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Radio Philco Century 100 with FM band extender

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The General
Posts: 10
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Mrs General spotted this Philco in an antique shop in Hitchin. It's a fairly basic AM/FM AC/DC 6 valve set from 1959 but I do like the contemporary styling. On opening it up, it was clear that it had been looked after & had a good cabinet. Like most 1950s British FM radios the top end of the FM band stops at around 100MHz so I decided it would be a good candidate for trialling a little FM band extender which I was eager to try out.

Chassis resto was straightforward, a few resistors had gone high, all the caps were ceramic & not leaky although I did replace the coupling cap feeding the UL84 grid just to give me a warm squishy feeling. I have an Avo Mk4 so I tested the valves; the UCC85 was very weak & so (unusually) was the UABC80 & so these were replaced. The UF89 was a bit weak but I thought I'd leave it & see how it goes. Alignment was all pretty much spot on except the FM IF needed a tweak. An intermittent heater chain was a headscratcher, this turned out to be a small crack in the PCB hidden under the wavechange switch. AM was a bit lacklustre on its internal ferrite antenna & with no provision for an external antenna but FM was quite lively & I was more interested in good FM performance TBH.

Everything done, it was time to try out the FM band extender. I previously made up a few little 10MHz frequency shifters based on an SA612 balanced mixer & a 10MHz crystal oscillator mounted on some little prototype PCBs I ordered from Farnell. The output impedance of the SA612 is 1.5kohms, the radio antenna input is balanced so is probably about 300ohms so there's a little matching transformer wound on a tiny toroid core. Unfortunately the SA612 is only available as a surface mount part so assembling the boards was somewhat tedious (I have big hands). I pinched a few milliamps from the UL84 cathode to power the board which I mounted on the FM tuner head.

And... It works! With the oscillator running, incoming signals are shifted down by 10MHz giving you a tuning range of 98 - 110MHz. There's a switch on the back panel which puts a 56k resistor across the crystal to stop the oscillator, this then reverts to the normal range of 88 - 100MHz. FM reception is quite lively even with the internal tinfoil dipole. There is also some conversion gain from the SA612. Well pleased.

Hopefully the photos show everything ok, this one will be a keeper, I'll probably use it as my workshop radio.


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Posted : 19/04/2024 4:21 pm
LSmith, Doz, Lloyd and 1 people reacted
Posts: 6437
Famed Member Registered

This is brilliant. For about the last 30 years (yes, really!) I've toyed with the idea of doing this in order to be able to use a set I bought in Atlanta in 1996 - a Philco 42-350 with FM. This of course covers the "Armstrong band" of 42 to 50MHz. Another set with unusual coverage is a Polish "Diora", which covers 64 to 73MHz - the OIRT band, I think.

Posted : 19/04/2024 8:00 pm
Posts: 1509
Prominent Member Registered

Fabulous work. I have some converters here *somewhere* that move 76-90 MHz to cover the UK bands. Used to sell them to people who imported Japanese cars. 

Posted : 20/04/2024 9:47 am
The General
Posts: 10
Eminent Member Registered
Topic starter

Thanks for the kind words, both. I can't claim originality for the design, I found it ages ago on the intraweb as a pdf file, I can't even remember where. I've attached a screenshot of the circuit, my cct is essentially the same except I added a 6.2V zener between pin 8 of the IC & ground & my matching transformer has an 11:5 turns ratio which matches to 300ohms as near as dammit. As there's no 6.3V supply on the Philco 'cos it's got a series heater chain I powered it from the UL84 cathode via an 820ohm resistor. The original cathode resistor had gone a bit high anyway so that compensated...

Doz, you mention a frequency shifter for Japanese cars, I have something similar, a 100kHz shifter for American car radios which, as every schoolboy knows, have a 200kHz channel spacing & the (digital tuning) radios only receive odd numbered frequencies. Being me, I picked it apart & saw it had two frequency shifters, one shifting the frequency (I think) 10MHz one way & the other 9.9MHz back again. This done of course, to avoid horrendous image problems with a single 100kHz frequency shift.

I think the next candidate for a frequency shifter might be my Pye Fenman 2...


SA612 cct



Posted : 20/04/2024 4:22 pm