replacement rectifier doesn't really have to be a full wave bridge anyhow. With modern diodes I'm sure there will be something suitable for the job as a half wave. The unloaded AC voltage from the transformer is 13.5v which will obviously drop a bit when loaded and the plate on the speaker suggests that the field coil only requires 6v. From memory I *may* have some 10a10 diodes which are 1000v 10a which could be a suitable fit although I'd have to do some digging in some spares boxes to find them!

As for the ARC/ABC it'll be hard to tell exactly what it means, I suppose there were hundreds of places even in the early 30s that it could be related to, all I know is that it most likely didn't leave the BTH factory like that! It may be possible it was used in an ABC cinema and found its way here as a second hand unit.

[Radio Wrangler]

Watch out, it may not be just a simple half-wave rectifier. There may also be a flywheel diode to allow the speaker magnet winding inductance to keep current going in the 'off-time' to act as a smoothing arrangement. It could also be a full-wave arrangement, and still with a flywheel diode function.

A speaker whose field vanishes for 50% of the time, pulsing at 50Hz might sound rather peculiar.... Would particularly suit the word "Exterminate!"

I think that's a Westinghouse Brake and Signal Co. logo on the end of the copper oxide rectifier stack.

David

[EF80TVVALVE]

Hi David,

Yes that's worth considering for the replacement circuit, easily enough done. I was considering a small electrolytic if I went for a half wave for the same reason. The original rectifier is a Westinghouse Brake 'Style A4' but I can't seem to find anything about it. It may be worth me finding any old catalogues from the time to see if there is day information about it although this may be difficult to do.

[Analogue man]

Originally Posted by Radio Wrangler

I think that's a Westinghouse Brake and Signal Co. logo on the end of the copper oxide rectifier stack.

David

It most certainly is.

[PJL]

I have a similar amplifier chassis using Ferranti transformers that is built on a crudely folded metal chassis with a massive mains transformer mounted at one end. This one has an audio coupling transformer to a 354V then a PP driver transformer to a pair of bottle shaped PX4's driving a PP output transformer. It was mounted in an early 30's HMV radiogram cabinet and is definitely home construction probably from a design by Ferranti. The transformers do suffer from blue spot and have integral tone correction capacitors embedded in the mouldings.

The friction drive on your tuning condenser should be OK as they are normally tapered and will be spring loaded, it will be the condenser that needs work to re-lubricate.