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Pionner SX 424 buzzes like crazy.

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drdickinson

New Member
So a couple of months ago I got my first proper vintage audio equipment, a Pioneer SX 424, it was a little rough around the edges but it worked flawlessly for the most part, until this morning.When I turned it on I was greeted with a loud buzz from the speakers, I turned it off immediately and checked all the audio connections, then I turned it back on and the buzz was gone, for like 10 seconds, then it came back, I tried all the audio inputs but nothing, the buzz is still there, and the thing is, the amp itself is buzzing as well, even without anything plugged in it. I opened it and it seems that the buzzing is coming from the AC Transformer, could it be that the transformer have just given up? Or could it just be a cap or something minor like that?
 
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A transformer will buzz if it's tired or heavily loaded because of a near short, ie, providing significant current.
However speakers will buzz mostly due to excessive dc voltage, typically caused by a shorted power
transistor or... Most likely the shorted transistor causing both symptoms. Disconnect speakers. Do not
use headphones, do you have a multimeter?
 
The amp is cap coupled, do you have anything that could act as a dummy load, ideally an 6-12ohm
resistor, at least 2 watts, prefer 5W+.

Is the amp original or has it been recapped/refurbished at some stage?
 
mbz said:
A transformer will buzz if it's tired or heavily loaded because of a near short, ie, providing significant current.
However speakers will buzz mostly due to excessive dc voltage, typically caused by a shorted power
transistor or... Most likely the shorted transistor causing both symptoms. Disconnect speakers. Do not
use headphones, do you have a multimeter?
Click to expand...
I have a multimeter, what should I check?
It seems like it have never been restored, so I guess that the caps are the original ones, about the dummy load, I can buy some resistors from the local electronic shop, so not big deal there.
 
The suspicion is that there is abnormal dc voltage at the speaker terminals.
Connect dummy load (a resistor in the range 8-16 ohms, high wattages, 2 required, L&R channel)
to red/black speaker terminals, power up amp and measure the dc voltage between red/black
speaker terminals, repeat for L&R channel, expect less than 100mV.

Since the amp is unrestored suggest basic health check of power supply etc... will get some test points.
 
Suggest download the service manual and schematic from hifiengine,
need to register to prove you are not a robot, it's safe, no spam etc...
used by most AK members.

The power supply is reasonably simple, check dc voltages at pin 36 then 38,
bloack probe connect to phono ground or chassis, service manual has
24.7Vdc and 13Vdc.

upload_2018-7-5_12-7-6.png

Consider diode testing transistors, particularly Q11,12,13,14,15,16,17,18

Nice graphic from markthefixer on diode testing, red/black refer to multimeter
orobe orientation, 2 results expected, around 650mV and OL/open loop or
open circuit, use the diode function on your multimeter.

DiodeTest.JPG
 
Remember what I said about it seeming to be "unrestored"?
IMG_20180705_240929816.jpg


Yeah.... Not sure if I should even bother anymore.
 
Yikes! Suppose it comes back to how good it sounded before and how easily you can replace it.
Maybe start an AK poll, should it be saved? Parts unit is a last resort for me, I'd try and save it however
in the USA such treasures are plentiful... so leave it to you.
 
By "Not bothering anymore" I don't mean throwing it to the trash or something like that, but rather selling it as a parts machine or keeping it around and fix it once I gather more technical knowledge. I found a SX-650 for around $40 bucks with a faulty sound channel, according with the seller it has never been fixed (Something I forgot to ask to the seller of the 424), it might be an easy fix, but I'm not sure.
 
I think this is a problematic power switch. It MAY be the cause of your problems. I would consider either Ohming it out (power disconnected!), or jumping it temporarily.
 
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redk9258 said:
I think this is a problematic power switch. It MAY be the cause of your problems. I would consider either Ohming it out (power disconnected!), or jumping it temporarily.
Click to expand...
First hand knowledge is highly valued. Are you saying the power switch is going intermittent so not supplying constant power to the amp resulting in surges?
Simple task to ohming it out as suggested above.
 
What board is that that is all hacked up? Possible to find a replacement from a part out? Assuming that the fault with the unit is easy to find/repair, anyway.
 
mbz said:
First hand knowledge is highly valued. Are you saying the power switch is going intermittent so not supplying constant power to the amp resulting in surges?
Simple task to ohming it out as suggested above
Click to expand...
I don't really have first hand knowledge other than seeing others here on at AK with that style switch having burned contacts. Buzzing seems like it could be the arcing sound from the switch. I think Pioneer, Kenwood and others used that style switch. If this is the style of switch that I think it is,in some cases there is a spare set of (unused) contacts that can be moved into the working position.

Several threads about Pioneer power switches...


http://www.audiokarma.org/forums/in...sx-450-power-selector-switch-cleaning.173215/

http://audiokarma.org/forums/index.php?threads/pioneer-sx-450-power-switch.700078/

http://www.audiokarma.org/forums/in...ioneer-sx-650-power-switch.53495/#post-506121
 
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