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SX-828 Short

xratedpez

xratedpez

Super Member
Hi All, recently picked up an SX-828, and am really looking forward to hearing this thing sing.

It had been worked on many, many times by the looks of things... And the last guy who had it saw "smoke", so I expected to see some toasted resistors (and I did), R40 (and also replaced R38). Also replaced the emitters on this channel (R44 & R46)

I also replaced all transistors on this board per Mark's instructions here: http://www.audiokarma.org/forums/archive/index.php/t-383775.html
So, outputs, BIAS, drivers, and all other transistors on main amp board AWH-010 are brand spankin' new.

Fired up with the DBT (75W) and there still seems to be a short.

Any ideas on what to check first?
 
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I'm far from being a tech but I would check the diodes on the amp boards? Did you re-cap it? Also, I would double check your transistor installation for proper lay-out into the board.

What is the DBT doing?

What about the protection board? Anything done to it?
 
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Haven't touched the protection board. DBT staying bright.

Only 3 diodes on amp board, 2 are zeners. Checked those, they are OK. I did not recap, just replaced all transistors.

Tracing back to P/S board, looks like R12 is toast(y). Will pull that in the morning, test and replace. It's definitely seen some stress. Also, some remnants of black smoke under the center of the P/S board (carbon on chassis). Pulled 1 representative rectifier diode nearest the carbon trace, but tests OK. Going to replace those tomorrow too, if RadioShack carries them.

Wondering if there are any components on this board that will commonly fail. Either of the two larger heatsinked transistors, maybe? I'll test them in the morning, thanks for any suggestions.
 
BTW, I used this site when trying to determine pinouts of various transistors. Seem trustworthy enough? I tried to confirm w/diode tester (most times).
 
don't trust other people's work, make sure each component is the correct one for that position and each AND EVERY resistor MEASURES correct.

sound tedious? i do this each and EVERY TIME i work on something that is NOT stone cold factory, as evidenced by the condition of the solder joints and leftover wave solder flux.

BE SURE the off board thermal bias diode is undamaged and connected.

edit:

don't use "that site", ksa992 is ecb with flat facing you, leads down, from left to right!!!!
p.s. dn beat me to the Fairchild link...
 
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Yup- they are Fairchild. I try to get Fairchild as often as possible!

So, reading "that site's" pinouts, they just have the resistor flipped- looking back, I read it correctly, but for the future will not trust anything but my Fluke! :)

Also picked up a dynamic transistor checker (micronta) yesterday, hopefully this will help me diagnose bad transistors. Interesting old unit.

Mark, thanks for the reply. I really appreciate it considering your limited posting time! (and dn, too!!)

Are these off-board bias diodes (mounted to heatsink) unobtainium like the horseshoes on the 950 I just fixed? I'll check that tonight.

Thanks again, gents!
 
Here's a quick shot of the p/s board. A few wire wraps have certainly been broken and re-connected (poorly). Any wires, etc look out of place?

ImageUploadedByTapatalk1335218044.128481.jpg
 
i think you are going to have to test and check everything starting from power supply and working forwards .. anything could be the wrong component or installed incorrectly .
 
Whoops. Forgot to say I pulled that rectifier diode to test. And the metal oxide resistor in the upper corner. Both test fine as do the other rectifier diodes and the Zener on this board. And the diode pack too.
 
xratedpez said:
Am I correct in thinking due to it being a short I should be searching for a diode or transistor?
Click to expand...

both .and they may well be installed the wrong way round so will show as a short as its been worked on before . small chance of shorted capacitor/s also
might be more efficient to remove all transistors and diodes that have been replaced in the past and powering up to see what happens , i would check power supply first though by disconnecting everything . that's just my way .others may disagree . my thoughts are if power supply is good then that's half the battle ..
 
From what I can tell, I've already replaced every transistor/diode that's been touched by anyone else! :)

Most everything else looks original except sloppy wiring work (and not sure why that's been touched in the first place! I've replaced all transistors on amp pcb, and tested all of them on the p/s board. Any other board you suggest I look at?
 
from what I can tell, it looks like Q1 ~ Q4 look backwards. It looks like you have the base going to the emitter. I would re-check all of them. Q11 Also Look incorrect.

EDIT: This is based on the E-C-B pinout along with the SM foil side of board. Also, haven't looked at all of them.

Ok, if all the pinouts are ECB then this is what looks incorrect to me, Q1~Q4, Q9~Q12, Q5~Q8 is hard to tell what direction they are facing

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Did a little replacement searching on Q9,Q10 if you replaced with Fairchild KSC2073, that has a BCE pinout, The same with Q11, Q12 replaced with KSA940 also has a BCE pinout according to Fairchild specs and the ones I have on hand. These would be correctly installed. My apology for assuming they were all ECB.
 
To me it looks as if dn is correct with his assumption that Q1 to Q4 are in backwards:

IF Q1 to Q4 are now KSA992, THEN it is ECB config.
IF their precursors have been 2SA572, THEN it was BCE config.

See pic, this is my power amp board after redoing it - flat faces of mentioned transistors look opposite to those on your pic. My 828 still runs without any problem.

Please check!

Frank
 

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