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SX 950 power board issues, and issues down stream

R

Rathwine

Member
Got an SX 950, and when I plugged it in, I had dimming and brightening of the lighting, as well as popping noise on the A speaker output. I found a blown cap on the EQ amp board, and recapped and replaced all transistors on that board. Before I went further I checked the numbers on the power board, after hearing that you have to start there for these units. Here are the numbers. Most are good from what I can see, however one is way out. I have purchased all recommended caps/transistors for the whole reciever as at some point I want to replace them all. However, I would like to fix the problem first, then tackle each board on it's own so that I do not cause problems that are new, without fixing the existing problem. Pin 15 is way out. These are all DC measurements. If I need to change to AC I can do it. Please let me know what you think.

2 &3=5.2v
4 & 5=13.5
6 & 7=50.4
8-9-10=35.9
11=27.7
12=0
13= -14
14= -54.1
15=5.1 (yes this one is positive, not negative)

Please let me know your thoughts. I purchased my replacements from another large thread that included a full parts list for caps and transistors. I have not purchased any of the Zener Diodes.

Thanks a ton for your help. Let me know if you want any other testing. Thanks Troy.
 
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I would first check that all fuses and output transistors are good, then restore the power supply #1 caps,#2 transistors etc. , check it with a DBT after caps then after transistors etc.. . I don't agree with "fixing the problem first" I know a lot of people do. I like to get the old weak caps out first ,turning the unit on/ off looking for the problem could create more
 
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Q6 is definitely out of spec. I will pull the board tomorrow and remove it, and the c23. I will test those out of board, and just plan on replacing them since I have replacements. I have tested all of my new transistors and caps already.

Should I remove the resistors and test them also?

Thanks a ton. I have all of the caps and transistors to rebuild the power supply board. I can do that whenever.
 
I am definitely in the 'fix it first' group.
Was the blown cap on the EQ board C18? It is directly connected to the -19vdc supply that is now bad. If Q6 failed first it may have taken out that cap.
While Q6 is out measure dc volts on the base pad for Q6 with power on, it should be close to -20vdc.
Before you reinstall Q6, with power off measure the resistance of R19, 150Ω. This resistor is a ballast for Q6, if Q6 is damaged R19 was probably stressed.
Replace C23 when you replace Q6. If it was C18 on the EQ board, and that supply is positive (pin 15) it has reverse biased c18 and may have damaged it again. I would also replace it again before powering up after Q6 is replaced
 
It was Q7 on the eq board that blew. I can test the cap that came out of c18. I can even remove the new c18 and test it also.

I will test the pad for q6 base while out and report. I will also test r19 with the power out while q6 is out.

I will report what I find.

On a side note, I have not had it powered on much since fixing the eq board. I have not turned on any speakers and only tested with cheap headphones. What are the chances I have damaged c18? I am happy to replace it if so, just want to try and narrow down parts for the next order and have them come at one time.

Thanks for all of the patience and help.
 
Rathwine said:
It was Q7 on the eq board that blew. I can test the cap that came out of c18. I can even remove the new c18 and test it also.

I will test the pad for q6 base while out and report. I will also test r19 with the power out while q6 is out.

I will report what I find.

On a side note, I have not had it powered on much since fixing the eq board. I have not turned on any speakers and only tested with cheap headphones. What are the chances I have damaged c18? I am happy to replace it if so, just want to try and narrow down parts for the next order and have them come at one time.

Thanks for all of the patience and help.
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I don't see a Q7 on the EQ board, AWF-011. ??
Any time a polarized cap (electrolytic) has a reversed dc voltage applied to it, it can receive damage, more time in reverse damages it more, but I do not (personally) ever trust a polarized cap that had reverse voltage applied. For the small price it cost to replace, why risk putting a unit back in service with a potential known failure. YMMV.
 
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Sorry typo. C7 blew on the eq amp board. Did not find any of the transistors as damaged and all tested ok on my tester out of the board.

I will add that to my list to replace. I would rather be safe. I do not wish to do this again too soon.

Thanks.
 
Typos happen :rolleyes:. Describe how C7 'blew', please. Did it literally come apart? Do you have a photo? You said you already replaced all the transistors and caps on the EQ board?
 
The Q-6 on the power board tested as a diode from pin 2-3. Did not show anything on pin 1. Obviously it had died.

Resistance on R-19 while Q-6 is out is 152 ohms which should be good.

R-18 only shows 13.2K ohms resistance but I am not sure if it is because it is still in the board. One end is attached to C-23 and R-17 which may be altering my read.

I will clean up, make sure that board is isolated and test the pad for Q-6 with power while using a DBT for power.
 
merlynski said:
Has this unit ever worked correctly for you? Is there evidence someone worked on it before?
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No it has not worked for me. I do think someone has been in here before as I found a cut leg from another cap while I was cleaning, and I saw someone soldered a ground wire twist on the flat amp board. 15910456731205884173156609621319.jpg
 
merlynski said:
Typos happen :rolleyes:. Describe how C7 'blew', please. Did it literally come apart? Do you have a photo? You said you already replaced all the transistors and caps on the EQ board?
Click to expand...
I posted the pic above for the blown cap. When I got this and checked bias, it was not far off, so I tested the unit. Started getting a popping noise so I powered down and started checking. When I saw I had a blown cap on the eq board I went ahead and ordered all of the parts for a recap and replacing the needed transistors. I just finished that and tested the power with the DBT attached and when I saw power was way off on pin 15 I reached out here. I did not want to do more damage.
 
Rathwine said:
I posted the pic above for the blown cap. When I got this and checked bias, it was not far off, so I tested the unit. Started getting a popping noise so I powered down and started checking. When I saw I had a blown cap on the eq board I went ahead and ordered all of the parts for a recap and replacing the needed transistors. I just finished that and tested the power with the DBT attached and when I saw power was way off on pin 15 I reached out here. I did not want to do more damage.
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Good on you for the dbt use!

I was expecting to see an exploded capacitor, caused by a large current leak or overload. That looks like a leaky seal oozed some electrolyte. Good call to replace it, but it relieves me a bit to see something small.

Rathwine said:
Just tested Q-6 base with power on and DBT in place and got -17.6VDC which seems right for having the DBT in place.
Click to expand...
What does R19 measure? If it is 150Ω replace Q6 and the caps. What voltage do you have on AWR-101 pin 15 now?
 
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