Pioneer SX 2500 autotuning 19KHz pilot tone? Please help!

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I am the proud owner of a Pioneer SX2500. A few days ago, repaired the L channel as it was straight forward. What is not straightforward is the autotuning feature. The motor runs back and forth as does the dial pointer without stopping on stations.I do receive about 48 stations in stereo if the antenna behaves. I have scoured the internet and read about the Motorola patent application in 1966 and a more complicated patent in 1971. However I think the 1966 patent is more applicable and has a 19Khz pilot signal that is amplified and terminates the dial pointer search when it reaches a station, as signified by the amplified 19Khz signal which in turn turns off the motor. I have a
Telequipment S51E 3MHz oscilloscope, Eico 377 function generator, Sencore SG165 AM-FM-stereo analyzer,Iwatsu SS-5702 20MHz dual channel oscilloscope
and visual analyzer as tools. Also have replaced about 5 transistors, and three capacitors on the muting, autotuning, and the indicator unit.

 

[backmd]

I am the proud owner of a Pioneer SX2500 and repaired the L channel as it was straight forward. What is not straightforward is the autotuning feature. The motor runs back and forth as does the dial pointer without stopping on stations.I do receive about 48 stations in stereo. I have scoured and thoroughly searched the internet and read about the Motorola patent application in 1966 and a more complicated patent in 1971. However I think the 1966 patent is more applicable. The patent circuit has a 19Khz pilot signal that is amplified and terminates the dial pointer search when it reaches a station. I do have a scope, signal generator, Sencore SG165 stereo tester, and visual analyzer as tools. Also have replaced about 5 transistors, and three capacitors on the muting, autotuning, and the indicator unit.

 

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OK a clue is that when I press the local station button, upon decompression, the autotuning motor stops. I cannot upload the schematic or the service manual because they are too large, but they are on Audiokarma. Now we have a 30Mhz scope hooked up. On the FM IF board W12-030 the
FM IF output has a signal, I think it varies with the music; whereas AFC and M2 have no output. Check C24 and C25 on the FM IF board W12-030
C24 measures 1.26uf and 1uf is the spec.
C25 is shorted, replaced with 100uf 35 volt nichicon which I had in the goody locker.
On W18-015, the autotuning board no waveform at TP. Definite waveform at input M. OK now the waveform is gone as if it was shorted. Connecting M to M2 on the FM IF board W12-030 eliminates the stereo signal as if it were shorting it out. M to ground measures 580 ohms. M2 to ground (FM IF board W12-030) also is 580 ohms. Pin 11 to ground is 12k ohms.
Desolder C25 positive and remeasure. Ok the problem is not C25, but the center tuning meter which measures about 600 ohms to ground.

 

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Ok treat the autotuning board as an amplifier and inject 1Khz sine wave and see if we have a sine wave at the test point. Both the Eico and scope have capacitor protected outputs and inputs respectively. OK we have two sine waves with flat bottoms. I may have to service the generator later. OK when I decompress the local station switch there is a dc pulse sent to the test point. I hear a 1000 Hz tone on the headphones but very faint. Inject at base of Q1. Injecting at pin 10, no output from test point. Inject the base of Q11, no output! Caution Q9 is PNP


Check voltages of Q1, Q10, and Q11

Transistor Emitter Base Collector

Q1 3.6 4.2 9.0


Q10 -4.8 -4.9 10.7!


Q11 -4.6 -3.9 -4.6!

 

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OK uncle, I give up!
Ok now I have a clear almost too light copy of the foil diagram; let’s try again.

Ok I have a beautiful 2 cycle sine wave at 19Khz from the Sencore SG165 signal generator. Let’s apply such at M on the autotuning board. Signal gets eliminated at 12k resistor R10. Let’s apply 400hz signal now. It goes from 4 divisions to 1 ¾ divisions across the 12k resistor R10. Reduces signal 1 ¾ division at D1 and clipped signal at D2 anode (arrow). R6 has markedly reduced amplitudes on one side and markedly reduced amplitudes and a clipped waveform on the opposite side which is connected to the transformer primary center tap. No signal at R5 680 ohms. No signal at R4 560 ohms.


Switching to 19Khz, no signal makes it to R5 or R4. No signal at D1 or D2.
C6 measures 15pf spec is 680pf

C7 is shorted! Removed C7 and out of circuit analysis shows a shorted ceramic capacitor. No wonder no signal went through the transformer.
Schematics are here: http://akdatabase.org/AKview/albums/userpics/10004/Pioneer SX2500 Schematic.pdf
for those of you waiting for your Mouser.com deliveries.

 

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T1 had the following part number
AM IFT T71-022-O
and many am IF transformers that fit other receivers have the part number T71. So I was incorrect to try a 19Khz stereo tone for signal injection. It stands to reason that if T1 is an AM IF transformer, that one should use 455Khz as a signal injection frequency.
Injecting both 10.7 Mhz and 455Khz cause no waveform at the opposite end of R10.


OK when we inject 455Khz at pin 10 we get a huge output at the collector of Q1


The output when we inject at pin 10 is present based upon am reception and changes with the tuning of stations. When we are on a strong station the 455KHz output peaks. Let’s peak transformer T1(T72-021-O) Nearly 3 divisions at 780am WBBM. After peaking nearly 3.1 divisions.
Q11 checks well and good on the Sencore TF26 transistor tester.

Q10 checks no good! C15 is not shorted but measures about 5uf. Reinstall another KSC1845!

