Need help with Pioneer SX-780_random power up and off

alanr

New Member
Hi all,
First post here, need a little help. I have a Pioneer SX-780 that I recently found in my Dad's old stuff after he passed away. I'd like to get it working and use it for FM stereo listening and it's been about 25 years since I heard it play.

The unit powers on and lights up on the front, all the controls appear to be working. When hitting the power switch I do hear a prominent relay click. Once the unit is on at a random time thereafter it will start playing sound...usually around 2 minutes or so and then it will stay on for anywhere between 5 seconds and 10 minutes but then the sound will shut off(front lights stay on always). When it does this the wattage meters for the speakers go to zero as well. When the unit is working I can swap between AM/FM/AUX, etc without issue and without any scratchyness, I don't think there are any control issues...it sounds like something internal and non-physical. I've tested all the fuses, I've done the preliminary voltage checks on the darlingtons and I've inspected for buldged capacitors...that is about the extent of my knowledge from googling on this forum. I've also re-soldered the transistors with the silver aluminum heatsinks that appear to be prone to failure from other posts. The symptoms have persisted throughout this troubleshooting. Also I might be crazy but it seems the unit is more likely to start playing sound if upside down or on the side lol...I know probably crazy. Tapping on the components and bumping the chassis doesn't seem to affect anything. I would imagine this is something with the 'protection circuitry'?

Any help would be greatly appreciated.

Thanks,
Alan
 
A few things to check

The tape monitor, and other lever switches can get very dirty and cause this. Get some deoxit D5 or D100 and spray those switches, as well as the function switch.

Tap on top of the protect relay, and see if sound comes and goes. Those relays can get very spotty. If you hear popping and the sound comes and goes, either replace the relay, or take it out and apart, and clean and polish the contacts

Check the solder connections on the regulator transistors (TO220 style). Issues here will shut down AM and FM.

Check the 430 ohm resistors feeding the + and - 24V regulators for signs of overheating, or verify they read 430 ohm in circuit with power off. If they get flakey you can have intermittant sound, and then off. I had one 780 recently that was burning up these resistors. I found the fault to be one of the phono preamp IC's (HA1457) was shorted at its + and - 24 volt pins.

Lastly check for overheated resistors in the power supply area, other possible cold solder joints etc. And check for heat on the heatsink developing after a while. You could have a bad STK0050 heating up and causing protect, but doubt this is the issue, because if it was you wouldn't hear the relay clink on power-on.
 
Tom B,
Wow thanks so much for the info. I will check everything and report back.

Uncle Bambi,
I will have to double check and confirm on that.
 
Ok I have some new info. I was wrong about the relay click. The switch itself is pretty loud so I was thinking that sound was the relay. I've now realized that best case scenario the relay comes on about 5 seconds after the power switch is flipped. So...that said...my symptoms are that I will hit the power switch and then randomly later the relay will click and sound will play...then randomly later it will click off and the sound goes off of course.

Tonight I tested the voltage on the 3 transistors based on the readings in another thread and they were all good. I also confirmed by checking the voltages on downstream traces that the solder on those transistors is making good connection(I've also re-soldered them multiple times too).

I de-soldered and removed the relay and the connections were extremely dirty(the rest of the relay looked very clean and pristine). I cleaned it up real good and I was really thinking that was going to be the fix but no. Same symptoms after reinstalled. I did a little troubleshooting on the relay however...I was able to install it without the plastic case and check operation. What I did find is that if I tap the relay when the relay is already engaged and music is playing it will typically de-engage and stop playing. So I'm thinking the relay is bad. If I physically engage the relay the music starts playing without any scratchiness or anything so I think the connections are indeed clean but I think the bad part of the relay is the motor\magnet that engages it. One thing that makes me think the relay is good however is that I am not able to tap the relay to make it come on...only to make it turn off. I would think that assuming all electronics are functioning normally that the little motor\magnet would be attempting to engage the relay constantly...so if I physically engage it I would think that it would stay engaged as if I were assisting it from the physical aspect...but that does not appear to be the case. I've also noticed that tapping other areas on the board while the relay is engaged causes the relay to disengage...so it could be something loose somewhere and I'm just diagnosing it to be the relay as it's the only physical component.

