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SX-939 worth the effort?

zebulon1 said:
So, your PS is back on line and up to specs?
The amps are back together and double checked for correctness?
The protection is working.
DBT is dim?
Did you check the DC Balance and idle adjustments using the DBT? Is the DC Balance and idles stable?
Remember you can't operate the receiver on the DBT but you can set the balance and check the idles for adjustment.
Once the idles are checked on the DBT, run the trimmers back for a minimum reading and reset them to specs when on house power. 25mV's is the correct setting.
If you have any questions ask now.
If all the above has a yes answer you can try it.
Maybe give us another rundown on the days work.
Click to expand...
Bob: Power amps are not done. I will get all transistors on Monday and will finish it.
As of now with both power amps disconnected and powered on DBT relay clicks. All the Pin voltages in PS, protection, and control amp are looking fine.
My question: If I finish one amp and connect it (other one remain disconnected) can I set the bias and DC offset of the finished side? Or I have to finish both amp and connect both them at the same time to set DC offset and bias?
 
Remember "Tickling the Dragon". Use the DBT, bring it up with the one side, and set the trimmers to spec. (BIAS to 25mv). If it's stable, BACK OFF THE BIAS to a very low level. You can leave the offset lone at this point. Don't touch it again until you prove out the functioning of the other amp, and then bring them BOTH UP at the same time . in stages. Offset to 0.0v (+ or -) and Bias to 25mv.
 
sanjib said:
Bob: Power amps are not done. I will get all transistors on Monday and will finish it.
As of now with both power amps disconnected and powered on DBT relay clicks. All the Pin voltages in PS, protection, and control amp are looking fine.
My question: If I finish one amp and connect it (other one remain disconnected) can I set the bias and DC offset of the finished side? Or I have to finish both amp and connect both them at the same time to set DC offset and bias?
Click to expand...
With both power amps disconnected can I connect something to auxiliary and listen through headphones?
 
larryderouin said:
Remember "Tickling the Dragon". Use the DBT, bring it up with the one side, and set the trimmers to spec. (BIAS to 25mv). If it's stable, BACK OFF THE BIAS to a very low level. You can leave the offset lone at this point. Don't touch it again until you prove out the functioning of the other amp, and then bring them BOTH UP at the same time . in stages. Offset to 0.0v (+ or -) and Bias to 25mv.
Click to expand...
I think I must wait until Monday to touch it again!!!!
 
No, because the headphones got thru the amp too. Plus Mr. MURPHY is beating down your door. The less you mess with it, the longer he beats his head on the wall.
 
I will get the output transistors today and after installing it I will post the update and wait for guidance. At the moment only right amp is connected. I took out 4 output transistors. All four tested good. But as Bob suggested I will replace those and then I will power it on DBT. Here is the list of my all transistors in right amp board so far
Q1 - 2SA726 - KSA992FBU
Q2 - 2SA726 - KSA992FBU
Q3 - 2SA726 - KSA992FBU
Q4 - 2SB528 - KSA940TU (I don't have KSA1220AYS)
Q5 - 2SC1451 - KSC3503DSTU
Q5 - 2SC1451 - KSC3503DSTU
Q7 - 2SD358 - KSC2073TU (Complementary to KSA940TU)
Q8 - 2SB528 - KSA940TU (I don't have KSA1220AYS)
Q9 - 2SC1448 - KSC2073TU
Q10 - 2SA740 - KSA940TU
Q11 - 2SC945 - KSC945GTA
Q12 - 2SA733 - KSC733GBU

Power supply and protection circuit voltages are looking good and DBT dim with relay clicking.
If you have any advice before putting new output transistors please let me know now.
 
I sent the 1220's
Make sure you have the PNP and NPN's in the correct locations.
Refer to the service manual.
Double check all the wire wraps for connection.
If it passes the DBT, go for it.
Remember the trimmers. Idles to min.
 
UPDATE NEED HELP!!