Really weird installed another nichicon cap at C15, 10 uf at 15volts and replaced with 100uf at 35 volts. Both the “bad” nichicons and KSC1845 test good out of circuit!


Why of course, Q10 and Q11 need to be matched pair of NPN differentials!
OK soldered in another KSC1845 with similar if not identical gain; but no change in the autotuning. C12 is shorted. C13 is shorted. C14 is good. C10 is good. C11 is shorted

C11 .001 1000pf 50 volts short verified out of circuit replaced with 820pf mylar

C12 .04uf 50 volts replaced with .047 polypropylene
C13 .001 1000pf 50 volts old one shattered replaced with 820pf mylar

Still same behavior autotuning goes from R side to L side without stopping even though signal strength hits a 5 with many stations tuned.
Q10 -4.8 -4.9 10.7!


Q11 -4.6 -3.9 -4.6! Same as before. I just do not understand. Maybe someone with more experience and more intelligence can figure it out, but I cannot.

 

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If pin 12 goes positive it may shut off motor; by turning on the base of

Q11, current flows through the collector of Q11, turning on the base of

Q12 current flows through the collector of Q12, causing multiple transistors to turn on/off, turning off the motor. Recheck Q4 and Q6 on the indicator unit, if no good, check associated caps.

Yes indeed a 1.5 volt stimulus stops the motor dead in its tracks applied at pin 12!
Weird Q4 is good on one range and bad on another.

Q6 checks bad on all ranges. Must be a shorted cap somewhere.


Both C2 (.22) and C3 (.47)are shorted to ground!

0.33uf mylar for C2 and C3; all connections verified!


Now both Q4 and Q6 check well and good on one range only?


Still does not work on autotuning even rotating VR1 and VR2?


Constant 0.17 volts at pin 12 no matter what the station; check the voltage at the fm front end agc post. Negative one volt at AGC on the IF board. Pin 7 on the am unit measures -.115volts.


Pin 12 to AGC2 is 47kohms. It should be 133kohms according to the schematic. AGC to maybe pin 7 is 60k and should be 100k according to the schematic. Pin 7 on the switch goes to pin 12 on the autotuning board with 30k as resistance and it should be 33k; close enough. Pin 7 goes to AGC with 55kohms and it should be 100k ohms. Diode checks well and good!

 

[backmd]

Constant 0.17 volts at pin 12 no matter what the station; check the voltage at the fm front end agc post. Negative one volt at AGC on the IF board. Pin 7 on the am unit measures -.115volts.


Pin 12 to AGC2 is 47kohms. It should be 133kohms according to the schematic. AGC to maybe pin 7 is 60k and should be 100k according to the schematic. Pin 7 on the switch goes to pin 12 on the autotuning board with 30k as resistance and it should be 33k; close enough. Pin 7 goes to AGC with 55kohms and it should be 100k ohms. Diode checks well and good!


OK the voltage at pin M should be zero at center channel tuning and it is at zero. Pin 1 is 9.9 volts irregardless of tuning. Pins 8,7,6, and 5 are 11 volts. Pins 10, 11, and M are zero volts. Pins 8 and 7 go to the motor. Pins 5 and 6 go to the lamps.

When one presses the autotune:

Pin 8

9 volts when the tuning pointer moves to the R and 1 volt when it moves to the L side


Pin 7

9 volts when the tuning pointer moves to the Land 1 volt when it moves to the R side


Since they are bilaterally symmetric it is reasonable to assume that that part of the circuit is working well.

 

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Does pin 3 of the muting unit go positive with station tuned in and turn off the motor? See if tuning lamp is functional! Positive voltage from pin 6 of the indicator unit should hit pin 12 from the autotuning unit to turn off motor!
Check all caps on indicator board now! C4 is 4.3uf spec is 2.2uf therefore it is good. C2 and C3 are new. C5 is shorted! C5 is on the foil diagram but not on the schematic, so lets cut it and see what happens.

Nothing happened. Replaced with 6.8uf cap. 0.5 volts at positive lead. Hooking pin 6 on indicator board to pin 12 on autotuning board cuts voltage from 1.38 to 0.77 volts. We must get pin 6 to a higher current.

V3 to V1 is VR2 according to the schematic; and when the wiper gets close to V1 we have the maximum voltage at pin 6! R6 is 2.2k at V1.Let’s change V1 to 1kohm and see the voltage change. There is no voltage change even going to zero ohms.


Check ebc voltages at Q1, Q2, and Q3 on indicator board.


Transistor Emitter Base Collector

Q1 .02 .57 .06 too low!


Q2 0.0 0.5 13.5


Q3 12.8 13.5 13.0


D3 does not test like a diode. All other resistors and diodes are reasonable. Recheck Q1-Q3 with Sencore. D3 is 1S188 NTE110A 30 volt 150ma

1N5402 NTE5802 200 volt 3 amp, now it behaves as a diode.

1.07 volts at pin 6 only.

Q1 checks well and good on the Sencore TF26, with two ranges with chirps and excellent meter readings. Check resistance from B+ to collector; 30.6kohm.


V1 voltage varies with tuning from zero to 2.2 volts when centered on a station.


OK connecting V1 from the indicator board to pin 12 on the autotuning board, we get 19 stops on fm as we go from the L side to the R side of the dial scale. I am finished now as the autotuning works! That was about as hard as acing the quantum physics final at the University of Illinois down at Champaign IL