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SX950 restoration issue

J

jboerger3

Member
I appreciate everyone’s input on this forum and I have learned a lot from reading about other people’s issues. I was inspired to take on my first project, of this type, knowing there was a lot of knowledgeable people on the forum. I am restoring an SX950 that I bought new in 1977. It worked when last turned on - 5+ years ago and the only thing that wasn’t original is a fuse that was replaced in 1978. As I have read about others working on SX-x50 series receivers, the initial inspection of my unit indicated that I was, likely, starting with a unit that was in better shape than most. I hadn’t used it much since purchasing an AV receiver 20 years ago. No bulging caps, no evidence of smoke or anything else. And the exterior is mint.

In the last couple of weeks I replaced all the caps and transistors in the power board, the protection board, the phono board, the flat amp and tone boards. I also replaced all the diodes, including the two zeners, on the power board as well as the main (22,000mF) filter caps. I deoxited the pots and switches and replaced the power cord. WIMA film caps went in for the 2.2mF and smaller caps. Nichicon PWs in the power lines and KT and KL’s in the signal path. All are rated at the same or higher voltage and/or temperature when compared to the oem caps. I purchased most of these components, in kits from Peace.Love.and.Music, from Milwaukee, and he included good instructions. After all that, the 100w bulb dimmed, but the unit didn’t come out of protection. I plugged the unit in directly and was very happy when the protection relay clicked. Now on to the power amp.

I replaced the twelve transistors, the eight electrolytics, and the four trimpots (multi-turns). KSA992s for the A726, KSA1381 for the A818, KSC3503 for the C1628, KSC2073 for the D381 and KSA940 for the B536. The 992’s went on the board backwards compared to the original 726s and the rest of the transistors went in the same way the old ones came out. The only issue that I had working on the amp board was that the legs of the KSC3503 were too wide to fit through the board so I used a 1mm hand drill to ream out the holes. No visible damage to the pads or traces.

At that point, I shudda tried it on the dim bulb - when I finished the board, but, I wanted to test the output transistors. I mucked up the first mica trying to remove it and although the other 7 are intact, I replaced all 8 of them with pink Keratherm insulators (https://www.amazon.com/gp/product/B08HZF342L/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1). Using my brand new $15 component tester, the gain was 77, 55, 45, 70 for the 2SB557-R, and 39, 42, 47, 32 for the 2SD427. I remounted the board and heatsink, and installed (carefully) the thermo-diodes (a great bit of advice from this forum).

But, the bulb didn’t dim this time and the lights in the unit didn’t light up at all. I pulled and verified that all the fuses are good. So I am seeking guidance on where to start. Am I wrong to think the problem is likely in the power amp? Could the Keratherm insulators be causing a short? Should I order replacement output transistors - given the range of differences in gain?
 
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jboerger3 said:
But, the bulb didn’t dim this time and the lights in the unit didn’t light up at all. I pulled and verified that all the fuses are good. So I am seeking guidance on where to start. Am I wrong to think the problem is likely in the power amp? Could the Keratherm insulators be causing a short? Should I order replacement output transistors - given the range of differences in gain?
Click to expand...

The outputs were fine when you tested them no need to replace because of gain.
Now if the insulators you used are shorting out then it could damage the outputs. Easy to tell if they are shorting out measure from the outside of case of the transistor to the heatsink
make sure you get a good connection on the heatsink if it is shorted you will have 0 ohms from the case to the heat sink

Verify all the transistors are installed correctly you mentioned that the 992s went in "backwards" this would be the case if the pin orientation is different between the two transistors.
Did you verify each transistor pin orientation when you installed the new ones?

You say the bulb does not dim and you do not get any lights either?
 
Thanks Goat. No continuity from outputs to the heatsink. I dug the aligator clip through the paint and confirmed I had good contact. And good to know the outputs are ok. I will have to "unwrap" the heatsink and board, tomorrow, to check the orientation of the 992's as I can`t read the board. The 100w bulb does not dim and no lights on the unit. I measured 15v at the bus at the end of the power cord, which makes sense (115-100).
 
merlynski said:
:lurk:
Click to expand...
Step in I just answered an easy question he had, you are much better at troubleshooting these over the internet.
I have a hard time asking the right questions. Like in this case I know we have an issue and it was working before he worked on the main amp board
sooo it was be something he did when working on this board.

