Steve
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vintageradio.co.nz
Posts: 732
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Post by Steve on Jun 26, 2022 17:56:34 GMT 12
I might be having a blonde moment and missing something obvious, but I have two dial glasses that both look like model 75 dials.  The top one is the standard model 75 dial (and has '75' etched up at the top edge of the glass), the bottom one is the same size and looks like one at first glance - but the broadcast band upper frequencies are more squished, and instead of a 16M band, it has a standard shortwave band, 6-18MHz. It has no identifying etching at all. Can anyone hazard a guess as to what model it comes from, or if an oddball variant of a 75 - which one? Cheers, Steve |
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Post by DHockey on Jun 27, 2022 9:27:31 GMT 12
That is a bit of an odd one, the order of the shortwave bands is different in the lower pictured dial too, it is the same as used in the model 90.
My guess is that it is a very late model 75 using the coil set of the "new" 90. The 90 omitted the 16M band and replaced it with a general coverage shortwave band.
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Post by Peter Walsham on Jun 27, 2022 10:22:30 GMT 12
The 'crowded' HF end of the dial (top dial glass) suggests the use of a Plessey (or similar) tuning capacitor using semi-circular moving vanes rather than the 'normal' RCNC tuning capacitor (suggesting that RCNZ made their own tuning capacitors in later sets?).
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Steve
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vintageradio.co.nz
Posts: 732
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Post by Steve on Jun 27, 2022 10:58:19 GMT 12
That is a bit of an odd one, the order of the shortwave bands is different in the lower pictured dial too, it is the same as used in the model 90. My guess is that it is a very late model 75 using the coil set of the "new" 90. The 90 omitted the 16M band and replaced it with a general coverage shortwave band.
Ah... I'd discounted the model 90 because it had no tone control markings even though it does, now you mention it, look exactly like a model 90 dial - but your suggestion led me into my archives of photos and to I picture I have (came from Trade Me I think) of a model 75L, from 1942 (the very end of the model 75's - and a reasonably high serial number as well)
So I think you're right - they must have changed the coils over to the model 90 ones towards the end.
I wonder if there were technical reasons for dropping the 16M band, or if it just wasn't required by most users - or maybe a general shortwave band was more useful since it gave full coverage rather than the 'spot' coverage of a bandspread band...
And Peter, regarding RCNZ making its own tuning gangs - not to my knowledge. They purchased from a number of manufacturers - Plessey, RCC, Polar etc. Often the variants of any given model would exist only because they ran out of one tuning gang and had to switch to another - necessitating a new dial, and possibly also new coils to suit. Because they were manufacturing in such large numbers, this would often happen - either because they ran out and couldn't get more - or because they found a different model worked better.
A good example of this, which they documented, is the model 66. The service docs cover the 66, 66E and 66J. The 66 used a Plessey type K tuner, while the E and J variants used a Plessey type E tuner (possibly why the 66E has the E suffix). The J used a coil box assembly rather than individual coil cans.
Later models of the 66 (A and W) are also documented as having different gangs. The A used the Plessey E type, while the W used an RCC type. Someone (JWS?) also noted that a 66A he worked on had a Polar tuning gang in it - and that one 66A had an RCC gang in it - this is messy, but it probably happened. I know some models were 'tweaked' for different regions - the model 60 and 65 pushbutton sets I think had a complete engineering change of their tuning setup for sets sent to Chch...
Anyway, I think this solves the mystery for now - its likely a model 75L (L... late?) dial. Thanks 🙂
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