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

[greenstar]

It looks as if it could be home made to me, but good quality. Bigger sets like this were more common in the States, but they certainly existed here (Decca Deccola 1948, 16 valves!). A big house perhaps. It's a lucky survivor - people would strip transformers from such sets and weigh them in for the copper. You could buy cabinets like that - worth looking in early 30's magazines.

[EF80TVVALVE]

Thanks all for the advice regarding the capacitor, I was hoping to get a look this evening but I've been busy with one of the televisions. I'll see if it is definitely the cap or if something has dripped onto it. Greenstar- it's crossed my mind as to whether it is home made, it wouldn't surprise me if the person was very competent but I'd have to agree that the cabinet would be purchased if this was the case, or the builder was a professional in cabinetry as it is just too good for the typical 1930s home constructor.

As I'm going through it I'm hoping to find some clues as to who made it. The only thing I have found so far is 'abc' or 'arc' hand-written to the side of the speaker in pencil, whether that means anything or not I don't know.

[EF80TVVALVE]

I've inspected the leaky Parmeko PIO capacitor today and it is definitely leaky at the top corner, I'll clean off what I can and seal with epoxy. I presume it's probably been tipped up in the past as I can't really see how it has leaked from the top. Just out of curiosity I've put a meter across them and measured their capacitance and they look okay so I'll bring them up slowly.

There are also these Ferranti capacitors fitted, I presume these are normal wax paper types. They measure okay on my meter, ie no shorts but if they are wax then maybe it is best to restuff these ones as I've had trouble with similar in the past when operating at their working voltage. There are a few similar block wax paper types throughout that I, will also restuff.

[Ed_Dinning]

Hi Bren, good news on the can cap. The Ferranti will be waxed paper so should be easy to restuff

Ed

[EF80TVVALVE]

Thanks Ed, I'm just looking at a way to get them open, unlike the usual type with wax sealed bottom these look go be soldered shut around the seams. They do however test okay with a multimeter so I may run them on a separate PSU for a few days and see how reliable they are.

I've been looking further today at the psu and it uses two full wave rectifiers in parallel. There are also two further paper in oil capacitors wired in series of each other between each anode connection (basically across the 425-0-425 winding) with a connection from the centre of the two capacitors to one end of the 4v rectifier winding (I'm hoping I've explained that well enough). I'm just wondering what purpose these capacitors actually have, I've never seen this before on a power supply and I'm just curious as to their job in the circuit.

[EF80TVVALVE]

The power supply has been removed today, very simple as all of the wires are held on by threaded nuts. Here are some photos now it's had a clean up. I'm sure this is where most of the weight comes from, it's awfully heavy.

[Ed_Dinning]

Hi Bren, yes a typical PSU of the period, built like a brick outhouse.
I may have some of the dogbone resistors if you need any.

If the caps are soldered in to cans then they may be OK. A bit messy but they are easily unsoldered with a "heat in the fire" type of irn

Ed

[EF80TVVALVE]

Thanks Ed, I'll give these a test and see how they go. Whilst the wires are disconnected I decided to take the amplifier unit out, it looks as though both chassis are constructed with wood and a layer of metal attached to the tops and sides. Construction is fairly basic really as to be expected from something of this age, looks as though it has a hum pot and tone control. I've also attached a somewhat crude schematic as drawn by yours truly, just so anyone interested can see the type of circuit employed in the amplifier stage.

[EF80TVVALVE]

I've now removed the radio section for a clean and to see what will be needed. It looks as though it was built to a certain design as a lot of capacitors are marked C1, C2, C3 etc. It looks quite impressive circuit-wise but I can't help but think that the part wood construction does make it look a bit cheap, but I suppose this would have been a common construction method in this time. I'm beginning to think that this is looking more and more home-made.

[Ed_Dinning]

Hi Bren, I have a couple of sets built like that, a wooden chassis that was covered with thin metal or copper foil for screening and earth continuity purposes. Quite standard home build/ as recommended by mag practice.

Ed

[Uncle Bulgaria]

I'm not convinced - remember that at the time, the possibilities for a home builder were not nearly as divorced from the 'professional' as today.

[Herald1360]

Originally Posted by EF80TVVALVE

The power supply has been removed today, very simple as all of the wires are held on by threaded nuts. Here are some photos now it's had a clean up. I'm sure this is where most of the weight comes from, it's awfully heavy.

If the PSU tops 100 lbs in its own right I think it would qualify as a boat anchor all on its own

I suspect the "hum pot" or humdinger was pretty much de rigeur for any directly heated output valves in a half decent amplifier if the manufacturer didn't bother with a proper centre tapped heater winding.

[EF80TVVALVE]

I'm actually going to weigh the PSU and see what it does come to weigh as it is a heavy bit of kit! I'm going to look through some old publications and see if there is anything like the receiver published around that time but there doesn't seem to be many with 7 valves, it could be someone's own design and never published but chances are that it must exist somewhere than not.

[G6Tanuki]

Originally Posted by Herald1360

I suspect the "hum pot" or humdinger was pretty much de rigeur for any directly heated output valves in a half decent amplifier if the manufacturer didn't bother with a proper centre tapped heater winding.

In some respects an adjustable 'humdinger' as seen here is _better_ than the fixed centre-tap in the output-stage-valves' heater-primary, since it allows you to dial-out any imbalance caused by stray capacitive/inductive effects. A fixed centre-tap on the primary means you have to live with such construction/layout imbalances and their associated hum.

