dfarrington
Member
I am going to pull R259 and R260 and check them. I may have blown them while working to fix Q22 and Q24. Will check tomorrow night.
SX-780 Q20 concern with no sound or relay click
- Thread starter dfarrington
- Start date
I am going to pull R259 and R260 and check them. I may have blown them while working to fix Q22 and Q24. Will check tomorrow night.
merlynski
Curmudgeon Electronicist
I would like to suggest narrowing the focus and working on one channel at a time, methodically, step-by-step, from left to right. There have been so many parts changes and measurements recorded it is getting impossible (for me, anyway) to keep track of them all.
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petehall347
the brandy coffee man
late to the party again . just checking if power supply voltages are all good right now ?
merlynski
Curmudgeon Electronicist
I believe so, and it also looks like the Protection circuit is working correctly with the current inputs. We are seeing major (different) DC offset from both channels. Still have 1KΩ in place of STKs.
petehall347
the brandy coffee man
might be worth checking if the 1k resistors are in the correct places . something to rule out .
dfarrington
Member
in
pins 1 and 3; 0 and 8 on both STK's
petehall347
the brandy coffee man
0.22 ohm resistors in tolerance ?
petehall347
the brandy coffee man
for sanity sake they are numbered like this .
dfarrington
Member
I will do this tomorrow night. just desoldered .22 ohm and soldering in another set. Measures are .3 to .4 ohms on each resistor. I will solder back in tomorrow and then check if I can adjust to 17.1vdc. Yes all the same as you state for Q5. I had just remeasured Q5 before pulling the .22 ohm resistors. it wont be until tomorrow night.
merlynski
Curmudgeon Electronicist
OK. Note that when measuring very low resistances the leads contribute to the reading you get. You can compensate by touching the probes together and noting that reading, then measure the resistor, subtract the lead resistance you noted before, then the result gives you a closer, more accurate measurement of the very small resistor.
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dfarrington
Member
I can only get it to -.6v. I can adjust to -17v but not positive 17v
dfarrington
Member
I set it at -17.1v
merlynski
Curmudgeon Electronicist
OK, that is good, and it adjusts up and down. The schematic I have shows an arrow to that point labeled "-17.1V". Is the common emitter on Q7 +0.6vdc as marked on schematic? Or are the voltages still as you previously reported:
Q7
B = -1.7v right side on schematic?
C = -38.5v
E = -1.1v <<==
C = -39.2v
B = 0v left side on schematic?
dfarrington
Member
merlynski
Curmudgeon Electronicist
Should be +0.6vdc, let's eliminate the feedback path. Remove Q11 and lift one end of R255. Then power up again and recheck the voltages on Q7 and Q9. Without the feedback they should balance out. This will verify the transistors and other components in the differential amp (Q7) and the current mirror (Q9).
dfarrington
Member
Okay, I am sorry and would love to do more tonight, but I have to get some sleep. I will start again tomorrow night and should be able to stay up later. Will do then and as always, thanks for the help.
merlynski
Curmudgeon Electronicist
, you're welcome and goodnight 
3
You're going to drive yourself crazy at this arm. The -1.7 and -1.1 is because of the negative voltage on the output. Lifting the feedback will only make measurements more impossible.
You're going to drive yourself crazy at this arm. The -1.7 and -1.1 is because of the negative voltage on the output. Lifting the feedback will only make measurements more impossible.
merlynski
Curmudgeon Electronicist
We eliminated the STK by removing it and putting in the 1kΩ resistors. Lifting the feedback will verify that the diff amp and current mirror are not the problem, unless they are not in balance without the feedback, which will then verify that the problem is in the diff amp and current mirror.
We have been going in circles (pun intended) so breaking the loop will help. A feedback loop is difficult to troubleshoot because it is a loop. Each stage of a servo has a response to its input that can only be seen by breaking the loop. The shotgun method has not resolved this, the OP has changed (or checked) transistors, checked (or changed) resistors so I am trying another method.
We know the input is good because there is 0vdc on the input base of Q5 and Q7. And the varying of VR5 varies the current through Q5 as the voltage at the junction of Vr5 and R237 changes when VR5 is adjusted. Thus, since the left side b-e junction of Q7 is reverse biased, and the right side b-e junction is forward biased, the current that imbalances the Q7/Q9 circuit is coming through R255 (seems most likely to me) if Q7 and Q9 are good.
