Pioneer SX-780 problems

[Jeff Harrell]

Greetings all!

I recently inherited a broken Pioneer sx-780. I feel pretty confident that the Darlington power packs are fried.
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Back story:
Several years ago the previous owner built some custom speakers and had a short on both speakers when the unit was powered on, ever since the unit's protection relay would not click on. He had it diagnosed and was told the "output transformer" was blown. I suspect he meant output transceiver which should indicate the power packs.
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I opened up the unit today and both 1a fuses @ FU3 and FU2 were blown. I replaced them and they immediately blew again upon powering on. I've removed the power packs and powered on the unit with only a fuse in FU3. The protection relay will now click on and it appears the tuner works because the tuning needle will move and the source light is now active.

Should the headphone jack work with the darlington's removed? I figured it would but it does not.

I'm not sure where to go from here, i'm afraid to install a new set of packs and potentially fry them also.

I have a basic multimeter but the fuse is bad so i dont have any readings at the moment and i'm not sure what / where to measure. I'm not that great with electronics and the schematic i'm staring at might as well be in another language.

Any help would be greatly appreciated!

Thanks!!

 

[zebulon1]

You will need the Modules.
The forum is beginning to finalize a universal module that can be assembled as a kit. You would have to order the components and hardware. Fabricate a heatsink.
The design by a few members is an open source concept so there is research to do.
Start here:
http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
If you couldn't assemble it yourself someone could be found that can.
If original is desired:
Don't get to worried about all the rumors of counterfeits. It is real but members here all have good sources albeit - if they have any left.

 

[Jeff Harrell]

I'll grab my other multimeter from my office tomorrow. Is there anything I can check to make sure there isn't a problem further up the circuit? I've read about some fusible resistors but don't know the locations. I think I have the voltage numbers and pins for the darlington packs written somewhere.

The open concept might be something I'd look into in the future, but right now my plans are to keep this as close to stock as possible.

Thanks again

 

[Awesomeaudio]

This guy is also a great resource.

There is a repair thread somewhere here on this very unit.
I'll see if I can find it.
Maybe someone else will beat me to this.

 

[Jeff Harrell]

Lol, yea I've watched kev's video many times, it's very good

I've been lurking here for a while and it seems that replacing the darlingtons is the common fix. I'm just concerned since parts are scarce I don't want to damage the ones I've ordered

 

[Awesomeaudio]

Unfortunately that's the Achilles heal with the 80 series units.
Zebulon hit on something about the "replacement" STKs.
Replacement or sometimes referred to as "Counterfeits" are every where, but I guess it depends on the supplier. If they back up their product then that's half the battle.
Originals are no longer manufactured and I'm not sure too many will be willing to part with their genuine old stock items.
Rick's (rcs16) STK replacement looks like the way forward as these STKs keep burning out.

 

[larryderouin]

There are either 2 double emitter resistors, or 4 singles in the front of the packs on the main board. IIRC they are .22ohm reisistors. Replace these when you replace the packs. Good insurance.

The Voltage Regulators that you are confusing with Sansui fusible resistors are on the left side of the board all in a straight line (left being as you face the unit from the FRONT!). These regulators get hot enough to desolder the legs. Get some larger malleable heatsinks along with some nylon screws and nuts (a TO-220 installation kit) with a mica insulator. The case is the collector of the unit and electrically HOT! Install the heatsinks so they don't contact ANY OTHER component, and re-solder the legs. Also look at the power supply and check the solder joints on the LARGE and small DIODES. Re solder as necessary.

I agree about the up-rated board for the 780. As there are more hens teeth in the world as there are working original STK-0050 packs, keeping it as original as possible is a noble but futile gesture. (We are AK....Resistance is Futile) PUN Intended. I'm in the process of getting the parts together for my 790 which has original working packs, which will be stored in doubled plastic bags with it . My understanding is, is that mounting the new board and the wiring /installation is reversible. So if Panasonic (who bought out Sanyo in 2014 an closed it down shortly thereafter) decides to make new STK's (which is about as likely as them going full production of the SL-100mk 2 or MK 5 @ $600.00 a unit) you and I will be long gone wearing pine peacoats.

Either way is fine. But I would think that this is a PRIME Example of needing to go with the uprated board and being done with it. Reliability should go way up with it, and if you shoul blow an output, it would be infinately cheaper and easier to replace than trying to find a dying format.

 

[Jeff Harrell]

I just got home with the meter from work. Grounded the meter on the chassis, put it on DC volts and checked pin 1 on the left channel... Something on the top side of the board started to smoke. I turned off the unit and turned it back on, the relay still clicks on but I'm worried!

