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SX-939 worth the effort?

larryderouin said:
If you follow the outbound side of F5/F6 you'll find that it's a 13V 100ma(Pin 4 on P.S.) to the TUNER BOARD(AWE-041). The wire splits and one side and goes to PIN 20 on the Tuner board and also Pin 3 on the Muting AssyBoard (AWM-063). The other side goes to Switch Assy Bd (AWS-069 3/4) where it enters at pin 32 , it goes thru switch S2F (FM) to pin 28 (off) or Pin 29 (On). Pin 28 (AWS-069) goes to pin 4 on the (AWE-041) Tuner board and supplies 13V. It's knocked down to approx 1.9V by R20 and then to PIN 2 of Q6 (HA1137). Back to pin 28 on the Switch board (AWS-069 3/4). Pin 28 takes the 13V (switch is ON) and hands it off to the Tuner Board @ Pin 3 for 13V to power the Oscillator and Mixer for FM.

Suggest you go back over the last parts you replaced before the STV broke. USE the DBT until it shows no brightness in any mode. Disconnect the F5 & F6 fuses and prove out the Audio section of the receiver(phono preamp to amp output). Use all inputs EXCEPT AM, FM,MIC. Then Disconnect the lead from Pin 4 on the P.S. and check for the correct DC voltage. It may be a volt or 2 high, that's ok as there is no load. If correct, then disconnect Pins 3 and 4 on the tuner board (insulate the ends) reconnect pin 4 on power supply, and check pin 4 on P.S. for correct voltage with S1F IN and OUT. If no problems the voltage should NOT CHANGE. If it does on either, look for a pinched wire(broken insulation), a solder blob at the connections crossing connections, etc. Once that proves out, connect pin 3 and power up. Check voltage at pin 4 P.S. if it drops precipitously, you have a bad FM Tuner Front end. Disconnect Pin 3 and repeat checks on pin 4 of the tuner board. If it drops you've probably got a shorted HA1137 Chip. In either case, you'll probably be better off replacing the complete board. Both the SX-838 and SX-939 share the same tuner board (AWE-041).
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The wires connected to the transformer and the fuses are on the RIGHT SIDE of this excerpt of the FULL SX-939 schematic. TOP TO BOTTOM with the Wire color to the Fuse panel from the transformer.
View attachment 1545537


Pin 22 Blue F3
Pin 23 Violet F4
PIn 20 Green F6
Pin 19 Green F5
Pin 12 RED
PIn 15 RED
Pin 17 ORANGE F7
Pin 18 ORANGE F8

These are all low voltage circuits. AND EACH WINDING IS CENTER-TAPPED to Ground, so you'll get extremely low ohms or what apprears to be a dead short. Pull all the fuses, and power up then check the fuse pairs (like colors) and you'll get varying AC voltages depending on what it's powering. Nothing wrong with the transformer as long as it's putting out voltage.
Click to expand...
Larry: Sorry, the title of this thread is making this confusion. I am working on Pioneer SX-1010 not SX-939 although the schematic looks very similar!
I will follow your general advice. It would be great if you confirm that your above advice would be excatly same in case of SX-1010. Thanks
 
Larry:
In case of SX-1010 the outbound side of F5/F6 goes to Pin 1 and 2 of power supply board feeding the rectifier circuit (D1-D4) and ultimately connected to Pin 4 and Pin 3,7 of power amp. So my assumption I have some problem either in rectifier circuit (D1-D4) on PS board or in power amp. Am I right?
 
Actually we should have had you START a NEW Thread as it would have the correct info for the unit you are working on. So we;ll start all over.
Pins 1 thru 8 are on the transformer side. Same thing as the 939 only pinned differently. Note that they are in pairs by color. Each wire corresponds to a fuse and each lead (Same Color)is on an end of the same winding.

Now as for the F5, F6 problems. More than likely in the AMP. Pull Each output Transistor in turn (put it back before pulling the next one) and do the 6way test (see this link....
https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/. ). Replace bad ones in pairs (NPN and PNP). You can also disconnect pins 3-7 (-56VDC) and pin 4 (+56V) from the amps and power up. If the amps are bad you should have the DBT flash and dim. Check the voltages for those pins from the P.S. (check all voltages on the left side of the P.S. on the schematic). The amp voltages(amp pins 3-7,4) will be about 5V high (unloaded) if the P.S. is ok. I'm betting at this point you've got some output's and maybe a driver or two that are blown, maybe a resistor or 2 in the amp board.
 
Larry: Please read below

Connections on schematic:

Fuse F5 connects Pin 7&9 on board AWR-053-0 (fuse board); fuse F6 connects Pin 8&10 on board AWR-053-0. Pin 9 on board AWR-053-0 connects to Pin 1 of PS board (AWR-054-A) and Pin 10 on board AWR-053-0 connects Pin 2 of PS board.

Observation 1:
With both fuses F5 and F6 connected Pin 9 &10 on board AWR-053-0 showing short in multimeter.
Observation 2:
With both fuses F5 and F6 connected Pin 1 & 2 of PS board showing short.
Observation 3:
If I take out any one of fuse F5 or F6 no short between Pin 9 &10 on board AWR-053-0 and no short between Pin 1 & 2 in PS board.
Observation 4:
Pin 7 & 9 on board AWR-053-0 are always showing short irrespective of fuse connected or disconnected.

According to schematic the Pin 1 and Pin 2 of PS board connected to Pin 4 and Pin 3,7 of power amp. Pin 4 never shows any short with Pin 3 or Pin 7 irrespective of fuse F5 and F6 connected or disconnected.
 
