SA-8100 protection circuit issue

Left q5 base out. Pulled d5, Q4 C=+2.2V. Left q5 base and d5 out, pulled d6, Q4 C=+1.1V

Both times the amp came out of protection.
 
get voltages right across r13 r14 r18 .. 3 separate readings .
measure them on the legs rather than the solder pads .
 
R13: 0.7V
R14: 33V
R18: 0.3V

I take it R14 voltage should be much lower. Should R13 be higher?
 
r14 could well be shot . try a new one ..if you dont have one try swapping r13 and r14 .
at this stage i would expect them both to read about same voltage across them .
 
I would think 15k is a common resistor, so I should be able to get my hands on a new one tomorrow at work.
 
I can't see an obvious problem, but it may also be worth re-flowing all the solder joints in that area, maybe with a bit more flux - and double checking there's no problems on that side of the board while you're there (it's hard to follow the traces etc. from a photo)....
 
if that doesn't fix it we need to start looking at q4 . even though you changed it .
i need to look at r13 voltage too as a quick look says its not correct . maybe bad solder or board crack .
 
q3 collector 18.6 . plus 0.7v is not even close to 35v .
r14 might be warm if you touch it .
voltages from pin 16 to r13 and r14 would be helpful . one side of each resistor should read 0v .
 
petehall347 said:
q3 collector 18.6 . plus 0.7v is not even close to 35v .
r14 might be warm if you touch it .
voltages from pin 16 to r13 and r14 would be helpful . one side of each resistor should read 0v .
Click to expand...
Pin 16 to R13: 0.3 to 0.1V (varies a lot)
Pin 16 to R14: 33.3V
R14 did not feel warm.

For reference, pin 16 measures 34.1v to ground.

What should the q3 collector voltage be? The schematic seems to indicate 20V.

Edit: pin 16 to ground voltage corrected.
 
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if that is both sides r14 there is a break in the board .
try from r13 to r14 voltage .. if not zero on pin 16 side the break is there .
1.9v to account for but might just be one of those things . was just noting this for reference .
am thinking we are close now.
 
If it helps at all, I have a fully working SA-8100. I rebuilt the protection board on that so if you want a reference voltage or what parts were used for which transistors etc, I can provide that?
 
No, sorry, I was not clear. R13 and 14 register 0V on the side connected to pin 16 by the board trace. The voltages I gave you are measured on the resistor sides connected to the respective collectors. No break in board.
 
FYI, I ran a simulation on this circuit earlier today and here are all of the calculated voltages and currents under normal conditions. This is based on a pin 16 voltage of +35 VDC.

upload_2017-8-9_20-36-37.png
 
I also checked continuity between the solder pools on the foil side and the connecting wires on the opposite side for R13, 14, 18. All connections are good, so I don't see a bad solder joint on any of those.
 
KeithD said:
Will also reflow the solder on the resistor joints.
Click to expand...

While you are reflowing, you should also go over the solder connections for Q3 and Q4. They look like they could use a little tidying up based on your photo.
 
LesE said:
FYI, I ran a simulation on this circuit earlier today and here are all of the calculated voltages and currents under normal conditions. This is based on a pin 16 voltage of +35 VDC.

View attachment 982801
Click to expand...
Nice, so the values I have for Q3 are pretty close to your values. It's q4 E and C that are off.

Q5 other than base (which is currently disconnected) are correct. Q6 and 7 are good.

I'm a novice at this, but there must be something wrong with R14.
 
Yes, Q3 is looking better but I recall that Q3 was the problem yesterday. That's why I suggest the reflow of Q3 & Q4.
 
LesE said:
Yes, Q3 is looking better but I recall that Q3 was the problem yesterday. That's why I suggest the reflow of Q3 & Q4.
Click to expand...
Checked continuity between solder pads and respective legs of each transistor. All was good, but I did reflow all the solder joints anyway to make sure there were no gaps between the transistor legs and the board holes.

Also, just to be clear, D5 and 6 are disconnected. Also, base of Q5.
 
I believe that D5 was lifted to isolate Q1 & Q2 which didn't indicate a problem. Having D6 & D7 lifted shouldn't have a significant effect on Q3 & Q4 collector voltages. Both collector voltages should be balanced at around 18 to 19V.
 
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