Pioneer SX-1050 receiver restoration

[electra225]

This may be my next project. Mine has a bum channel. It will work fine sometimes, sometimes the left channel cuts out, sometimes the left channel has a growl like you get putting your finger on the hot end of the volume control. I have watched videos on how to go thru one of these. It looks complicated, yet doable. The front ends seems to be okay. If I connect an amp to the "main in-pre out" connections, it works fine. So I am going to concentrate on the audio section. I need to find a schematic first thing. I am a total novice at solid state devices, so this will certainly be a different effort. I suspect it needs a good recap first thing. Has any of you gone thru something similar to this and do you have any tidbits of wisdom you'd like to offer?

[danrclem]

I have a non-working SX-1050 but I suspect that mine has blown output transistors. Maybe someday I'll get around to checking it out.

I'm just a tech wannabe so I'm not likely to be of much help. My first though was dirty controls but you said the preamp worked ok. Maybe a cold solder joint that separates when it gets hot? I also have a SX-750 that I quit using a few months ago because it would quit working when it got hot. I could let it cool off and then it would work again. I could smell it but couldn't pinpoint what was going on with just a visual check.

Hopefully somebody will chime in that actually knows what they're doing.

[electra225]

What little I have read and according to videos I have seen tells me the transistors are typically reliable. Solid state devices use electrolytic caps for coupling devices. I believe also that I have seen "rebuild kits" that include all the necessary capacitors for these receivers. I wonder if Sams has schematics for things like this?

[danrclem]

These links may help you some. I have done image searches for schematics but some of them are hard to see.

I was adjusting the DC Offset on my SX-1050 when it blew so it was 100% my fault. I don't think I shorted anything out with the screwdriver but may have. I now have a small coated screwdriver.

https://www.hifiengine.com/manual_libra ... 1050.shtml

https://audiokarma.org/forums/index.php ... ct.925512/

[Motorola minion]

Much better quality electrolytic caps were used in most receivers like this including Sansui, Marantz, Kenwood, etc.

Also note! that direct coupling is used between transistors. Complex feedback and bias networks do use capacitors and low ESR should be confirmed before embarking on a wholesale recap.

That said, I generally do not work on this level of equipment beyond cleaning switches, pots, connectors, etc. That task alone is nearly always sufficient. I just did a Marantz 2252, a fantastic piece. The last Marantz was a 2238, which one channel prematurely distorted slightly at higher volume. I was not able to repair it and returned to customer, citing poor availability of replacement transistors. Did not want the hassle of finding them!

The NPN and PNP silicon transistors (2SA, B, C, D prefixes) used by Pioneer and others are often particular designs that NTE, SK, etc. ABSOLUTELY cannot substitute. Ask anyone who has repaired a Sony TV

Sams is very little help with this type of electronics, hit or miss coverage so HW Sams' MHF series of books is not worth exploring. The websites in the last post are the best resources I have found.

There are two easy methods to suss out a sometimes-bad transistor; Freeze spray and a heat gun with small nozzle to heat only the suspect part.

[electra225]

Mine would probably benefit from some detailed troubleshooting. Your comment about dirty controls is well-taken. For a long time, when the left channel would cut out, I could move a speaker switch or the function switch or sometimes even the volume control and the left channel would cut back in. I have been watching YouTube videos on them, but they don't really help that much. I wish Uncle Doug would do a video on a SX-1050. If I do decide to dig into this thing, I may need a good ESR meter, then try to figure out what ESR actually is and what relationship to reality it has. At this moment, I believe ESR is an audiophool term. Each video mentions ESR and idle current. Research goes on. I can't hurt it and it will still sell unrestored and not working for pretty good money, so I don't have much to lose.

[electra225]

I've been doing some research on this amp. The more I read, the more confusing it becomes. First, I need to identify the issue. Second, I need to do some directed testing. Like you said, Dave, service literature is scarce, so I may have to just wing it. Where do I start? I could buy an ESR meter if I really need one. Maybe just recap the entire amp section? Would any of you guys like to embark on this journey and would be willing to walk me thru it?

