Pioneer SX-780 need help signal tracing. Powers on no sound. Darlington packs are good.

Kingbleu

Well-Known Member
I have a Pioneer SX-780 that powers up but produces no sound and no relay click. The STK's (Darlington power packs) are good. I would like some help tracing the signal. I have test equipment that I have picked up over the years. Mostly at garage sales or Craigslist.

I have two SX-780's one that works and one that doesn't. I was thinking I could probe around and trace the signal, which would lead me closer to the issue. I thought it would be easy because I could use the working unit as a base line. However this is much harder than I expected because I don't know where to put the probe.

I know for sure that the Darlington Power Packs are good because I swapped them out of a working SX-780.
The unit passes the Dim bulb test.
The unit powers up, the three lamps, source, and tape monitor lights come on but the relay doesn't click
No voltage to the VU Meters
FM needle is dead
The rail voltages are +40.6v and -40.6v
I tested the three transistors on the heatsinks Q25, Q19, Q20 and resoldered them. They all tested good.

Q25
E 13.7v
C 27.3v
B 14.4v

Q19
E 40.4v
C 51.5v
B 41v

Q20
E -40.4v
C -53.3v
B -41.1v

R1 The big ceramic resistor mounted on the chasis tests 26.7v and 51.9v
The emitter resistors all measure .4 ohms
NO signal when probing at R233/R234 (the input to the power amps)

Opamps TA7136 Q3 and Q4 are suspect not even close to specs.

Q3
pin7 -21.8v
pin6 -24.6v
pin5 -21.2v
pin4 -24.8v
pin3 -21.3v
pin2 -20.6v
pin1 -21.2v

Q4
pin7 -21.8v
pin6 -23.9v
pin5 -21.2v
pin4 -24.8v
pin3 -21.2v
pin2 -20.6v
pin1 -21.2v

STK-0050

Left STK (looking at front of unit)
pin1 39.2v
pin2 -41.3v
pin3 39.2v
pin8 39.2v
pin9 41.3v
pin0 40.3v

Right STK (looking at front of unit)
pin1 -1.1v
pin2 -41.4v
pin3 4-9mv
pin8 18-25mv
pin9 41.3v
pin0 1.2v

Side note when I first got the unit it powered up the relay clicked but no sound. Now after replacing the STK's the relay wont click.
 
Last edited:
Test the regulated voltages at Q17/18. It would appear they may be O.K. due to your readings at the OP amps Pin 7, but that doesn't show the -23V supply.

What signal source is being used?
 
Last edited:
Test the regulated voltages at Q17/18. It would appear they may be O.K. due to your readings at the OP amps Pin 7, but that doesn't show the -23V supply.

What signal source is being used?

Q17 and Q18 are both showing negative values?

Q17
B -21.1v
C -21.9v
E -21.9v

Q18
B -25.4v
C -36.6v
E -25v

The signal source being used is a Circuitmate Function Generator FG2
 
Opamps TA7136 Q3 and Q4 are suspect not even close to specs.
Because its supplies are wrong.

q17 is the positive +24v regulator, which is toast (-21.9V) so you are going to have to figure what is blown, test the comps assoc with the reg, Q17
q19 emitter is the supply, should be close to scat ~ +39.5VDC
 
Last edited:
Because its supplies are wrong.

q17 is the positive +24v regulator, which is toast (-21.9V) so you are going to have to figure what is blown, test the comps assoc with the reg, Q17
q19 emitter is the supply, should be close to scat ~ +39.5VDC

After testing the components associated with Q17 I found Resistor 305 was blown. It is a 430 ohm resistor. I replaced the resistor and wrote down the before and after values. Is it possible, that after the resistor blew, it toasted Q17, Q18? Should I go ahead and replace Q17 and Q18 at this time? Or should I look further for other faulty components?

rcs16 can you please explain in more detail what you mean "q19 emitter is the supply, should be close to scat ~ +39.5VDC" thank you.

Q17 blown resistor values
B -21.1
C -21.9
E -21.9

Q17 values after replacing resistor
B 15.7
C 15.1
E 15.1

Q17 actual values in service manual
B 24.6
C 34
E 24


Q18 blown resistor values
B -25.4
C -36.6
E *-25

Q18 values after replacing resistor
B -10.9
C -10.5
E-10.3

Q18 actual values in service manual
B -24.6
C -34
E -24


Q19 blown resistor values
B 41
C 51.5
E 40.4

Q19 values after replacing resistor
B 41.1
C 49.3
E 40.5

Q19 actual values in service manual
B 40.1
C 51.6
E 39.5
 
Last edited:
Q17 seems to be hurt. It is likely that is what caused the resistor to fail, rather than the other way around. Something may be pulling down the +24V at the emitter.
 
