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 Post subject: Pioneer SX-780 turn on problem
PostPosted: Aug Sun 23, 2015 4:29 pm 
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Posts: 427
I am the original owner of this receiver, circa 1978, just use it in the garage... Sometimes it will not click on, I know there is a delay circuit in it. If I whack the side with a rubber mallet it sometimes comes on... Is there anything replaceable in this thing, and can parts be obtained any more? I know I could buy something better for less than it might take to fix this, but it has value to me as the first thing I bought with my own money after getting my first job...


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Sun 23, 2015 6:32 pm 
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Location: Utica, NY 13502 (USA)
It sounds like the quiescent current and/or balance might have drifted. A service manual will tell you how to adjust. But I have a Pioneer SA-7100 integrated amplifier whose protection circuit had a leaky transistor which caused the same problem that you describe.

Dave


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Sun 23, 2015 6:49 pm 
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Could be just poor solder connections. Check around the regulator transistor also.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Mon 24, 2015 3:06 am 
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Location: Aurora, CO 80013
Or one of your STK output ICs could be going bad- common problem.

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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 12:45 am 
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Location: Hillsboro, Oregon
Whacking on the side... Oh boy.

If you are intent on learning solid-state electronics, some of us can lead you through a few things, and you'll end up knowing more and possibly fixing it. But it's hard to diagnose by "remote control" if you don't know what you're looking at and don't speak the language. If that's the case, I recommend you find someone knowledgeable in your locale who can actually sit down with it at a workbench and debug it. Maybe there's an ARFer down the block, you never know. Where are you?

To tell the truth, most of us here are at home in the tube era. There are other forums with more people specializing in solid-state audio. AudioKarma is a good one, with a sub-forum devoted specifically to Pioneer, and several resident veterans for the task you're looking at.

Regards,
Dave


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 1:06 pm 
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I think I will check the solder joints, I do have a schematic for it, though it is very small. I understand solid-state stuff enough to know the use of ICs and transistors, and also the packaging of some things into replaceable units... I will also seek help on that web site you suggest, thanks... Just not sure of parts availability for something that old, especially with ICs...


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 4:32 pm 
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There is probably a scan at www.hifiengine.com . It's worth a look.

The protection circuit monitors both channels for (a) average DC output voltage, and (b) average output transistor emitter current. If (a) is nonzero or (b) is greater than a certain amount, the designers assume something is deadly wrong and opens the relay to keep the amp from burning out your speakers. (a) and (b) can be measured with a DMM. The manual will show test points. Measure and report.

The protection criteria can be violated by either normal drift in settings, or catastrophe, so you may have nothing more to do than readjust one or both channels' OFFSET pot.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 5:37 pm 
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Location: Minnesota
Dave Wise wrote:
Whacking on the side... Oh boy.

If you are intent on learning solid-state electronics, some of us can lead you through a few things, and you'll end up knowing more and possibly fixing it. But it's hard to diagnose by "remote control" if you don't know what you're looking at and don't speak the language. If that's the case, I recommend you find someone knowledgeable in your locale who can actually sit down with it at a workbench and debug it. Maybe there's an ARFer down the block, you never know. Where are you?

To tell the truth, most of us here are at home in the tube era. There are other forums with more people specializing in solid-state audio. AudioKarma is a good one, with a sub-forum devoted specifically to Pioneer, and several resident veterans for the task you're looking at.

Regards,
Dave


Dave,
While I agree with you generally that most of us are into tube gear, that does not really hold true in this corner of the forum. Threads here seem to be evenly split between tube and ss, possibly even more so solid state. Also the folks that hang out here are just as knowledgeable as many on AK if not even more so (it seems many folks on AK like to talk about their "techs"). Not to downgrade AK, plenty of really smart guys there, but there are plenty of really smart guys here also.

That being said, whacking the thing with a rubber mallet isn't the smartest thing to do. I would suspect a bad switch connection (power maybe), dirty relay, and/or cracked/cold solder joints. The modules usually either work or they don't.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 5:51 pm 
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Location: Utica, NY 13502 (USA)
The fact that you can give it a whack and it starts working is probably a good thing. The problem might be more mechanical in nature rather than electrical.

Dave


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 6:07 pm 
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I downloaded the service manual, I'll report back once I get some time to play with it... Thanks for the suggestions


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 9:36 pm 
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Location: Fernandina Beach, FL
Is there a relay in the circuit anywhere?

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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 9:50 pm 
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Yes, labeled S7. It's energized by Q26 (2SC1384) on command from "Q27", a SIP IC? Transistor array? type PA3004. The schematic in HiFiEngine's service manual has a small gap between scan pages, you'll have to tape it together with whitespace and guess.

Pin 4 of Q27 is listening to the DC voltage at the amp output, averaged over both channels. There's no emitter current monitoring on the output transistors.
If pin 4 is not zero (like you'd get if one or both channels was out of balance), the relay will drop, disconnecting the speakers.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 10:20 pm 
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I only ask about a relay should there be a sticky or intermittent contact?

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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Tue 25, 2015 10:25 pm 
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Sure, although that would be more likely to kill just one channel. My guess is the relay is not being energized, and for good reason.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 6:04 pm 
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I redid the DC balance adjustment in the service manual, relay still does not pick sometimes, intermittent. I pulled it off the board so I can pop the cover and clean the contacts.. Would a replacement be made anymore? the markings on it are FRL-264 D048/04A1-01, made by Fujitsu.. I do see that special IC PA3004, I assume that is not available anymore..


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 6:18 pm 
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Good; you're already making progress. Divide and conquer. If you can get a voltmeter across the coil, it will be easy to determine whether the protection circuit is failing to energize the relay, or the energized relay is failing to pick. If the former, put your meter on the DC average going into the PA3004*. This will alert you to intermittent problems in the amplifier that are causing one or both channels to lose DC balance. For example, the balance trimmers could be scratchy. If you have a signal tracer, hook it to the wiper and listen while turning the pot. My favorite chemical for pots is polyphenyl ether.

* It's possible that the voltage there is too low to measure. You might need two meters, one on each channel before the mixing resistors. Or two sessions with one meter.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 7:24 pm 
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Waiting on relay replacement info.. If none available, I will clean it and reinstall, and see where it is at... I really think the hammer smack is pointing to the relay.. I also noticed they use a lot of the old wire-wrap connections (not soldered), not sure if they would degrade over 37 years or not...


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 7:27 pm 
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Wire wrap is a very good long-lasting connection if nobody unwraps it.


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 8:04 pm 
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More research has found this:

2.) On the MAIN BOARD Behind the power switch, are 3 voltage regulators with heatsinks on them (look like transistors). They are in a row, front to back. DO NOT TOUCH WHEN POWERED UP as they are electrically LIVE. They get extremely hot and the solder can break down causing intermittents. DeSolder each lead, and Resolder the leads on the boards, and clean off the old heatsink grease and add new (take each one out, one at a time, separate the heatsink from the regulator, clean and regrease, re-assemble, and solder in place.

The rearmost one in mine has burnt looking connections to the board, I am going to remove, clean, and resolder... Does anyone know what heatsink grease is? I have an old tube of something called Dielectric tuneup grease, which I think I bought years ago when replacing the distributor module on my 77 Pontiac...


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 Post subject: Re: Pioneer SX-780 turn on problem
PostPosted: Aug Wed 26, 2015 8:28 pm 
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After cleaning and reinstalling the relay (probably not needed), and removing and reinstalling that power regulator, she appears to be good to go... Did about 25 on/off cycles, worked every time... Thanks for the dialog.... My first stereo is back!!!


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