 

[Awesomeaudio]

There are either 2 double emitter resistors, or 4 singles in the front of the packs on the main board. IIRC they are .22ohm reisistors. Replace these when you replace the packs. Good insurance.

The Voltage Regulators that you are confusing with Sansui fusible resistors are on the left side of the board all in a straight line (left being as you face the unit from the FRONT!). These regulators get hot enough to desolder the legs. Get some larger malleable heatsinks along with some nylon screws and nuts (a TO-220 installation kit) with a mica insulator. The case is the collector of the unit and electrically HOT! Install the heatsinks so they don't contact ANY OTHER component, and re-solder the legs. Also look at the power supply and check the solder joints on the LARGE and small DIODES. Re solder as necessary.

I agree about the up-rated board for the 780. As there are more hens teeth in the world as there are working original STK-0050 packs, keeping it as original as possible is a noble but futile gesture. (We are AK....Resistance is Futile) PUN Intended. I'm in the process of getting the parts together for my 790 which has original working packs, which will be stored in doubled plastic bags with it . My understanding is, is that mounting the new board and the wiring /installation is reversible. So if Panasonic (who bought out Sanyo in 2014 an closed it down shortly thereafter) decides to make new STK's (which is about as likely as them going full production of the SL-100mk 2 or MK 5 @ $600.00 a unit) you and I will be long gone wearing pine peacoats.

Either way is fine. But I would think that this is a PRIME Example of needing to go with the uprated board and being done with it. Reliability should go way up with it, and if you shoul blow an output, it would be infinately cheaper and easier to replace than trying to find a dying format.

Well said. Larry.
I totally agree with you.

 

[Jeff Harrell]

There are either 2 double emitter resistors, or 4 singles in the front of the packs on the main board. IIRC they are .22ohm reisistors. Replace these when you replace the packs. Good insurance.

These are the 2 white rectangular modules on the top side?

 

[petehall347]

I just got home with the meter from work. Grounded the meter on the chassis, put it on DC volts and checked pin 1 on the left channel... Something on the top side of the board started to smoke. I turned off the unit and turned it back on, the relay still clicks on but I'm worried!

your meter might be set up wrong .. make sure one probe is in common the other in volts/ohms . be absolutely sure nothing is plugged into amps ..
bearing in mind i have never seen your meter .i am taking a guess .

 

[Jeff Harrell]

I've only used this meter a few times but here's a pic of the settings. Nothing is plugged into the amp other than the power. Packs are removed

 

[petehall347]

I've only used this meter a few times but here's a pic of the settings. Nothing is plugged into the amp other than the power. Packs are removed

just as i thought ..you have it set up to read amps . with the probes plugged in like that it is like a short circuit between the probes . that is how amp measurement is measured by your meter . it doesnt matter that it is set on volts it is the way the probes are plugged in ..
move the red one to the middle .
will need to look at schematic to see likely casualties .. am thinking right now of a transistor and 1 or 2 resistors have taken a hit .. was it left or right side looking from from front of amp ?

p.s you are not alone ..i have slipped up like this many times and i know better .. or i should do by now ..

 

[Jeff Harrell]

OMG I didn't even see the V on the middle probe, I only saw Ohm

I actually checked both but the right channel did not show a reading, maybe I wasn't grounded. The left channel I believe is where it smoked, on pin 1. Smoke appeared to be in the center of the board.

With the probe in the CORRECT port now, I'm reading ~-39.5 & ~+39.5 v on both channels (pins 1 and 0)

 

[petehall347]

your meter fuse was likely blown at your first reading .
things to test/replace are q11 q13 r257 r259 . also check for burned copper tracks on the board .

trouble is with this design of amp with the module removed even if no other problems pins 1 and 0 will give wacky readings .same as if something is wrong ... best check what i said . in theory it is just those that may or not be damaged .

 

[Awesomeaudio]

These are the 2 white rectangular modules on the top side?

[Post edit] That's them yes. There are two per channel.

 

[Jeff Harrell]

things to test/replace are q11 q13 r257 r259 . also check for burned copper tracks on the board .

I see them on the schematic but I'm unable to orient myself on the PCB. still looking

 

[Jeff Harrell]

I believe I've found
r259 = red
r257 = blue
but the schematic traces looks slightly different for q11 & q13

The traces look good and I think the regulators and solders look good also

 

[Awesomeaudio]

 

[petehall347]

q11 2sc1775a q13 2sc1885
r259 200 ohms red black brown
r257 510k .. 510 thousand ohms . green brown yellow