Pioneer SX-535 120Hz hum

[Phil_S]

Don't know where else to post this. I'd be grateful for your counsel and/or advice. We have begun the terrible process of getting rid of stuff in anticipation of selling the house to downsize. We encountered a Pioneer SX-535 carefully bagged and boxed. It was working fine when it was retired, I guess in the mid 1980's. By that time, decent stereo receivers/amplifier units could be purchased for very reasonable prices and they had a much more compact footprint. Sleek, all black plastic cabs were all the rage. But I diverge.

I thought maybe I'd sell this historic beast on eBay, so I plugged it in only to hear the dreaded 120 Hz hum and it's loud. The hum is present when the power is turned on and it is mostly unaffected by the volume control. It is present on speaker connections A and B, and on both channels. If I dime the volume, there is an itty-bitty increase in hum volume between 9 and 10. The hum doesn't change with the selector switch (AM, FM, phono, aux.) AM an FM are both working. I didn't test Phono and Aux, but there is no reason to think they are bad as they were also working at retirement.

Service manual is here, but it is a PITA to read. https://www.audioservicemanuals.com/p/p ... ice-manual

So, here's the question. What should I do?
1) Should I bother to attempt to fix it? I assume it is PC board hell inside. I don't have solid state or meaningful PC board experience.
2) Sell it on eBay and disclose the hum; returns not accepted?
3) Does anyone here want it? You can have it for the cost of postage or shipping, which I imagine will be $25-$30 due to the size of the box, which is 23" x 19" x 10". Yes, it is the original box, but the original packing materials are long gone.
4) Give it to charity, e.g., Restore, Goodwill, etc.?
5) Take it to the dump?

Thanks for your thoughts!

[sluckey]

Most likely suspects are the big filter caps.

[martin manning]

Power supply rebuild kits are available for around $20, if you need a project. Asking prices for working ones are $150-$200.

[Phil_S]

Thanks to both Steve and Martin. I had already checked eBait to see if it is worth selling. I agree that it is, even with the modest hassle factor. People seem to want these.

Here you can see, the only parts not mounted on a PCB are the two 4700u 35v caps. I'd happily change those. The biggest issue is size, 35mm diameter. Looking at eBay or Amazon as a source to avoid getting burned on postage, looks impossible to find 35mm to fit the mounting clamps. Would it be shoddy workmanship to get inexpensive radial caps and zip tie them to the existing clamps? Can you suggest an alternative? I'd still have the same problem with other snap-in caps that seem to be in the range of 20-25mm.

I don't know about the rest of the ecaps. Martin, I hear what you are saying...not sure I want to get a kit. Kit includes a transistor. I don't have any heat sink paste. my inclination is to start with the two big caps and see where to go from there. Does that make sense?

[martin manning]

Need a bigger cap? Go up in voltage rating: https://www.mouser.com/ProductDetail/Ni ... 7iTw%3D%3D

The big filters are probably the place to start, but some of the others may be going after sitting that long. The kit I saw didn't include many parts, so it probably wouldn't be too difficult. I see nine radial electrolytics and a transistor with a bolt-on heat sink on the power supply board in your picture. Used to be you could stop by the local Radio Shack and get a tube of heat sink grease. Are there any electronics shops still around your area?

[Phil_S]

Radio Shack is gone. There was a local store. One day I drove by on the way to somewhere else and there was a Sonic there instead. I was blown away, just torn down and gone. I may have even shed a tear or two. I was so shocked. That store was an institution. It was there for at least 50 years as best I can tell.

I've decided I can tolerate the postage from Mouser or similar place and that I can recap the power supply board. It's the two snap on caps and 9 inexpensive radial caps on the board. I don't see why I need to bother the other parts, diodes, transistors, etc. Please tell me if this is not good thinking. If I were going to do all of the parts, I'd want to pull the board.

I don't want to pull the whole board for fear of not being able to properly reconnect it. Two adjacent sides are wired to somewhere else. If I break the wire connections on the short side, I can hinge the board up far enough to work on it. But look at this picture -- not of the board but nearby -- this is the type of wire connection I'm looking at on the board. It looks solderless. I'm reluctant to unwind the wire. I know I don't have the manual skills to put it back the way I found it. Would it be objectionable to clip the wires further down and then solder/splice and heatshrink to repair them? I followed some to the other end and there are more of those solderless connections ;-(. I thought I could break the wire connections at the other end, but I don't want to with those solderless connections.

[sluckey]

Those are called wire wrap connections. A special wire wrap gun is needed to install the wires on the posts. Very secure and reliable connection when done properly. You also need a special unwrap tool to properly remove the wires. And the wires cannot be reused once unwrapped. Most '60s and '70s data processors and computers had thousands of those connections.

I still think you should replace only those two big caps and re-evaluate.

[martin manning]

Radio Shack is gone. There was a local store. One day I drove by on the way to somewhere else and there was a Sonic there instead. I was blown away, just torn down and gone. I may have even shed a tear or two. I was so shocked. That store was an institution. It was there for at least 50 years as best I can tell.

