Pioneer SX-424 Troubleshooting

Spillmagic

New Member
My dad has a Pioneer SX-424 that he bought new in the early 70's. I remember it working when I was a kid, and he always had music playing in the house.

Now he went to hook it up after some years sitting and it won't power on.

I've taken to fixing it, and have searched through the existing forum posts on the SX-424 and have not found an answer to my particular issue.

I have checked the three fuses that screw out from the back with a continuity tester, and all three fuses are good. I also checked the soldered in fuse inside the unit and it is also good.

I haven't went very far in troubleshooting, as I seem to have hit trouble very early in the circuitry.

There are two 110v plugs on the back of the unit. One always on, and one switched. The always on plug always has power, and this works. When turning the power/input selector knob to one of the 'on' positions, the switched still gets no power.

I feel like this is telling me that the on knob is not functioning correctly. Would that be correct?

I'm also thinking that this would all happen before the transformer, so my power issue seems to be isolated to just the input/switch area, because from there it goes straight into the transformer to power the boards.

Attached are a picture of the unit as a whole, the wires in question are the white ones at the top, these carry the 110vAC. The bottom picture is of the power switch/knob.

Any advice is VERY appreciated. I would love to get this working again for him, but don't want to buy a $40 replacement knob if that isn't my issue.
 

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Sounds like the Power Switch on the back of the speaker Rotary switch assy is not making contact. This is a common problem on the PIONEER units that incoprporate the power switch on the Speaker selector. There are a few threads on how to disassemble the switch, clean/burnish the burned contacts, and re-assemble. Most will be on the SX-450 to SX-750 Receivers but they are very similar to the 424. Use them as a guide.
 
I got the power switch cleaned up and the unit powers on! This switch was quite a bit different from the SX-450. Pictures attached for reference, maybe they will help someone.

Took the switch off, desoldered the two white (AC) wires, removed the two screws to separate the pieces of the switch (after taking pictures), the piece you want is the brown plastic cylinder at the back of the unit, drilled out the rivets and opened the unit, bought a needle file kit and ground down the contacts (HOW IS THIS CONTACT SO SMALL?), replaced the rivets with 1-72 x 1/2 bolts and nuts, reassembled, soldered the wires back on.
 

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Now I am getting a hum, 60hz I think.

This hum is always active no matter the source, and is not effected by the volume knob.


I measured voltage DC across 35 and 37 = 39.5V

I measured voltage AC red on 35 and black on 37 = 87.8V

I measured voltage AC black on 35 and red on 37 = 0V

I only did this because of a thread from someone troubleshooting a 434 and Larry asked them to check these voltages. Does this mean a diode issue? I'm not sure what the AC one direction and not the other would mean.


I also checked resistance on all of the chassis grounds, and all points measure 0.5ohms.


Thoughts?
 
Check the diodes at Pins 33/34 and the 2200µF main filter capacitor in the power supply section. Also verify that all grounds are intact.

Take a close look at the schematic:

SX-424SchematicWide.jpg
 
Thank you @Watthour , I had the schematic, but didn't know how to link it like that.

I've checked the capacitor and diodes that you mentioned.

The cap (blue in my attached screenshot) appears to be bulging, I'm hoping someone could make a recommendation for a replacement. Part Number from Pioneer is C52-074-B. Specs are Electrolytic 2200uF 50V. The original unit is an ELNA brand. I take it this is the one to not skimp on the replacement, correct? (I'm super new to this)

The diode directly off of post 34 (red in my attached screenshot) is reading non-open in both directions. I did not remove one leg to test the diode, but I expected to find one of the failed shorted with the results I got on the AC voltage test. Does this seem like a correct diagnosis?

I have included the best picture that I can of the diode, could someone recommend a replacement? It doesn't look like the service manual includes any details about diodes used.



I've replaced the lights with soft white LEDs and cleaned up all of the accumulated dust, this thing is looking fantastic! Now if we can just get rid of this hum my dad will be absolutely thrilled.

And now that I've heard this thing in action, I'm going to have to find myself a garage sale unit or the like ;)
 

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From the service manual the diode is a FR2-02 you can use a 1N5404
for the cap use 647-UHE1H222MHD6 Mouser part number
 
Thank you all, I ordered the 63V capacitor that @Watthour linked.

Installed the capacitor and new diodes and I still get the hum.

