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thanks. so, on the bad tuna, when the station is in tune, the max reading i get on the far leg of r23 from vr1 is 1.12v; and that's when vr1 is at its max position; when there's no station in tune, i still get a reading of 0.55v - this is also indicative of Q10 failure? the good tuna, when in tune, gives 1.2v reading after vr1 is opened up ~1/3 its rotation; it measured 1.35v at 2/3rds rotation, which was where it was at before i touched it. out of tune, i get a reading of 1.18v at the 2/3rds rotation position of vr1 also, the reading from vr1 to the near leg r23 resistor in the bad tuna was almost half the voltage of the good tuna's reading.
and, just to be clear, the bad tuna's in-tune light is ON all the time, even when no station is tuned in. the stereo light goes in/out, when a station is tuned/de-tuned.
and, re: pinning of the transistors, if i get nos parts, the pins should be the same orientation as the originals?
On Mon, Sep 28, 2020 at 02:34 PM, Herb Ward wrote:
A few thoughts based upon your observations.
1 . Given your comment re the burnt resistor, I would replace both Q9 and Q10. Make sure you get a MPS-A13 not a sub. There are significant hfe ( gain )
differences in the Motorola series of NPN darlington transistors of that parts’s vintage. The Motorola GP series ( complimentary NPN and PNP ) of MPS - A06 and MPS-A56 may be appropriate for the other devices. Of course make sure the pinning of the original and replacement devices is checked.
2 . Q9 and Q10 both function as switches . Q10 is turned ON when adequate signal strength is received. If you measure the voltage at the junction of R23,
and VR1, the DC voltage output of IC1, a LA1235 limiter/detector IC made by Sanyo, should be at least 1.2 volts , as Q10 is a NPN darlington transistor, and there are TWO base emitter drops , The second emitter is at ground.
3 . Q9 is turned on when you are center tuned. Again, if the quadrature detector is off alignment , ( or the tuner is not properly tuned to the station ) the equal base voltages at Q6 and Q7 will not occur, and this will keep the base of Q9 low by means of Q8 being on. When Q8 is off, R35, the 10k base pull up resistor for Q9 will turn on Q9 by bringing the base of Q9 at least .6v above its emitter.
4 . So do keep in mind that from a logic standpoint, you have a two state condition for the In tune lamp to be lit. There must be adequate signal strength, AND the detector output must be in balance which will make the DC
inputs of PNP differential pair to be the same, or about 13.3 v as indicated on the schematic.
Hope this helps...