RS sure was was convenient for some things. There are a couple of independent shops in business near me, and the computer mega store Microcenter has a decent selection of electronic parts and tools. There are a couple of locations near you, I believe, and FYI they have heat sink compound.

I've decided I can tolerate the postage from Mouser or similar place and that I can recap the power supply board. It's the two snap on caps and 9 inexpensive radial caps on the board. I don't see why I need to bother the other parts, diodes, transistors, etc. Please tell me if this is not good thinking. If I were going to do all of the parts, I'd want to pull the board.

Sounds good to me. Semiconductors are not likely to die from disuse, and if you are ordering the big caps from Mouser you might as well get the others too.

[Phil_S]

The sounds like good advice. Steve, I hear you. I'll see what the economics look like. My concern is, if just buying the snap in caps doesn't fix it, I may get bit by the double postage. It may pay to buy all the caps even if I don't use them.

Thanks very much for the explanation of the wire wrap connections. I had a feeling it about it, so I'll need to cut the wires somewhere and then mend them. Meanwhile, I figured out where to mount a terminal strip, so I've got a solution to dealing with the wires! I still need to drill one hole to mount the other foot. If I have to splice on some extra length, so be it.

I think I'm good to go from here. It may be a little while until I get everything and do the job. I'll do my best to remember to post about the result.

As always, I really appreciate the help.

[sluckey]

Yes, I'd go ahead and buy all the caps, but still only install the two big ones first, then re-evaluate. I bet that if you spend some time on Mouser that you can find some caps that will be drop in replacements. Might be a bit more expensive than generic snap-ins. Start by measuring the diameter of the originals. OK to go with higher voltage rating.

[sluckey]

Oh, BTW, here's an example of some typical backplane wiring from the '70s. Google "backplane wiring" for some really scary pics...

https://i.redd.it/qmbbkzt4bqa31.jpg

[Phil_S]

Wow, that wiring, it's really something.

I decided to go the cheap route and take my chances. I bought 2x 4700uf 35v Nichicon radial caps on eBay for ~$8 total, USA seller. Should have them in a week I guess. I'll mount a terminal strip over the snap in holes and make it work. If that doesn't work out, I'll be no worse off going to Mouser for the rest of the power supply caps. It will be around the same total cost even with the shipping, and while I'm at it, I can buy a few other things I don't need.

I searched Mouser for longer than I should have. Even the cheaper 35mm diameter snap in caps are in the $4 range. No, surprise, there are many items out of stock.

My logic on this repair is that I'd like to sell it on eBay with no hum. If the buyer is repair savvy, he can do what he wants with it when he gets it and if he isn't, he won't be any the wiser. I don't need to disclose that "service" was done. I can make a decent and proper installation of the radial caps. I'll leave the old can clamps in it. This isn't like selling a guitar amp where the buyer wants to see the amp porn. It will be some pictures of the item showing the good condition of the wood cab, front and back panels. That's about it.

[Phil_S]

It turned out to be a butt ugly cap job. The problem is that I thought it would be OK to have 3x 20AWG solid wires, a resistor, and a stout cap lead on one terminal. I should have used two terminals with a bridge. In any case, I managed to get them soldered properly. Steve gets the prize for good instinct. At the get-go he said just change the 2 big caps. It's quiet now - zero hum. The only thing is that is saw a blue spark in the vicinity of the power supply board when I turned it on, and didn't like that one bit. However, both AM and FM seem to be running fine. I couldn't tell if volume wasn't what I'd have expected because, in the basement, with no antenna, I didn't get a very good signal. When I have a chance, upstairs, with decent speakers and antenna, I'm expecting all will be as expected. At least, that's what I hope. Either way, eBay, here I come!

Thanks very much to both of you for both advice and encouragement. I am very appreciative.

[ampbldr2]

I rebuilt my late dad's Pioneer SX-1050 (120 watter) during 2020 and it was manufactured in 1976-77 and still going strong. Some of the best years of Pioneer Engineering.

You should keep that SX-535 for your grandkids to enjoy.

[Phil_S]

I rebuilt my late dad's Pioneer SX-1050 (120 watter) during 2020 and it was manufactured in 1976-77 and still going strong. Some of the best years of Pioneer Engineering.

No question about it. These were great units. This one belonged to my wife's brother, who must have taken it to college in the mid 70's. The box still had a tag on it for delivery to his dorm room. I have the SX-424, which I bought sometime after 1976. I don't remember if it was new or used. Unfortunately, one channel seems to have bit the dust.

You should keep that SX-535 for your grandkids to enjoy.

LOL, my wife is getting ready to downsize and she's on the warpath to get rid of anything we don't use. I was able to rescue this from the pile for the dump! That's not one of the choices offered, and so, after 41 years of being married to the same woman, I may not be any smarter, but I am a bit wiser. I know when I'm licked. I still have to deal with the SX-424, too. I don't get to keep that one either.

She hasn't asked about any of the 12-15 guitar amps I have here. I won't be saying anything about those. I can hear it already, "Do you really need all those guitar amps?" We all know the answer to that one.