[IRCA] Send more boring technical stuff!

["Karl J. Zuk" <karlzuk@xxxxxxxxxxx>]

Steve:

Send us your work! Thank you very much for working on it!
I'd love to see what Sony is up to.

I wonder who will be the first to reverse engineer this into a super DX design.

Karl Zuk  N2KZ




> From: steveratz@xxxxxxxx
> To: IRCA@xxxxxxxxxxxxxxxx
> Date: Thu, 27 Dec 2007 15:00:19 -0800
> Subject: [IRCA] TECH: More SRF-59 Ultralight boring technical stuff
> 
> 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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