sx 535 tuner and its ha 1156

john stumpf

Super Member
ive a question that im hoping someone here can answer. this tuner and its ha 1156 have a circuit that i dont understand,pin 14 has a 2sc945 that shunts the stabilization circuit on pin 14t in what APPEARS to be all modes EXCEPT fm stereo. none of the later pioneers with the '1156 have this feature. does anyone care to share its purpose?
 
Pin 14 is the VCO input to the IC. The RC circuit built around Pin 14 (exclusive of the switching transistor) sets the time base for the oscillator at about 76 KHz for the quadrature detector stages. It may be that the transistor switching in the additional resistance in anything BUT FM stereo mode would alter the 76 KHz time base and de-tune the detector so no stereo signal is passed.

I believe the manual is also mis-marked, since there is only one "FM" mode for the selector switch, and there is power at Pin 33 of the tuner board in any mode but FM, and not "stereo" as the not indicates.
 
thanks watthour i have a reason for asking this. it must have been a waste of time as its not used in the the 50 series which uses the same dip.
 
The 2sc945 bjt turns off the VCO, by shorting it to ground. The VCO is turned off in AM mode because it generates noise and will interfere with the AM signal.
Turning off the VCO is also a method to place the part into mono mode.
x50 receivers use the HA1196, the sx950 sch shows a 2sc945 shorting out the VCO on pin16.
there is also a mono switch control input on pin12 that connects, thru the mute switch, to the HA1137 quad chips level detector/mute ckts.
 
by god you're correct! i guess i need my eyeballs checked the 750 radio card does indeed have a 1196 in it! but its not showing that shorting transistor, after the abortive 626 tuner fiasco i did some soul searching and tried out the 636 tuner awe046 in it. it was relatively painless and required absolutely no butchery of any kind. i can easily restore it if a proper board comes along. it plays really well in am and fm.but i was stuck without an unavailable in anything but fm stereo 13.5 v input for pin 14.i ended up with one wire with no home that went to pin 12 on the 626 tuner this was directly connected to pin5 on the ha1142 chip that i could not find a datasheet on,so its function is an unknown. the end result was a functioning am fm radio with all the functions working features. the only modifcation was i had replace the resistors in the cr board to get the output signal in the proper mv range as the tuner is fed through the equalizer board phono section. the only downside i can see is my fm tuner cap is now 3 gang instead of 4. that said,it seems to pull in quite well
 
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