1920's TRF Rx

[Phil G4SPZ]

Never mind the radio, David, just admiring your nice tidy workbench and lovely Marconi sig gen!

[David Simpson]

Thanks Phil. The TRF is now completely stripped down, and shall focus on the cabinet refurbishment. Spoke with the Pal who gave it to me last Sun - he thinks the grained ebonite front panel had the trade name - "LingaLonga" or something similar.
Regarding the workbench - I've been in love with CT394B's since the 60's, and aquired it's smaller sibling - the TF995A/5 back in the springtime. Tidyness-wise, two reasons - its a bit small, so I try & aviod clutter., and those folk who may have served in the RAF might recall that no-one went home at 5pm unless every tool was returned to shadow boards or tag boards - thus avoiding a bollocking by the Chief Tech.
Still waiting for someone to come up with a suggested circuit diagram. Pleanty of 2 valve c/d's on the go internet-wise, and found an "Ediswan RC Threesome" but its no use. Really need something that contains the same Lewcos Aerial & Reactance coils, the GECoPHONE tuning capacitor & so on as per the pictures. I've drawn up the c/d as its wired just now, but some of the coupling connections are wrong. All components have been measured for L,C or R, but they may well have detereorated over the last 70 years.

Regards, David

[Mr Moose]

Hello,
This set is quite an unusual design as the first two valves are resistance-capacity coupled (using a Lissen resistance-capacity unit) whilst the second two are the more normal transformer coupled. As you have a capacitance meter you might want to check the capacitance between the P. and G. terminals on the Lissen RCC unit.
If I have read the circuit rightly, it also seems to use a rather unusual reaction arrangement using a coupling coil in the anode circuit and using a differential condenser to variably bypass the RF from the anode of the first valve to earth or through the reaction coil to earth according to the setting of the condenser.
Personally I would retain this arrangement rather than changing it to something more normal unless you can't get it to work.
As to the valves to use I would have advised a PM1HL (or PM1HF or PM2HL) for the first valve a PM1LF (or PM1HL or PM2HL or PM2DX) for the second valve and a PM2 or PM2A (or PM2DX) for the output valve. (or Cossor 210HL, 210LF and 215P or 220P: or Osram HL2, L210, and P215 or LP2)
I suspect that the terminal strip at the back has been made from the ebonite panel from an older set where ATC would have denoted Aerial Tuning Condenser and AnTC, Anode Tuning Condenser etc.
I think that the middle terminal between aerial and earth is a second aerial terminal to be used if you are using an untapped coil rather than the tapped Lewcos coil.
There seems to be a flexible wire going to the LT. terminal of the Lissen RCC unit, although it has disappeared in the final lot of pictures, and I think this would have gone to the Grid Bias battery; there should also be a connection to the Grid Bias battery from the intervalve transformer.
Yours, Richard

[geofy]

Don't know if these are of any use, the first is a mains trf design. The second diagram has a swinging coil for the reactance against the fixed coil, this circuit from the 1922 Radio constuctors handbook.

Geof

[Herald1360]

1922?

More like 1942 I reckon I don't think octal bases were around in 1932 either.

[David Simpson]

Richard & Geof, thanks for your replies. This set certainly seems to be of a hybrid design. Not that I know much about TRF's anyway. As an RAF Boy Entrant trainee Wireless Mechanic nearly 50 years ago, we studied TRF theory for a short while before going on to superhets, then more in-depth studies on double superhets. And certainly have never been trained on civilian radios. Later fitters course training was just on double superhets.
Give me a day or two & I'll try & draw up a decent circuit diagram to attach.
Shall try & aquire the 3 recommended PM valves. Ed Dinning recommended them also. I see from the AVO Valve Data Manual that they require HT's between 100 & 150V, Heaters are 2V, & -veVg between 1 & 6V. I have bench PSU's that'll supply those voltages. But wonder what the original battery packs gave out ?
I'm just back from a friend's house. He's a retired woodwork teacher & his hobby is now cabinet/furniture making. He's given me a wee jamjar of white gunge which has worked wonders on the front grained bakelite panel after first giving it a thorough clean with acrylic thinners. He also says the front side panels are Oak.

Regards, David

[geofy]

Originally Posted by Herald1360

1922?

More like 1942 I reckon I don't think octal bases were around in 1932 either.

This was a Practical Wireless project to recreat a 1922 design, but using more modern components, hence the Octal valves instead of the original types. But the circuit is from 1922. The valve bases made from solder tags and coils being home wound.

Geof

[David Simpson]

Hello again Folks. Shall try & attach a copy of the circuit diagram.
I've drawn it out - as is.But obviously, will need some modification. ie No GB as yet for V3. Speaker connections will need connecting in V3's anode circuit. The coupling will need changing between V1 & V2. Out of spec & duff components will need replacing.
'Fraid my Rotring drawing ink has run dry, & some guy borrowed my stencils some time ago & never returned them. Hence inferior diagram.
I've been advised which PM valves to aquire. But advice on component values would be greatly appreciated.
Now that everything is stripped down, its obvious that V3 is a later addition. The circuitry is very similar to TRF's shown in "77 Selected Circuits" - a complilation of circuits from vintage era "The Wireless Constructor", which can be downloaded off the internet.

Regards, David

[Ed_Dinning]

Hi David, that now looks more straightforward; simply stick the GB battery in the negative of the AF transformer with the battery + to deck.

Ed

[FRANK.C]

Hi David
That resistor in the anode circuit of V1 (what is marked Lissen 1R (1.92 MR)) it is way too low in value if it is 1R and too high in value if 1.92MR to get any signal from to feed to V2's grid. I would try one of several hundred ohm's.

The resistor in the grid of V1 is a grid leak typical value about 2MR.

