Mystery 1930s 3 valve battery radio chassis
 

Hi, this is my first post on this forum, and my first experience with old radios.

I was hoping someone could help me either identify the model of this chassis I came by, and perhaps help me work out why it's unable to receive anything other than Radio 4 LW (even with a huge aerial and earth) It's a two band set, with RF AMP (Mullard SP2) Regen/detector (some sort of Mazda L2?) and a PM22 audio output valve.

It uses an inter-valve transformer to couple detector to audio amp, and there are also a pair of screened RF coil units will multiple taps.

I've had no luck identifying the chassis, so I have painstakingly reverse engineered the whole thing to the attached circuit diagram (chassis-schematic.jpg) I have also attached some physical images, just in case someone has seen it before.

Looking at the circuit, I don't understand why isn't the tuning varicap isn't connected to ground like any other design out there, instead it flies up the HT (literally a flying lead from the valve cap) I also can't understand why the grid of V1 is tied to HT 120v while it's anode can't see more than 80v. There wasn't any evidence that anyone tried to re-wire the internals, as you can see in the images.

I hope I haven't made you go to sleep now, hopefully someone with some proper knowledge can spot what's gone wrong.

Thanks and hopefully hear something soon

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

I am sorry I cannot identify the model, but one thing immediately noticed is the lack of any D.C. return (grid leak) for the grid of V2. This means V2 will most likely be at anode cut-off conditions if the grid drifts far enough negative. Assuming it is a leaky grid detector, the .1 uf cap in the grid circuit is far too large and should be more like 100pf. A suitable grid leak value would be 1 or 2 megohms. (From grid to negative line.)
If you have drawn the coils correctly, I suspect that they are mis-wired too.
Looks a nice little chassis though, I wish you success with it. Tony.

Re: Mystery 1930s 3 valve battery radio chassis
 

Is that a dial lamp I see? If so that suggests that it has a mains power supply - although the valves are 2 volt, perhaps the bulb is too.

Peter

Hi, thanks for looking. I just noticed that I drew the 1M resistor in the wrong place. It is actually connected to the grid of V2. So wouldn't that solve the grid leak problem?

I'm 90% sure I've drawn the coils correctly, in what way do you think are mis-wired?

Yes, it's only a little 3v lamp, running off the LT supply. There's also another low voltage lamp on the chassis, which I think it used as a HT fuse.

Thanks

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

The 1 meg resistor in the schematic is connected to the anode of V1 (SP2) but on the chassis it's connected to the grid of V2 (L2) as a grid leak resistor and is also connected to what looks like a PU input socket, the low value grid leak capacitor that should connect to it can be seen partly poking out from under the chassis cross bar, I would check the reversed schematic again.

So far as I can make out.

EDIT: Post crossed....!

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

Yeah, that was my goof. What is the PU input socket for? I was guessing it's a pick-up for a gramaphone?

Re: Mystery 1930s 3 valve battery radio chassis
 

Yes, gramophone.

Are you sure about the main tuning capacitor connections? Is it a single or twin gang? If it was twin gang there would be two connections to it, possibly one stator to the grid and one stator to the anode, the rotor connexion would be connected to the chassis via the frame.

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

Well, it is a twin gang. There are only two physical wires going into it though, from underneath. One to each stator. I continuity tested the rotor plates. They are not connected to anything else but each other. So it's effectively one big rotor and two sets of stator plates.

The anode flying lead goes to one of the stators, from a side solder tag. The other stator is connected to the grid of V1.

I'll check continuity of the rotor plates to chassis ground again tonight. I didn't think it was connected, but I'll check again...

Thanks

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

Physically, it's a twin-gang, with two connecting wires going to each set of stator plates from underneath. One of those wires goes off to the grid of V1, and the other to the coupling cap to V2. That same stator plate has a side solder tag that is also connected to the anode of V1.

You make a good point about the rotors being connected to earth, when I continuity tested them, I didn't find any connection to anything other than themselves. But can't remember continuity to the chassis. I will check again tonight to see if there is a connection.

Thanks

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

Agreed, difficult to tell what's going on from the pictures but the circuit still needs some work!

It looks like a fairly straightforward TRF with RF, detector, and output. One of the controls is for reaction the other is an aerial trimmer.

Re: Mystery 1930s 3 valve battery radio chassis
 

Hi, good point..

It's definitely a 2-gang tuning cap. There are two wire conections, one goes to the grid of V1 and the other to the anode on V1. I don't recall there being any continuity between the rotor vanes and the chassis, I'll check that tonight and update the circuit diagram.

Thanks

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

You can see it's twin gang in the last coil photo (I've only just noticed) So tuned grid, tuned anode.

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

So, if those rotor vanes are earthed, would the attached circuit make more sense to you?

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

Yes, reaction is back into the RF amps anode circuit, both grid and anode circuits of the RF amp are tuned, if the tuning gang isn't grounded properly to the chassis the tuning/sensitivity will be very poor. Looks manufacture made to me...Any cabinet/other bits etc?

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

Much better but it still looks slightly wrong around the reaction. The cap on the left looks to be the reaction control and the one on the right an RF filter. Do these controls do anything? I would expect it to only oscillate if the filter is set to it's lowest (fully open) position.

Normally there would be an inductor between the anode and the intervalve transformer and the reaction would be taken off the anode and the RF filter at the junction of the inductor and the transformer.

The problem with MW is likely to be a dirty switch. The switch shorts out the LW winding.

As it was probably a kit set you can't never sure it was ever wired correctly!

Re: Mystery 1930s 3 valve battery radio chassis
 

The reaction capacitor might need altering in the schematic, it looks like a differential capacitor to me, if it is then redraw as a differential variable capacitor as in this schematic:

https://www.vintage-radio.net/forum/...ad.php?t=62849

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

Hi,

Are you referring to C4 and C5 in the attached photo (reaction-control-schematic.jpg) ?

It seems to be a twin ganged trimmer control with the common connection going to earth. While rotating the shaft, C4 seems to go down as C5 goes up, and vica-versa.

I've also attached a photo of the device so that you can see it, maybe I've analysed it wrong. (reaction-control.jpg)

I think the switches are ok, but I'll check again.

Tam

Re: Mystery 1930s 3 valve battery radio chassis
 

Yes, it's a differential capacitor as per Post#16 and it's link.

Lawrence.

Re: Mystery 1930s 3 valve battery radio chassis
 

Great spot, and that link is really good, as it that schematic. Thanks
T.

Re: Mystery 1930s 3 valve battery radio chassis
 

I'll try and find a better circuit symbol representation of a differential type.

Lawrence.