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SX950 restoration issue

Q3 is better as a KSC1845, for its gain and speed, if you have them (OOS most everywhere), if you don't have them KSC2383 is OK (that one is OOS at Mouser now too).
Q10 is better as a KSC2383, for its higher current capability. You should replace Q9 also, with KSC2073.
One those are done check to see if you have 5.4vdc on pins 2 & 3.
 
Check dc offset for adjustability, leave it at zero vdc on DBT.
Do the bias setting on DBT to see if it adjusts/increases, just as a circuit function test. The set them back to minimum. <-very important.
If both adjustments work on both channels, you can go off the DBT and recheck the dc offset, set bias, recheck offset, let it warm up, just sitting there, for 15 minutes or so, the recheck bias. Bias and offset will wander some, that is normal. Once it is settled I will play some music to warm it up a bit more, then recheck bias (music off).
 
I powered on with the dim bulb to go through the offset and bias procedure, on the power amp, as instructed. Again, I had upgraded the trimmer pots to multi-turns. Adjusting VR1 did nothing to change the measured voltage on pins 9&10, which, varied between 4mv and 5mv. Not a good start, but, I had previously watched Mark’s (Blueglow on youtube) video on the procedure and knew 5mv was not terrible (he was only able to get to 9mv on a standard trimmer pot). So, I moved on to VR2 and was able to get a steady 0mv. I was able to get VR3 and VR4 to a steady 20mv after warm up for several minutes. So I turned down VR3 & VR4, took it off the dim bulb, powered it up and was able to get the same offset and bias settings as I had when on the dim bulb. I checked my power supply voltages and got the following:


Pin 1 SB 0 vdc 0
Pin 2 SB 5.4 vdc 6.2
Pin 3 SB 5.4 vdc 6.2
Pin 4 SB 13.9 vdc 13.32
Pin 5 SB 13.9 vdc 13.32

Pin 6 SB 51.5 vdc 49.48
Pin 7 SB 51.5 vdc 49.48
Pin 8 SB 36.5 vdc 35.55
Pin 9 SB 36.5 vdc 35.55
Pin 10 SB 36.5 vdc 35.56
Pin 11 SB 28.0 vdc 26.81
Pin 12 SB 0 vdc 0
Pin 13 SB -13.5 vdc -13.51
Pin 14 SB -51.5 vdc -51.77
Pin 15 SB -19 vdc -19.25

Pin 22 7.5 vac 7.29
Pin 23 7.5 vac 7.28
Pin 29 7.5 vac 7.24


So I’m feeling pretty good about the project status. I unplug the echo dot and speakers - from my shop unit and plug them into the sx950. I turn it on and it sounds real good - on the right channel. Nothing on the left. I switch source inputs and then speaker outputs left to right. I check all three sets of speaker outputs. Switch the source to AM and then FM. Flip back and forth between stereo and mono. Rotate the balance switch back and forth (I’m happy that all the switches and pots are quiet.) But, the left channel remains silent. I unplug source and speakers thinking that I have just an issue with the left channel of the power amp.


I notice that my speaker lamps are not lit as I switch them. And the source lamps are out too. The stereo indicator light works on FM. I hear the protection relay click open so I shut down the unit. I turn it on again and the protection relay doesn’t click. I reinsert the dim bulb and the bulb dims at power up. I go back to the power supply board and get the following readings:


Pin 1 SB 0 vdc 0
Pin 2 SB 5.4 vdc 0.013
Pin 3 SB 5.4 vdc 0.013
Pin 4 SB 13.9 vdc 0
Pin 5 SB 13.9 vdc 0
Pin 6 SB 51.5 vdc 3.25
Pin 7 SB 51.5 vdc 3.25
Pin 8 SB 36.5 vdc 1.79
Pin 9 SB 36.5 vdc 1.79
Pin 10 SB 36.5 vdc 1.79

Pin 11 SB 28.0 vdc 9.58
Pin 12 SB 0 vdc 0
Pin 13 SB -13.5 vdc -13.41
Pin 14 SB -51.5 vdc -6.32
Pin 15 SB -19 vdc -1.99

