Pioneer SX-434 still humming after recap?

[weplowman]

Hi there,
This is my first question on AK so bear with me.
I recently got hold of a Pioneer SX-434 from Gumtree as my first project. It cleaned up pretty well and I was able to replace the blown lamps. I connected up a CD player and it played music However there was a noticeable hum on both speakers after about 3-5minutes. I turned it off, disconnected the CD player and powered it up again. The hum came back :thumbsdn: This is with volume at 0 and on all inputs (am, fm, phono and aux).
I thought a recap would sort it. I recapped the power board and replaced the filter caps. I still get the hum. I then replaced the transistors on the power board. Still the hum!
I have ordered the parts to recap the amp board. Do you think this will sort it out?
If not what's the next step. Is the transformer dodgy?

 

[weplowman]

I was reluctant to recap the amp board straight away as I struggled to get the board out far enough to work on. I've worked out how to do that now - I need to disconnect the input selector switch. earth lead and headphone socket to get enough slack on the cables.
I'm just waiting for the parts to be delivered.
I didn't order the Nichicon KL caps as listed in the AK doc (couldn't find a UK seller). I plan to use Nichicon FW caps. Will they be OK?

 

[Watthour]

Frequently, low frequency humming at 60-120 Hz in both channels and all modes is related to the power supply. In addition to the capacitors, diodes in the PS rectifier bridges should be checked. At the same time connections/solder joints can be inspected.

 

[larryderouin]

Watthour's right on the power supply being suspect. Check pins 4 & 5 on the power supply with meter set for DC and again for AC. Write down voltages and report back. A bad diode will cause an AC voltage on a DC circuit which in turn causes a hum. Anything more than a few millivolts AC is no good and you need to troubleshoot the diodes.

Check and replace as needed all parts that are not in spec., Including Diodes. You can use 1n4004 diodes for the power supply diodes, although I used UF4004 in mine. The UF's are supposedly quieter, but in the 434 i don't think it make's any difference.

There are some chassis grounds on the 434. Make sure they measure 0 ohms and are tight. If a rivet is loose, drill it out, clean the metal on the chassis and lead ends to bright metal, and use a nut and bolt with a lock nut (nylock), or if you have a blind rivet tool, rivet it. spray the hole and the lead ends with DeOxit before assembly to enhance the electric contact.

What's the AWR # on the POWER SUPPLY? There should be a round sticker on the back of the unit with one of 3 designations.....KCU, FV, or GN. GN is 5voltage general Export. KCU is US and Canada, and FV is Denmark, Norway, and Sweden approved. As you are in the UK the power supply board is probably different than in the colonies(if you have the GN version). Do you have the Service manual and or the board schematics? Manual @ hifiengine.com . The individual board drawings/schematics are hereand here.

 

[jheu02]

The problem with diagnosing this is there really wasn't enough information... The OP states he had hum, and assumed capacitors right away. No information on pitch of the hum...60Hz pre rectifier, 120Hz...after rectification. It makes a big difference as to what should be the first things to look at. If the 60Hz, I'd suspect a bad ground somewhere, as Larry was alluding to. Check all ground wires and soldered connections.

I just chased down a hum on an SA-8500II that only appeared when I touched the volume control. Quiet room, unit powered on and volume set so I could hear the hum. Then, while touching the volume control so the hum would show itself, I CAREFULLY started moving ground wires a little. I found that moving a ground jumper wire between two wire-wrap posts sticking out of the volume pot was the culprit. Soldering the wire to the post made no difference in the hum...I think the wire was acting like a little antenna. By moving it, the hum went away.

 

[weplowman]

Thanks for the tips. I won't be able to check this evening but will let you know what I find.
I already have the schematics. I can see there are a couple of four prong ground strips. I will check those plus the rectifier diodes when I get a chance.

 

[weplowman]

The power supply board is AWR062. I measured the voltage as suggested.
Pin 4 -23.2V DC
Pin 5 23.1V DC
No AC voltage at either pin.

I checked the grounding strips (see images) The one nearest the transformer has had solder run round it. I can still loosen the screw though. Is this how it would have been shipped from the factory or has somebody tried a repair?

Do you think this could be the source of my trouble. I am thinking of removing the solder and cleaning up the area around this strip.

 

[jheu02]

Have you tried moving the various ground wires while the unit is powered up? You can always use a plastic or otherwise insulated stick/wand/etc if you don't want to stick your fingers in the unit while powered up. If you do use your hands, make sure you only stick one inside, the other in a back pocket or such. More applicable to the high voltages of tube amps, but good practice nonetheless. If there's issues with the ground wires, this is one way to find which ones have bad connections. Hook up "disposable" speakers or turn the treble down and don't have the volume set too high...you can get static-type spikes, scratchiness, etc which will come through the speakers.

 

[larryderouin]

OK. You've got the standard single voltage US version (KCU). I take it your house power is at least 220 and you are using a step down transformer.

Check all 5 diodes on the power supply board. 1 bad diode can cause a hum as you describe. Set your meter for diode, and meter it both ways with the probes on either end of the diode. One way you should get a voltage reading, the other will show an open (for a good diode). A bad diode will show voltage both ways.

FW caps are ok. I prefer the UKL's. But if they aren't available the FW's will do.

