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Pioneer SX-880 2SK34 FET Replacement

Brivan said:
Well, I'm back at this, again, with a different SX-880. I've got +58v & -58v in to each of the 2SK34 JFETs, but only about +3v & -3v out, respectively. I replaced both JFETs with NTE312 JFETs and E202 diodes and am getting the same result. I've checked for proper grounding, etc. but am baffled as to why my output voltages are so low. Obviously, the receiver is dead. Can anyone shed some light on this? Thanks!
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Found it. D28 was shorted. Pretty sure I checked D28 yesterday, but I may have been testing the wrong traces. Transistor heat sink glue had oozed over it during the assembly process (not sure if this aided it its demise) and it was kind of a pain to get out. If anyone wants to take a stab at explaining why a shorted MZ-140 zener diode would cause loss of voltage on the output-side of both 2SK34 FETs (and not pop any fuses or destroy other components), I'm all ears!
 
ItsGarand said:
Good find!
So you went with the FETs instead of the CRDs?
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I installed E202 diodes in my last attempt at eliminating the FETs as the problem so I simply left them in place after finding the bad MZ-140 zener diode. I saved the original 2SK34 FETs as good used replacements.

Which brings up another thing - using my Peak DCA55 semiconductor component analyzer to test the FETs, it would always come back with "source and drain not identified" errors - no matter if I tested an original 2SK34 FET or a new NTE312 from my stock. Not sure if that's a normal reading with this analyzer or not.
 
Brivan said:
Well, I'm back at this, again, with a different SX-880. I've got +58v & -58v in to each of the 2SK34 JFETs, but only about +3v & -3v out, respectively. I replaced both JFETs with NTE312 JFETs and E202 diodes and am getting the same result. I've checked for proper grounding, etc. but am baffled as to why my output voltages are so low. Obviously, the receiver is dead. Can anyone shed some light on this? Thanks!
Click to expand...
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[COLOR=revert-layer]I have an SX 880 with the same problem

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[COLOR=revert-layer]I picked up a pair of 2sk34 43B's. the ones that came out were 2sk34 91C
the service manuel says the part should be 2sk34 C or D. ( is there a difference)
I'm only getting out+3v and a -3v the print shows +42v/-42v to drive protection circuit and
beyond.
0n audiokarma there was a thread of the same thing and the person found his D28 diode MZ-140 zener was bad and replaced and he got his voltages. so I changed my MZ-140 with a 1N4743A 13v and
the voltages stayed the same. no change

Before this problem came up I was tripping the protection circuit at 1/3 volume. so I started replaceing caps and everything was working fine and powering up. I ran the reciever for 5 min and some thing went and the protection circuit did its thing.
I checked all of my caps and soldier joints. Q24 (2sk34) ties into Q30,Q28 at a junction point and should be +42 v but its only +3v . Q25 (2sk34) ties to Q29,Q27 and it should be -42v but its only -3v.
all the transistors seem to be ok
any suggetions ... is the something to the type of sk34 or cap passing something through.[/COLOR]​

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I have a sx 880 with the same problem

I picked up a pair of 2sk34 43B's. the ones that came out were 2sk34 91C
the service manuel says the part should be 2sk34 C or D. ( is there a difference)
I'm only getting out+3v and a -3v the print shows +42v/-42v to drive protection circuit and
beyond.
0n audiokarma there was a thread of the same thing and the person found his D28 diode MZ-140 zener was bad and replaced and he got his voltages. so I changed my MZ-140 with a1N4743A 13v and
the voltages stayed the same. no change
Before this problem came up I was tripping the protection circuit at 1/3 volume. so I started replaceing caps and everything was working fine and powering up. I ran the reciever for 5 min and some thing went and the protection circuit did its thing.
I checked all of my caps and soldier joints. Q24 (2sk34) ties into Q30,Q28 at a junction point and should be +42 v but its only +3v . Q25 (2sk34) ties to Q29,Q27 and it should be -42v but its only -3v.
all the transistors seem to be ok
any suggetions ... is the something to the type of sk34 or cap passing something through.
 
