Pioneer SX-780 Problems Need Help Please

[Young46]

Hello all, a friend of mine dropped off a SX-780 that was in protection and ask if I could fix it for him. Having repaired/restored I think 3 of these before, I agreed. I powered up with the DBT and sure enough it had all the signs of a blown STK0050 module. I had a good spare and installed it. DBT power up again and it was better but in protection mode. I started looking at components around the PA3004 chip and went ahead and replaced the electrolytic caps. I checked the voltages on the chip and decided to pull the PA3004 and do quick test which revealed it might be the culprit. I failed to record the voltages, sorry. After some research I was able to get information for the new replacement board for the 3004 from some fellow Ak'ers. I also pulled Q26 and it checked ok. After receiving the new replacement board and components I built and installed it. DBT power up was good so on to full power. Full power, few seconds and relay click! Good, but then after a few more seconds another click, the RCh VU meter jumps up a bit and stops. Low power on the RCh during playback. I shut it off. Fiddled with the volume knob and it played fine on both channels. It would do the same when fiddling with the speaker buttons like leaving the button pushed in (A on) while powering up. I cleaned the pots and switches. Still no fix. The DCO won't adjust either and it is getting 4+ volts on the RCh, and 4mv on the left. Any help is appreciated.

 

[markthefixer]

Q12, Q14, Q16. check R260 200 ohms, R262 100 ohms.

that is, as long as Q6, Q8 & Q10 are behaving.

4v is a valid protect reaction.

q11,12 2sc1775a ksc1845
q13,14 2sc1885 (hot? ~ 0.12w) ksc2383
q15,16 2sa912 (hot? ~ 0.12w) ksa1013

 

[Young46]

S

Q12, Q14, Q16. check R260 200 ohms, R262 100 ohms.

that is, as long as Q6, Q8 & Q10 are behaving.

4v is a valid protect reaction.

q11,12 2sc1775a ksc1845
q13,14 2sc1885 (hot? ~ 0.12w) ksc2383
q15,16 2sa912 (hot? ~ 0.12w) ksa1013

Sorry, doctor appointment today kept me from making much progress today. R260 supposed to be 200ohms but tested 150ohms. R262 tested ok. Mark are you saying to check for q13-16 to be hot during operation? Not quite understanding the ~ (around) 0.12w comment. 0.12v? Where is this measured? Are you listing the equivalent transistors for the originals?

 

[markthefixer]

replacements, just do em if you have em.

Q13 thru Q16 get hot - I was calculating what wattage each was shedding, and what the transistors are supposed to handle
they are larger than normal, but transistors do better when cool.

R260 - color codes? could be factory change.
https://www.digikey.com/en/resource...ors/conversion-calculator-resistor-color-code

You MAY be able to spot a bad transistor, when the emitter to base voltage is different than the expected 0.6v,
pos 0.6v for npn,
neg 0.6v for pnp

 

[Young46]

replacements, just do em if you have em.

Q13 thru Q16 get hot - I was calculating what wattage each was shedding, and what the transistors are supposed to handle
they are larger than normal, but transistors do better when cool.

R260 - color codes? could be factory change.
https://www.digikey.com/en/resource...ors/conversion-calculator-resistor-color-code

You MAY be able to spot a bad transistor, when the emitter to base voltage is different than the expected 0.6v,
pos 0.6v for npn,
neg 0.6v for pnp

Thanks, I understand now. VR5 and VR6 I pulled to try getting some contact cleaner into them. Neither would actually adjust to 0 ohms but they are real sensitive when the adjustment gets to around 25 ohms. Do you recommend changing them for new bournes?

 

[markthefixer]

Thanks, I understand now. VR5 and VR6 I pulled to try getting some contact cleaner into them. Neither would actually adjust to 0 ohms but they are real sensitive when the adjustment gets to around 25 ohms. Do you recommend changing them for new bournes?

while multi-turn trim pots are nice on DC offset adjusts, they are not mandatory.
DC offset adjust on the sx-780 is 150k ohms, so these adjustments are not in 0 ohm territory.
If these opened up, it wouldn't be good, but NOT a disaster.


"0 ohm" territory is with 100 ohm trim pots in series with thermal bias diodes. These have the potential for disaster.

and, after contact cleaner, you ARE relubricating the pot, right?

 

[Young46]

while multi-turn trim pots are nice on DC offset adjusts, they are not mandatory.
DC offset adjust on the sx-780 is 150k ohms, so these adjustments are not in 0 ohm territory.
If these opened up, it wouldn't be good, but NOT a disaster.


"0 ohm" territory is with 100 ohm trim pots in series with thermal bias diodes. These have the potential for disaster.

and, after contact cleaner, you ARE relubricating the pot, right?

Yes, I use Fader Lube after the contact cleaner.
Thanks for the explanation of the trim pots.

