SX-780 Q20 concern with no sound or relay click

Thank you all. Stk's pulled and 1K's put in their place. Q20 is back to what I tested as a good 2sc536a. Still no relay click, but I am retaking readings.

Q20
B = -18.2v
C = -54.1v
E = -26.6v

More to follow.

Q20 is off hard because the B-E junction is reverse biased. The only source of current to pull Q20B negative (to -40vdc) like it should be is through Q24. I suspect an open trace or a blown Q24. You indicated trace damage in that area from the past?

What are the CBE voltages on Q22? If it is blown or installed wrong it could pull Q20B positive.
 
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C310 = 7.38v across it. To ground the negative side is -27.1v and the positive side is -18.7v
R310 = 17.95k ohms
R311 = 558 ohms
R312 = 18.1k ohms

Q20
B = -18v
C = -55v
E = -27v

Q24
D = -18.8v
G = -55v
S = -55v

Q22
E = 0v
C = -18V
B = -18v
 
What is the voltage across C310?

C310 = 7.38v across it. To ground the negative side is -27.1v and the positive side is -18.7v <-positive side should be -0.6vdc*

R310 = 17.95k ohms <-schematic shows 12k Ω, did you mean R313, 18k Ω?
R311 = 558 ohms <-schematic shows 18k Ω, typo/swapped R312?
R312 = 18.1k ohms <-schematic shows 560 Ω, typo/swapped R311?

Q20
B = -18v
C = -55v
E = -27v

Q24
D = -18.8v
G = -55v
S = -55v

Q22
E = 0v
C = -18V
B = -18v

*Base of Q22 should be -0.6vdc, it appears to be open B-E or inserted wrong, or/and shorted B-C
Make sure there are no solder bridges, shorted or open traces in the area.
Q24 is an FET and is static sensitive, easily damaged without proper handling.

Verify R311 & R312 are correct, not swapped
Check/Replace Q20, Replace Q22, and if possible Q24.
You are in the right area, the voltage divider R323 and R324 provide the negative supply voltage sense (-6.9vdc) to the Protection chip pin 6. Until the voltage at Q20E is correct (~ -39.5vdc) the relay will not pull in.
 
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I just pulled Q22 from the pcb and did a continuity of the pins and found the collector and base are shorted. If there is not supposed to be continuity then the only 2 I have are toast. I have 2 others in working 780's but not pulling them. The pcb has no continuity between grommets. So pcb is okay in this area.. Do you test a bipolar transistor differently, as I checked for ohms and only got a reading between base and collector regardless negative and positive meter lead position? What is a quality replacement for 2SA904a? In addition, since the second one came out of a working receiver and I did not short it out with meter pins, then I fear something else may have toasted it. As a result, is there any further testing I can do until replacements arrive?
 
Just read your last reply Merlynsky and I simply pulled it from one unit and placed it in the other and no ground strap, so maybe I did fry both with poor handling, because early on in this chaos of fun I had pulled the original, attempted testing and reinstalled it. What is the best way to handle these?
 
Here is a link to Echowars' bipolar transistor testing: http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Just read your last reply Merlynsky and I simply pulled it from one unit and placed it in the other and no ground strap, so maybe I did fry both with poor handling, because early on in this chaos of fun I had pulled the original, attempted testing and reinstalled it. What is the best way to handle these?
You can toast an FET while testing it with a DVM, sadly,
You need a static ground strap for your wrist, have the chassis grounded, and/or a static conductive mat to work on. Look on the internet for static handling precautions, one link I found : http://machineryequipmentonline.com...ct-transistors-fetsmosfet-safety-precautions/

One of our other members suggested a current regulator diode from Mouser to replace the unobtainium 2SK34 jFET:

954-E-202

But they are currently out of stock :(
I will dig out a suggestion for the 2SA904 replacement, somebody here probably has it listed.
 
when testing fets you can manage to switch them on with your meter . so you might see a short that is supposed to be there .
 
*Base of Q22 should be -0.6vdc, it appears to be open B-E or inserted wrong, or/and shorted B-C
Make sure there are no solder bridges, shorted or open traces in the area.
Q24 is an FET and is static sensitive, easily damaged without proper handling.

Verify R311 & R312 are correct, not swapped
Check/Replace Q20, Replace Q22, and if possible Q24.
You are in the right area, the voltage divider R323 and R324 provide the negative supply voltage sense (-6.9vdc) to the Protection chip pin 6. Until the voltage at Q20E is correct (~ -39.5vdc) the relay will not pull in.