[Paul_RK]

Originally Posted by EF80TVVALVE

I'm going to look through some old publications and see if there is anything like the receiver published around that time but there doesn't seem to be many with 7 valves...

True - Scott-Taggart's Super-Gram is the only real home-constructor beast of a domestic wireless that comes to my mind, nine valves (PP5/400s again) plus thermionic and metal rectifiers!

https://worldradiohistory.com/UK/Wir...4-09-S-OCR.pdf

Paul

[Radio Wrangler]

It might be a home made copy of a commercial unit rather than a magazine project.


David

[Lucien Nunes]

If the speaker field is 6V, the rectifier is probably copper oxide rather than selenium. Their failure modes are different and they do not emit the same nasty smoke. It might be serviceable, if so I would leave it in circuit and just discreetly fuse upstream of it with an inline wire-ended fuse in some sleeving to protect the transformer.

Echoing the comments above, I am still finding mid-30s PIO capacitors in a number of chassis reliable even at high HT. Again, one can work miracles at protecting the transformers and rectifiers from shorted C's, with some carefully chosen wire-ended fuses between wires and tags, also by secreting low-temperature thermal fuses in the transformers themselves.

FWIW the only significant PIO block failure I have had in the last few years was a German-made 18μF component where the construction of its leadouts was unusual and IMO unreliable. I can well imagine it had been open to the air for a long time. It was also subject to electrically arduous duty in the phase-converter circuit for a 1/6hp 3-phase motor, handling an amp of ripple at 350V AC. Its conventionally-sealed TCC neighbours all have good insulation.

Overall I think it's a wonderful thing and look forward to seeing your progress. With the sword of Damocles hanging over every triode-output chassis these days, it's heartwarming to hear of one that is going to be enjoyed in original form.

I would add my usual caution that by making it work again, you are altering and deleting parts of a rare historic record of practices from 90 years ago. Some of the decisions you have to make have no single correct answer. A stuffed component is neither original nor modern, nor a commercially available part. It's a weird hybrid of your own making that no radio maker ever saw the likes of. OTOH you could say that the casing of the original cap was sound, but the wound element was no good, so you have retained the casing but changed the element - a perfectly reasonable course of action. Document everything, keep the old bits, have fun, play loud!

[turretslug]

Originally Posted by EF80TVVALVE

There are also two further paper in oil capacitors wired in series of each other between each anode connection (basically across the 425-0-425 winding) with a connection from the centre of the two capacitors to one end of the 4v rectifier winding (I'm hoping I've explained that well enough). I'm just wondering what purpose these capacitors actually have, I've never seen this before on a power supply and I'm just curious as to their job in the circuit.

They look as though they've been squeezed in where they could be, one in particular right on the edge of the chassis, the other tucked under the transformer top plate on something where everything else has reasonable breathing room- I wonder if they were an afterthought to overcome modulation hum, or maybe if the primary/secondary screening was insufficient to keep the general HF mains rubbish at bay? Whilst the general consensus seems to be that they are a reliable type, one going short could kill the transformer.

[100series]

I have seen something similar which was used at Glyndebourne but that was 1940's. I am guessing here but this might have been for somewhere where people who wanted to listen to music, with the best quality of reproduction, gathered.

It looks too specialist to be just stuck in a hotel for people passing through. This was for people who really cared about music and unless a very affluent individual or family would be for an opera house, or a really exclusive club./music hall.

And I am certain this was built in one go and not placed in a piece of furniture with a different original purpose. It is beautiful, wonderful and I wish it was mine. I am green with envy. It is beyond impressive. Lucky man.

I really hope you find room for it at your home or can arrange for it to be kept somewhere else where it is appreciated if that is not possible.

[EF80TVVALVE]

Thanks all for your replies and apologies for neglecting the thread for the past few days. The Scott-Taggart design is interesting and I did wonder before if this was anything to do with any of his publications, I know that in the 1930s he was known to publish very advanced designs. I'll bring the speaker supply up on a variac and see how it performs, I personally think it may be okay to leave as is, that is should it be in good working condition.

Just to update everyone on what is happening, the past couple of evenings I've been looking at the radio section, particularly the tuning drive. This is going to need some serious attention, as can be seen in the photo it's driven from the centre spindle/control knob directly to a metal wheel that drives the tuning condensers and indicator needle. Over the years the metal on metal contact has worn down the larger wheel and now it doesn't operate very well and loses grip all too easily

My plan is to get a very thin piece of copper and cut/file down to the right shape and size then affix to the wheel where it has worn away, allowing the centre spindle to have a good grip on the larger drive wheel again.

For the power supply I have some modern black solid core wire on the way to replace what is there currently. The original stuff is awfully brittle and the insulation is so poor that it needs to be replaced. If this was all low voltage circuits then I'd be happy to keep it as is, but I'd rather keep things as safe as can be in this area, especially if it can cause damage to the components.

I'm trying to keep it as original and correct as possible, the only exceptions I am making is with safety. I know this sort of stuff will never be truly safe in the modern sense, but I'm sure other members will understand that some aspects of early circuitry methods are downright dangerous. I'll be re-stuffing caps where possible, but any that test okay in low voltage circuits I'm happy to leave in place. This is by no means going to be a quick project, but I'm hoping to have it playing to some degree by next month as it'll be my birthday and it would be rather a nice treat to hear it by then!