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Pioneer SX-780 Repair help

I can make a mouser order if needed. Im not finding CRD E-202 available from Digikey.

Will E-202 work for replacing 2SK34 at Q28, Q29 as well?

I plan to restock up on spare transistors to cover all bases for further trouble shooting of this problem. KSC2690AYS, KSA1013YBU, KSC2383YTA, KSC1845FTA, KSA992, KSA1220AYS. Q20, Q19, Q25: KSA940TU, KSC2073TU. Any other suggestions or better suited replacements to add to the list?
 
Yes, look at the linked thread. The voltage regulator circuits that use a constant current source at 2mA can use that diode. There are more varieties of current available, as well.
 
Is there any other components I should be checking or reporting back with voltages to further troubleshoot before placing an order? I imagine it will take 1-2 weeks.
Will a bad or lack of 2SK34 at Q24 cause all pins on Q20 to = -51v?
 
The voltage control transistor (Q22) has to have the ability to control current to the base of Q20 in order to regulate the voltage at the emitter of Q20. It works with the constant current source, which passes only a specific amount of current, by shunting away that current from the base to decrease conduction in Q20 and lower the voltage, when the voltage is too high. When the voltage is too low it allows more current into the base of Q20, allowing the emitter voltage to rise. If the constant current source does not limit the current properly it damages Q22. If Q22 fails open, then there is no control of the current into the base of Q20 and it saturates because of all the current available from Q24. That is lacking in some detail, and is way simplified. So a bad Q24 can cause -51vdc on the emitter of Q20. That may have been TMI, I got on a roll :rolleyes::blah:.
 
Not at all, thanks for taking the time to explain!

I will place an order for some E-202 CRD's and a few spare transistors from Mouser tomorrow AM. I wanted to try Digikey but I didnt find any Semitec E-202 or equivalents in stock.
 
I had the same experience, I was making a DigiKey order, but ended up doing another Mouser order with some of those. It should be here soon.
 
I finally received my parts from Mouser and installed them last night. The CRD solution worked and Q20 is now showing correct voltages!

There still is a problem with some of failed test point initially reported. A couple readings have improved

Q20 = B: -39.45v, C: -50.3v, E: -38.7v
Q19 = B: +39.2v, C: +48v, E: +38.6v
Q25 = B: +13.87v, C: +23.6v, E: +13.27v

C307: +38v (good)
C308: -38.8v (good)

R238 top: -38.8v (good)
R238 bottom: -30.01v (s/b -17.1v)

R237 top: -38.8v (good)
R237 bottom: -29.57v (s/b -17.1v)

Q6 = B: -0.549v, C: +38.7v, E: +0.01v
Q5 = B: -0.549v, C: +38.7v, E: +0.01v
*Q6 & Q5 were previously replaced with KSC1845FTA before Q20 repair

R240 bottom: -0.549v (s/b -13.5v)
R239 bottom: -0.547v (s/b -13.5v)

Q10 = E1: -37.7v, C1: -36.6v, B: -37.1v, C2: -37.1v, E2: -37.7v
Q9 = E1: -37.7v, C1: -37.7v, B: -37.1v, C2: -37.7v, E2: -37.85v
(*Q10 &Q9 are the original 2SC2291)

Q7 = B2: +0.01v, C2: +0.01v, E: +0.6v, C1: -37.1v, B1: +0.01v
Q8 = B2: +0.01v, C2: +0.01v, E: +0.6v, C1: -37.1v, B1: +0.01v
(*Q7&Q8 are the original 2SA979)

Q15 = B: +38v, C: +6.77v, E: +37.6v
Q16= B: +38v, C: +6.61v, E: +37.61v
(*Both 2SA912 were replaced with KSA1013YBU before Q20 repair)

Q13= B: -38v, C: -4.17v, E: -37.2v
Q14= B: -37.8V, C: -4.19v, E: -37.2v
(*Both 2SC1885 were replaced with KSC2383 before Q20 repair)

Q11= B: -36.95v, C: -4.17v, E: -37v
Q12= B: -36.8, C: -4.20, E: -37.2v
(*Both 2SC1775A were replaced with KSC1845 before Q20 repair)

Q28 & Q29 (2SK34): one was replaced with J113, the other with BF256B which is what i had onhand before Q20 problem was fixed. I dont have any more BF256B. Should i replace both to match with new J113 or CRD202? Would this cause the problem with some of the incorrect voltages above?
 