I just realized Merlynski added questions to the resistor readings post as I am new to the site, but thanks for asking. I pulling the resistors again to double check.
 
I had just typed C310, so I must have had 310 still on my mind when typing resistor number start and I just continued.

All are in proper placement on PCB and are in tolerance.

R311 is 17.95k ohms
R312 is 558 ohms
R313 is 18.1k ohms

I have new capacitors already on their way, but they were actually for the other receiver, but since I've already pulled other parts, what's one more I may need? However,I was informed this receiver with the Q20 issue was re-capped. I had pulled C310 earlier in the week and it measured in spec.
 
I had just typed C310, so I must have had 310 still on my mind when typing resistor number start and I just continued.

All are in proper placement on PCB and are in tolerance.

R311 is 17.95k ohms
R312 is 558 ohms
R313 is 18.1k ohms

I have new capacitors already on their way, but they were actually for the other receiver, but since I've already pulled other parts, what's one more I may need? However,I was informed this receiver with the Q20 issue was re-capped. I had pulled C310 earlier in the week and it measured in spec.

Those three resistors are the voltage divider so that the negative regulated voltage "mirrors" the positive regulated voltage.

'Recapping' means there may have been mistakes made in component placement. Do you know if it worked correctly after it was recapped but before you got it?
If not, the recap may have created a problem, i.e. reversed polarized caps, wrong parts, improperly replaced transistors, etc. More work for you . . . look closely for this possibility. Good fun for getting back into electronics :bigok::rolleyes:.
 
I am attempting to look it up, but I can't recall what type of capacitor is C313? I am also looking how to tell which direction it goes in. Last time I had one of these in my hands was 1986.
 
You are welcome! See you then :thumbsup:
Another static handling link:
http://www.informit.com/articles/article.aspx?p=21704&seqNum=3

I'm back! Good news. I have Q25., Q19, Q20, Q23, Q24, Q21and Q22 with all voltages correct. Replacing Q22 and 24 did the trick. I still have the 1k resistors in place of the STK's. Unfortunately, I still have no relay click;

PA3004
1. 13.5v
3. 0v should be .9v
4. -.9 v should be 0v. Very odd and confusing pin 3 and 4 are swapped in a negative form.
5. 0v
6. -7.1v
7. 0v
8. .5v

Q26
E = 0v
B = 0v should be .9 from pin 3
C = 51.4v

In addition to these concerns; the 1.4 and -1.4 voltages on the STK's and collectors of Q15, Q16, Q13, Q14, Q11 and Q12 are way off;

Left STK
1. -40.6v
2. -40.5v
3. -23.7v

7. 40.8v
8. -23.7v
9. 40.7v
0. -16.6v

Right STK
1. -39.8v
2. -40.2v
3. -40.2v

7. 40.3v
8. -40.2v
9. 40.3v
0. -33.5v

Collectors

Q15 = -30.4v
Q13 = 40v
Q11 = -40.1v

Q16 = -13.7v
Q14 = -40.6v
Q12 = -40.6v

Where should I start from here?
 
I'm back! Good news. I have Q25., Q19, Q20, Q23, Q24, Q21and Q22 with all voltages correct. Replacing Q22 and 24 did the trick. I still have the 1k resistors in place of the STK's. Unfortunately, I still have no relay click;

PA3004
1. 13.5v
3. 0v should be .9v
4. -.9 v should be 0v. Very odd and confusing pin 3 and 4 are swapped in a negative form.
5. 0v
6. -7.1v
7. 0v
8. .5v

Q26
E = 0v
B = 0v should be .9 from pin 3
C = 51.4v

In addition to these concerns; the 1.4 and -1.4 voltages on the STK's and collectors of Q15, Q16, Q13, Q14, Q11 and Q12 are way off;

Left STK
1. -40.6v
2. -40.5v
3. -23.7v

7. 40.8v
8. -23.7v
9. 40.7v
0. -16.6v

Right STK
1. -39.8v
2. -40.2v
3. -40.2v

7. 40.3v
8. -40.2v
9. 40.3v
0. -33.5v

Collectors

Q15 = -30.4v
Q13 = 40v
Q11 = -40.1v

Q16 = -13.7v
Q14 = -40.6v
Q12 = -40.6v

Where should I start from here?
Q13 is -40v
 
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