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 _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