Can someone please guide me in what to do next? Any other voltages I should report, what transistors I should replace first?

Thanks!
 
I’m no expert, but if you’re not getting the expected voltage drops across the resistors, you might want to verify that they’re good and providing the right resistance. You can lift one leg to remove it from the circuit and test it with your ohm meter.
 
Greetings,
Q6 = B: -0.549v, C: +38.7v, E: +0.01v I think you have base and emitter measurements reversed here
Q5 = B: -0.549v, C: +38.7v, E: +0.01v I think you have base and emitter measurements reversed here
*Q6 & Q5 were previously replaced with KSC1845FTA before Q20 repair

Q28 & Q29 (2SK34): one was replaced with J113, the other with BF256B which is what i had on hand before Q20 problem was fixed. I dont have any more BF256B. Should i replace both to match with new J113 or CRD202? Would this cause the problem with some of the incorrect voltages above? CRD202 cannot be used in this application, Q28 and Q29 are used as jFET switch devices here, not as constant current devices as in Q23 and Q24 location. but they should both be the same, J113 (switch application) or BF256 (RF amp application), if those are suitable.
 
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I double checked all the resistors in that area by lifting one leg and they are all in spec. I did get R237, R238 to show -39v on one side and -17.1v on the other by adjusting VR5, VR6 so that's no longer an issue.

My problem is now bottom of R239 & R240 = -0.62v because Q5 & Q6 emitter = -0.62v when it should be 13.5v.
Q5&Q6 (2SC1775A) were replaced with KSC1845FTA before my power supply Q20 was fixed.
Can someone confirm KSC1845 is a suitable replacement or suggest a better one? I only have 3 more KSC1845 onhand, once confirmed I will replace.
 
Yes i do have emitter and base readings reversed!

Ok i will replace Q23, Q24 with matching J113's.

Thanks!
 
Power supplies look good. KSC1845 is good for Q5 and Q6.
Some of the other voltages you listed are less important in themselves, than is their effect on the rest of the circuit, and may be different than the schematic because of different than original component replacements..
Significant dcv measurement points are:
Bases of Q5, Q6, Q7, Q8: should be very close to 0vdc
Pins 1 and 0 on the STKs, should be about equal voltage but opposite polarities, e.g. +1.4vdc and -1.4 vdc. They are never exactly equal, but close.
Pins 6 and 9 on the main PCB: should be very close to 0vdc.
Do you have original STK modules installed, or aftermarket replacements? They should only be about 3v different, but, if I read your data correctly they are about 10vdc different.
All these measurements should be taken with input set to Aux, no input signal, volume to minimum, tone controls centered or off, not on DBT.
What dc voltage do you have at:
Pin 6:
Pin 9:
Q1 (left channel STK)
pin 1:
and pin 0:
Q2 (right channel STK)
pin 1:
and pin 0:
 
I replaced Q5, Q6, no change on emitter voltage.
Q5= B: -0.003v, C: 38.5v, E: -0.572v
Q6= B: 0.001v, C: 38.4v, E: -0.565v
Q7,Q8 both base are 0v

It should be noted i dont have STK modules in yet, the 1k resistors are still installed. I installed and blew a new pair of STKs after the relay finally clicked but before Q20 voltages were correct. Now im a little hestitant to install another aftermarket pair until i know its safe!

Pin 6: 0.03v
Pin 9: -0.07v
Q1 (left channel STK)
pin 1: -4.64v
and pin 0: 4.37v
Q2 (right channel STK)
pin 1: -4.56v
and pin 0: 4.63v
 
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Q5 and Q6 bases are good, very close to 0vdc.
Those numbers look very good for having the 1kΩ resistors in place. Notice how the pins 0 and 1 are nicely balanced.
And the outputs at PCB pins 6 and 9 are good, the diff amp is working to control dc offset. You should be able to adjust VR5 and VR6 to move above and below 0vdc (dc offset adjust).
I bet the protection relay pulled in, yes?
If so, I would say from those readings you are ready to install the repaired (or new) modules.
I have not seen a good STK (DIY or otherwise) fail to work correctly once those conditions are met.
Power up on DBT, when it passes, recheck power supplies and dc offset (pins 6 & 9), check bias for adjustment functionality. Then reduce bias setting to minimum, and power up on full mains. Relay should pull in. Recheck dc offset, set bias, let it warm up, recheck bias is stable. Offset and bias will always wander a bit. :bigok:
Then try some music.