Because its supplies are wrong.

q17 is the positive +24v regulator, which is toast (-21.9V) so you are going to have to figure what is blown, test the comps assoc with the reg, Q17
q19 emitter is the supply, should be close to scat ~ +39.5VDC

In the above statement about q19 what is scat. Is that a typo. What do you mean it should be close to scat
 
Q17 seems to be hurt. It is likely that is what caused the resistor to fail, rather than the other way around. Something may be pulling down the +24V at the emitter.
Are you suggesting that I keep hunting for the thing that may be pulling down the +24v at the emitter of q17? If so any advice on where to look. Thank you
 
A first step:
Check for continuity between pins 4, 6 and 7 of the TA7136 IC?
Check for continuity between pins 1, 4 and 8 of the HA1457 IC?
It could be the EQ or the Pre amp IC's. Or something else.
 
Last edited:
A first step:
Check for continuity between pins 4, 6 and 7 of the TA7136 IC?
Check for continuity between pins 1, 4 and 8 of the HA1157 IC?
It could be the EQ or the Pre amp IC's. Or something else.

Do you mean HA1457.
No continuity between pins
 
scat = schematic
HA1457, you need to check all the places that the +24VDC regulated voltage goes too.
Q1,2,3,4
First thing to do is remove Q17 and measure the resistance from the Q17-emitter to chassis ground.
check resistance of R301,302,305,309,310
check &/or change C301,303,305
Q17, use 512-KSC2383YTA (Vce=160V,Ic=1A,Pd=900mW,Ft=100M,hfe=60-320,TO-92L,ECB)
 
Thank you very much. I am grateful for this knowledge that you share. Now I got some work to do on this receiver.
You say to remove Q17 and measure resistance from the Q17 emitter to chassis ground. I am very confused on how to do this. If I remove Q17 from the circuit board how do I measure resistance from emitter to chassis ground?
 
There are series resistors & ecaps to ground, running to each load (supplies for Q1-4), these act as supply noise filters to remove hum/noise
If you remove these R's you can isolate the loads from the supply.
get the supply up and running at the correct voltage, then you can figure out if/which load is drawing too much current, (try one set of series R's +/-24V at a time) to figure out if/why the regulator has failed or is out of spec V.
 
:banana: This is so fun. Its like a drug. I love it.

It is the HA1457 that is drawing all the current. It took me awhile to figure it out but here is the process.
Removed Q17 and measured resistance from chassis to ground 46.5k

Checked the mentioned resistors. All check ok except for R309
R301 391
R302 219
R305 425
R309 13k Should be 22k
R310 9.3k

Checked the mentioned Capacitors as well as C213, C214 they all tested very bad and I replaced them all.
C301 57
C303 62.5
C305 35
C213 16
C214 15

Replacing the caps did nothing for Q17, Q18 so I started to pull the IC's. I started with the Q3, Q4 TA7136P I compared them to each other and tested resistance from pin to pin. They both had the same values so I figured either both are good or both are blown. Next I tested Q17, Q18 while the TA7136P's were pulled and no love there. So I moved on to Q1, Q2 HA1457 I pulled them out and compared them to each other, testing resistance from pin to pin I saw that they were not matching. So I pulled a HA1457 from my working SX-780 to use a base line and found out one of the HA1457's was blown.

I replaced the blown HA1457 and powered her up on a dim bulb and Voila Q17, Q16 are closer to spec but still no relay click. I have made some forward progress and it feels good. Now what should I do to get the relay working? Below are the readings while on dim bulb.
Q17
B 24.6v
C 32.8v
E 24.2v

Q16
B -24.6v
C -29.2v
E -24.2v
 
Left STK (looking at front of unit)
pin1 39.2v
pin2 -41.3v
pin3 39.2v
pin7 39.2v
pin8 41.3v
pin9 40.3v

This is so fun. Its like a drug. I love it.

but still no relay click.

Looks like you need another fix...,
Until you solve the reason for pins 3 and 7 being 39.2 volts, you won't get a relay click.


Edit: Looks like pins 7,8,9 should be 8,9,0 (thnx merlynski)
 
Last edited:
Back
Top Bottom