Another SX838 help thread!

[Bill S]

Hi Gents,
Recapped about 5 years ago (all caps except filter caps) and good until recently. Protection kicks in and goes in and out every few seconds. After a while, it stays in protection until I leave it off and check voltages the next day. Some of the problem transistors replaced on the EQ board (Q1,2,3,4,5,6) Mic board (1&2) Power amp board (Q1,2,3,4,5,6) and protection board (Q6&7). I believe I changed Q3,4,5,6 on the control amp board as well. DC offset hard to set to zero - bounced around but if I recall I have it around 15-20 mv (hard to zero).

Power supply board voltages:
Pin 1. 38AC
2. 32AC
3. 32AC
4. 44 .8
5. -43
6. 0
7. -44
8. -14
9. -13
10. 21.5
11. 28
12. 31
13. 34.5
14. 12.9
15. 0
16. 0
17. 51.5
18. 40AC
19.40AC
20. 0
21. 7.6
22. 8.2AC
23. 8.3 AC
24. 6.4 AC

Protection board (this is where it gets wonky)
1. 0
2. -13.15
3. .001
4. 0
5. -17.5 mv
6. 16.8 mv
7. 15.5 mv
8. -15 mv
9. 34.4
10. 34
11. 34.4
12. .45mv

Voltages on protection board seem suspect to me. Unit was in protect when voltages were taken. I should add I burnished the contacts in the protection switch. I noticed a wire had broken off one of the pins when I pulled the switch off and I soldered it back on. I think it's in the right pin but I don't have a picture of one wired up to confirm. Your thoughts on what to check at this point. Thanks in advance.

 

[zebulon1]

The voltages look good.
One thing to re-check is pin 12. It should be read in AC volts, looking for 7.5vAC.
If you have the correct voltage. Something on the board is at fault.

How bad is the DC Balance fluctuating and is it on both amp boards?
Try removing the jumpers and recheck the Balance.

 

[Bill S]

Sorry - missed my note Pin 12 is AC. Voltage there is 8.18 AC
Transistors on PS board:
Q1= 35.5B 34.5E 51C
Q2= 13.9B 13.4E 29.1C
Q3= 14.4B 35E 14C

DC voltage balance adjustment bounces as follows:
R channel -4 to 1.0 mv. Starts higher, drifts lower and stabilizes between 1 & 2 mv.
Left channel - big DC inrush at start up to 430mv for 1 sec, drops to 300, then 80 then 4-5 mv and settles in between 1&2 mv. No difference with jumpers removed. Unit stayed in protection during this time. Looks like issue with left channel but after a while, it doesn't even come out of protection even with DC gone at terminals.

 

[zebulon1]

On the protection board, I would replace a transistor at a time starting at Q7 and work back.
Use the recommended equivalents listed here on AK
The protection is simular too the sx-1010.
There are troubleshooting procedures for the protection on the 1010's here on AK that MTF has posted. You can use that or start swapping out the transistors as mentioned above.
I'm out of town and have limited access to my notes.

 

[larryderouin]

Take a picture of the Protection switch (relay?????) and indicate the wire you soldered back on. Let's see what you did and indicate correct position of the wires.

 

[Bill S]

Take a picture of the Protection switch (relay?????) and indicate the wire you soldered back on. Let's see what you did and indicate correct position of the wires.

Hi Larry - thanks for weighing in. The wire that came loose from the protection switch when I removed it for burnishing was the green wire (top) in the picture which goes to pin 20 on the power amp board. It's soldered to the second pin down on the left side of the protection switch.

 

[larryderouin]

Green one looks ok. Redo the BRN WIRE on the right. Looks like it burnt and spearated in the covering. Could very well be your problem

 

[Bill S]

Green one looks ok. Redo the BRN WIRE on the right. Looks like it burnt and spearated in the covering. Could very well be your problem

Thanks Larry. BRN wire (it's really orange) clipped and resoldered. Still no go - doesn't come out of protection at all. I am going to repair a couple other burnt wires that are on the protection board and try it again tomorrow. Protect board did receive new caps and transistors back when I overhauled the unit about 6 years ago.

 

[Bill S]

Went through and checked wires, unit still going into protection. Obtained some voltages from protection board. Some readings wouldn't stabilize so I provided a range. Looks like I'll be replacing Q6 & Q7 at this time as problem appears to be with Q6. Any thoughts??
Q1 C 19.03 E -.017 B.016
Q2 C 19.04 E -.013 B.019
Q3 C 18.88 E-.654 B -12
Q4 C 18.85 E -.652 B-.012
Q5 C .49 to .56 (ranged) B 19.48 E. 14.20 to 15.50 (ranged)
Q6 C. .054 to .106 (ranged) B .556 to .396 (ranged) E 0v
Q7 C 34.67 E 0v B .015 to .022 (ranged)

 

[Bill S]

I replaced Q6 & 7 today and powered the unit on. It came out of protection as it normally should, long enough for me to set offset and idle. I got idle at 30vdc and offset on each channel was 1-3 mv. Then it went back into protect and cycled in and out. Voltage readings on Q6 & 7 vary as the unit goes in and out of protection but are similar to what I show in the post above. Any ideas for me as to what to try next??