EDITED
 
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Here's a little update, some good news and some bad.

STK's installed and it passed DBT. Power up on mains, relay pulls in. RIGHT STK looking good, LEFT STK major problem!

RIGHT STK:
1: -1.04v
2: -41.1v
3:-0.005v
8: +0.007
9: +41.2v
0: +1.51v

LEFT STK:
1: -38.2v
2: -40.7v
3: 0v
8: 0v
9: +40.6v
0: +38.5v
(Voltages remain the same with STK installed or removed)

These results lead me to Q15, Q13, Q11. All 3 transistors have bad collector voltages:

Q15 = B: +38.43v, C: +38.43v, E: +37.92v
Q13 = B: -38.6v, C: -38.2v, E: -38.2v
Q11 = B: -38.2v, C: -38.2v, E: -37.7v

*Resistor R259 smoked a tiny bit a couple mins in after initial power up on full mains - POWER OFF, wait. Was able to power on after that with no issues and it still tests within spec.

What could have caused this to happen, bad (new) STK module?

With the LEFT STK disconnected I am still getting the above Q11, 13, 15 voltages. Are all 3 transistor blown and need to be replaced or is another component responsible for this?

At this point I have no idea if the LEFT STK is still good or not or if it could survive getting +/-38v on pins 0 & 1. I know none of the pins are shorted to each other.

I dont have the time or budget to build the DIY STK right now, I know they are the best solution. I listen to my SX-780 daily and I successfuly used knockoff modules to repair.

I really the appreciate the help and AK support!
 
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You are welcome, we are happy to help!
My immediate conclusion from your description and symptoms is that the 'NEW' knockoff STK is (and was) bad from the beginning.
Without a current path through the bias spreader (STK pins 0 & 1) the voltages at Q11, Q13, and Q15 will be wrong, whether they are good or bad.
Lift one end or R259 and measure its resistance, or just replace it with the original value. Measure R261. Replace Q11, Q13, and Q15, they have likely been over-stressed by the current that smoked R259. Even if they seem to test OK with a DMM they are now unreliable IMHO.
Q11: 512-KSC1845FTA if you have one
Q13: 512-KSC2383YTA
Q15: 512-KSA1013YBU
Reinstall the 1kΩ resistors in the place of the LEFT STK and remeasure pins 1 & 0. If they are still balanced around 0v and about equal in value you should be good.
 
The SX-780 is finally back up and running!! :) Thanks Merlynski

I replaced everything you suggested Q11, Q13, Q15. For good measure I replaced Q19 & Q25 with KSC2073TU and did the heat sink mod others have outlined. I reinstalled 1k test resistors and all the voltages returned to normal. New STK, bias adjusted and I got clear sound in both channels on AUX. I will hook up a turntable tonight to test phono.

I noticed Q25 heatsink gets hot to the touch after 5-10 mins, whereas Q19 & Q20 does not. Is that normal?

AM tuner works. FM tuner has issues. The stations are there but reception is very poor even with an antenna hooked up and the sound is very poor. Strange garbled or muffled sound. I read the tuning fins need to be blown out with compressed air and the brass contacts cleaned (syringe drip of contact cleaner on points only) working tuner band back and forth 30-40 times. I started this process last night but didn't really start to see any results yet.
Any advice? Test and look for a bad resistor, diode or capacitor on tuner board?
 
Cycle FM between MONO and STEREO. Mono bypasses the Stereo Multiplexer. If the sound clears up in MONO, and sounds garbled in STEREO, the Multiplex circuit is either crapped or the chip(PA-1001A) is bad. Go ahead and blow out the fins, and clean the ground rings. It can't hurt. The tuna may need an alignment. There are a bunch of small value (<1uf) Baby Blue Sanyo capistors on the tuner board. Replace with WIMA films, Same value, 50V. These caps are Solid aluminum and start to lose their capacitance and gain resistance. Hence capsitors. (Credit for terms goes to ECHOWARS EDIT: Actually Fred Longworth about 20 years ago. WHOOPS!). 2sa725 and or 726 on the board can be replaced with KSA992.
 
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Biscayne said:
The SX-780 is finally back up and running!! :) Thanks Merlynski

I noticed Q25 heatsink gets hot to the touch after 5-10 mins, whereas Q19 & Q20 does not. Is that normal?
Click to expand...

You are welcome.
That is normal, and why more heatsinking is better for Q25.
Glad you got it working!
 
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