Hello everyone, I have a sx-780 that i have been working on for some time.I am at the point where i don't know what to try next. At first the protection relay would not click so i replaced the stk0050 modules with new ones. Still no click I checked the voltages at the module pins and had some bad readings. i removed the modules and checked the voltages at the module pin location and here's what i found pin#1=-40.5v pin#2=-41.0 pin#3=0.0 pin#8=0.0 pin#9=41.0 pin#10=41.0 these are on the right side. the left side read pin#1=.004v pin#2=-41.0 pin#3=0.0 pin#8=0.0 pin#9=41.0 pin#10=40.0 I dont know if you can get good readings with the modules removed like this but I was afraid that I would fry my new modules. Also with the modules removed the protection will click in. I have resoldered Q25, Q19 and Q20. I replaced Q12 q14 and q15 with transistors from another board and still the same voltages It is unknown if transistors i installed are any good. Does anyone have any ideas why my voltages are so high on pin 1 on the right side? any help would be greatly appreciated. Thanks , Bill
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SX-780 problems
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Brad Muller
Super Member
It sounds like another job for the man of fix it......Markthefixer!!
post the three voltages EACH of the three power transistors in a row before installing the modules.
The readings at the module sockets are mostly useless without the modules.
If the new ones were going to pop, they would have popped immediately.
The readings at the module sockets are mostly useless without the modules.
If the new ones were going to pop, they would have popped immediately.
Is Q20 emitter 40.6(bad) or -40.6(good but typo? )
Yes, we need the modules IN, to figure out what's next... it COULD be the protection circuit. That's why I always advocate checking the output voltages at the square pins near the protection relay before they are interrupted by the relay. I just haven't memorized their numbers, but the same is available at the modules.
Ok Mark, The modules are in place. When i powered up the unit , R 259 burned out. checked pins 1 and 2 on the module and they were dead shorted. I replaced the module with another one, condition unknown, and replaced the resistor and powered the unit back up. No smoke this time. What should I look for next? I am going to call it a day and will check back tomorrow Thanks, Bill
I had asked for the pin 1,2,3 8,9 & 0 voltages on both modules.
Edit: You didn't mention burned resistors. IF R259 (left channel) is burned, you have other transistors to test. Q11 & Q13 out of circuit. This is not going to be a simple repair. NO power yet.
I will have to post replacements later.
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Hi Mark, Here are my voltages. Left module pin 1=39.5 pin 2=-40.6 pin 3=38.8 pin 8=39.2 pin 9=40.7 pin 10=40.4 right module pin 1=-34.6 pin 2=-40.7 pin 3=-33.4 pin 8= -33.3 pin 9=40.8 pin 10=-32.2 pin 25=39.3 pin 26=0 pin 27=-33.4 pin 28=0 pin 29=0 pin 30=0 pin 31=0 I hope this helps. Do you have part numbers for Q11 & Q12, Q13 & Q14? thanks Bill
Ok, since transistors haven't been tested, and BOTH sides are screwy:
just replace Q11, Q12, Q13 & Q14 (2sc1775, 2sc1885) with :
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
and Q15 & 16 (2sa912) with:
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
2sa912 to-92 ecb 150v 50mA 0.75w 65-450hfe 200mhz current source
2sc1775 to-92 ecb 120v 50mA 0.3w 400-1200hfe 200mhz gain side of vas darlington
2sc1885 to-92 ecb 150v 50mA 0.75w 150hfe 200mhz drive side of vas darlington
check the resistors around them as well, looking for burnouts.
these are mouser.com parts numbers...
just replace Q11, Q12, Q13 & Q14 (2sc1775, 2sc1885) with :
512-KSC1845EBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
and Q15 & 16 (2sa912) with:
512-KSA992FBU (ln)to-92 ecb 120v .05a .5w 100mhz 150-800hfe $0.05 ea
2sa912 to-92 ecb 150v 50mA 0.75w 65-450hfe 200mhz current source
2sc1775 to-92 ecb 120v 50mA 0.3w 400-1200hfe 200mhz gain side of vas darlington
2sc1885 to-92 ecb 150v 50mA 0.75w 150hfe 200mhz drive side of vas darlington
check the resistors around them as well, looking for burnouts.
these are mouser.com parts numbers...
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I added the old transistor's lead layouts - luckily they are the same as the originals - be careful getting them in right...
jsarsfield
The Pioneer Pirate
Don't feel alone Bmac R259 on my sx-780 fried just like yours. I'll be following your thread and I'm hoping it will be very helpful in getting mine fixed. Mark is a Jedi at this s#$t.
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Hi Mark, I have replaced Q13 and Q14 with ksc2310ybu transistors. You recommended 512-ksc1845ebu but Mouser had them backordered and I had these onhand. I replaced Q15 and Q16 with 512-ksa992fbu which you recommended. I turned the unit on to check the voltages and the protection relay engaged. I hooked up my test speakers and i have sound!! The right channel sounds great but the left side has a high pitched buzz along with the sound. The buzz or squeal is not affected by the volume control. You can hear it with the volume all the way down. I replaced the module with a new one but this had no effect on the problem. It almost sounds as if ac is leaking through somewhere. here are my module voltages left side pin#1 -0.6v pin#2 -40v pin#3 0.1v pin #8 -.1v pin#9 39.9v pin#10 -1.2v right side module volts, pin#1 --0.9v pin#2 -40.1v pin#3 .2v pin#8 .2v pin#9 40.4v pin#10 1.5v dc offsets are.2v left and .2 v right. your help would be greatly appreciated. thanks Bill
mouser has thousands of 1845's in stock
512-KSC1845UBU
p - f - e - u are increasing gain suffixes,
bu means bulk as opposed to ta
ta means mounted on a cardboard tape
_q means non-RoHS compliant
Test the transistors you pulled, and the two you intend to replace, you should find two that test good of the four. Use them