I installed 4 new output transistors. Order of transistors NPN, NPN, PNP, PNP from left to right viewing from back. Checked the orientation of all transistors and I think they are correct. Powered on DBT with 85W bulb. Bulb dim and relay clicked. Put the red lead of VMM on 14 and black on 16 voltage was 0.1 mV but couldn't change the reading by rotating the trimmer. Then I checked voltages at all pins of right power amp (only this one is connected now). I got weird values (red lead on pin and black on chasis ground). Please look below

AWR-032-0-D
Pin 1 = -41.1V
Pin 2= -41.1V
Pin 3 = -55.2 V
Pin 4 = 54.7V
Pin 5 = 0
Pin 6 = 0
Pin 7 = -55.2V
Pin 8= 43.9V
Pin 9 = 43.9V
Pin 10 = -42.2V
Pin 11 = -43.9V
Pin 12 = -43.9V
Pin 13 = -41.6V
Pin 14 = -38,8V
Pin 15 = -41.1v
Pin 16 = -41v
Pin 17 = -41.1
Pin 18 = 0
Pin 19 = -41.1v
Pin 20 = -41.1v

What I should do next?


 
UPDATE 2 NEED HELP!!
I was trying to measure the voltages across emitter, base and collector of different transistors in the amp board. Looking in to schematic I found the collector and base of Q4 were both connected to positive rail. I measured the voltage at emitter with reference to chassis ground (black lead on chassis) got 54.5V and red lead on base got 53.7V which is very similar to what is in SM. But when I measure voltage at collector I got -42.1V which should be 1.8V. My first question my measurement method is correct or not?
After that I pulled out the Q4 and my transistor tester showing it as diode. So I replaced it with new one and after replacing it DBT stays bright! I have no clue what is happening here!!
So really in need of help!
 
If the collector of Q4 disconnected from circuit it passes DBT and relay clicking!!! I hope this info will be helpful.
 
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My next step is to finish the left board and connect it and then check it. It may need both boards to be connected to PS in order to work!
 
Double check pin layout and actual implementation on the board. You may have 2 of the leads crossed on Q4.
 
This thread changed to an SX-1010 around 100 posts ago didn't it? (just making sure I'm following it correctly)
 
Right amp done DC balance and idle current can be set!! Q1 and Q4 wrongly placed!!! How many times I will do these mistakes???
Decided to wait for right replacements of Q4 (KSA1220), Q7(KSC2690) and Q8(KSA1220) and then finish both amp.
 
sanjib said:
Right amp done DC balance and idle current can be set!! Q1 and Q4 wrongly placed!!! How many times I will do these mistakes???
Click to expand...

Please take your time. I spend more time checking my work than the actual work.
This is the kind of thing makes this crazy thread more like torture than help.
Now don't get the wrong idea. Just make sure you double check everything. A lot of work goes into a repair and every time you mess with it you risk damaging something else. Remember I said that once before!
 
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zebulon1 said:
Please take your time. I spend more time checking my work than the actual work.
This is the kind of thing makes this crazy thread more like torture than help.
Now don't get the wrong idea. Just make sure you double check everything. A lot of work goes into a repair and every time you mess with it you risk damaging something else. Remember I said that once before!
Click to expand...
Bob, yes you are absolutely correct! At last it is singing!!!:music: I have used KSA940 at Q4, Q8 and KSC2073 at Q7 whereas it is recommended to use KSA1220 and KSC2690. Amplifier is sounding good with these replacement. But it is not buttoned up yet. Should I use those KSA1220 and KSC2690 when I get them or it is fine to go with installed KSA940 and KSC2073? Regarding specifications they are very much same but general opinion KSA1220 is faster than KSA940. How much of that criteria is important in this case?
Please let me know.
 
I would swap them out but with all the work you put in getting this far, I'm not sure.
Only if you promise to abide by all the cautions we mentioned and the experience you gained through your own fault.
That's all I'll say about this matter in post# 159.
Take a break from it for a few days.
Oh - Good news you got it running!!!!
Your a good sport.
 
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