SX-1980 Low frequency hum after off/on power

[sx1980pioner]

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Hi.
I have a Pioneer SX-1980 and after an to fast off/on power I got a low frequency hum.
Anybody had that problem?
What can be the problem?

Regards from Sweden
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[rcs16]

Possible Power supply. Measure the dc supplies for Ac voltage. Could be a bad ecap in that supply’s. Hard to say exactly without testing

 

[sx1980pioner]

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I have a repairman who say I need to change the four main 22000uF capacitors.

But this repairman "tapeheads" on this link, have repaired many Pioneer SX-1980 and he say he has never changed these four main 22000uF capacitors.
http://www.tapeheads.net/showthread.php?t=89007&highlight=sx-1980

So does anybody know what else it can be,
if it is not the four main 22000uF capacitors?


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Here is a nice link to "tapeheads" , where he repair a SX-1980 and have taken lots of nice pictures.
http://www.tapeheads.net/showthread.php?t=26368&highlight=sx-1980
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Regards
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[sx1980pioner]

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If I need to get 4 new main capacitors.
Is my data right on them?

Anybody know where to get new ones?

On my original capacitors it say 85°C
http://www.tapeheads.net/attachment.php?attachmentid=117246&d=1592852680

I guess I can get them with higher "100 volts notice voltage"
and higher "105°C temp"

Data
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Electrolyte capacitor
Aluminum beaker
Maximum height excluding terminals 120
Max diameter 75
Raster measures 28.58
Number 4
Colour: Black
22,000 uF
100 volts notice voltage
105°C degree temp
Max ripple 15A
Tim 20,000 or more
Ring Clip mounting kit 4pcs
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Regards from Sweden
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[rcs16]

you continue to not take the sound advise in the link and ask the same questions again here. So I’ll say it again, It is near impossible to say exactly what part(s) it could be without doing some measurements. If the power supply is original then it is #1 suspect.

 

[zebulon1]

I fair to slim chance it’s the big filters but it an easy check to see if AC is present.
If the hum is in both speakers it’s something common to both channels, which would be a bad Ecap in the power supply’s.
I had a few pieces that just had loose connections.
Buy this time in the 1980’s life cycle, the power supply is going to need more than just a repair.

 

[rcs16]

I had a tech say to me just last week that a new ps solved an under lying hum issue in the 1980 that he was servicing. He went through the bother to re-build the old supply just to have the jfets fail again

 

[sx1980pioner]

I had a tech say to me just last week that a new ps solved an under lying hum issue in the 1980 that he was servicing.
He went through the bother to re-build the old supply just to have the jfets fail again

Can you explain this ?
Did your tech build a new power supply to fix a hum?
Did the new power supply not fix the hum?
Did it destroy the JFETs (Junction Field Effect Transistor) ?

Regards

 

[EastPoint]

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I have a Pioneer SX-1980 and after an to fast off/on power I got a low frequency hum.
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Don't turn it on and off fast then. Problem solved.

 

[sx1980pioner]

Don't turn it on and off fast then. Problem solved.

I managed to avoid that for about 30 years..

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[rcs16]

I have very little info from the tech. I believe he attempted to service a 1980 power supply. It ran for awhile and then it failed, lead to believe it was the jfets in the 80v regulators. Bought a brand new supply lots of questions for me, tech installed a new supply, problems solved.
Bottom line those old power supplies are ticking time bombs. they can be serviced but are a PITA because all the connections are hard wired. As a minimum you have to remove the transformer connections in order to get at the back. There are a service bulletin to be verified too. I gave up on the old ps and designed a new one that is much easier to service and incorporates the improvements

 

[sx1980pioner]

I have very little info from the tech. I believe he attempted to service a 1980 power supply. It ran for awhile and then it failed, lead to believe it was the jfets in the 80v regulators. Bought a brand new supply lots of questions for me, tech installed a new supply, problems solved.
Bottom line those old power supplies are ticking time bombs. they can be serviced but are a PITA because all the connections are hard wired. As a minimum you have to remove the transformer connections in order to get at the back. There are a service bulletin to be verified too. I gave up on the old ps and designed a new one that is much easier to service and incorporates the improvements

Do you have drawings on your power supply , that you can share ?

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[EastPoint]

The power supply is available here. No affiliation. The seller also has blank boards available.

 

[rcs16]

Complete docs are made available upon purchase. But the basic design is the same as the original, adding in the new ring of led constant current sources, many other modern and current production parts. Basically everything is beefed up to increase reliability, ease of install and service.
Your unit needs to be checked out, see what the supply voltages are running at, test for noise on the supplies etc. Fairly easy to do, with the bottom plate off. It is a slug to move around be careful.