SX-950 Project

[Motnick]

I picked up an SX-950 a month or so back from a local guy that I have bought equipment from in the past. I was actually picking up a set of speakers from him for a friend and he mentioned that he had recently lost a channel in his SX-950. He didn't want to bother with having it repaired since he has other receivers that he could easily put in the SX-950s place, and he offered it to me as is. The overall condition was very nice so I grabbed it.

I have removed all of the outputs and tested them with the 6-way test. 2 of the 8 tested bad with the 6-way test. I can also see some resistors were cooked, and I expect that some transistors on the amp board may also be toast.

My plan is to completely rebuild the Power Amp board and replace all 8 outputs. I may also rebuild the power supply, but I was thinking of fixing the amp board first to see if I can get the unit working correctly.

By searching previous threads on the forum I have put together the following list for the Power Amp board. Can you guys please let me know if I'm on the right track with this? Also what is the usual cause of this type of failure? Anything else that I should be on the look out for as a cause of the blown channel? I certainly don't want to rebuild the entire amp board only to have the channel blow again because I overlooked something.


Power Amp Board: (AWH 050)

C1: 2.2uF 25v CSSA: 647-UKL1H2R2MDDANA 2.2uF 50v
C2: 2.2uF 25v CSSA: 647-UKL1H2R2MDDANA 2.2uF 50v
C3: 1uF 25v CSSA: 647-UPW1H010MDD 1uF 50v
C4: 1uF 25v CSSA: 647-UPW1H010MDD 1uF 50v
C5: 33uF 16v CEA: 647-UPW1H330MED 33uF 35v
C6: 33uF 16v CEA: 647-UPW1H330MED 33uF 35v
C11: 330uF 10v CEA: 647-UPW1E331MPD6TD 330uF 16v
C12: 330uF 10v CEA: 647-UPW1E331MPD6TD 330uF 16v

Q1: 2SA726: 512-KSA992FBU
Q2: 2SA726: 512-KSA992FBU
Q3: 2SA726: 512-KSA992FBU
Q4: 2SA726: 512-KSA992FBU
Q5: 2SA818: 512-KSA1381ESTU
Q6: 2SA818: 512-KSA1381ESTU
Q7: 2SC1628: 512-KSC3503DSTU
Q8: 2SC1628: 512-KSC3503DSTU
Q9: 2SD381: 512-KSC2073TU
Q10: 2SD381: 512-KSC2073TU
Q11: 2SB536: 512-KSA940TU
Q12: 2SB536: 512-KSA940TU

R37: 594-5083NW150R0J, 150ohm, 2watt
R38: 594-5083NW150R0J, 150ohm, 2watt
R39: 594-5083NW150R0J, 150ohm, 2watt
R40: 594-5083NW150R0J, 150ohm, 2watt
R43: 594-5083NW4R700J, 4.7ohm, 2watt
R44: 594-5083NW4R700J, 4.7ohm, 2watt
R45: 594-5083NW4R700J, 4.7ohm, 2watt
R46: 594-5083NW4R700J, 4.7ohm, 2watt
R47: 594-5083NW4R700J, 4.7ohm, 2watt
R48: 594-5083NW4R700J, 4.7ohm, 2watt
R49: 594-5083NW4R700J, 4.7ohm, 2watt
R50: 594-5083NW4R700J, 4.7ohm, 2watt
R51: 280-CR5-0.5-RC, 0.5ohm, 5watt
R52: 280-CR5-0.5-RC, 0.5ohm, 5watt
R53: 280-CR5-0.5-RC, 0.5ohm, 5watt
R54: 280-CR5-0.5-RC, 0.5ohm, 5watt
R55: 280-CR5-0.5-RC, 0.5ohm, 5watt
R56: 280-CR5-0.5-RC, 0.5ohm, 5watt
R57: 280-CR5-0.5-RC, 0.5ohm, 5watt
R58: 280-CR5-0.5-RC, 0.5ohm, 5watt

D1: 1S2473: 512-1N4148
D2: 1S2473: 512-1N4148
D3: 1S2473: 512-1N4148
D4: 1S2473: 512-1N4148
D7: 1S1885: 512-UF4004
D8: 1S1885: 512-UF4004
D9: 1S1885: 512-UF4004
D10: 1S1885: 512-UF4004

MJ21193G (x4) (output)
MJ21194G (x4) (output)

VR1: 10K ohm multi offset: 652-3266P-1-103LF VR1,2 offset 10k 12 turn - correct
VR2: 10K ohm multi offset: 652-3266P-1-103LF VR1,2 offset 10k 12 turn - correct
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

Output transistor Mica: (8 needed): 534-4662

 

[rcs16]

You can inspect/test those other bjts/resistors you seem to ?. can also run the amp (just as a test) with one set of o/p bjts/amp, to confirm that there is something else not right, suggest to power up on a dbt while do this troubleshooting.
I changed my o/p's years ago and still working okay so it not imperative to do a complete rebuild but of corse doing so would increase its reliability.

 

[markthefixer]

list looks ok, won't need all those resistors - predict to-220 drivers and 150 ohm emitter resistors are toast.
check the 0.5's with one leg lifted.

cause? overstressed? idle current trimpot opened up? 2sa726 failure?

we have a march availability date on the 512-uf4004 so use 833-UF4004-TP they have 12000 of THEM.

power up on dbt without outputs connected to check voltages safely after work.
The higher the DBT wattage, the closer the test voltages will be to normal.

100% one leg lifted resistor tests is your BEST insurance.

 

[w1jim]

Me - I’d also recap and replace the transistors on the phono EQ board. Easy / cheap to do and a common failure point.
BTW - great receiver, I’ve got one i resurrected in the bedroom.

 

[Motnick]

predict to-220 drivers and 150 ohm emitter resistors are toast.

