Sanyo 9990 right channel hum

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tweeters

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Mar 17, 2012
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Hi all,

I have a moderate hum (probably mains ac Hz) on the right channel of my 9990, and it's too loud to live with. :sad:

I have tried a new 1000uF cap across the supply because i cant work out how to get to the smoothing capacitor that i presume is on the main board (the PSU board has no cap). -it didn't help

There is no hum with radio, only the line-in (when the tape player is set to record). It is here i think i am getting closer to the problem, because playing the tape (with very perished belts) there is a much louder hum(and/or oscillattion,approx8Hz). but 'recording' from the radio there is no hum, so now i'm confused and stuck :huh: :huh: :huh:

There is no hum with batteries.
using the light button changes the hum a little.
and it does increase when turning up the volume, same with headphones.

Any help or pointers would be very much apreciated as it's getting beyond me and my tools, and i really like this one. :-)
 

tweeters

Member (SA)
Mar 17, 2012
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uk, central
Thanks Monchito
I now have an alternative (cheaper) switch cleaner spray.
and i'm guessing the record bar is the multi-pole VERY long switch (that is difficult to access under the PCB).
Now i have some spare time, i guess it's time to 'get in there' and blow some dirt out and catch it in a cloth hopefully.
I will report back when the deed is done.
 

monchito

Member (SA)
May 5, 2009
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pythonville florida
u can even clean the function switch also while u have it apart and see if that helps ...thats always your first resort ,then if that does not work then we know its on a componet level :-)
 

tweeters

Member (SA)
Mar 17, 2012
11
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uk, central
ok, so it's now back together with no change :sad:

I worked lots of switch cleaner spray into the record bar and function switch, and i'm begining to think it was wasted effort because it's fine on DC supply, but not knowing enough about the subject, it may or may not have helped in some other way :huh:

Still, at least i'm getting more familiar with taking it apart and together again...
Which may now help because i'm starting to suspect some aspect of the power supply circuitry, which appears to be partly on the main PCB and partly on the transformer PCB. I had kinda expected all the convertion from AC to smooth DC to take place on the same PCB, but it appears not.

So if anyone can explain these power supplies, in general, or for this machine that may help fix the hummmmm,
but until then i'll go do more reading...
Hummmmmmmmmmmmmmmmmmmmmmmmmmmmmm...
 

k2j

Member (SA)
Mar 28, 2011
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Try with radio on so you have a buzz sound - don't shock yourself on the transformer or ac cord.... and see if there is a loose solder joint. Try wiggling/flexing the PCB boards. Also check each boards ground connections/circuit.

Is that a big enough filter cap? I used a cap the size of a coke can to smooth the PS on a humming Tron arcade machine before.

Your on the right track, good luck!! :thumbsup:
 
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