Upon further inspection, there are quite a few capacitors that have physically leaked on the board. Specifically the 25V 1000uF caps (circled purple in first pic) at C57 and C58 on the AF board, and every styrol cap on the tuner board (circled red, second pic). There is also a metal oxide resister in the power area that appears to have leaked (circled blue in first pic). Also, there are two thermistors that appear to have leaked (circled red in the first pic), and I can't find a replacement for them.

I've made a cart on Mouser with all of the electrolytic caps on the AF board, along with all of the styrol caps on the tuner board, I even took the time to make sure they are sized correctly. I've also ordered a new 270ohm metal-oxide resistor. 1.) Does anyone know what a suitable replacement would be for the thermistors that have leaked? part number: 22D45

2.) How do I clean the capacitor goo off of the board?

3.) What am I missing?
I tried my best to figure the rest of this out myself and not ask any more dumb questions (I spent two weeks looking at the logical diagram for the AF board and tracing because I didn't scroll down farther to find the physical diagram and the full parts list, OOPS live and learn right?), but I concede to the knowledge of the forum to help keep me from making a 'gotcha' mistake.
 

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I suspect some of what you see is not leaking components but glue used to hold them in place for wave soldering. That glue has been reported sometimes to become corrosive over time. You can remove the component and scrape the glue (or goo) off with a sharp wood or plastic tool, then finish clean up with a q-tip and acetone (or similar solvent).
Also you can use this mod to protect your switch from future failure: http://www.audiokarma.org/forums/in...orth-5-and-an-hour-or-so-of-your-time.504673/
 
That actually makes a ton of sense! I would have never thought this was glue, I just assumed they were leaking. So this would be glue on any of the parts that wouldn't stay in place well through the machine. I couldn't figure out what would have leaked on the side/bottom of the transistor heat sink, this is a great explanation.

Do the Styrol caps still look bad though, with the rusty tops/rusty components next to them?

This still leaves me with a hum, I've replaced the main cap and rectifier diodes, where should I go from here if not replacing the other caps (I may do this anyway at this point).

Thank you for the link to the power switch fix, I think I will just open it back up and fix it if it happens again, it was a pretty easy process.
 
If the component leads are corroded they should be replaced if possible, so they do not fail in the future. Clean up the glue on the AF board (AWK-014), as it is no longer needed to hold the components in place, and can become conductive, causing current leakage to flow in the wrong places. Don't mess with the Tuner board (AWE-008) unless and until you are sure you have a problem with the tuner. It requires some special equipment and skills to realign it once it has been misadjusted or the components changed. It may then require a trip to a tech and $$ to realign it.
 
Is the hum there on all input selections? Does the hum vary with the volume control? Is it present and the same volume in both channels?
 
merlynski. The glue corrosion is NOT a corrosion problem on PIONEERS in the x2x and x3x series. Not sure about it in later X50 and X80 series. However it IS a DEATH knell for SANSUI's and some IIRC, Kenwoods or Yammies. Cleaning it off is advisable, but it just isn't the problem on PIONEER's that it is on SANSUI's.

As for the 22D45 Varistors, lift one leg out of circuit, and ohmmeter it using a set of mini grabbers. Use a blow drier on LOW HEAT and watch the ohmmeter for change whne heat applied. NOT A LOT OF HEAT. A couple of Seconds is all you need to see a change. Once heat if taken off, the reading should go back to the original reading. If that's the case, leave them in. These rarely if ever go bad in these units (at least I've not seen one go bad in 10-14 units I've done that had them).
 
@merlynski The hum is on all inputs, does not vary with volume, and is present on both channels. Everything I've found points this to the rectifier bridge or capacitors. I just ordered new ceramic capacitors, the ones that are parallel with the rectifier diodes, they are slightly cracked at the top of the leads. They may not be bad, but for less than $.50 a piece...

@larryderouin It's good to know that the glue probably isn't my issue. I'll still work on cleaning the glue up when replacing the caps.

I ordered all of the caps for the AF board and a desoldering gun (I've wanted one for a long time, nice to have an excuse to buy one finally). This thing is 45 years old, and there is a slight bulge to most of the caps.

The varistors may not be a problem now that I know the goo is actually glue and not leaking from components. I only thought that they were bad because I thought the glue underneath them had leaked out of them.

I'll report back in a week or so to let y'all know how successful the recap is.
 
merlynski. The glue corrosion is NOT a corrosion problem on PIONEERS in the x2x and x3x series. Not sure about it in later X50 and X80 series. However it IS a DEATH knell for SANSUI's and some IIRC, Kenwoods or Yammies. Cleaning it off is advisable, but it just isn't the problem on PIONEER's that it is on SANSUI's.
Thanks Larry! :thumbsup:
 
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