[electra225]

I have the SX-1050 on the bench, with the covers off. THAT I can do competently. From here on, I'm a total noob. I am going to try to figure out how to clean the controls and go from there. Any ideas would be sincerely appreciated. Criticism will work as well....

[electra225]

Since I've received no input otherwise, I have forged ahead and have found this parts source:

https://www.ebay.com/itm/125241353287?_ ... %3A2332490
This may not be the one I work with, but the seller seems to have a full line of parts and claims to have restored these receivers more than once. This kit has everything but the driver and output transistors, those are available from the same source for $20 a pair. Thoughts?

I also just read where these amps can have a 150 AMP surge current! Whoa!

Here is another vendor for parts. They offer technical assistance which might be a plus for a noob. They also package their parts identified for specific PCB's. This might also be a plus for a noob. Vinyl Engine will be no help. They have suspended registration due to the spambot invasion. None of their information will be available to me. Their admin seems to be a really touchy guy....

https://www.ebay.com/itm/155126491108

[danrclem]

Cleaning the controls first would be a good start since they would need to be cleaned at some point anyway. It's possible that it may cure some problems. I cleaned mine a long time ago and I don't remember it being very difficult. Even if cleaning gets it working better, I think by replacing all these parts you'll be better off in the long run. I think this receiver was made in the late 70s, so it has some age on it.

Some people tend to shy away from kits, but this guy seems to have all of his ducks in a row. He has a 100% rating with lots of sales, and he gets very good reviews. It's hard to say how much time you would spend figuring out what to buy on your own so even if this does cost more it should be worth it in the long run.

I think if was going to tackle a project this big I'd have to at least consider buying a desoldering gun. It could keep you from lifting some traces and it would be a lot faster. I don't remember the brand but there is a cheap desoldering gun out there that gets fairly good reviews.

I'm fairly certain you already know this but do one board at a time and check it out before going to another one. I'd wish you luck, but I don't think you'll need it.

[electra225]

Thanks, Danny. I appreciate your input. Your comments were well-considered and are well-taken. Thank you.

This will be the largest project I ever got involved in, mostly due to its being new territory for me. It may take a year or more for me to wade thru this project and not mess something up. My hands are not as steady as they need to be. I have NO idea what I'm doing. A step at a time. YouTube videos are not much help. Most assume I know more than I do. Each one gives little tidbits I write down for reference later. That last vendor I listed sends his technical information via Zoom. I have no idea what that is, nor how I would use it. So I have plenty of things to learn besides how to work on the device under test. This will be a bigger project than the Stereo Theater was. That project took three years....

Your suggestion of buying a desoldering iron is well-taken....

At my age, I may be totally daft for attempting something like this. The wife said "Well, it's broke now, so you don't have much to lose. And, it will keep you out of the bars and off the streets". Thanks, dear.....

[electra225]

Would something like this work?

https://www.ebay.com/itm/317485601662?_ ... R4KNwL_0Zg

[electra225]

To you guys who understand solid state electronics, my hat's off to you!

I have learned the following, that may make this make more sense. The base of a transistor is like the cathode in a vacuum tube. It controls the current that flows thru the transistor. The collector is like the grid of a tube. The signal comes in on the collector. The emitter is like the plate or anode of a tube. The output of the stage would be the emitter. There are two basic types, NPN and PNP. You can identify them on a schematic by which way the arrow is pointing. NPN arrow points out, PNP arrow points in. You can check a transistor with an ohmmeter. There are other kinds of transistors that I may not have to deal with at this point. I think my next step is to treat this like a tube chassis and get a good baseline on it. I have a graphic equalizer I can hook up to see if the issue is ahead of the amplifier or not. Then I can develop a plan. I'll clean controls first to see if that improves matters. This receiver is death on tweeters, so I'll connect open spare speakers to it, so if I blow one, oh well....

There are four little blue pots on the driver board that are apparently always found bad. I'm trying to find out what they do and how to adjust them. These pots are included in the repair kits I listed above, but information on what they are and what they do is scarce as hen's teeth. Do the instructions included in the kits explain this?

