Pioneer SX-780

Clay77

New Member
Hello,Nice forum yall have here.
I have experince working on tube guitar amps and I've managed to fix a few SS devices but far from profecient,

I have a sx-780 that was gave to me.It lights up but no sound and no click.
I have manage to find that one of the .22 ohm emmiter resistors is open.
I'll have to write down voltages to remember them but the one STK-0050 voltages measured way different from the other.Should I assume the one Stk-0050 is fried.

What else should I be looking for?

Should I just replace the pair while i'm at it?

Where can I find replacements?

Thanks,cbass
 
Hello,Nice forum yall have here.

And now, you're a part, so it's a forum WE have here.

Welcome Aboard!

In your 780, the failed emitter resistor is likely due to a failed power pack (hybrid output module). It could be due to other factors as well. It seems that most often the power outputs in these fail from shorted speaker leads or speaker voice coils/crossovers. They can also fail from overheating due to poor heat conduction to the sink, improper bias voltage/open bias networks allowing runaway, incorrect bus voltages, and probably numerous other potential problems.

What else should I be looking for?

It would be good to test both pairs (all four) of the 0.22Ω emitter resistors. It would also be good to desolder and lift the connection pins for both output modules for testing both the resistors and the modules themselves. Chances are that one of the modules will need replacement, and the other can benefit from a fresh layer of heat sink paste (silicone grease, non-conductive).

With the modules removed it may be safe to power the unit on and check a few voltages. This is safest to do with a current limiting device like a dim bulb tester (DBT) but can be done at normal line voltage so long as the proper fuses are in place. Building a DBT is inexpensive and easy, and provides cheap/effective insurance. Your can find several examples by searching this site.

If all is O.K. the unit should power up normally and should close the protection relay after about a 3-6 second delay. If that happens without blowing fuses or overheating, it would be good to measure the voltages of the DC power supplies. Between the leads of the two main filter capacitors should be +40V on one side and -40V on the opposite capacitor (meter negative referenced to chassis ground). The other places to test are the three larger transistors near the left end of the chassis (viewed from the top), Q25, Q19, and Q20. These three have a "U' shaped aluminum heat sink attached to their collector tabs. You can measure DC voltage directly from the collector tabs.

The rear transistor (Q25) should have about 25V, the middle one (Q19) should have about 50V, and the front one (Q20) should have about -50V. If all those are present and the relay clicks, you may only need to replace the failed output module(s).

Should I just replace the pair while i'm at it?

While maybe not necessary, it's not a bad idea, and it would be good to keep the working original as a spare/backup.

Where can I find replacements?

The original Sanyo output modules are no longer available from the manufacturer. There are several places offering replacements. I've used modules from B&D Enterprises, Electronix, and Consolidated with success. Some are good quality, some are poor quality. A little research might be helpful before ordering.
 
Thanks Watthour

I will check voltages and dissolder the packs one at a time starting with the suspected bad one to see if the relay clicks.

It looks like the tuner works so thats goods news.

I've been using my bulb limiter but thanks for reminding me.When I first powered it up.There was a weak distorted output and it did click on.
 
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It appears that the regulated voltages are pretty close. Just for the sake of argument, and while you wait for your replacement device(s), take a close look at the solder pads and foil circuit traces around the Q25 transistor - The one closest to the main heat sink. That particular transistor usually gets the hottest of all three of those. They can even get hot enough to un-solder themselves. As a result, there may be a bit of a resistive connection there dropping voltage slightly, or maybe your line voltage is slightly low. It's still worth a look, and possibly cleanup and re-flow of the solder joints.

If the transistor is getting excessively warm, you may want to make a modification, either by adding an insulation washer and screw hardware then clipping on some additional heat sink, or something like this:

http://www.audiokarma.org/forums/showthread.php?t=496080
 
Thanks for the help.

I had already reflowed those joints.I'll poke around and see if I can find the voltage drop.MY wall voltage is typically around 123VAC

The companies you listed I either couldn't find the STK's or they were out of stock.

I will probably replace all the .22 resistors and all those trannys and do the heatsink mod.

Should I be considering replacing the power supply caps?I often replace them on tube amps over 30 years old but those are running close to 500V on them.
 
Not sure why but i am getting 27 volts on Q25 now.

What should i have at pin 1 of the power packs? Iam gettin 39VDC at both of them
 
pin 1 should be at -1.4 volts . i seem to remember the reading will be wacky without the stk module in place
 
It is dangerous to operate without the packs.

AND with no feedback, while the protect relay may click, the rest of the circuit can "rail" to either supply.
POSITIVE OR NEGATIVE.
Making voltage measurements other than power supplies inconclusive.

Test the other transistors out of circuit and check resistors if you want to do further tests.

You would have to arrange diode drops totaling 2.4v between the pins (facing the right way no less)
and then establish a point near zero volts to "call" zero volts for your feedback point.
 
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Sorry, Clay. It seems I should have been clearer in my earlier post. I mentioned that the power supply voltages can be checked, and that it's safest on some form of current limiter, like a DBT. I didn't specifically mention that other testing would be difficult or impossible. With 27VDC on your Q25, the rest of the circuit loading may pull that down a little and it could be just about right when reassembled and running. ±10% is usually acceptable, so even 22½V is O.K.

Have you also checked the littlediode.com site? Amazon?
 
Ok I am not sure if I understood what Mark was saying but I put 2 diodes in series between pins 1&3 and 8&10 for a total 2.4v drop per module.or should it be four diodes in series on each side?

I replaced the open.22 resistor with a .47 because thats all I had on hand.

I am now getting 1.3 ,-1.3 on pins 1 &10 with my current limiter plugged in.So that seems good?

Can I hook up the module I suspect is good and leave the diodes on the other side to test it?
 
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I should NOT have said ANYTHING about how to spoof the presence of a module - I have NEVER needed to do it, thus cannot speak as to direct experience AND if I were to attempt it, it would be IN-PERSON FIRST and I would tread VERY cautiously - even though the consequences to me and my well stocked parts pile would just mean an inconvenience as I replaced possibly burned up parts.

You STILL need to establish a zero volts point to tap off to close the feedback loop.

but + and - 1.3v is as far as you need to go until you have two modules - you have proved that the three closest transistors to the module are ok. Might as well "quit" while you are ahead, I really don't have the hand holding time to unravel a "disaster". I have deadlines and promises breathing down my neck.
 
Be careful what you say around me I know just enough to be dangerous :stupid:

I'll put this aside till I come up with some packs and other parts.

Thanks for the help guys
 
If you haven't had any success in locating modules, this place lists them as available:

http://www.hbelects.com

I have no idea what they are like, and no experience with them. I just ordered some Ge transistors from them today because they were one of the few domestic places listing them as available.

Caveat emptor.
 
So 5 years later i pulled the old pioneer out and decided it was time to get it going. I really appreciate the help i received from y'all.
I see that there is a kit available now to replace the power packs with discreet components. Think i will go this route.
I'm also going to do a full recap.Filling up my cart at mouser now.
My question is. Is it worth the hassle to replace the smaller electrolytics with film caps?
Thanks
 
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