[IRCA] TECH: More SRF-59 Ultralight boring technical stuff

["Steve Ratzlaff" <steveratz@xxxxxxxx>]

I have traced out the SRF-59 printed circuit board and made a rough
handdrawn schematic of it. It's more or less straightforward as to inputs
and outputs.
As Gary DeBock has already mentioned, there are only two integrated
circuits--a 30-pin main IC1 and a 10-pin audio amplifier headphone driver
IC2.
A trifilar transformer isolates the dual headphone audio channels from the
FM antenna signal on the same lines (the headphone cable outer shield acts
as the FM antenna). The FM signal is passed through a series resistor which
is shorted in the DX/Local switch, through a monolithic thinfilm FM bandpass
filter, then into IC1.
The 1.75 inch long small AM ferrite rod antenna has two windings; one goes
directly to IC1, the other to one section of the 4-section variable tuning
capacitor. It appears both the RF and the local oscillator are tuned, for
both FM and AM modes. L3, the metal can transformer that Gary has found
peaks AM for best sensitivity appears to be part of the AM local oscillator
circuit, not strictly in the input signal path. I'm not sure at this point
what that transformer is actually doing to peak sensitivity. By touching
sections of the variable cap, I was able to severely detune stations in both
AM and FM, thus I believe those two sections are indeed the local
oscillators for both modes.
There are various pins that have bypass caps to ground. There is one other
metal can slug-tuned transformer, which by experimentation I found affects
the audio only in FM mode. I believe this is the FM stereo multiplex
transformer.
Dual audio outputs go to the dual-section volume control, then directly to
the headphone driver audio amplifier, IC2. Dual outputs from IC2 go to the
headphone jack, for the stereo headphones.
There are no other components, and no feedback from IC2 to IC1. There are no
ceramic IF filters, no AM detector diodes or FM quadrature detector diodes;
no external agc feedback lines. Everything having to do with processing the
AM and FM signals and deriving the audio outputs happens inside IC1. And
everything runs off a single 1.5 volt cell.
It would be nice to find a data sheet and block diagram for that IC!
I can send a 350kB .jpg file of my handdrawn diagram to anyone who would
like to examine it. If I get a number of requests, I will wait and send the
file at one time to everyone, due to my slow diallup connection.
Thanks to Gary DeBock for getting me interested in this very unusual little 
radio--I've certainly had fun using and examining it!

Best regards,
Steve AA7U
NE Oregon

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