Barrakkuda's SX-1250 Restoration Project

[barrakkuda]

Time for an update.
So I thought it was surprising that after I did the Flat Amp board AWG-042 there was no improvements on the right channel. I don't have much experience with this, but I thought this board surely was causing the issues. Anyways I continued with the Protection Circuit Assembly AWM-091, but didn't see any improvements after doing that one either! Surprised again, I decided to go back and triple check the Flat Amp board, and what do you know, I found a couple of issues. I found a mistake I made on the PCB design, one of the traces was not connected to the rest of the net, but fixing this didn't fix the audio issues. Then I found a broken wire connection at pin 44 of the Function Switch Assembly. AWS-094, that goes to pin 26 of the Flat Amp Assembly. I wonder if someone did all the damage to the boards looking for this issue! Fixing that fixed the crackling and popping on the right channel, and it's now as high as the left channel. The sound is still not great, but It was a relief to fix these issues before moving on to other boards.

Now, I've started doing the Power Supply Assembly AWR-107, and unfortunately someone has messed with this board too, one of the corners of the PCB is broken. Also, it looks like they unsoldered one end of all the diodes and some capacitors on this board to test them out-of-circuit? The broken piece of the PCB had been glued back but it was falling off. So decided to just replicate this board too, same as the Flat Amp. I suppose replacing boards, with non-original ones, lowers the value of the receiver, but I want a working receiver, and I can do these things, so I'm doing them. I'll save the original boards and put them on a frame or something...

Tonight I'm submitting the PCB files for printing, I'll probably get them in a week or so I'll have to wait to continue until then.

The more I get to know this receiver the more I notice its long history. Here are some more pics of the progress.

New Flat Amp AWG-042 PCB with a bit of jumper wire fix. I've corrected this error in the cad files too.


New AWR-107 PCB. If you spot any issues, let me know!



Power Supply Assembly AWR-107, with a broken corner.




Scars from the past:

 

[Mr.Heatsink]

Good stuff Barrakkuda!

 

[Mr.Heatsink]

 

[barrakkuda]

So before doing the Power Supply Board (AWR-107) I had the left channel go out a couple times, and noticed that the volume control seems to affect it, When I raise the volume quickly sometimes it comes back, I'm not sure if it's a mechanical issue with the variable resistor on it or some electrical issue on one of the boards. I went ahead and did the Power supply board and both Power Amp boards (AWH-048), but had no improvements on the left channel. I was hoping not to have to open up the volume control but now I will.

First I'm going to do the bad one that the receiver came with and see if it can be repaired. I think I'll have to use my Dremel to grind down the metal post that holds it together, it's not the type that has the pressure clip. But I kind of doubt that it can be repaired. While cleaning it I noticed something rattling inside, and when I opened it up to spray QD contact cleaner into it, some pieces of plastic or PCB came out, not sure what it is. Then I'll remove the volume control, I got from ebay, from the new Flat Amp board and open that one up and see if it needs more cleaning. Hopefully that will fix the issue with the left channel.

One other thing going on is that the Stabilizer board (AWR-106) is as bad as the Flat Amp board. I decided to get the new redesigned one from delta9electronics on ebay. I also go the new redesigned Protection board (AWM-091) to try it out, even though I've already rebuilt that one. So waiting on those to arrive.

New Power Supply PCB AWR-107 all done:



This, what looks like a burn mark, was on the metal bracket by the Power Supply board. Someone must have done it while working on this board. ...


Rebuilt Left and Right Power Amp boards AWH-048:





Terrible condition of the Stabilizer board AWR-106:



Particles of something that came out of the bad volume control when I cleaned it:

 

[Sonytubes]

Fascinating!!!

 

[barrakkuda]

I was able to open the volume control (ACV-169), the faulty one that came with the receiver. I used a Dremel with a small cutoff wheel to grind off the metal holding down the plastic disc, the one you can see in the picture above. Then, I used straight and pointy electronic tweezers, inserting them very carefully between the black plastic case pieces to separate them, completely pushing out the white plastic wiper discs. This allowed me to fully disassemble it.

Afterward, I cleaned the carbon pads very carefully with QD contact cleaner and applied Fader lube to them. I reassembled everything, ensuring that the wipers made proper contact with the carbon pads. Unfortunately, after testing it, I observed no change, it still produced poor resistance measurements. I believe the carbon pads are worn in some areas where the wipers make contact. I've been researching the topic of repairing resistive carbon pads and found a couple of posts where people suggested using colloidal graphite liquid to fix them. So, I've ordered some of that to experiment with.