Frank

[PJL]

It's roughly as I had thought...

1) The grid bias for V3 is placed between the AF transformer secondary and earth (you are showing a solid line.
2) The markings are different on these and the 1 'ohm' is really 1M but it has drifted to 1.92M. As Frank says it is wrong anyway and I would suggest nearer 50K as the audio output will be very low otherwise.
3) The dual resistor holder may already have the audio coupling capacitor built into it. Take a carefull look at any markings.
4) If not then the black capacitor should be as you have drawn. Otherwise, the black capacitor may have been further RF filtering and should be connected to the junction of the inductor and resistor to earth.
5) The green capacitor 1uF is probably leaky and will need replacing. It should go to earth as you have shown.

You will need a high impedance moving iron speaker or an output transformer and conventional speaker. If you fit a low power output valve you can use high impedance headphones or a horn speaker but they won't take the 20mA of say a PM202.

[Mr Moose]

Hello,
The lissen RCC unit simply has a capacitor between P. and G. as shown by the dotted line in your circuit.
The resistor between P. and H.T. on the Lissen unit should be about 20K to 100K (depending on the valve and HT voltage used) and not about 2Meg. If it is 2Meg then I suspect that someone has fitted a grid leak rather than an anode resistance. You could use it instead of the O/C grid leak for V1.
The resistor between L.T. and G. on the Lissen unit is a grid leak and should be about 1Meg to 2Meg
The grid bias for V3 goes between the secondary of the intervalve transformer and earth with the -ve to the transformer instead of the direct connection on your drawing.
T10 does not go to the HT battery. The high impedance speaker or headphones or low impedance speaker with transformer would go between T9 and T10.
Yours, Richard

[David Simpson]

Thanks Guys, for several most compehensive & helpful replies. Once I've fitted the correct components & altered the wiring, shall re-draw the modified circuitry.
Aquiring the PM valves is in hand, hopefully. Shall concentrate on the cabinet refurbishment meantime. The "Gunge" I refered to in a previous post - for cleaning the front grained ebonite panal - is "FARECLA" G6 Rapid Grade Paste Compound. And for a final polish - "Greygate" Plastic Polish" - which was used 40 years ago to polish "Lightning" Perspex canopies. I've an old tin of "Shellack" somewhere, and a few years ago - made some lovely furniture polish with sieved beeswax & high grade linseed oil.

Regards, David

[Oldtimer74]

Hi David. Get some Bakobrite from Chas Miller to polish up the ebonite front panel.
I agree I am sure it is a home brew set. The valves would have been 2 v filament types run off an accumulator. The resistance capacity coupling unit between V1 & V2 is not very common. Many designs used two transformer coupled stages. Trouble is they tend to become unstable unless well spaced and well decoupled. The green 1 mfd cap is part of the decoupling and will almost certainly be leaky. So replace with a much smaller modern component inside the case. I wonder how many coils you have? Those plug-in coils were made to cover from about 100 m to about 20,000 m. There were many manufacturers and the usual marking indicated the number of turns. Because of the wave-winding their self capacity is quite low and the Q is quite good. I am surprised that the middle coil is the reaction one. The main tuning sets the waveband and should be at least one size larger than the other two. Use the smallest coil you can to get sufficient reaction and the size of the aerial coupling coil will to some extent determine the selectivity and sensitivity of the set.
Good luck. These very old TRF sets are easy to work on and get going. You want to try some of the 1930s Phillips for real problems!
Pat G3IKR

[David Simpson]

Hi Pat,Have already done the front panel with Farecla G6, and finished off with Greygate Plastic Polish. (Have also done the complete Bakelite case of a Ferranti 145, and its come up great).
Now there is two of us working on very similar TRF's, which is great.
I have one Lewcos 60X aerial coil & a selection of 3 reactance coils.

Regards, David

[Oldtimer74]

Hi David. I guess the 60 x coil will cover MW OK. Fine on the polishes. I wonder where you got them from? Particularly the plastic polish. I remember some 55 years ago when I was in the RAF we had some perspex polish for the aircraft canopies but have never seen it for sale.
Pat G3IKR

[jayfar1]

Hello David

Looking through my box of pre-war bits and pieces I found a Lewcos coil marked 60X. It differs from the usual coils in that it is centre tapped with a terminal on top. If my memory serves me right this allows the aerial to be connected to it to "improve" selectivity.

If this is of any use to you let me know.

Jim

[Oldtimer74]

David. the reaction circuit is quite OK. This design was evolved as an improvement on the earlier circuit with either swinging coils or an ordinary variable capacitor in series with the coil to earth. It has the advantage that the total capacity across the coil is constant so adjustment does not affect the main tuning. It is also somewhat easier to control. You will get better selectivity as you move the aerial tapping towards the ground end of the coil but sensitivity will be reduced. You cannot have both together in a TRF!
Pat G3IKR

[David Simpson]

Thought I'd answer Peter Scott's post in "Information Wanted" Forum, relating to GeorgeB's similar 1920's TRF post, in this Forum.
Enamelled copper wire would be a better & more authentic alternative to the bare t/c/w or insulated s/c wire. I used to have a decent roll on a wooden bobbin, but for the life of me cant find it. Wonder if RS sell it ? There are enough hazards as it is with bare terminals galore, so will have to do something.
Next question - could someone recommend a suitable speaker transformer to put in V3's anode circuit, please? And where could one be obtained ?

Regards, David

ps. Many thanks to Jayfar1 for the spare Lewcos coil

[Jim McLay]

Hi David,

Did you ever complete your TRF, and if so can you post an "after" pic and perhaps the final circuit?

I am starting down this road but with a lot less to work with.

Regards

Jim