Pioneer SX-1980 help needed

rcaman

rcaman

cornbread george
Ok so i rarely ask for help on here. I have a sx-1980 in for repair this has had previous repairs done to it the problem is the left channel it's burning resistors r57 and r59. the outputs are disconnected and they are good it has a brand new power supply which is putting the correct voltages out. i have replaced all the transistors on the left board checked and rechecked the pin outs and they are correct. here is what i have replaced q1,q11,q17,q19,q23,q34 with ksc1845. q3,q5,q7,q21 with ksa992 . q13,q25 with ksc3503
q15,q27 with ksa1381 q29 with mje15032 q31 with mje15033 . d1 ,d19 with uf4007 d1,d17,d13 with 14138.'
r57 and r59 with 100 ohm 1 watt. they start to burn at about 70 volts on the variac with the outputs disconnected. i have checked for cracked traces i cannot find anything wrong. also replace vr3 with 100 ohm and vr1 with a 200k ohm and both are centered. the board is a gwh-109a. all and any help very much wanted.
 
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As I usually recommend, with power off do a nodal ohmmeter comparison of the two amp boards to identify any differences as a place to start. Better to check this way before applying juice.

Good luck
 
I have already done that and the replacement transistors are all top quality from mouser.
 
vr3 should start out at zero ohms for minimum idle current!!! You are fortunate that the output transistors were not connected.

Now, assuming that vr3 is dialled back to zero ohms and the resistors still burn:

The string of these components below are VITAL, if there is a break in it anywhere, the drivers get turned FULL ON and massive damage with plenty of smoke occurs.
YOUR SMOKE - R57 & R59
To that end, I do ohmmeter tests from COMPONENT LEAD to COMPONENT LEAD (thus traces and solder joints ALSO get tested as a matter of course)

Let's say "a thermal diode's lead breaks off.
Between D5 cathode to D7 anode
OR
D7 cathode to VR3 bottom
is now an open circuit. BLOOIE!!!

+80v regulated
R37 (100 ohms) sensing element of 7 mA current source
Q15 emitter of 7 mA current source
Q15 collector of 7 mA current source
D3 anode (the arrow side of the diode)
D3 cathode (the pointy end of the diode with the band)
D5 anode
D5 cathode
D7 anode STV-4H diode on heat sink
D7 cathode STV-4H diode on heat sink
VR3 "bottom" idle current trimmer
VR3 "top" & wiper idle current trimmer
D9 anode
D9 cathode
Q13 collector voltage amplifier stage
Q13 emitter voltage amplifier stage
R35 (220 ohms)
-80v regulated

Once this circuit is confirmed, the next step

The D3 anode to D5 cathode +1.8v drives the positive part of the push pull circuit
and
the D9 cathode to Q13 collector -1.8v drives the negative part of the push pull circuit
These are critical as they run the show - including idle current. That's what Vr3 is for...

Looking objectively, Q15 is a current source and calculations indicate it should be sourcing 7 Milliamps from the collector.
0.7v across R37 (100 ohms). (80v - 78.7) - 0.6v (D1) = 0.7v then divided by 100 ohms is 7 milliamps.
The 7ma goes through that string you checked, ending up in Q13.

Q13 is the voltage amplifier stage in the circuit, and somewhere further forward
there is a feedback point that the system will try to keep at 0.000 volts.
Which then splits the 3.6v across D5, D7 & VR3 into +1.8v and -1.8v .


What would I do?
pull q17 q19 q21 q23 current limiters
pull q29 q31 drivers that are melting R57 & R59

substitute for r55 1000 ohms 1/2 watt a
pair of 470 ohm resistors and at the center
between the two resistors, that point becomes
my new feedback point. Connected to "A".

Start debugging voltages to get +1.8v and -1.8v at the appropriate point.
 