The ground points on the 434 (terminal strips) are screwed in ONLY. No solder used on them. A screw that can be turned when it's supposed to be tight has stripped threads and no possibility of a good ground is remote at best. Suggest getting rid of the solder, take off the terminal strip (undo the screw, possibly stripped threads)and lift up. Use a dremel or similar power tool to clean up the bottom of the strip and the chassis area. clean the area with Alcohol, or your favorite non volatile solvent, and some lint free toweling. Do not use the original screw. Re-Attach the terminal strip with a screw and locking nut (nylock), or double nutting. Re-check continuity with a meter. Also clean up any ground wire connections that have broken off and have been soldered on. Originally they were wire wrapped. Wire wrapping will give you 4 airtight connections per revolution of the wire around the post. BUT!!! It takes a special tool and lots of practice. Just make sure the connections from the terminal to the chassis are tight and ohm out good (less than 2 ohms or so. The closer to "0" the better, but the difference isn't really that noticable.)

One thing you can do is to reverse the 434's power cord on the step-down. Use whichever position that gives you the least hum.

I'll open my 434 and take a look at those ground points and compare them to yours. The one with the solder on it, looks a bit funny as far as wiring is concerned.
Larry

 

[weplowman]

Thanks for your advice. I already had a look at removing the soldered ground strip. The solder is very thick and I think I risk causing damage if I try. The screw isn't stripped. I have been able to remove it and tighten it as much as I can.
I measured at each of the grounds and they were at 1 ohm so good enough I guess.

BTW, I do have a general export model (220v). Maybe I misread the board number? I have already tried reversing L/N in the 3 pin mains plug but this had no effect.

I think I need to test the diodes as suggested. Do I need to remove from board to do this?

Bill

 

[jheu02]

I think I need to test the diodes as suggested. Do I need to remove from board to do this?

Bill

For an accurate test, at least one leg should be removed. Visually check to make sure they are oriented correctly even before getting the soldering iron out.

 

[two.dogs]

I'm rooting for you. Keep at it - the sx434 is a little jewel of a receiver. I bought one to practice recapping then gave it to a friend. Missed it so much that I bought another.

 

[larryderouin]

Thanks for your advice. I already had a look at removing the soldered ground strip. The solder is very thick and I think I risk causing damage if I try. The screw isn't stripped. I have been able to remove it and tighten it as much as I can.
I measured at each of the grounds and they were at 1 ohm so good enough I guess.

BTW, I do have a general export model (220v). Maybe I misread the board number? I have already tried reversing L/N in the 3 pin mains plug but this had no effect.

I think I need to test the diodes as suggested. Do I need to remove from board to do this?

Bill

Bill; Mine is a KCU US version. The General export and KCU version have the same # P.S. You won't do any damage to the strip by de-soldering the area. Just move the wiring out of the way and hold them back (use some heavy thread or string) while using the iron. Looks like a couple of the wires broke off at one point and they soldered them back on (No problem). If you feel more comfortable remove the soldered wires (tag them as to which pin) and then clean up the junk solder. You can refer to your photo's also.

Diodes ideally should be checked (like resistors, and caps) with one leg lifted. This gets them out of circuit and you'd be reading the part under test only instead of the circuit too.

Larry

 

[weplowman]

Well, I had the parts so I thought I would recap the amp board and replace the four SA763 transistors before tackling the ground strip. Once complete I powered up the amp and left it for 10 minutes. Lo and behold the hum had gone! :thmbsp:
There were a couple of caps (can't remember which) - same one from each channel - which look OK from above but once I took them out I thought the base had a slight bulge. I assume these were the culprits.
I left it powered on for the afternoon playing FM and no problems.
All lamps replaced (even fixed the FM stereo indicator), fascia cleaned up and wood waxed. It looks and sounds sweet!
It's currently hooked up to a Denon CD player, Raspberry Pi with HiFiBerry DAC running piCorePlayer and Mission 731 speakers.
Now to sort out the Rotel RP855 t/t I picked up last week!! (Could be another thread).
Thanks for all the help and support!

 

[Alienz]

:thmbsp:

...and also a big :thmbsp: for the Substance CD

 

[Edward Musci]

I'm rooting for you. Keep at it - the sx434 is a little jewel of a receiver. I bought one to practice recapping then gave it to a friend. Missed it so much that I bought another.

Hello, Another electronics rookie here. I have aquired a pritty nice sx434. She did sound nice until I was trying to diagnose a hum on both channels with a crackle type on one. Now with the speaker fuses connected fuse #7 and #8 on the power supply board blow. If I remove the speaker (external ) fuses it powers up and ps fuses are ok. I would love some help. I really like this pioneer, reminds me of my first Technics back in the early 80s.
Thanks so much.

 

[markthefixer]

The SX-434 has output capacitors in the power amplifiers. IF one were shorted, that could explain the symptoms.

When the speaker fuses and internal power supply fuses blow, you have speakers connected, correct?
You need to disconnect the speakers, and see if they stop popping.

If you have a DMM, measure the voltage at the speaker connections (but STILL without speakers connected)

 

[Edward Musci]

Thanks for the help.
Disconnected speaker wires, plugged in all fuses, turned unit on. All good, no smoke and fuses are not blowing.
Measured 24 v on the positive side of the speaker terms. That seems high right?
Also I am getting similar readings at tp1 and tp2.

 

[markthefixer]

Do one or both channels have the 24v at the speaker terminals (tip: that's NOT supposed to be there)

 

[Edward Musci]

Yes,both.