Pioneer SX-780 Receiver Yes Click, No Sound

jplainview

New Member
I just picked up this receiver (its actually the black dial SX-790 but otherwise identical to SX-780) and have it hooked up to some KLH Model 6's. There is absolutely no output sound. The relay clicks about 4 seconds in and the dial bulbs work. I've tried the FM as well as phono input. I've tried both A and B. The only bit of sound is a little flutter when adjusting the bass knob. Caps don't look busted at all. Does anyone have any troubleshooting advice?
 
I've tried every permutation of the two tape monitor switches. Its resting in both up position, where tape monitor light is off and source light is on.
 
First check the regulated voltages at the three heat sunk power regulator transistors.
Then the power at the op amps in the phono amp and preamp sections.

Does the FM tuning affect the meters at all?

don't know how? speak up and ask :D

there are no dumb questions, and a LOT of what to do is found in many many previous 780 threads.
 
I've been scouring a bunch of AK threads with similar issues but so far I haven't found one where theres no sound but relay clicks on. The meters don't really budge unless I fiddle the bass knob and get a tiny bit of distortion/flutter noise. I am cranking volume all the way up and down.

I am definitely new to this stuff so if anyone can help me out I'd really appreciate it. I need to run and buy a volt meter and probably solder/iron since this may be a tough one!
 
When in FM/AM does the set seem to tune in any stations? You may not hear anything but you should see movement on the tuning Signal meters. We are wondering if the tuner is getting any power from the power supply.
 
You DON'T look for similar symptoms, you LOOK for instructions on how and where to read the power supply's regulated voltages. THAT is the key to diagnosis. Anything else is just social "how are you's"...

The 13v that runs the tuner also runs the protect circuit, so the relay pulling in without the tuner working points to suspicion of "enemy action"

It ALSO shuts down the muting circuit designed to stop pops when the sources are changed...

I KNOW what I am doing and talking about...
 
see my signature line...

some fool may have been fooling around with it.
 
While you are waiting for your meter to arrive, have a look at the circuit board in the power supply section. You will see some transistors with aluminum heat sinks on them. Examine the solder joints on these transistors carefully. Look for a dark ring in the solder, around the component pin. If you see that, re-solder them. Those transistors get hot and de-solder themselves. A common problem with Pioneer equipment.
 
I KNOW what I am doing and talking about...

Are you sure? I'm not so sure. Anyone have any opinions? ;)



jplainview - You asked a fair and straight question, and got a straight answer. Mark eats, sleeps, breathes, and dreams Pioneer. For some reason unknown to many of us he seems to have an affliction with the brand, specifically, but his broad background gives him a a solid base for diagnosing this kind of thing via keyboard and monitor regardless of the name on the front panel. You're lucky he takes the time to look at all these threads and share his accumulated knowledge. We're all lucky that he spends so much of his limited free time here.

We're also very fortunate to have other highly skilled and experienced resources, and you've been lucky enough to have several of those also chime in. Like MTF, Dr. Audio, Pete, and Zeb1 will not steer you wrong. The rest of us will tag along with the thread and help out with the common and easier things, but when the real head-scratching begins, those are the resources you want helping out. There are others, but you have already met some of the best.

As for the meter, your model should be adequate for the diagnosis and setup of the unit once (not "if") it is repaired. The hFE (gain) measurement feature might also com in handy if this process goes a little deeper.

Regarding the stereo, the 790 is a keeper. There are LOTS of 780s still out and around, still singing along. There are a lot fewer 790s. While they are functionally identical, the 790, being more rare, is somewhat more valuable.

If you haven't already done so, visit the AK database or other resources on the WWW and download the service manual. One thing you may find is that the diagrams in most of the scanned service manuals available for download are unclear, cropped, or sectioned in such a way that they may be difficult to read. Having had five of these units I got so tired of fighting it I reconstructed a clear copy. You can download the cleaned up copy HERE:

http://www.wwdsltd.com/files/SX-780Schematic-Wide.jpg

There is also a cleaned and complete version of the board artwork for download HERE:

http://www.wwdsltd.com/files/SX-780PCBoardLayout.jpg

Hopefully that will make it easier to follow the manual and AK suggestions.

A pencil-type solder iron of 30-40W should be sufficient for most of what will be needed here. Anything larger can be dangerous due to the potential for excessive heat. It isn't critical to get anything more than a rudimentary iron, some solder paste, a bit of 60/40 solder (preferably flux-cored), and possibly a desolder bulb and/or solder wick to help cleanly remove old components. There's no benefit to a grounded-tip iron in these devices, although it wouldn't be a real problem if that's all you could find. If you haven't been there before, practice a bit on scrap wire, boards, and components. Desoldering and resoldering on old, discarded devices is a cheap way to develop good habits.

The rest should be basic hand tools, your time, and a willingness to succeed.

That should be a pleasant unit once it is returned to service - Well worth your time and attention.
 
JP - Doc Audio covered it in words in his last reply. This is a 780, and a classic example of what the Doctor described. It might help you visually:

SX-780Q25Overheating.jpg
 
Wow you guys are so awesome. Its been a rough work week but coming home to these replies is like Xmas!

So I'm in a technical profession and I've soldered a bit in the past on school projects and did some small projects outside of school back in the day --Playstation 1 mod chip anyone? :) so I can solder a bit and I understand electronics a bit.

I'm new to vintage audio/vinyl but its already become my favorite past time in the last 3 weeks. I'll post pictures of my whole unit soon; it was a Goodwill find. It's been kept really clean; even the boards inside were relatively dust free and the Darlingtons look like they may have already been swapped once. The transistors seem ok-- one of 6 solder joints looks a little brown but not too bad. Pic attached below, also of power pack.

Thanks everyone for the replies so far and I will follow up soon with some voltage readings!
 

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