I pulled all 6 transistors from the failed channel tonight. Q9 and Q11 both fail the 6-way test. R37 and R39 are both badly burnt. I’m sure they’re toast.

cause? overstressed? idle current trimpot opened up? 2sa726 failure?

I would have bet that at least one of the differential pair transistors would be bad, but they both pass the 6-way test. Hopefully I will find an open trim pot. It would be nice to find the smoking gun.

we have a march availability date on the 512-uf4004 so use 833-UF4004-TP they have 12000 of THEM.

I have a bag of twenty UF4004FSCT-ND that I bought from Digikey awhile back. Are these the same as Mouser 512-UF4004?

 

[w1jim]

I have a bag of twenty UF4004FSCT-ND that I bought from Digikey awhile back. Are these the same as Mouser 512-UF4004?

Same thing - on-semi diodes.
I’d replace the trim pots with multiturn Bourne’s parts.

 

[Motnick]

Me - I’d also recap and replace the transistors on the phono EQ board. Easy / cheap to do and a common failure point.
BTW - great receiver, I’ve got one i resurrected in the bedroom.

My plan for now is to get the unit working again as a first step. Once its working I will go through the other boards to complete the unit. This is a nice unit and well worth redoing completely.

 

[zebulon1]

Good plan
I had several 950's these last few months and all had main board issues on one channel. MTF has down, his advice is spot on.
You need to check the diodes but mine survived as well as the pre stage resistors. All I did was replace the transistors, trimmers and a few driver stage resistors.
Double check your work several times. The easier the set, the easier it is to make a mistake.

 

[SaturationPt]

<<snip>>The easier the set, the easier it is to make a mistake.

Ain't it the truth.

 

[Motnick]

Thanks for the tip. I am starting to get much more confident now that I have a couple of other units under my belt. This advice probably couldn’t be more timely! It’s a good reminder to continue to take things slow and double check everything before moving forward.

 

[Motnick]

cause? overstressed? idle current trimpot opened up? 2sa726 failure?

It looks to me like VR3 is the likely culprit here.

VR3 is measuring 135 ohms in the position it was set at. VR4 in the good channel is set very close to the same physical position but only measures 16 ohms.

I took VR3 out of the circuit and measured it. It will adjust down to 0 ohms, but it jumps from 0 to around 40 ohms as soon as you start adjusting it. If I adjust it all the way to the max it measures about 675 ohms.

 

[Motnick]

I completed the work on the failed power amp channel tonight. I found VR3 to have failed and both Q9 and Q11 were blown. The two 150 ohm resistors were also bad. R37 measured 1.25k ohms, and R39 measured 23 ohms.

All 4 diodes tested fine but I replaced them anyway.

The 0.5 ohm and 4.7 ohm resistors tested fine, but I replaced them all anyway since I had half way removed them in order to test them.

I powered up on the dbt and the amp does not have a short any longer. The protection relay even clicked in after a few seconds.

I plan to rebuild the other channel next, and then have a look at the PS voltages.

 

[markthefixer]

Congratulations!! A bright spot to a snowy day!!!

Such a pretty picture, I recognize quite a few familiar new components.

for reference: 71-CP5J-0.5

280-CR5-0.5-RC

 

[zebulon1]

I bet you had another big smile

 

[Motnick]

Since we are snowed in up here today in the midwest, I decided to spend some more time on the sx-950. I was able to complete the other channel of the power amp today. I powered up on the dbt with the outputs removed and all seemed well.

Voltages on the dbt (100watt bulb):

Pin 2 & 3 (5.4) 4.8
Pin 4 & 5 (13.9) 13.1
Pin 6 & 7 (51.5) 50.4
Pin 8, 9, & 10 (36.5) 36.0
Pin 11 (28) 27.6
Pin 13 (-13.5) -13.0
Pin 14 (-51.5) -50.2
Pin 15 (-19) -17.9

Since the voltages seemed alright, I went ahead and replaced all eight of the outputs with the new set that I had bought.

I would like to set the offset and bias next, but the manual has me a bit confused. The manual that I have states that the jumpers should be removed and 5.1k ohm resistors should be connected to the POWER IN jacks. Is this necessary? I have not had to do this with any of my other pioneer receivers.

 

[markthefixer]

5.1k's not necessary, nor are 8 ohm dummy loads. Just minimize the volume.

5.1k's are for magazine reviews and formal power / engineering tests for standard conditions.
8 ohm loads were for output capacitor amplifiers, modern 10 meg input resistance dmm's would read enough leakage voltage to give you a heart attack.

 

[Motnick]

Thank you Mark! That’s what I figured but I thought I’d stop and ask before blazing ahead.

Let me dial things in and I’ll report back.

 

[petehall347]

looks like the drivers killed the outputs but not the fault of the drivers as they were being told what to do .

 

[Motnick]

looks like the drivers killed the outputs but not the fault of the drivers as they were being told what to do .

It seems like good motivation to replace the bias and offset pots whenever you restore one of these units.

 

[Motnick]

I cleaned all of the pots and switches with deoxit, and I have dialed in the bias and offset without any issues at all. Everything seems to be working correctly with the power amp board now.

I’ve let the amp play a bit and I have discovered that the volume in the right channel is lower using FM stereo mode. I pulled the jumpers and used RCA cables to cross over the channels and the issue moved to the left. I think this means that the issue is not on the power amp board?

If I switch from stereo to mono while playing FM the issue goes away, but returns as soon as I flip the switch back to stereo. I do not have the issue using AM. I also hooked up a TT to Phono 1 and the issue is not there. Changing between stereo and mono using phono and AM works fine. The issue seems to only show up using FM and stereo.

Any suggestions on what to look for as a cause for this? I would like to fix this before moving forward with recap/restoration work.