While waiting for the graphite to arrive, I'll continue working on the rest of the boards. The new Stabilizer and Protection boards I ordered will take another 2 weeks to arrive from Canada. I'm hopeful that the volume control isn't the root cause of the issues with the left channel. Hopefully, working on the other boards will resolve the problem.

I've also been curious about how these variable resistor PCBs are manufactured. They don't seem very complex to replicate. I've read that this ACV-169 is a logarithmic volume control. I'll need to research that further and find out how the carbon pads are made. Something interesting to note is, if you look closely at the PCBs in the pictures, you can see thin, 1-2mm lines on the carbon pads along the sides. These are areas where some of the carbon coating has been removed, presumably to adjust the resistance of each step of the potentiometer.

Volume Control ACV-169 fully disassembled:

 

[Sonytubes]

Sounds like a breakthrough plan, resurfacing with colloidal graphite liquid.

I hope it works !

 

[Colleton]

Making your own replacement boards via PCBWay is very, very cool. Impressive!

Why's it gotta be a C64? Atari 800's were where it was at...

I got into this the same way - rebuilding old 80's/90's computers (Ataris, and Macs) before trying a rebuild on a Marantz 2250B. I've been having fun rebuilding old stereo equipment ever since. Best of luck with your rebuild, I think you're doing great.

 

[barrakkuda]

So the new Stabilizer and Protection boards, from delta9electronics, arrived. They look good and came with clear instructions, and supposedly work better that the original boards. So far I've only installed the new Stabilizer board (AWR-106), the old original one was in really bad shape, as you can see in the pictures I posted previously. I won't replace the Protection board with the new one yet. Then I tested the receiver and checked/adjusted the voltages on the new board, everything worked. No change on the volume issues.

Regarding the issues with the left channel, I'm not sure yet that volume control is causing the issue or not, but I've gotten interested in figuring out how to fix these things, or finding a modern replacement. If you recall, from one of my previous posts on this thread, the original volume control (ACV-169) that came on the receiver was giving me, what I believe are, bad resistance readings, so I bought an original replacement on eBay. This replacement did give good resistance readings so I installed it, and after testing the receiver I found that the left channel would go out sometimes and come back after fiddling with the volume control. I think it would come back more often when I lowered and raised the volume quickly. Why? I ended up buying another original replacement volume control from eBay to compare, not cheap, and this one gives me bad readings too, as the one that came with the receiver. That has gotten me a little confused. Those readings can't be normal, right? I shared them on this post previously. I'm hoping that the volume control currently installed is not the cause of the issue, but if I can't find otherwise, I'll have to try cleaning it again.

While working on that, I've also been researching solutions for a modern replacement for this pot. I haven't found much about other people doing it for this particular receiver though, but I have learnt about other types of volume controls used in audio, like logarithmic resistor ladders and stepped attenuators. It's something interesting I'll probably research more after I'm done restoring this receiver. If anyone has any advice for me on this topic, I'd really appreciate it.

Another board I got to do, since my last update, is the Equalizer Amp board AWF-021. Almost everything went well with this one, just a solder pad lifted a bit, from one of the wire posts. These posts go on very tight, I think it would not be a bad idea to enlarge the holes a little, the solder pad probably got lifted when pushing the post through. The receiver tested fine after restoring this board. No change on the left channel issue, and I think sometimes the right channel does the same thing, unless I plugged in the speakers reversed.

Here are some pics of my latest progress.

New Stabilizer board AWR-106:




Equalizer Amplifier board AWF-021 done:
(forgot to take pictures before reinstalling it)

 

[barrakkuda]

Great news: the issue with the left channel, which was intermittently cutting in and out, seems to have been resolved. Contrary to my initial suspicion, the problem wasn't caused by the volume control. Instead, it was due to a cold solder joint in the Control Amp Assembly AWG-041 and likely a broken wire connection at the Function Switch Assembly AWS-094.