[electra225]

The issue appears to be in the audio section. One channel is totally dead. It will pop when I turn the power on and off, but that's it. The other channel is extremely weak. I can barely hear the FM radio with the volume turned all the way up, a dangerous situation for my hearing and for the speakers I have connected if the volume ever returns. I need to verify how the "main in-pre out" shorting plugs are supposed to be connected. I have manipulated all the controls, so I don't think this is a control issue. I am now going to connect another amp to the "main in" terminals to see if I can hear the radio then. The tuning meters move, so I think the front end is working. This thing is really broke! Stay tuned....

[electra225]

I've got another, more immediate problem. The soft-start relay is clicking, turning power on and off, although the dial lights always stay on. I'm suspecting a short in the power supply. The amp meter on my Variac/isolation transformer spikes every time that relay clicks. That can't be good. I have checked all the fuses I can see, they are all okay. I haven't checked any voltages. It's been at least a couple of years since I worked with this receiver, so maybe some thing has failed in that time. The rectifier diodes all check good. I'm not getting any indication of excessive current draw on the amp meter of my Variac. I'm kind of afraid to connect my Kill-A-Watt to this thing because when you first turn it on, it draws more current than the KAW is designed to handle.

[electra225]

The next step is finding and obtaining a schematic. I'm chasing my tail otherwise. The label on the receiver says it draws 490 watts. My KAW says it is drawing 22 watts. It's a good bet I don't have any B+. I disconnected the filter caps with no improvement. I measured 20 volts on the filter caps. The reason the relay was clicking was that I had the chassis turned onto its side. It's not happy like that.

[William]

If the suppliers of the two kits have parts, and one has rebuilt SX1050's several times, do either have a schematic or any documentation that might lead you in the right direction? It might be a question to ask either of them.

Bill

[electra225]

I found a 92 page service manual online that I can download for free. I'm assuming then that I can print whatever pages I need and not have to print the entire 92 page document. I found a relay that energizes the primary of the PT that appears to not be working. My concern now is the condition of the PT. There is a series of fuses downstream from the PT that are all good. The fuse ahead of the PT is good. I didn't see any magic smoke nor do I smell anything that's been hot. Resistance checks on wires coming out of the PT indicate no problem. The dial lights work, but I'm not sure if they go thru the PT or not, typically they do. Something took a dump while I was doing my baseline check. I have more than one problem. I remembered that back when this thing was running somewhat correctly that, occasionally, when I turned the receiver on, it would come on, then go back off. I would shut off the power switch, turn it on again, then the receiver came on and worked. I may have run this old receiver past its expiration date.....

[William]

Have faith, if the PT is good, I am sure it is totally repairable.

Bill

[TC Chris]

That solder sucker is a more elegant version of my old Radio Shack model, which has a rubber squeeze bulb, like on a bike horn. They are really useful devices.

There are two kids of protection circuits. One deals with DC offsets in the output, shutting things down when the output is passing DC (not at all good for speakers). Another relay turns speakers off when starting up or turning off to avoid big thumps, which can also be unfriendly to speakers. My Denon integrated amp, my current main source, has a wonky thump relay which needs its right-channel contacts cleaned.

I like the kit idea because you don't have to make a list of all the caps you need, hunt through the catalogs, select the right values, shapes, and quality (like temp rating). Somebody else has done all that work and probably bought in bulk as well.

Maybe other have different experience, but I do wonder of shotgunning all the transistors is as necessary as the first guy claims. I was doing that diagnosis & repair of the Heath console electronics last winter, if you remember. The problem was elusive so I started replacing transistors. Turns out that the problem was one resistor, one of the first things I repaired, but I replaced a red-red-red with red-red-orange, which kept the transistor biased wrong. In my defense, the color difference was barely discernible--I only figured it out when I applied an ohmmeter and then examined the colors with a magnifying glass. But the lesson is start simple. We know that electrolytic caps are the most likely things to fail with age. And you can do a lot f diagnosis if you have a schematic with voltage readings and an ohmmeter.

I've got my Dad's old Pioneer SX-626 to work on some day, and an even older SX-1000 that's dead. Let us know what you discover in Pioneer land.

Chris Campbell