Repair whooshy/scratchy KR-4500


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Hello,
I have a KR-4500 which I purchased some 25 years ago. It was used a little initially when kids were learning on a piano, and then served as a piece of furniture most of the years.
Opened it a few days ago and wanted to play and instead of hearing music notes, I heard whooshing sounds with faint notes in the background, and it's scratchy too.
Started to read about it in various forums and saw that these keyboards suffered from bad electrolytic capacitors.
So, being an engineer, I opened it up and removed and inspected the motherboard - it looks 100% clean. No capacitor leakage at all, no burnt or rusty signal paths. Looks brand new.
I don't mind embarking on a project replacing the capacitors, but wanted to hear from others who have experience with this keyboard as to other things that might cause this behavior.

Any help would be appreciated.
 
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happyrat1

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Electrolytic Caps can look perfectly fine and still be leaking internally.

The best way to know for sure is to use an ESR Meter to check their values in circuit.



Gary ;)
 

happyrat1

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BTW, the scratchy sounds are probably resistance noise from dirty pots. A few squirts of DeOxit will clear that right up.


Gary ;)
Gary,
Thanks so much for the reply!
I learn something new every day.

As to cleaning pots- do you include also the two small ones on the motherboard?
 
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Welcome.

You can give anything where there are contacts a light squirt of the spray.

These type of sprays generally do not leave a residue and are non conducting.

Now resist the temptation, from one Engineer to another, to twiddle the pots on the Motherboard as they are set where they are set fir a reason, the clean will penetrate and clean.

You could also carefully remove any ribbon or other connectors and give both a spray before reattaching.

Gary’s meter is $50 and if you are sure you want to repair it then this is the way to go or for that you can buy a lot of Caps so you may be better just changing them all.

Or at $50 plus the cost of parts you could buy a keyboard that actually works for that.
 

happyrat1

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The procedure for cleaning pots is to give them a squirt, then twist them back and forth so that the built in wiping action cleans the contacts.

With the trimmer pots on the board, however, this procedure is not advisable unless you have measured their resistance setting and recorded it to restore afterward. OK to give them a squirt but don't twiddle them unless absolutely necessary is my advice. Even then proceed with caution.

And the $50 investment in an ESR meter is a solid investment in any tech's toolkit.

Gary ;)
 
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Thanks all,
Got the ESR Gary recommended - unfortunately it was defective... These look like very cheap quality, or I was really unlucky.
Will try a different ESR meter and report on results. In the meantime I ordered the caps as I know I will be changing them anyways.
 

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