 

[12344321]

C1384 next to the relay can be damaged from the surge of a blown STK, and its also the one that powers the relay. Easy enough to replace it with a 2690 but you may need to slightly enlarge the hole. I just did this to a unit as a means to prolong the lifespan of rhe unit.

 

[Young46]

C1384 next to the relay can be damaged from the surge of a blown STK, and its also the one that powers the relay. Easy enough to replace it with a 2690 but you may need to slightly enlarge the hole. I just did this to a unit as a means to prolong the lifespan of rhe unit.

Thanks, Q26 could be damaged I suppose. I did test it and it tested ok.

 

[Young46]

I had the replacements and installed them.

R260, color code revealed a 150 ohm resistor as you suggest, a production change.

Q6, Q8, Q10 are close to behaving. Q10 emitter 39.78vdc, Q8 base 16.2mvdc. R236 17.3mvdc. Protection still acting the same. I can put DMM to pos leg of Q16, turn on the unit and it stays in protection. More work to do.

 

[Young46]

This thing is acting really weird. It still goes in and out of protection on the right channel. Today I was moving it around on the bench and it came out of protection on both channels. It briefly stayed out of protection and for a brief period played strong on both channels. I had test speakers hooked up but then the relay started clicking on and off. I could fiddle with the am, and fm antenna connections on the back and the relay would kick in and out. Grounding issue?

 

[larryderouin]

Board(s) are flexing too much Loosing connection or ground. Cracked pc board. Check ALL the boards, every inch of all boards for a crack across the traces. If you find cracks impinging on the traces, scrape traces back from both sides of crack. lay a cap or resistor scrap lead across the break, and full length solder. Do this for all joints. Old cap leads or old resistor leads of sufficient length. Ohm out any ground, tighten or solder the ground to chassis to get <1ohm,

 

[Young46]

Board(s) are flexing too much Loosing connection or ground. Cracked pc board. Check ALL the boards, every inch of all boards for a crack across the traces. If you find cracks impinging on the traces, scrape traces back from both sides of crack. lay a cap or resistor scrap lead across the break, and full length solder. Do this for all joints. Old cap leads or old resistor leads of sufficient length. Ohm out any ground, tighten or solder the ground to chassis to get <1ohm,

I once had a Nikko that exhibited the same behavior and it did turn out to be a cracked board and I repaired it just as you are describing. I’ll go over it tomorrow and hope I can spot something. Thanks

 

[Young46]

I went over the boards today and then went over them again. Looking for any cracks I didn’t find any and I looked very closely. Sometimes it will start up and after a few seconds the relay clicks. Other times it won’t. I can turn it on lying flat and after a few seconds the relay clicks and the Rch Vu meter jumps up to 1w and stays there. It works on both channels and the Rch Vu meter reacts to volume increases but will not return to 0 when turned down. After I turn it onto it’s side from lying flat it will stay in protection. I can touch various spots inside and outside the unit and sometimes it will come out of protection. It is not consistent. I reflowed a whole bunch of solder joints. Now it’s in protection and wont come out. I may go out there in the morning and turn it on and it might come out of protection. Driving me crazy. No single point to pin it on.

 

[Young46]

replacements, just do em if you have em.

Q13 thru Q16 get hot - I was calculating what wattage each was shedding, and what the transistors are supposed to handle
they are larger than normal, but transistors do better when cool.

R260 - color codes? could be factory change.
https://www.digikey.com/en/resource...ors/conversion-calculator-resistor-color-code

You MAY be able to spot a bad transistor, when the emitter to base voltage is different than the expected 0.6v,
pos 0.6v for npn,
neg 0.6v for pnp

Mark, I had the replacement transistors and changed them out.
R260 color code revealed 150ohms, not 200 as per the SM.

Transistors that haven’t been changed all seem to be good.

please see some of my other posts if you haven’t read them.

 

[12344321]

When the distortion happens, do you see anything on meters? Does it change if you jiggle the speaker wire for that channel?

 

[Young46]

When the distortion happens, do you see anything on meters? Does it change if you jiggle the speaker wire for that channel?

When it’s out of protection and the meter jumps up to about 1w and increase the volume the needle moves as normal. When the volume is dialed down the needle goes to about 1w and stops. No it doesn’t change if the speaker wire is jiggled.

 

[12344321]

There's a pair of small caps for that circuit, and some resistors.... I have a 680 that did something similar. When started, the left meter would climb and quickly spike the meter, with no input. It ended up being a 100 ohm resistor in power supply that caused it. I'd start checking those, could be one has drifted high.

 

[Young46]

There's a pair of small caps for that circuit, and some resistors.... I have a 680 that did something similar. When started, the left meter would climb and quickly spike the meter, with no input. It ended up being a 100 ohm resistor in power supply that caused it. I'd start checking those, could be one has drifted high.

Thank you, I’ll check it out.

 

[Young46]

I pulled one leg of the resistors and they all are within spec. I had already replaced the two small caps.