Pioneer SX-636 Intermittent DC Offset How to Diagnose?

[Johnny_Law]

Have an SX-636 on the bench. It was acting like an output transistor had shorted, giving me ~20VDC offset across the right channel at one point. Got it to the bench (a couple days later) and checked all the pins on the Power Amplifier board (AWH-034) and the Power Supply board (AWR-060). The only voltage that looked strange was 13.9V at pin 14 of the power supply - but the resistor right there tested 270ohms on the nose. All other pin voltages were within 10% of spec.

Confused by the correct pin voltages, I wanted to double check my DC offset. I measured again and it was much better. Around 8mV on the left channel and 30mV on the right. Hooked it back up and it started singing sweetly again. Even pin 14 on the power supply is back at 6V where it's supposed to be.

Question is - how do I diagnose this if it's not acting up any more? Should I just keep it on until it happens again? Is this weird or does this happen normally?

Thanks,
Ryan

 

[markthefixer]

Have you gotten rid of the (weird and problematic) 2sc1451 transistors in the power amp? No less the 2sa725/2sa726 transistors.

There are unconditional preventative replacements, the 2sc1451 can blow out the power amp circuits when it finally fails instead of operating intermittently.

I know, I know the schematic says 2sa763, but the 2sa726 part number is sitting right there after the "OR" that follows the 2sa763 number....

 

[Johnny_Law]

Hi, Mark - thanks for stopping by. I read some of your posts in the past concerning those transistors (e.g. http://audiokarma.org/forums/index.php?threads/sx-636-hiss-crackle-pop.373254/, also, a thread I found with a similar DC offset issue http://audiokarma.org/forums/index.php?threads/pioneer-sx-636-30v-on-left-output.726515/) and was thinking it'd be a good idea to replace those regardless. I'll likely do so and see if the DC offset ever spikes again.

You had listed the following as replacements:

ksa992 for the a726

512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea

And larry had this list:

Look for 2SA725, 2sa726, and 2sc1451's on your schematic and boards. The following lists two different transistors for the 636. Either may be there. But they won't be mixed. It's one or the other. If it has 725, and 726's REPLACE ALL OF THEM. These transistors are problematic and cause these symptoms. I'm with Mark on the 763's. I have a unit with them and haven't had problems with it. but every unit that had 725,726, 1451's I've had to replace them to get rid of the crackling, pops, etc.

Taken from another recent 636 thread and parsed.

# in parenthesis is the board Number. The new transistor numbers are Mouser #'s.

(AWH-034)Q5+Q6 2SC1451 = KSC3503's, 512-KSC3503ESTU to-126 ecb 300v .1a 1.2/7w 150mhz 40-320hfe

2sa725
TONE AMP BOARD (AWE-046-0)
Q8 Transistor 2SA763P-5 or 6,(2SA725-F or G)
Q9 Transistor 2SA763P-5 or 6,(2SA725-F or G)

2sa726
AF Amp BOARD (AWK-035-0)
Q1 Transistor 2SA726-F or G
Q2 Transistor 2SA726-F or G

PA Amp (AWH-034-0)
Q1 Transistor 2SA763F-6 or 5,(2SA726-F or G)
Q2 Transistor 2SA763F-6 or 5,(2SA726-F or G)
Q3 Transistor 2SA763F-6 or 5,(2SA726-F or G)(2SA640-K or L)
Q4 Transistor 2SA763F-6 or 5,(2SA726-F or G)(2SA640-K or L)

Q1-Q4 2SA726 = 512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe

2sc1312 use a KSC1845 This is the complement to the ksa992.

2sa725 and 2sa726 use the ksa992

2sc1451 use the ksc3503

Larry

 

[Johnny_Law]

To resolve this thread. I replaced the following components which solved the issue. Like Mark mentioned, I imagine one or more of the 2SA726/2SC1451 was flaking out as is their propensity, according to their reputation.

Power Amp AWH-034:

• Q1, Q2, Q3, Q4 2SA726 with 512-KSA992FTA
• Q5, Q6 2SC1451 with 512-KSC3503DS
• Q7, Q8 2SC1509 with 512-KSC2690AYS
• C1, C2 0.33uF, 25V 598-DSF050J334 use 581-TAP334K050SCS
• C7, C8 220uF, 6V 647-UFG1A221MPM
• C9, C10 47uF, 35V 647-UPW1V470MED1TD