SX-980 missing negative dc voltage

[coreycgate]

I rebuilt a non working sx-980 power supply a few months ago. It was working great and decided to stop. I am missing the negative dc voltages from pin 25, 27, and 28. I replaced q3, q4 and d9 without luck. My background is IT, so this is a new leaning experience for me. I did check R8 and have -72v. Here is list of some voltages I checked. I am looking for guidance, and any help is appricated.

Pins:
16: 13.1
18: 7.6
20: 34.8
22: 51.8
24: 23.2
25: 0
27: 0
28: 0

Q1-
E: 52.0
B: 51.5
C: 69.1

Q2-
E: 14.4
B: 15.0
C: 52.0

Q3:
E: 1.2
B: 120mv
C: 440 mv

Q4:
E: 1.1
B: 1.1
C: 500 mv

Q5:
E: 13
B: 13.8
C: 24.4

 

[soccerfan]

Hope you have a DBT (Dim bulb tester). I am still learning but looks like R8 is open (measure and replace) as Q4 collector is missing the voltage. Measure R6, R7. Check the zener D9 as well

Edit - List the replacement part numbers as well that you used

 

[mrk229]

I was just going to suggest the same thing as @soccerfan . I bet you only checked one end of R8 for voltage. (blue box)

Also the collector of Q3 has no voltage suggesting one or both of the 1.5 k resistors is open. If resistors have failed the question to ask is why?

 

[coreycgate]

I was just going to suggest the same thing as @soccerfan . I bet you only checked one end of R8 for voltage. (blue box)

Also the collector of Q3 has no voltage suggesting one or both of the 1.5 k resistors is open. If resistors have failed the question to ask is why?


View attachment 3306998

Can you suggest a part number for R6, R7 and R8?

 

[mrk229]

Can you suggest a part number for R6, R7 and R8?

MFR-25FTE52-1K5 | DigiKey Electronics

Order today, ships today. MFR-25FTE52-1K5 – 1.5 kOhms ±1% 0.25W, 1/4W Through Hole Resistor Axial Metal Film from YAGEO. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.com

MFR-25FTE52-6R8 | DigiKey Electronics

Order today, ships today. MFR-25FTE52-6R8 – 6.8 Ohms ±1% 0.25W, 1/4W Through Hole Resistor Axial Metal Film from YAGEO. Pricing and Availability on millions of electronic components from Digi-Key Electronics.

www.digikey.com

 

[Colleton]

I had a similar issue with my 980. Both R3 and R8 were open. markthefixer helped with the diagnosis, those resistors function like fuses when transistors or diodes in the PS short out. Hopefully I got that right. At any rate, the point I was getting around to is, as others have also asked, what made the resistors pop?

In my case I rebuilt the PS board, all caps, transistors and diodes, then replaced R3 and R8. You can buy or make a dim bulb tester with a 100w incandescent bulb, then replace the resistors and power the receiver on using the dim bulb tester to limit the current flow and see if the bulb is bright or dim. Bright means a short, dim means it's probably okay. There are videos on how to use a dim bulb tester on youtube.

Good luck!