Since my last post, I had been questioning whether the volume control was really the source of the issue. It seemed more like a broken connection, especially since the left channel would sometimes return to normal when I quickly adjusted the volume. This seemed to cause the receiver to vibrate just enough to momentarily re-establish the broken connection. Consequently, I started inspecting the other boards I hadn't yet checked, using a magnifying glass to search for cold solder joints without removing them. Eventually, I found one on the Control Amp board and fixed it (I'll address the rest of this board later). This discovery led me to test the receiver by gently pushing and pulling on various boards to see if the left channel would react. Sure enough, when I manipulated the Function Switch board, the left channel flickered in and out. If I had more experience, perhaps I would have thought to check this sooner.

Moving on, I've now tackled the Function Switch Assembly AWS-094. I completely removed it from the receiver, cleaned the switches, and replaced the sole capacitor on this board. After reinstalling it, the left channel issue has vanished. I'm super happy about the progress in the restoration.

The next task is the Tuner Assembly (AWE-068), which is a major component. I'll be removing it entirely from the receiver, just as I did with the other boards, and then I'll move on to the few remaining ones.

Another issue with this receiver is the wiring. Some of the wire ends are in really bad shape, so I'll need to address them somehow. It looks like some may have been shortened in the past. I'll likely splice in new ends where necessary and use heat shrink tubing where needed.

Not many pictures today. I'm not that good at documenting the process.

Function Switch board (AWS-094) before and after. Forgot to take a pic of it back on the receiver and now it's blocked by the Flat Amp board.
Before:


After. Not much difference to see visually but resoldering all those posts and wires to this board contributed to fixing the issue.

 

[Mr.Heatsink]

Your documentation is just fine Barrakkuda. Keep up the good work.

 

[barrakkuda]

Happy New Year, everyone! I took a break during the holidays and have just returned to working on this project.

So far, since my last update, I've restored the Tuner Assembly AWE-068, which turned out well (see the pictures below). I plan to attempt the tuner alignment later on. However, after working on this board, I tested the unit and encountered issues with both channels, including very low volume and some crackling sounds. I conducted several of the same tests as before, which involved pushing and turning all buttons and knobs . I discovered that the buttons on the Filter Amp Assembly AWM-089 were the source of the problem this time. It's odd that I hadn't noticed this before, as I'm sure I had tested those. Anyway, after removing the Filter Amp Assembly, I found that this board also had some damage from previous repairs. While not as severe as the other boards, several through-hole pads were lifted. I proceeded to remove all the components from this board, scanned it, and replicated it in KiCad to have some new boards made. It will take a week or so for those to arrive. In the meantime, since this board is not too far gone, I decided to attempt to repair it. I acquired some through-hole repair eyelet rivets from an online source and achieved pretty good results. I'm undecided on whether I should use the repaired board or the new one. What would you do? At this point, several boards have been replaced, so what does it matter?

After I finish with the Filter Amp, I'll work on the other two on that side: the Speakers Switch and the Control Amp boards.

Here's just a cool pic of the receiver after I removed the Tuner board:



Tuner Assembly AWE-068 before:


After:



Filter Amp Assembly AWM-089, damaged pads:





Pad repair with tin plated copper rivets. Good enough to salvage this board:

 

[Mr.Heatsink]

Nice work again Barrakkuda, may i ask what method(s) do you use to remove your wire wrap post? Wire cutting and rewrap or desoldering the post altogether? It's quite nice.

 

[barrakkuda]

Nice work again Barrakkuda, may i ask what method(s) do you use to remove your wire wrap post? Wire cutting and rewrap or desoldering the post altogether? It's quite nice.

Hello @Mr.Heatsink, I desolder the whole post without unwrapping the wires, making sure that the solder is melted enough so there's less risk of lifting the though-hole pads.

 

[barrakkuda]

Short update. I'm done with the Filter Amp Assembly AWM-089. I reused the repaired original board and not the new PCB I created because if the original is still good after repairing it, why change it, right? But it was a good opportunity to get that board recreated in case anyone needs one.

I've also finished cleaning and reflowing the Speaker Switch Assy. AWS-095. This one doesn't have any components to change so it was an easy one. After that, I'm doing the Control Amp. Assy. AWG-041.

Cleaning the switches of the Filter Amp Assembly AWM-089. While cleaning one of them, one of the small metal contacts got dislodged from its position and the switch got stuck. I had to take it completely apart to make fix it.


After soldering most of the components back onto the Filter Amp Assembly AWM-089, I fixed several through-hole pads (as seen in pictures from my previous post). Additionally, I had to repair a broken trace using a piece of copper wire. See if you can spot it in this picture (it's not yet fixed). This board was a bit beat up (nos as bad as the others) but you can see the amount of scratches from previous repairs...