Where to start......

What do you mean unwrap the heatsink?
Where are you measuring and what are you measuring ACV regarding this 15v?

I want to verify power supply voltages even though this is not likely an issue.
Measure between ground and the following pins

On the power supply board

Pin 1 SB 0 vdc
Pin 2 SB 5.4 vdc
Pin 3 SB 5.4 vdc
Pin 4 SB 13.9 vdc
Pin 5 SB 13.9 vdc
Pin 6 SB 51.5 vdc
Pin 7 SB 51.5 vdc
Pin 8 SB 36.5 vdc
Pin 9 SB 36.5 vdc
Pin 10 SB 36.5 vdc
Pin 11 SB 28.0 vdc
Pin 12 SB 0 vdc
Pin 13 SB -13.5 vdc
Pin 14 SB -51.5 vdc
Pin 15 SB -19 vdc

Pin 22 7.5 vac
Pin 23 7.5 vac
Pin 29 7.5 vac

Also if you can get a good image of the amp board showing the transistors I can verify pin orientation.
I am going to lookup your replacements as you specified in the first post
 
Verify the installation of the transistors

2SA726 BCE ---- KSA992 ECB Install reversed

2SA818 EBC ---- KSA1381 ECB Must Rotate too match pins
2SC1628 EBC ---- KSC3503 ECB Must Rotate too match pins

2SD381 BCE ---- KSC2073 BCE
2SB536 BCE ---- KSA940 BCE
 
goat67 said:
Verify the installation of the transistors

2SA726 BCE ---- KSA992 ECB Install reversed

2SA818 EBC ---- KSA1381 ECB Must Rotate too match pins
2SC1628 EBC ---- KSC3503 ECB Must Rotate too match pins

2SD381 BCE ---- KSC2073 BCE
2SB536 BCE ---- KSA940 BCE
Click to expand...

I fear that I did not rotate the 1381 and 940 properly, but cannot read the board when it is mounted in the chassis. I have attached pics of the power amp board as well as the power supply, protection and phono boards. When I say "unwrapping the heat sink" I mean detaching it and the board from the chasis. If the pics are good enough for someone to be able to determine if the orientation is correct or incorrect, without "unwrapping" - that would be nice. But, I will likely have to unwrap to fix anyway. The 15v is measured at the end of the power cord - inside the chasis, where it is soldered to the buss's. This is when powered through the 100w dim bulb. Working on this while I am cooking Sunday dinner.

If I do the measurements at the power board, I will want to do that while using the dim bulb, won't I?



P1090713.JPG P1090714.JPG P1090715.JPG P1090716.JPG P1090717.JPG P1090718.JPG P1090719.JPG P1090720.JPG P1090721.JPG
 
My fears were well-placed as those medium-sized transistors were oriented incorrectly. I pulled two of the four and the two are still operating. I will get back to it later today.
 
I could not tell for sure from the pictures. Don't reinstall/reuse any that have been powered up while installed incorrectly.
No point in measuring power supplies at this time. The DBT is doing its job correctly, but there is a large overload if the AC input is choked down that hard.
Pull the output transistors off the heatsink, but keep track of which one was where. We can do most of the troubleshooting with them out. Turn the bias adjust pots to minimum resistance (VR3 & VR4) on the power amp pcb, AWH-050.
Recheck all the transistors you changed for correct orientation. When discussing parts please use the designations, like Q9, Q11, C3, etc.
Then. when you are ready power up on the DBT.
 
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Yes Q7 Q8 wrong for sure I can not make out the others. But I bet Q5 and Q6 are also wrong. The board has the markings for the transistors now that you have the pin outs it is just a mater of putting the leads in the right holes. You do now how to read the markings on the board? The long straight line is the Base the line with the arrow is the Emitter and the last one is the Collector. The base is the common junction for the emitter and collector, if the arrow is Not Pointing iN it is a NPN

After you get the transistors all in right try and see if the bulb dims.. and You can take the voltage reading from the power supply board when on the DBT

Where are you putting your DMM leads when measuring the 15v? I do not think you are reading the ac main. And it is AC?
 
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The transistors on the equalizer board are positioned wrong, they should all face the same way on one side and also facing the other way on the other side. Replacements are E-C-B
 
These may be useful:

transistor_images.png PNP, Pointing iN Pointer; NPN, Not Pointing iN pointer.