Thoughts?

If you guys do believe it to be the relay...where would you recommend purchasing one? I checked ebay and most of them are the wrong number of pins. This is a Fujitsu FRL-264, D048-/04AL 01, ASR-032-A, FV2, 10 pins.
 
I doubt it's the relay. Check the voltage on the coil side while you power it up, then watch it. Push on the board where you got the relay to drop out before and watch the voltage.

Rob
 
Check for a cracked trace on the under side of the relay board. The crack may be widening as it warms up.
 
Tom B,
Image 0383, are here the 430ohm resistors in question? If so yes they test good.


Robisme,
I'll have to research this and get back to you, not 100% which contact is which.

Bluescat,
I don't see anything abnormal there, also my symptoms can occur within 10 seconds of power on, everything is ice cold.

All,
Does everything look healthy in pictures 0384 and 0385? I don't see anything overheated, the one capacitor I circled looks odd.

I've noticed this evening that if the unit is on it's right side vertically in the air...it seems to stay on nearly indefinitely. Note that is the side where gravity is assisting the relay into the closed position for what it's worth. Also could be pure luck of course.

Note sure if related but one thing interesting about the FM, it actually sounds pretty good right when you are close to being directly on a station. But once I am directly tuned in and the red stereo light comes on the sounds gets waaaay louder and gets waaaay distorted...like it's getting a super strong signal or something. Does this on all stations that can tune in stereo. Is that common?

Thanks all for help thus far.
 

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Here are my darlington voltages from looking down at the bottom board. Note that my left channel is very distorted and when the speaker selectors are off the watts gauge is stuck in position towards the center where as the right speaker gauge moves with the music and volume control...again with speakers selectors off.


Left:
1=-1.26
2=-40.87
3=-0.08
8=-0.06
9=41.0
0=1.16

Right:
1=-21.41
2=-40.8
3=-0.16
8=-0.17
9=41.0
0=-18.85
 
Just a thought, if the unit always stays on while on the right side, I would take a pencil and gently push each component up( towards the left) and see if that tells you anything. Or replace the relay first, and then try that, if the problem persists.
 
Ok, worth a try...I'll let you know. Do those darlington voltages look normal? I'd like to confirm those before I replace the relay, the scenario with the left channel sounding terrible has got me worried one of those is down.
 
It's been 4 or 5 years since I replaced the power packs in my sx780 and I forget the values, but I will look around for any notes I took. At first glance I would say that the right channel numbers look out of whack on yours, but that's not to say that something else upstream is not causing those readings ( I'm not a tech). You could search the forums for posts by "mark the fixer" on the Pioneer forum or maybe pm him. Another possibility, is to move this thread to the Pioneer forum, and maybe he will chime in. When I was working on my 780 his advise was invaluable. Another option is to remove the right power pack and see if the unit still shuts down, and if it does reinstall it and remove the left one and try it again. Another thing is that the power packs are getting harder to find and there are fakes out there, and while they may allow your unit to function, there maybe distortion or other problems with them. If it is the power pack, try to get an "original" used one. I bought a couple fakes, (which caused problems) before I found an original.
 
Didn't look thru all of these posts,but you might find some useful info. At least it is a starting point.

http://audiokarma.org/forums/index.php?search/8136658/&q=pioneer+stk+0050&t=post&o=date

BTW I just looked thru my parts box and found 3 stk0050's. They are probably bad or fakes, I don't know,(cancer drugs have affected my memory) but not sure why I would have kept them, if bad. You are welcome to them for free, if you want to try them, they say Darlington on them, for whatever that's worth.
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