Here's the reproduction Filter Amp Assembly AWM-089 I created.
It came out good. I'll probably post them on eBay. If any of you are interested. Let me know.



This is the Speaker Switch Assembly AWS-095, all clean, reflowed and ready to go back on the receiver.

 

[Mr.Heatsink]

Hello @Mr.Heatsink, I desolder the whole post without unwrapping the wires, making sure that the solder is melted enough so there's less risk of lifting the though-hole pads.

Thanks for sharing, I'll give it a go. Once again beautiful work so far.

 

[barrakkuda]

So I've been enjoying listening to my SX-1250 for the past couple of weeks after having completed its electronic restoration. Here's what I've done since my last update.

The next board I did was the Control Amp Assembly AWG-041, big one but uneventful, only several caps and transistors got replaced.

Here it is already done:



The last board I did was the Microphone Jack Assembly AWX-097, Nothing to be replaced here, just some cleaning:



After that, I removed and cleaned the big power transistor heatsinks with soap and water, then reinstalled the original transistors with fresh thermal grease. I didn't take pics of that process but here's one of one of the heatsinks already cleaned. The finish is not in the best condition but for now I'll leave it alone. I'll be sandblasting and powder coating them soon.


At this point the electronic restoration is mostly done but I still need to fix a bunch of issues with the wiring. As you can see in previous pictures, several wires have burned insulation at the ends and just look bad. I also replaced all the lights with LED types. The rubber brackets AEB-086 and AEB-085 for the lights are very dry and brittle so I took the time to replicate them. I modeled them in then computer and 3D printed some copies but decided to use the original ones because they're still holding up, even after replacing all the lights.

Before:

After:


3D printed replacements for anyone that needs them:


All put back together with new LED lights:


Enjoying it for a while before I start the rest of the restoration:

 

[barrakkuda]

Just a quick demo.

 

[dapbmw]

Here's the new board, almost fully populated with components from the flat amp board restore kit, along with any items that are not being replaced. There's something worth noting about the two FETs on Q5/Q6: the service manual specifies 2SK30, but I found 2SK34's on the original (damaged) board. The replacement flat amp board I purchased from eBay came with the correct two 2SK30 FETs, so I installed those on the new board.

Another observation I made on the original board was that one of the electrolytic capacitors (C22) had been incorrectly installed in reverse polarity. The only thing that's currently missing is the volume control pot (ACV-169). I've found a replacement on eBay, so I'll wait for it to arrive before testing and comparing resistances between it and the one I already have.

Some pics:
View attachment 3024027View attachment 3024026View attachment 3024024View attachment 3024025
I'll give it a good clean after I install the volume control.
View attachment 3024023Do you still have the files for board? That I can send so pcbway? Cheers!

 

[dapbmw]

So I've been enjoying listening to my SX-1250 for the past couple of weeks after having completed its electronic restoration. Here's what I've done since my last update.

The next board I did was the Control Amp Assembly AWG-041, big one but uneventful, only several caps and transistors got replaced.

Here it is already done:
View attachment 3156044
View attachment 3156045

The last board I did was the Microphone Jack Assembly AWX-097, Nothing to be replaced here, just some cleaning:
View attachment 3156049
View attachment 3156050

After that, I removed and cleaned the big power transistor heatsinks with soap and water, then reinstalled the original transistors with fresh thermal grease. I didn't take pics of that process but here's one of one of the heatsinks already cleaned. The finish is not in the best condition but for now I'll leave it alone. I'll be sandblasting and powder coating them soon.
View attachment 3156053

At this point the electronic restoration is mostly done but I still need to fix a bunch of issues with the wiring. As you can see in previous pictures, several wires have burned insulation at the ends and just look bad. I also replaced all the lights with LED types. The rubber brackets AEB-086 and AEB-085 for the lights are very dry and brittle so I took the time to replicate them. I modeled them in then computer and 3D printed some copies but decided to use the original ones because they're still holding up, even after replacing all the lights.

Before:
View attachment 3156075
After:
View attachment 3156077

3D printed replacements for anyone that needs them:
View attachment 3156076

All put back together with new LED lights:
View attachment 3156078

Enjoying it for a while before I start the rest of the restoration:
View attachment 3156080

I want one, do you have the stl file for
the bulb holder?