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New Pioneer SX 780 Troubleshooting

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philoc

New Member
I'm the proud owner of a lightly used Pioneer SX 780. I purchased it off of Ebay and was psyched to see it arrive today looking nearly new. A very small amount of dust on some of the knobs and plugs on the back, almost no dust inside and the only cosmetic issue is some sticker residue I'll be removing shortly. Otherwise no chips or scratches, fully working lights and like new faceplate.

My issue is it's operation, however, which is much more important.

After hooking up my turntable, an aux source, (my phone), and my Snell Type E's I can't get any connection. The lights come on, so I know I've got power, but the meters show nothing regardless of using A and B speakers and changing inputs. There's also no movement in the tuning meter for AM or FM when I use those as sources.

So far I've troubleshot in the following ways:
- Tested the Snells in both A and B speaker inputs
- Tested other smaller speakers with smaller gauge wires (also in A and B)
- Tested four inputs 1) AM 2) FM 3) Phono 4) Aux
- Tested all of those inputs with both speakers in each A and B speaker input
- Done all of the above tests while changing settings for all other buttons, (Stereo/Mono, Loudness, etc.)

Other notes:
- None of the indicator lights come on (Source, Tape Monitor, Stereo)
- I haven't hooked up anything to the tape monitor inputs so that's no surprise, but I assumed Stereo was tied to the stereo vs. mono setting, and that doesn't change based on the switch setting
- In the photos from the ebay listing those indicator lights are on
- Right before hooking up the Pioneer I was using all the same equipment to listen to records through my turntable without issue, so all other components are in working condition

I was hoping someone could help me figure out what might be wrong before I harass the ebay seller about this. Any other obvious tests I'm missing?

Any thoughts? Thanks in advance.
 
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So, you have source audio on the Aux port. I've seen the speaker cut out switch fail on the headphone jack; try plugging in headphones to see if you have any audio. If you do then you may have to clean the headphone jack or replace it.
 
Tuning meter doesn't sound promising. Do you have an antenna connected? I wouldn't worry about AM, just find a strong local FM station and see if you can detect a response on tuning.
 
Unplug, remove the wood case and check the four internal fuses. One or more may be blown.
EDIT: Don't remove the case, remove the bottom plate. Do you hear the relay "click" a few seconds after power on?
 
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Thanks for the quick responses infullview and eiraved.

To clarify, the turntable was in the phono input. I also tested playing mp3s through my iPhone in the aux input using an aux cable. Also, I’ve tested on all frequencies of AM and FM without any audio coming through or needle movement on the meter.

One thing I haven’t tested is headphones. I seem to have lost my 1/8” to 1/4” converter. Going to have to pick one up before testing, but that’s a great idea. I’ll try that next, as well as listening again for the relay’s click after opening the bottom plate. Hopefully that helps to narrow down the diagnosis.

Thank you!
 
Welcome to Audiokarma :thumbsup:, lots of help available here for SX-780 repair.
Some basic questions: Do you have any experience troubleshooting electronics? Do you have a digital multi-meter (DMM)? Can you solder and unsolder components on printed circuit boards? Do you have the schematic and Service Manual? Do you have a DBT (dim bulb tester)?
The SX-780 is notorious for power supply problem and blown STK power output modules, but are usually (IMHO) easy to repair. YMMV
Until you get sound through it from the Aux input to the speakers don't worry about the Tuner or Phono sections.
And make sure the Tape Monitor is Off, lots of us have been caught out on that one . . .
Two to seven seconds after power on you should hear the protection relay click, as eiraved asked. Does it click? You will never get sound if it does not click, as it connects the output of the power amp (internally) to the speakers and headphone jack.
 
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Thanks for the additional responses.

To answer some of those questions:
1. I have a good amount of experience troubleshooting hardware at the user level, but almost none troubleshooting hardware at the circuit board level
2. I DO have a digital multi-meter
3. I don't have experience soldering but am willing to ask some acquaintances for help
4. I DO have the schematic and Service Manual
5. I don't have a dim bulb tester
6. I've double and triple checked the tape monitors
7. I DO NOT hear the protection relay click even after removing the bottom plate

I've used the DMM to test each of the 4A fuses and all seem to be fine. I also see two Darlington STK-0050 power packs which I'm hoping are originals.

Should my next move be testing whether there are issues with the power packs?

I did see an interesting suggestion from Tim Schwarz on an Electronics Point forum suggesting, "An easy way to see if one of the STK's failed is to put your voltmeter into the lowest OHM range (x1 ideally) and measure adjacent pins like 1-2, 2-3, 8-9, etc. and compare the 2 IC's. If one of them has a dead short where the other one does not, then you've likely figured out which one is bad."

Does this ^ make sense as the next logical thing to do? Also, is fixing those STKs, (if they're the problem), going to resolve the lack of a click sound from the protection relay?
 
philoc said:
Thanks for the additional responses.

To answer some of those questions:
1. I have a good amount of experience troubleshooting hardware at the user level, but almost none troubleshooting hardware at the circuit board level
2. I DO have a digital multi-meter
3. I don't have experience soldering but am willing to ask some acquaintances for help
4. I DO have the schematic and Service Manual
5. I don't have a dim bulb tester
6. I've double and triple checked the tape monitors
7. I DO NOT hear the protection relay click even after removing the bottom plate

I've used the DMM to test each of the 4A fuses and all seem to be fine. I also see two Darlington STK-0050 power packs which I'm hoping are originals.

Should my next move be testing whether there are issues with the power packs?

I did see an interesting suggestion from Tim Schwarz on an Electronics Point forum suggesting, "An easy way to see if one of the STK's failed is to put your voltmeter into the lowest OHM range (x1 ideally) and measure adjacent pins like 1-2, 2-3, 8-9, etc. and compare the 2 IC's. If one of them has a dead short where the other one does not, then you've likely figured out which one is bad."

Does this ^ make sense as the next logical thing to do? Also, is fixing those STKs, (if they're the problem), going to resolve the lack of a click sound from the protection relay?
Click to expand...

The Tim Schwarz advice is good but not complete or definitive, IMHO. There may be other failures, related to the STKs or not.
The next thing to do, in my opinion, is to build a dim bulb tester as that will protect your SX-780 from further damage during troubleshooting.
After that is connected checking and fixing the power supplies and any other failures that are causing the unit to stay in protection (not click) is next.
If the STKs themselves are bad they are un-repairable and must be replaced. There are very few sources of 'new old stock (NOS)' STK units out in the world. The big auction site is not a reliable source, as those are mostly fake or counterfeit.
However, if the fuses are intact your STKs may not be bad but other problems are causing the failure. Build a DBT, it may protect your STKs during troubleshooting, but no promises.
There is another option for solving the STK replacement issue if it arises, you can build and install a custom pcb with modern discrete components. Here is the link to the process on AK: https://www.audiokarma.org/forums/i...050-replacement-for-sx-780-and-others.721181/ The pcb and heatsinks can be found on the auction site, or you can craft your own. The parts list is available, you must order them yourself, and then assemble and install the units. The cost for all the parts, inc pcb & 'sink wiil be between $75 and $100 (USD). It works great, I have done it myself.
A cosmetically beautiful SX-780, like you describe, is a perfect place to start, unless you paid too much for a working unit that did not arrive in working condition. That would be your call.
You tell us where you want to go, many here on AK can and will help :cool:
Once again forgive me for being verbose :blah:.

EDIT: spelling, grammer
 
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