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SX-750 Stuck in protection, smoking resistor.

alexprollock

alexprollock

New Member
Howdy everyone. As mentioned, I've managed to get my 750 stuck in protection mode. Full disclosure, I am a rookie that now has huge respect for what you guys do.

I recapped the AWR-099 board to get rid of the hum in both channels...it worked, no issues there. But I had the transistors as well, so I figured I may as well replace those too. I replaced all transistors on the board, testing it after each one. I had no issues until the final two that I replaced (Q8 and Q10), my big rookie mistake here is that I desoldered Q8 with the unit still powered on...this is before I knew about DBTs.
I replaced the last two, hoping I didn't fry anything, but sure enough, the relay won't click.
Now R29 (the big 150 ohm insulated resistor) is smoking when I power on the unit.

I've since built a dim bulb tester, and checked the resistance of all resistors, as well as the diodes.
The light on the DBT is 100W and lights up bright, then dims to nothing when the unit is powered on.

I've gone as far as I'm comfortable with and am hoping to find where the problem lies. I suspected that I may have fried the Zener at D10 and ordered one, but I wanted to reach out to you folks first before I start shotgunning parts and making things worse.

Thanks!
 
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you missed the point of the question.

WHAT PINOUT did you use when connecting the c945 transistor(s)?

You see,
the KSC945YBU transistor is an E B C pinout (ALSO THE KSC945YTA)
while
the ksc945CYTA transistor is an E C B pinout.



Which is PRECISELY why MY lists NEVER include the 2sc945 or 2sa733 transistors or any of their variants.
Too much time lost to straightening out which pinout to use to install them.

also
My public sx-750 recap list from 2010 didn't include transistors.
That never made public sx-750 transistor list, has Q9 as a 512-ksc2383yta, so it definitely wasn't mine...

Your picture of Q8 leads me to believe you used an ECB pinout arrangement, the center lead goes to the collector (+30v) position. if a ksc945ybu was used it has a base <> collector reversal.
 
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I believe your KSC945YBU is EBC and you have it installed wrong. The emitter and collector legs are backwards.
 
Thanks for the correction guys, I did in fact have the base and collector reversed.

I have replaced Q8 and Q9 in what is now the correct orientation. The resistor is no longer getting hot, but it’s still in protection.
 
mikiekimi said:
How did you test it and what were the readings?
Click to expand...

Hi mikiekimi,

I’m sorry, completely had a brain fart yesterday and was answering to testing Q10 rather than D10 :boring:

Testing D10 with the diode mode on a multimeter, I get .003V for both forward and reverse bias.
 
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alexprollock said:
Hi mikiekimi,

I tested on the diode setting on my multimeter. Here’s what I got:
+ on B, - on E: .636V
+ on B, - on C: .634V
- on B, + on E: Open
- on B, + on C: Open
- on E, + on C: Open
+ on E, - on C: Open
Click to expand...

That's a 6-Way transistor test. D10 is a Zener diode...
 
I have one ordered that should be here by Friday.
I wanted to reach out and get others opinions before I started shotgunning different parts.

Now at the risk of sounding stupid here… should the light on a dim bulb tester put out any sort of light upon power up? Or should it remain off at all times unless a problem arises?
 
alexprollock said:
I have one ordered that should be here by Friday.
I wanted to reach out and get others opinions before I started shotgunning different parts.

Now at the risk of sounding stupid here… should the light on a dim bulb tester put out any sort of light upon power up? Or should it remain off at all times unless a problem arises?
Click to expand...
Flash of light, then it should dim down, but this depends on the size of the receiver/amp and the bulb wattage. I use a bulb that's 1.5x or so the rated WPC of the receiver - for example, a 75w incandescent for a 50wpc receiver. I use clear bulbs so there's no doubt.

The DBT glowing bright indicates a short. Flashing, then going completely dark (or glowing very, very faintly) is/can be normal. It will also start to light up if you drive the amp too much (and can damage things), so after your initial DBT test, you'll want to move it to regular power to do any adjustments or use the amp. Naturally there are use cases for keeping it connected to the DBT as well for testing.
 
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508C45D6-D112-4085-9472-0B533A45F226.jpeg
mikiekimi said:
Looks to me like D10 might be bad...
Click to expand...

Alright guys, I got the new Zener, and the relay now clicks. The DBT (100w bulb) now flashes and dims to a glow. I believe I’m on the right track.

I really appreciate you guys helping out a new guy!
 
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insulate those probes!!!! they slip, we ALL slip!!!

insulating them protects against shorts that can be devastating.
 
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