r405 and r406 checked at 11 ohms . replaced with new 2.2ohm resistors
the voltages for the2sk34's changed from +3/-3 volts to +38/-38 volts
protection circuit clicked in . 12.3vat pin 1 PA3004. its supposed to be 13.1volts
but works drops in and out every voltages down stream uses +41/-41 volts ????
any idea !!!! thanks Steve
 
Sjohannsen said:
R405 and R406 checked at 11 ohms. Replaced with new 2.2ohm resistors;

Voltages for the2sk34's changed from +3/-3 volts to +38/-38 volts;

Protection circuit clicked in.

12.3V at pin 1 PA3004. its supposed to be 13.1volts, but works - drops in and out
Click to expand...

That should be close enough, as evidenced by Q26 now being satisfied. The fact that you are measuring 12.3V could indicate problems with Q26, or D28 not maintaining 14V. What is the voltage at each lead of Q26?

FWIW, the threshold for the PA3004 12-14V. If that DC is dropping below 12 at some point, that could be a reason for the protection being intermittent.

Sjohannsen said:
Every voltages down stream uses +41/-41 volts ????
any idea !!!! thanks Steve
Click to expand...

I interpret that as stating that other downstream circuits specified at ± 41.5V are now at ±38V. If the line voltage is within reasonable range (no DBT or low line) it might be good to report the voltages at all three legs of both Q27 & Q30.
 
Q26 12.2v , 12.85v, 34.7v
Q30 56.6v, 38.8v, 38.1v
Q28 38.8v, 13.0v, 12.85v
Q29 -38.2v, -.57v, .53mv
Q27 -58.7v, -38.2v, -37.6v

I had replaced D28 , the MZ-140 zener with IN473A rated at 13v before the changes

R405,R406 were replaced with 1 watt 2.2ohm resistors, the service manual shows 1/4 watt RD1/4PSF...J
could this have affected the 2SK34 voltages
 
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The 13V zener at D28, coupled with the typical 500-600mV forward drop of the B-E junction on Q26 is going to result in exactly what your measurement indicated, 12.5V. If that is dipping to 12V on occasion, the PA3001 could drop the relay. Try to find a 14V zener to see if that helps. 1N5351 should work.
 
Sjohannsen said:
Q26 12.2v , 12.85v, 34.7v
Q30 56.6v, 38.8v, 38.1v
Q28 38.8v, 13.0v, 12.85v
Q29 -38.2v, -.57v, .53mv
Q27 -58.7v, -38.2v, -37.6v

I had replaced D28 , the MZ-140 zener with IN473A rated at 13v before the changes

R405,R406 were replaced with 1 watt 2.2ohm resistors, the service manual shows 1/4 watt RD1/4PSF...J
could this have affected the 2SK34 voltages
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You really do want to put 1/4 resistors back in. You want those to fail fist vs more expensive silicone.
 
thanks . I replaced the R405 and R406 with 1/4 watt resistors.
I ordered the IN5351B zener diodes. through EBAY I will get the them in a couple of days
then install I will let you know
 
ItsGarand said:
If you are on a dim bulb tester, the voltages will be a little lower.
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While generally true, if the source voltage is within about ±10% the zener should still be trying to keep anything below its reverse voltage. Of course, if the source voltage is down below 14, all bets are off.
 
Sjohannsen said:
thanks . I replaced the R405 and R406 with 1/4 watt resistors.
I ordered the IN5351B zener diodes. through EBAY I will get the them in a couple of days
then install I will let you know
Click to expand...
I have purchased stuff on ebay myself, but it's worth saying that you must be careful about the source. Many part on ebay are Chinese counterfeits. Better to stay with the likes of Mouser/Digikey/Arrow etc.
 
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