Thanks for your response i will do as you say. i,m sick with something tested negative for covid but was admitted to the hospital thursday with a 104.9 fever. Got up Thursday got ready for work and started feeling bad to worse don't remember getting back in bed with my clothes on. Went to the hospital via ambulance i have never been so sick so quick in my life. came home the same night the doctor says covid I'm going in the morning for another test. I will let you know the test on the pioneer turns out i will check connection from component to component. i think i have already done this but will do again this is number 3 on a SX-1980 rebuilds i have done. i do have an extra left amp board i can rebuild but i do want this one repaired .
 
You concentrate on getting yourself back to health. The beast is not a forgiving adversary.

I don't have any particular rush.... neither should you. Without power on, the beast can sit for a while.
 
What’s the odds of both STV4H being bad ? Both are bad in this sx-1980 replacement of them with 4 1n4148 cures my problems. Now what to permanently replace them with. Help Mark
 
The 1N4148's will work but you need to get them into a form factor that you can pot with epoxy or JB Weld and mount on the heatsink. I recommend putting them next to each other, connected like batteries in series in a battery compartment (to give you a visual image), pot them with epoxy and allow to dry, then pot them into a ring terminal for 10 or 12 AWG wire.
AlternateImage_8846_isimageThomasBettsa184_sk1%24datasheet_200%24product_image_idproduct_image_0-native.png
 
Your STV-4H's are most likely NOT bad. Most DMM's don't push enough voltage across them to light them up.

STV-3H 1.80v @ 7mA
STV-4H 2.35v @ 7mA

Their specification is 2.35v at 7 mA. Most meters with diode test give up the ghost at 1.999v

Please Do not rush into precipitous action as I am attempting to gather together a cautionary and exploratory post!!!


Merylinski posts a test circuit. I'm trying to FIND it.

I developed a 7mA current source that I have NOT had time to check my array of diodes against.
Putting out forest fires like this precludes me from a plan of posts for prevention while dealing with the demands of real life. It's really frustrating.

ALL BELOW Quoted from merlynski
merlynski said:
STV diodes cannot be checked with most meters on their diode or ohms range, as there is not enough voltage drive to forward bias the junctions.
Click to expand...
You can build a simple test jig and use a dc voltage range on your meter to test them:

upload_2021-8-11_19-38-9.png


One nine volt battery and a 1.2KΩ resistor (not a critical value) should work, voltage on a good STV3 will be about 1.2vdc.
Marked leads on diodes usually indicate the cathode.

I put heatsink compound on STVs when remounting them, but lots of people believe it is not necessary (It satisfies my OCD perfectionism). The STVs job is to be warmed up by the heatsink to control the bias, and it is a very slow acting circuit, anyway.
 
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i didn't check them with a multimeter i pulled them and put the string of 1n4148 in there and the problem or the resistors burning stopped. i have done nothing further and the outputs are still disconnected i will dig further this weekend. thanks everyone for the great information.
 
So if the STV-4H had been installed backwards in that channel, you would have had very little to clue you into the problem.

It has been my experience so far, that other than the obvious lead breakage issues, little seems to harm them.
Who knows, that might change. :dunno:
 
markthefixer said:
So if the STV-4H had been installed backwards in that channel, you would have had very little to clue you into the problem.

It has been my experience so far, that other than the obvious lead breakage issues, little seems to harm them.
Who knows, that might change. :dunno:
Click to expand...
I don't think I've ever seen a bad one either, just broken leads.
 
I am going to check them tonight with the 9 volt battery and resistor. Will let you know the outcome i have never replaced them either or found a bad one.
 
Well Mark you were 100% correct. Both bias diodes were in backwards. Installed the the correct way and the smoke stays in. No more burning resistors going to do some voltage checks and connect the outputs and using a varic and dim bulb. Going to see if I can get some music out of her. I do hear the relays click on the power supply at about 90 volts. Forgot to add this has been worked on before it came to me straightening out previous work can be a chore.
 
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Glad the unit is yielding to your care. :)

I learned one thing with this, my marking of repaired STV diode leads is going to be a LOT clearer.
 
Well the SX-1980 is alive and making great music. Thanks everyone for all the helpful advice. Going to go through the other boards and recap. The ONLY problem I have now is the fm is nothing but rushing noise . Absolutely no fm
 
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