Help with information on old shortwave converter

[Nostaticatall]

I have a 'Magnum' shortwave converter which I don't know anything about and wondered if anyone had seen one of these before and could throw some light on it. It has a single Mullard 2 volt valve - a PM1HD and the plug-in coil has 40-80 lambda marked on it. The captive power lead has a 4 pin plug on the end for HT, LT and GB so it looks like it was designed as a companion to a non shortwave broadcast receiver.

Maybe this should be with the domestic receiver threads, I'm not sure.
Steve

[Station X]

I like it. Presumably the coil has one set of windings for IF and another for the local oscillator? If you can determine the LO coverage you should be able to work out the bands covered and the IF. The IF could be variable of course with all tuning done on the main receiver.

[John M0GLN]

A scan from a 1930-31 catalogue.

John

[Skywave]

ISTR that the Greek letter lambda was once used to signify wavelength. Hence, "40 - 80 lambda" implies 40 - 80 metres wavelength tuning range.

Al. / Skywave.

[Nostaticatall]

Thanks John - that's great! It answers the question on how it couples to the main receiver.

Steve

[Nostaticatall]

Originally Posted by Skywave

ISTR that the Greek letter lambda was once used to signify wavelength. Hence, "40 - 80 lambda" implies 40 - 80 metres wavelength tuning range.

Al. / Skywave.

That's right and having now seen the text in the scanned brochure from John it seems that it originally came with two coils, one for 20 - 40 metres and the other for 40 - 80 metres. I'll maybe have to wind one up for the missing one.

Steve

[Nostaticatall]

Originally Posted by Station X

I like it. Presumably the coil has one set of windings for IF and another for the local oscillator? If you can determine the LO coverage you should be able to work out the bands covered and the IF. The IF could be variable of course with all tuning done on the main receiver.

I've taken it apart and drawn the circuit out Graham. It turns out that it's a little TRF set in its own right. Circuit enclosed.

Steve

[AlanBeckett]

Hmmmm.
The circuit doesn't match the write-up in the advert, although it's logical.
The interesting thing is that if it was meant to be 'plugged into the radio detector socket' as per the advert how would you elininate the L and C which were presumably already there? The existing coil might be unplugged, although the advert doesn't say so, but what about the tuning and perhaps the reaction condensor?
Alan

[anthonys radios]

The frequency changer stage of a superhet used to be called the "First Detector", if that helps.

[Nostaticatall]

Originally Posted by AlanBeckett

Hmmmm.
The circuit doesn't match the write-up in the advert, although it's logical.
The interesting thing is that if it was meant to be 'plugged into the radio detector socket' as per the advert how would you elininate the L and C which were presumably already there? The existing coil might be unplugged, although the advert doesn't say so, but what about the tuning and perhaps the reaction condensor?
Alan

Alan, initially I presumed that the 4 pin plug carried HT,LT and GB but, now having dismantled the set and drawn out the circuit, there is no grid bias voltage needed as it's a straight triode with a grid leak. There is a fourth core to the cable which isn't connected to a pin at the plug end or to anywhere inside at the receiver end. I am beginning wonder whether someone has been playing around with it at some stage. I need to power this up and see if it works before doing anything else!
Steve

[Nostaticatall]

A view of the underside with the base removed

[Station X]

Wouldn't you need common, LT, HT and a connection to what was the anode so as to get the signal through to the next stage of the radio?

[Nostaticatall]

Originally Posted by Station X

Wouldn't you need common, LT, HT and a connection to what was the anode so as to get the signal through to the next stage of the radio?

I've just given it some more thought and, as the circuit is doing its own leaky grid detection, then it's just the AF that needs feeding into the detector valve socket of the receiver so the HT will be fed from what was the anode connection and also the AF signal will appear there too which is fine - the RF choke in the converter will block the RF from the signal and the existing decoupling cap in the receiver will take the audio to the next stage. Does that make sense or do I need a lie down?

Steve

[Oldtimer74]

I think one has to assume that it was intended for use with a straight set not a superhet.
Battery superhets were not common in 1930
Pat G3IKR

[Herald1360]

Surely this is just a complete "short wave" front end in a box which is designed to be plugged in to the detector socket of an existing receiver to provide short wave coverage. The existing receiver would then simply be used as the audio amplifier. The advert even suggests using the detector valve removed from the existing receiver in the converter, which would make the converter cheaper as it could be sold without a valve included. The advert seems to make perfect sense that way.

There's no question of it being a "converter" in the sense of the term as used now, where a mixer circuit is used to transpose a frequency band outside the coverage of a receiver into a range that is covered, either broadband (fixed local oscillator with station selection provided by the receiver tuning over the output band of the converter) eg 2m VHF ham band converter or fixed frequency output with a variable local oscillator in the converter eg Band III to band I TV converter.

[Nostaticatall]

Originally Posted by Herald1360

Surely this is just a complete "short wave" front end in a box which is designed to be plugged in to the detector socket of an existing receiver to provide short wave coverage. The existing receiver would then simply be used as the audio amplifier. The advert even suggests using the detector valve removed from the existing receiver in the converter, which would make the converter cheaper as it could be sold without a valve included. The advert seems to make perfect sense that way.

There's no question of it being a "converter" in the sense of the term as used now, where a mixer circuit is used to transpose a frequency band outside the coverage of a receiver into a range that is covered, either broadband (fixed local oscillator with station selection provided by the receiver tuning over the output band of the converter) eg 2m VHF ham band converter or fixed frequency output with a variable local oscillator in the converter eg Band III to band I TV converter.

That's exactly what it is - no down conversion going on, just a little SW TRF receiver in a box.
Steve

[Skywave]

Originally Posted by Herald1360

Surely this is just a complete "short wave" front end in a box which is designed to be plugged in to the detector socket of an existing receiver to provide short wave coverage. . . . etc. . .

The News Chronicle Wireless Constructor's Encyclopaedia (F. J. Camm) 2nd. ed. (page 91, fig. 104), describes a Short-Wave Adapter Unit, with layout, that is a very near match to the cct. diag. provided. The method of use and connection to the existing (TRF) receiver is exactly as Herald1360 has mentioned above.

HTH

Al. / Skywave.