Panasonic SG-5090 constant high volume noise - UK Vintage Radio Repair and Restoration Discussion Forum

digitaltoss · 16th Dec 2017, 2:35 pm

Hello all,

I have been trying to fix the mentioned music centre for a while. I could not get too much useful information about the issue neither the music centre, but I hope this forum will bring me luck.

I got this music centre in the summer from an old guy who lives in a farm outside the city. The place was really dirty, so this piece of equipment was covered in dust, and I even found rat poison and sunflower seed hulls inside of it. He have not used it for a long while, so I did a deep cleaning before turning it on.

It worked perfectly for about 50 hours of uptime, but then noise started. It is really hard to describe the issue since it is changing somehow... It is a high volume noise, that comes on immediately after turning the centre on, regardless of the audio input. Sometimes it is just random noise, but sometimes it has a pattern, like 5 impulses per second. It used to be the left channel, now both. Sometimes it changes when I touch the equipment's housing even so slightly (not to mention the circuits, or anything inside the housing), sometimes I need to use more force. And of course sometimes I turn it on and the noise is gone, and I have to wait a minute for the noise to come.

These are the things I have tried:

cleaning all the potentiometers, all of them, with alcohol
bypassed the speaker group selector (it has A, B and A+B options, and 4 speaker in connectors)
changed all the big (>1 mF) capacitors on the power amp's circuit
desoldered the power amp IC (STK463) and tested the connections (seems normal)
checked all the soldering on the board with a magnifying glass, fixing those which could be iffy
desoldered and measured the bridge rectifiers (looks good)

Now I am at the point where my only idea is to check the power supply, and that is it.

Do you have any ideas on this issue? What would you check? Keep in mind that I only have a cheap digital multimeter, so I cannot even get readings from the noise that reaches the speaker... (and I know, no household is a real household without an oscilloscope)

Thanks in advance.

Diabolical Artificer · 17th Dec 2017, 7:23 am

Welcome to the forum. It does sound like a dry joint or poor ground connection but could also be resistor noise but less likely so. Next is cleaning up any switches. You seem to have covered most of the likely culprits.

I non conducting prodder like a pencil is useful for tapping the PCB and components to try and find the source of the issue. Did this on a Marantz amp I had that had odd issue's; I'd fix one and another would pop up.

Best of luck, Andy.

GRID LEAK · 17th Dec 2017, 1:23 pm

Hello and welcome to the forum. I have found with these sort of faults that a hairdryer as a heat source and a freezer aerosol can sometimes be very helpful. Warming the panel with the hairdryer and then cooling a small area at a time with the freezer spray can sometimes identify the component or dry joint etc. which is causing the problem. Good luck with your problem and have a happy Christmas. John.

16th Dec 2017, 2:35 pm	#1
digitaltoss Retired Dormant Member Join Date: Dec 2017 Location: Budapest, Hungary Posts: 1	Panasonic SG-5090 constant high volume noise Hello all, I have been trying to fix the mentioned music centre for a while. I could not get too much useful information about the issue neither the music centre, but I hope this forum will bring me luck. I got this music centre in the summer from an old guy who lives in a farm outside the city. The place was really dirty, so this piece of equipment was covered in dust, and I even found rat poison and sunflower seed hulls inside of it. He have not used it for a long while, so I did a deep cleaning before turning it on. It worked perfectly for about 50 hours of uptime, but then noise started. It is really hard to describe the issue since it is changing somehow... It is a high volume noise, that comes on immediately after turning the centre on, regardless of the audio input. Sometimes it is just random noise, but sometimes it has a pattern, like 5 impulses per second. It used to be the left channel, now both. Sometimes it changes when I touch the equipment's housing even so slightly (not to mention the circuits, or anything inside the housing), sometimes I need to use more force. And of course sometimes I turn it on and the noise is gone, and I have to wait a minute for the noise to come. These are the things I have tried: cleaning all the potentiometers, all of them, with alcohol bypassed the speaker group selector (it has A, B and A+B options, and 4 speaker in connectors) changed all the big (>1 mF) capacitors on the power amp's circuit desoldered the power amp IC (STK463) and tested the connections (seems normal) checked all the soldering on the board with a magnifying glass, fixing those which could be iffy desoldered and measured the bridge rectifiers (looks good) Now I am at the point where my only idea is to check the power supply, and that is it. Do you have any ideas on this issue? What would you check? Keep in mind that I only have a cheap digital multimeter, so I cannot even get readings from the noise that reaches the speaker... (and I know, no household is a real household without an oscilloscope) Thanks in advance.

17th Dec 2017, 7:23 am	#2
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	Re: Panasonic SG-5090 constant high volume noise Welcome to the forum. It does sound like a dry joint or poor ground connection but could also be resistor noise but less likely so. Next is cleaning up any switches. You seem to have covered most of the likely culprits. I non conducting prodder like a pencil is useful for tapping the PCB and components to try and find the source of the issue. Did this on a Marantz amp I had that had odd issue's; I'd fix one and another would pop up. Best of luck, Andy. __________________ Curiosity hasn't killed this cat...so far.

17th Dec 2017, 1:23 pm	#3
GRID LEAK Triode Join Date: Jun 2009 Location: Lancashire, UK. Posts: 46	Re: Panasonic SG-5090 constant high volume noise Hello and welcome to the forum. I have found with these sort of faults that a hairdryer as a heat source and a freezer aerosol can sometimes be very helpful. Warming the panel with the hairdryer and then cooling a small area at a time with the freezer spray can sometimes identify the component or dry joint etc. which is causing the problem. Good luck with your problem and have a happy Christmas. John.