Pioneer SX-950 burnt resistors and shorted outputs

Ignore all readings provided as I discovered + Fluke probe had an intermittent short. Since my post on the weekend I found a short on STV-4H and replaced it as well as a disconnected primary wire to one of the outputs which has also been repaired. DBT test now working fine with no smoke but no protection click either. Have retaken all readings with new probes for reference:

Output board readings:
Pin 1 -48.5v
pin 2 -48.5
pin 3 0
pin 4 & 5 34.9
pin 6 0
pin 7 & 8 48.5
pins 9-13 0
pins 14 & 15 -41.2
pins 16 & 17 -48.4
pin 18 0
pins 19 & 20 34.9
pin 21 0
pins 22 & 23 48.4
pin 24 0
pins 25 & 26 -4.2
pin 27 -2.3
pin 28 -4.2
pins 29 & 30 40.7
pin 31 -40.9
pin 32 -4.3
pin 33 -4.8
pin 34 -4.3
pin 35 -4.8
pin 36 40.8
pin 37 -2.3
pin 38 -3.2
pin 39 -2.3
pin 40 -3.2
pin 41 -40.6
pin 43 0
pin 44 -.6
pin 45 -.6
pin 46 40.6
pin 47 .1
pin 48 .6
pin 49 0
pin 50 .6
 
taineasy said:
Ignore all readings provided as I discovered + Fluke probe had an intermittent short. Since my post on the weekend I found a short on STV-4H and replaced it as well as a disconnected primary wire to one of the outputs which has also been repaired. DBT test now working fine with no smoke but no protection click either. Have retaken all readings with new probes for reference:

Output board readings:
Pin 1 -48.5v
pin 2 -48.5
pin 3 0
pin 4 & 5 34.9
pin 6 0
pin 7 & 8 48.5
pins 9-13 0
pins 14 & 15 -41.2
pins 16 & 17 -48.4
pin 18 0
pins 19 & 20 34.9
pin 21 0
pins 22 & 23 48.4
pin 24 0
pins 25 & 26 -4.2
pin 27 -2.3
pin 28 -4.2
pins 29 & 30 40.7
pin 31 -40.9
pin 32 -4.3
pin 33 -4.8
pin 34 -4.3
pin 35 -4.8
pin 36 40.8
pin 37 -2.3
pin 38 -3.2
pin 39 -2.3
pin 40 -3.2
pin 41 -40.6
pin 43 0
pin 44 -.6
pin 45 -.6
pin 46 40.6
pin 47 .1
pin 48 .6
pin 49 0
pin 50 .6
Click to expand...
Power supply voltages are all also all good. Would the bias differential cause my "no click" in the protection circuit?
 
I suggest you study very carefully how the protection circuit works. The service manual explains it very well. This will help you in this diagnosis.
 
I am usually on the phone reading these posts so putting voltage measurements down requires one to find a print out or get on the computer or I can’t respond. It’s helpful if you would markup the schematic and post the results to make it easier for me at least.
To comment, if one of the stv diode stack is shorted you should get no ops bias. I do not think I have heard of a stv being short, if so one heck of a lot of current went through it for that to happen or there is a short on the pcb causing it. Isolate and measure it.
 
Still trying to isolate the problem. What ever it is it took out both the new trim pots I installed on the right side. Both are now not working and will have to order a few more to continue. Tracing it back but can't find a faulty component that may be causing it. DC offset and bias on the left are set to factory specs with no issues.
 
That’s really strange that the trimpots that you replaced are now faulty. The dc offset or balance pot is used to setup a voltage divider. There should be little current going through it. Check the bjt that’s used in the dc offset circuit for proper type and orientation.
Something is causing excessive current in the voltage amp stage(vas) to take out the stv diode and the bias pot that is in series with it. Sometimes it’s defective components, components installed incorrectly, could be open or broken copper trace(s) or bad soldering etc.
Before you power up again, compare each component on both channels using the ohmmeter, the readings should be close to the same. Do not power up until both read the same.
 
Cannot get trim pot VR4 to trim out correctly. Have tried a couple and still the lowest it will go is 330mv. VR1 (0mv), 2(0mv) and 3(20mv) all trim in to spec.
 
Compare the dc voltage across r35 and r36 both being 270 ohm. Q5 and q6 are constant current generators. You can also compare the dc voltage spread across q9/11 bases with q6/8
 
Both r35 and r36 check out at 270 ohms. Unfortunately I'm not the man I used to be building and testing Heathkit stuff in the late 1960's early 1970's. My memory does not serve me as well as it did and worry about putting probes on the wrong leads. Where do I place the +/- probes on q6 (PNP right channel) and q8 (NPN right channel)
 
rcs16 said:
Compare the dc voltage across r35 and r36 both being 270 ohm. Q5 and q6 are constant current generators. You can also compare the dc voltage spread across q9/11 bases with q6/8
Click to expand...
Went thru some of my old transistor inventory and replaced the q6 and q8 with new ones. Somewhat better result. DC is now 0 on the right but I can only get bias down to 27mv (should be 20) if I turn the trim pot all the way to the right. Wondering if this is liveable or do I keep looking.
 
rcs16 said:
Compare the dc voltage across r35 and r36 both being 270 ohm. Q5 and q6 are constant current generators. You can also compare the dc voltage spread across q9/11 bases with q6/8
Click to expand...
Unit turns on and I am getting protection click after 5-6 seconds but can't get the bias trim pot to drop below 27 millivolts (turned all the way to the right) on the right side. am I safe at 27 mv when correct bias is 20mv?
 
OK my curiosity got the better of me and I am back on the sx-950. Installed new transistors including new dead-on matching complementary transistors and all of the small ceramic caps with nice high voltage mica's. Now I can set bias with no problem on both channels but the DC offset is motorboating on both channels and will not settle in. Isn't this fun?
 
You must log in or register to reply here.
Back
Top Bottom