Pin 22 7.5 vac 7.29
Pin 23 7.5 vac 7.28
Pin 29 7.5 vac 7.24


I pulled and verified that all the fuses are good. I have a lot of emotional history with this receiver and will keep at it until it works properly. I have a stock of replacement transistors for everything but the outputs and the KSA940’s that are in Q11 and Q12 of the power amp board and Q2 of power supply board. Where do I go from here?
 
jboerger3 said:
6.3v d/c on each of pins 2&3! I did replace both Q9 and Q10 with KSC2073 and KSC2383 respectively.
Click to expand...
6,3v is/was high for the +5.4vdc of pins 2 and 3. My suspicion is one of them is installed wrong. This could mess up the entire +v side of the regulator. Check fuses FU4 and FU5. If they are blown pull Q9 and Q10 and test them.
Power up while they are out and see if the other + voltages come up. If not check the dcv on the cathode of ZD11, sb about +14vdc.
 
Pulling and checking each fuse was the last thing I did before posting today at 3. But, I just verified that FU4 and FU5 are good. Q10's 2383 is installed as an ECB. Q9's 2073 is installed as a BCE (it can only go in one way as it is heatsink mounted). I powered on the DBT and measured only 0.086v dc at ZD 1's cathode and 0v dc at ZD 2.
 
Dang, Fuzzy schematic, 61.5 is good voltages. Looks like the primary +v regulator is blown.
Replace Q8 with KSC2073
Replace Q1 with KSC2073
Replace Q3 with KSC1845 if you have one, otherwise KSC2383
Replace ZD11 with 512-1N5244B, 14v zener diode
Replace them all at one time, without powering up between changes. Close dc coupled devices, like in this circuit, tend to take each other out, in a circular failure if you do them one at a time.
 
I replaced Q1 and Q8 with KSC2073. Replaced Q3 with KSC1845. Replaced Z1 with 1N5244B. I powered up under dim bulb and the protection relay did not click. Power supply board voltages:


Pin 1 SB 0 vdc 0
Pin 2 SB 5.4 vdc 6.3
Pin 3 SB 5.4 vdc 6.3

Pin 4 SB 13.9 vdc 13.0
Pin 5 SB 13.9 vdc 13.0
Pin 6 SB 51.5 vdc 48.45

Pin 7 SB 51.5 vdc 48.45
Pin 8 SB 36.5 vdc 35.05
Pin 9 SB 36.5 vdc 35.05
Pin 10 SB 36.5 vdc 35.05
Pin 11 SB 28.0 vdc 27.0
Pin 12 SB 0 vdc 0
Pin 13 SB -13.5 vdc -0.75
Pin 14 SB -51.5 vdc -5.84
Pin 15 SB -19 vdc -1.31

Pin 22 7.5 vac 6.67
Pin 23 7.5 vac 6.67
Pin 29 7.5 vac 6.63


A few observations:

I measured 21.2v dc at Z1’s cathode.

When I went back through the above list, voltage on pins 2 and 3 had dropped, and were swinging back and forth between 2.6 and 5.2 v dc. Could not get a steady reading as the unit warmed up.

I appreciate the concept of false economy in replacing components and understand that transistors might test ok, but, not function ok. No problem throwing them out. I tested the four removed components and the three transistors tested ok. But the tester thinks the removed zener is now a resistor.

Although my unit appears to be pristine, a couple of the standoffs, finally, broke on the power supply board (ordered 100 from Amazon yesterday). And I have two pads that are gone and the traces lifted - on the power supply board (Q1 and Q8). I bent the legs of the new transistors down toward the traces and did my best to get a good solder there. My iron technique is improving, but, you were absolutely correct about how delicate these old boards are.
 
