That sounds a bit like the Astor joke. HMV built them properly & Astor built them & allegedly, someone took bits out until it stopped: They put that bit back & put the remainder into production.

My Thought: It is essentially a hack job & I would treat it as such. I would replace all of the old electrolytic caps, the remaining paper types, check the resistors as you go & put it back to comply with the original circuit. More often than not, there is a vast improvement in performance if its put back into spec & re-calibrated with accurate test equipment.

As I class this as a hack, I would check every bit of wiring; Based on experience the probability of it being wired wrongly, or having wrong parts is high. I constantly get sets like that.

Marc

Thanks Marc. Ian what is a double conversion, what does or would have this achieved? I haven't started it yet, but I have a schematic so I

should be able to trace back the circuit. It the funniest (strange) thing I've encountered yet in a radio!

JL, double conversion is done to get around the image rejection problem at higher received frequencies while maintaining high gain and selectivity.

The idea briefly is that, instead of converting the incoming signal down to 455 kHz, you first convert it to a higher frequency, typically 10.7 MHz., then convert it to 455 kHz. A bandpass filter after the first mixer completely removes any images.

In a single conversion receiver with an IF of 455 kHz, the image frequency (local oscillator + incoming) is 910 kHz away. That's easy for the RF front end to reject in the broadcast band but not so easy when you get up to, say, 20MHz in the HF bands. Having a first IF of 10.7 MHz puts the image 21.4 MHz away - easy to reject even well into the VHF spectrum.

The reason for the 2nd conversion to 455 kHz is to make it easier to get high gain with a narrow passband. In a communications receiver this translates to lower noise and better ability to reject unwanted signals that are close to the wanted one in frequency.

So, double conversion gives you the best of both worlds - good image rejection and good selectivity. Very important in crowded communication bands where a signal on an adjacent channel can be very very much stronger that the wanted signal.

To a certain extent, the development of stable high gain high frequency amplifiers and crystal filters has made the complexity of double conversion unnecessary in some more modern communication receiver designs.

That reminds me of one of the most unusual sets I have never seen in production. Why I mention it is that it was a "special" based on all of the parts in an AWA R301 with an output & bias mod. (Transformer would also be bigger): Unbelievable. The added OP valve was a 6A6 (around 10 Watts) transformer coupled.

That one (and it base model) is the only set I have seen where the 1st IF is a band pass filter (2 cans) some one messed it up & there was no way a signal was going through.

A lot of newer stuff used Wadley loops & ain't they a bundle of fun (sic).

Back in the mid 70s when I started the business I bought some valve two-way radios for my small "fleet".and so I could keep in touch with my wife and office manager at home with the babies. The radios were obsolete and very cheap, but boy did they keep me busy keeping them running!

The valve lineup of the receiver (77.360 MHz AM) was as follows:

6ES8 RF amp
6EA8 1st LO and mixer
followed by two double-tuned transformers, critically top-coupled, on 10.7 MHz
6EA8 2nd LO and mixer
IF transformers on 455 kHz
6BA6 1st IF amp
6EA8 2nd IF amp (triode was a mic amp)
A couple of OA91 diodes wired as a rate-of-rise noise limiter and mute gate - very clever.
2 x 6GW8s audio amp / plate modulator.

These things, when tuned up, would break mute and resolve a signal down to 0.2 uV - very impressive - but once they were rattled around in a vehicle this deteriorated somewhat.

AM on VHF gets into EVERYTHING as we often found to our sometimes amusement, sometimes chagrin. Suffice to say you needed to be careful what you said!

AM on VHF gets into EVERYTHING as we often found to our sometimes amusement, sometimes chagrin. Suffice to say you needed to be careful what you said!

Pre-internet, several mates of mine and I would converse at night on the CB (27MHz) and, like many, we either tricked up or had tricked up our radios so we'd over-power anyone trying to hijack our channel - a common problem back in those days and we all had the good old Station Master antennas to make sure the bit of extra oomph was broadcasted well.

The downside for me was that I lived in the staff quarters of the hospital I worked at, at the time, and my half of the conversation often ended up on everyone's telly in the same building. Fitting the suppressors that were available at the time did no good and I ended up having to shift my antenna to avoid the fallout from my occasional bad language voicing over the 6PM news bulletins.

Very interesting, this radio has 4 short wave bands so it's an impressive animal compared to my other sets which only have one or two at the

most. Obviously in it's former life it was owned by somebody who made good use out of these bands! I remember my dad telling me him and

his mates all sat around listening to the cricket on short wave back in the day, so short wave was more than just a novelty in days gone by.

I think I'll restore it back to it's original state and then decide whether to include the double conversion, but if I do I'll do it from scratch and

aim for a neater and more permanent modification using better components. That little coil thingy looks way out of place!

My double conversion radio suffers from local oscillator drift. Another way to double convert, with less drift, is to block convert (the local osc here is crystal controlled) a SW broadcast band down to a block of lower frequencies (say the 540-1600kHz band) and use a "tunable" IF converter. Though strong local MW AM broadcast stations will leak through But as most SW BC bands are around 300kHz wide, we could convert them down to 100 to 400kHz, and use a local osc that goes from 555 to 955kHz to get them on 455kHz. The drift on this local osc should be low enough that a station would not be lost over time. Getting a local osc and signal ganged tuning cap to track should be "fun". You'd need to have a high pass filter on the antenna input, to keep any LW signals out.

boy did they keep me busy keeping them running!

A young Dick Smith can talk about that, too.

Yes, there was a reason these radios were designed to be quickly exchanged in and out of a vehicle. If you had a large fleet you'd have a few spare radios and regularly ship them back for repairs.

First job I had after school (1966) had 2 way radios. The channel was shared with a company that quarried gravel for cement etc. Hundreds of trucks, very busy network. At least every couple of days you'd hear an air check "Pye workshops to 2CL"! The dispatcher would be quite critical about how the radio sounded and often knock him back on the test!