For testing purposes on the Diode test range the red lead goes where the red line is marked, black with the black, and the expected reading is between the lines. Six tests, both polarities, for each pair of leads:
DiodeTest.JPG

EDIT: more info
 
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jboerger3 said:
My fears were well-placed as those medium-sized transistors were oriented incorrectly. I pulled two of the four and the two are still operating. I will get back to it later today.
Click to expand...

Q5-Q8 were all incorrectly installed. I pulled and tested them and reinstalled correctly. I also verified that Q1-Q4 were also correctly installed. The dim bulb dimmed, but did not come out of protection. I measured 47v ac at the buss, inside the chassis, where the power cord is attached. That was versus 15v ac when the bulb did not dim. Tested voltage at the Power board pins as follows:
Pin 1 SB 0 vdc 0
Pin 2 SB 5.4 vdc 6.0
Pin 3 SB 5.4 vdc 6.0
Pin 4 SB 13.9 vdc 0.1
Pin 5 SB 13.9 vdc 0.1
Pin 6 SB 51.5 vdc 15.5
Pin 7 SB 51.5 vdc 15.5
Pin 8 SB 36.5 vdc 10.8
Pin 9 SB 36.5 vdc 10.8
Pin 10 SB 36.5 vdc 10.8
Pin 11 SB 28.0 vdc 8.0
Pin 12 SB 0 vdc 0
Pin 13 SB -13.5 vdc -10.4
Pin 14 SB -51.5 vdc -15.9
Pin 15 SB -19 vdc -5.6

Pin 22 7.5 vac 0
Pin 23 7.5 vac 0
Pin 29 7.5 vac 0

I went through the pins 4 times and the above were from the last time. I noted that the voltages changed nearly every time on all pins except 1, 12, and 22-24. When first measured, pins 2 & 3 started at 5.2 and slowly declined as i probed. Pins 4 & 5 measured 4.3 the first time and 0.1 the last time.
 
merlynski said:
I could not tell for sure from the pictures. Don't reinstall/reuse any that have been powered up while installed incorrectly.
No point in measuring power supplies at this time. The DBT is doing its job correctly, but there is a large overload if the AC input is choked down that hard.
Pull the output transistors off the heatsink, but keep track of which one was where. We can do most of the troubleshooting with them out. Turn the bias adjust pots to minimum resistance (VR3 & VR4) on the power amp pcb, AWH-050.
Recheck all the transistors you changed for correct orientation. When discussing parts please use the designations, like Q9, Q11, C3, etc.
Then. when you are ready power up on the DBT.
Click to expand...

Thank you merlynski, but I reinstalled them before I saw your post. I did turn down the trimmer pots by several revolutions. I took the time to check all the small transistors on the power supply, protection and phono boards that had a change in orientation too. On the phono board, the 2SA725's (Q1 and Q2) were replaced with A992s and they were reversed in orientation. On the protection board, the 2SC869 (Q1 and Q2) were replaced with KSC1845 that were reversed in orientation. The only change in orientation on the power board was Q10 where the 2SC869 was replaced with a reversed KSC2383.
 
These are the trimmers that are in the BOM.
VR1: 10K ohm multi offset: 652-3296P-1-103LF 10K ohm 25 turn
VR2: 10K ohm multi offset: 652-3296P-1-103LF 10K ohm 25 turn
VR3: 100 ohm single bias: 652-3386H-1-101LF 100 ohm single turn
VR4: 100 ohm single bias: 652-3386H-1-101LF 100 ohm single turn

I looks like you used the 10K for VR3 and VR4?
I have a bad feeling about the outputs.

Did you install new filter caps with the screw terminals? If so you could remove those +- feeds and power up on the DBT. This would eliminate the outputs as a short.

I know you you have transistor polarity issues your sorting but if the muli-turn 10KΩ trimmers were at 5000Ω (centered from the factory) even the limited current on the DBT may of done some damage.
 
I would replace with new all of the transistors that you installed backwards. Verify pin orientation is correct. Just because they "test" ok does not mean they are ok under operating conditions.
What value of trimmer did you use for VR3 and VR4?
Remove the outputs from the heatsinks keep track of where they came from ie when you reinstall put them back where they came from.

Once you get all the transistors installed correctly lets recheck the power supply voltages.
 
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