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Pioneer SX-50 suddenly very unstable, going in and out of protection mode.

S

Saffire

Active Member
My SX-50 has suddenly started acting really weird. This afternoon, I turned on the stereo, and the relay took around 10 seconds to click on. About 5 seconds longer than usual. I hit the speaker A switch, and was greeted with loud, scary pops through my speakers as the relay started clicking off and on at random. I shut off the stereo and disconnected everything.

I thought that the bias may be at issue here, so I turned the bias pots to 0, sprayed them with some CRC contact cleaner, and readjusted them back to 5mV. That still didn't fix the issue.

I then checked the DC offset of both channels at the speaker terminals. The right channel sat at 25mV, while the left channel was rapidly fluctuating from the negatives all the way up to 60mV. Seems highly unusual.

Now, here's what I discovered. When the stereo is powered on, it just sits idle with a buzzing sound only on the right channel, indicated by the on-screen output level meters. However, when I turn the sub-sonic filter switch, the relay will click off and not turn back on until I restart the stereo with the sub-sonic switch off. I'm thinking that my problem lies in the tone section of the board. Any ideas?

EDIT: The relay NOW refuses to turn back on until the sub-sonic switch is depressed. There's a 3 second delay until the relay turns back on after I release the sub-sonic switch. It's not only the sub-sonic switch, either. The relay will also click off when I turn on the simulated stereo switch, and click back on after releasing the switch. Where do I start from here?
 
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Okay, NOW this is weird.

I enabled both of the sub-sonic and simulated stereo switches, and the relay clicked off. But after a few seconds, it suddenly turned on again. It went off and on twice. I gave it a few seconds to chill out, and after restarting the stereo, it's now working fine again? But I'm still suspicious. When this happens again, I'll upload a video of the problem. That will make things more clear.

I'm not going to plug my speakers back in, this stereo has an issue that needs to be tracked down.
 
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I just took a DC offset reading of both channels, and they're both calmly fluctuating under 10mV, they don't go any higher than that. Maybe this was just some sort of freak problem? I dunno. I'll wait for you guys to chime in before I start setting everything back up.

The stereo currently works perfectly fine, no more clicky clacky relay, and it plays music through my headphones. Very suspicious indeed. I assume that this is some kind of short circuit somewhere. It's powered on right now and just chilling quietly, nothing seems to be wrong with it now, but I know it's planning something... :bs:
 
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Day 2:

The receiver has been running for an hour now, playing music through headphones, everything seems perfect. I really don't know what to do here. Should I just leave it and let be?

I'm still thinking that this was a bias issue. When I adjusted the bias on this receiver, it was very, very touchy. One sudden movement would cause the bias to jump from 4.5mV to 8mV. I set both channels to 5mV, and gave them a few minutes to stabilize. They would fluctuate around 4.7mV to 5.4mV, but it seemed good enough.

I figure that the reason this receiver now works properly is because it just needed some time for the bias to stabilize.
 
Well, one point I would check first is soldering points, especially of voltage stabilizing transistors in the power supply. They tend to unsolder themselves over time at a lot of Pioneer amps and receivers. And this can lead to exactly those effects, especially in a non switching amp.
 
Thorsten R. said:
Well, one point I would check first is soldering points, especially of voltage stabilizing transistors in the power supply. They tend to unsolder themselves over time at a lot of Pioneer amps and receivers. And this can lead to exactly those effects, especially in a non switching amp.
Click to expand...

I've looked over the solder joints yesterday, and they all seemed alright. How can I tell if a joint is bad? I didn't spot any cracks or oddities in any of the joints.
 
Thorsten R. said:
Well, one point I would check first is soldering points, especially of voltage stabilizing transistors in the power supply. They tend to unsolder themselves over time at a lot of Pioneer amps and receivers. And this can lead to exactly those effects, especially in a non switching amp.
Click to expand...

Ohh man! I just checked out the voltage stabilizing transistors, and it's EXACTLY the reason you mentioned. The transistors felt kind of loose when I touched them. So I looked underneath, and found loose solder joints holding them in place. I'll re-flow them, and report back.
 
When I sold and serviced Pioneer amps in a prior life (I have learned this job in the typical German way beginning as apprentrice and some years late getting the master craftsman´s certificate) about 90 % of them had this fault Including my own SA 8500 II. And it will be the first thing I will check when in a few days my "new" SX-550 arrives. Before I try to power it up.
 
Thorsten R. said:
When I sold and serviced Pioneer amps in a prior life (I have learned this job in the typical German way beginning as apprentrice and some years late getting the master craftsman´s certificate) about 90 % of them had this fault Including my own SA 8500 II. And it will be the first thing I will check when in a few days my "new" SX-550 arrives. Before I try to power it up.
Click to expand...

Here's a before and after. Red is before, and white is after. The factory used too little solder, and you can see cracks in the pads of the "before" transistor.