If pins 2 and 3 are not stable there is another problem, and the +v regulator may blow up again. Have you replaced C14 and C13?
The other + dc voltages look OK, are they stable, or do they move around with pins 2 and 3? I think I would replace Q9 with KSC2073 and Q10 with KSC2383.
Looks like the -v supplies are bad now also. Measure dcv on the anode of ZD12, sb -13vdc. If it is bad, Q2 and Q4 are probably also bad. Again this is a place to change all three at the same time. You can use a TIP42C, in a power supply regulator, for Q2 if you do not have a KSA940. Q4 should be a KSA992.
 
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I just powered up on the DB. Pins 2&3 start at 9v and head south from there. The other pins appear to be stable. All electrolytic's and blue blobs have been replaced (outside of the tuner). Zener 12 is -0.76 going in and 0 going out. I can replace that. I can replace Q9, Q10, and Q4 too. I do not have a replacement for Q2. I will replace the others and pull and test the KSA940 in Q2. If it tests ok, I will power on the DB. If it does not test ok, I will test and put the oem 2SB507 back in and power up while I am waiting for an order of A940's. Does that sound like a plan?
 
That sounds like a plan to me. KSA940 is OOS at Mouser 'till 3/24, mine are on back-order. You can use a TIP42C (511-TIP42C), they have stock on those.
I have seen recommendation (I just remembered) that a 512-2N5551TF works better in place of Q10 than a KSC2383 on lamp circuit ripple killers. If you are placing an order get some of those.
 
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I installed a new Z12. The one that I pulled was cooked. Q4 got a new A1013, Q10 a new KSC2383 (that’s what I have) and Q9 a KSC2073. I did the diode check on the legs of Q2 (A940 that I installed a couple of weeks ago) and it appears to be working. I powered on with the DB and the protection relay clicked.


Pins 2&3 are still all over the place. One moment they are at 9v dc and declining and the next they are below a volt and climbing. I see that pins 4&5 are high. The rest of the readings look ok. I usually go through the pins three or four times when I start it up the unit. I only did it twice when I saw that 4&5 were so high. Pins 6-11 move a little as the unit warms up. Pins 13-15 don’t move a bit so that side of the buss seems solid.


Pin 1 SB 0 vdc 0
Pin 2 SB 5.4 vdc see above
Pin 3 SB 5.4 vdc see above
Pin 4 SB 13.9 vdc 21.0
Pin 5 SB 13.9 vdc 21.0

Pin 6 SB 51.5 vdc 48.5
Pin 7 SB 51.5 vdc 48.5
Pin 8 SB 36.5 vdc 34.4
Pin 9 SB 36.5 vdc 34.4
Pin 10 SB 36.5 vdc 34.4
Pin 11 SB 28.0 vdc 26.5
Pin 12 SB 0 vdc 0
Pin 13 SB -13.5 vdc -13.1
Pin 14 SB -51.5 vdc -50.1
Pin 15 SB -19 vdc -18.6

Pin 22 7.5 vac 6.97
Pin 23 7.5 vac 6.97
Pin 29 7.5 vac 6.95


At this point, I’ve replaced three of the four heat sinked transistors (Q1, 8 & 9) and all three of the ones that I pulled tested ok. I think all the smaller transistors pulled test ok too. Might I have a resistor or diode problem upstream of Q8, 9 and 10? What is killing the zeners?


I do have some A940’s on the way, but, won’t have them till, likely, the weekend. As always, I really appreciate your help.
 
Excess current kills Zeners. That just about has to come from the pass transistor and the control transistor to get to the zener.

Is the voltage on the input side of the Q8 and Q9 regulators stable? That's the + side of C17.
Is the voltage at pins 4 and 5 stable? That comes from the same source as the pins 2 and 3, rectifiers D9 and D10.
 
Monitor the dcv at the + side of C14. That should be completely stable there. Q9 and Q10 form a capacitance multiplier to kill ripple on their output. Somehow you are getting the opposite effect. Did you change any bulbs to LEDs on the pins 2 & 3 circuit? (AM, FM, Phonos, Aux, Stereo lamp).
The voltage reference comes from ZD11 cathode through voltage divider R21/R22, but the current that drives the load comes from the 23.8vdc supply through R20.

EDIT: more
 
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