Larry: There is progress!
I took out the Pins 3, 7 and 4 from both amps it passed DBT and relay clicks! So you are absolutely right! Problem is in amp. I checked all output transistors and they are fine. Next I will connect Pin 3, 7 and 4 on one amp and will check with DBT to isolate which side is bad.
 
Before, I replaced Q1-Q3 (SA726) with KSA992FBU and Q5-Q6 with KSC3503DSTU. Now I pulled Out Q4 (2SB528), Q7 (2SD358), Q8 (2SB528) and all three are bad! I also pulled out replaced Q5, Q6 but they tested good so I put them back. Still there are original Q9-Q12. Do I also need to take out those and check it?
Which one of these ksa940, mje15029, mje15031 will be the right replacement for 2SB528?
Which one of these ksc2073, mje15028, mje15030 will be the right replacement for 2SD358?
 
CORRECTIONS!!
It seems that my transistor tester has some loose connection. After fixing the tester connection, all the transistors I pulled out so far (Q5-Q11) are tested good. Should I look for resistors? Any particular? Also diodes?
Please advise.
Thanks
 
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Larry: I was trying to check the resistors and I noticed that the resistors layout is not the same as in the foil side picture in the manual I downloaded (it is little different). My amp board number read AWH-032-D. Have you ever saw a board with D suffix? The problem is how do I know the correct value of resistors at different positions? Near the Pin 7 area (at the bottom of the board) starting from the left in the downloaded manual it starts with a diode then 11 resistors in a row then a capacitor. In my board, the diode is missing instead, it has 12 resistors in a row then the capacitor. In manual first 6 resistors (after diode) all are in kohm range. In my case in that row I am having one resistor in low 80-ohm range (still connected to the circuit not pulled out) it is the 3rd from left. Is it possible to get foil side picture of this board with suffix D?
 
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I think it is better to summarize current status

1. From right main amp Pins 3, 4, 7 disconnected DBT failed.
2. From both right and left amp Pins 3. 4. 7 disconnected light dims and relay clicked all the power supply board test pin voltages are good. Conclusion: somewhere in main amp there is a short!
3. Pins 3. 4. 7 of right amp only reconnected to the board, switched on DBT but failed
4. From right amp transistors Q5, Q6 pulled out tested good and reinstalled.
5. From right amp transistors Q7-Q11 pulled out. All transistors tested good but not reinstalled.
6. Decided to power on through DBT (at this point transistors Q7-Q11 are not in right amp and left amp is disconnected). It passed DBT but the relay does not click.
So it is sure that the short is coming from any of these transistors although all of them are tested good. Is the no relay click at step 6 normal since many transistors are out of board?

Two things I can do, either I reinstall pulled out original transistors one by one and test with DBT on each occasion to find the real culprit OR I can install new replacement of all pulled out transistors one by one and check with DBT each time.

Please advise which is a better idea or give any other suggestion.
 
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UPDATE AND NEED SOME HELP
I am concentrating on one amp channel to bring it in working state
Please read the post before this as it is the continuation.
I put back all pulled out resistors and now it passes DBT but relay is not clicking!

First these are the voltage readings at different pins of power supply board when both of the amp were disconnected and relay clicks when powered on through DBT. Values in the parentheses are from manual.
Pin 8 : +13.2 VDC (13.4)
Pin 9: -12.3 VDC (-13)
Pin 10: -54.5 VDC (-55)
Pin 13: +32.5 VDC (+35)
Pin 14: +54.1 VDC (+55)
Pin 15: 24.5 VDC (+24)

Here are the voltage readings after putting back all of the transistors. It now passes DBT but relay is not clicking
Pin 8 : +13.1 VDC (13.4)
Pin 9: -10.7 VDC (-13)
Pin 10: +28.3 VDC (-55)
Pin 13: +33.8 VDC (+35)
Pin 14: +52.7 VDC (+55)
Pin 15: 24.5 VDC (+24)

Please look at Pin 10 voltage it is positive instead it should be negative! Please advise what I should look for?
Thank you!
 
What does Power supply Pin #11 say. Looks like you lost your Negative Rail somehow. But with Pin 9 having Normal Voltage, Not so sure. Need to see what Pin 11 is. Take voltages at the BASE of Q5 and Q6. Also the collector of Q3 and collector of Q2.

The relay isn't going to click in until it senses the correct DC Voltages from the amp. If the amp is giving wrong voltages or no voltages the protection circuit will stay open.

Let me get Zebulon1 on here. He's one of the 1010 Guru's.
 
Larry: Thanks!

Little more update now relay clicks but Pin 10 voltage is -1.8V and Pin 11 is -22.4
 
Measuring on one amp, base of Q5 and Q6 are at -2.4V (black lead in chassis ground). Collector of Q2 is -2.6V and collector of Q3 is -1.6V (black lead chassis ground)
 
Larry: Here are the voltages you want to see. Everything measured with respect to chassis ground while powered on through DBT and relay clicked

Base voltages.
Right Left
Q5 -2.56 V -2.53 V

Q6 -2.54 V -2.53 V

Collector voltages
Right Left
Q2 -2.54 V -2.54 V

Q3 0 V -0.264 V

I also measured the power supply pin voltages

Pin 8 : +13.3 VDC (13.4)
Pin 9: -12.4 VDC (-13)
Pin 10: -2.5 VDC (-55)
Pin 11: -22.4 VDC (-24)
Pin 13: +32.8 VDC (+35)
Pin 14: +54.5 VDC (+55)
Pin 15: 24.6 VDC (+24)

Pin 10 is still amiss!
Thank you
 
Check R9 and R10 on the Right Channel. (Both 16K). Lift one leg and check. Replace as needed. I have a funny feeling they either went High or Open.
 
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