20200813_152806.jpg
 

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Alright, everyone. I'm happy to say that the SX-50 is back up and running great. However, I still need to be on the look out for this issue. Here are some pics of my repair work, a before and after.

20200813_152026.jpg 20200813_172506.jpg

I had to connect a jumper wire to one of the solder joints, because it had a lifted pad. It wasn't that bad, only half the pad was lifted, but I still wanted to ensure proper contact and reliability. So I ran a jumper wire.
 
Thorsten R. said:
When I sold and serviced Pioneer amps in a prior life (I have learned this job in the typical German way beginning as apprentrice and some years late getting the master craftsman´s certificate) about 90 % of them had this fault Including my own SA 8500 II. And it will be the first thing I will check when in a few days my "new" SX-550 arrives. Before I try to power it up.
Click to expand...


Well, it's still doing it again. :(

I was letting it run for a little bit, playing music through headphones, and the issue happened again. The receiver clicked off the relay and sent a loud pop through the headphones, it clicked off and on until I restarted the receiver. It did it again, twice, then went back to normal. Got any more suggestions?
 
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Sorry for all the confusion. Right now, the receiver works, but it still occasionally goes into protection mode and starts clicking in and out of it. Let's leave it at that.

Current DC offset readings sit below 10mV. I can't get an exact reading because they bounce all over the place. They don't go over 10mV, though.
 
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When I look onto your pictures: There are obviously more internally cracked solder points. If the surface of a point is not absolutely smooth and shiny it is suspicious. When there is a sort of dull ring around the wire resolder it. The wire can already be oxydized in the connection even then the soldering point does not look that bad.

E. g. take a look on your last pic: Look at the lower edge, the first soldering point at the right side of the middle looks extremely suspicious.
 
Thorsten R. said:
When I look onto your pictures: There are obviously more internally cracked solder points. If the surface of a point is not absolutely smooth and shiny it is suspicious. When there is a sort of dull ring around the wire resolder it. The wire can already be oxydized in the connection even then the soldering point does not look that bad.

E. g. take a look on your last pic: Look at the lower edge, the first soldering point at the right side of the middle looks extremely suspicious.
Click to expand...


You're right, I didn't even notice that one. What about the solder joints that I re-soldered? The reason they look dirty is probably because of the solder I used, it's flux-core leaded solder.

EDIT: Man, oh man! I really should've been more thorough! Looking back at the photos I took, I'm seeing cracked solder joints galore!

What I'm going to do is buy some PCB flux, along with some epoxy to hold the voltage stabilizers in place, because those stabilizers are being held on only by the solder joints, no bueno. So it'll be a few days before I start working on this again, but I'll take the time to mark all broken joints I see with a sharpie.
 
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The usage of flux is what I wanted to recommend anyway. I would not use epoxy. 1) It would be better to control the soldering points regularly; 2) If a transistor has to be swapped epoxy makes the task perhaps difficult; 3) The epoxy might worsen the heat dissipation of the transistors. Please observe: a lot of the heat dissipation is done by the "legs" of the transistors, diodes, stabilizers and such stuff.

And you are right about the "dirt".
 
Thorsten R. said:
The usage of flux is what I wanted to recommend anyway. I would not use epoxy. 1) It would be better to control the soldering points regularly; 2) If a transistor has to be swapped epoxy makes the task perhaps difficult; 3) The epoxy might worsen the heat dissipation of the transistors. Please observe: a lot of the heat dissipation is done by the "legs" of the transistors, diodes, stabilizers and such stuff.

And you are right about the "dirt".
Click to expand...

The transistors are both glued down from the factory, though. However, due to the heat, the glue dried up and broke away from the transistors over time. If I wiggle the transistors around, you can see the solder joints move around very slightly on the PCB. There's no solid connection there. Plus, the transistors are both attached to metal heatsinks, and those heatsinks are glued down to the board. Well, they were glued down.
 
@Thorsten R. Ah, I've stumbled into a predicament.

Check out my last pic with the solder joint that has a jumper wire connecting it to the nearest point.

I de-soldered that joint because I felt that it could be better. When I used my solder sucking pump on the joint, it sucked up half of the solder pad! So I was left with one half of a solder pad on the joint, that isn't good.

So, what I did was I made a neat connection with that jumper wire to the nearest point, and then used some solder to bridge both solder joints, which covered up the jumper wire. See here:

20200815_210427.jpg

Now, my question is; should I be concerned about this solder bridge cracking and breaking over time? I know that the voltage stabilizer is a high heat, high stress component, and I don't want it to be breaking the joints again.
 
Yes it will break, If you have done it right in about 50 years :trebon:
You should consider whether the glue is a heat sinking compound.
 
Thorsten R. said:
Yes it will break, If you have done it right in about 50 years :trebon:
You should consider whether the glue is a heat sinking compound.
Click to expand...

Got her back up and running! I've reflowed every cracked solder joint I could find, and the receiver is now working flawlessly. Thanks for your help!
 
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