Mk134 high pitch whine

Having assembled the steam engine sound generator there is a high pitch squeal this increases with volume, the train noise itself works as does the whistle, any ideas??

Could you post a picture of the top and bottom of the board?

I have a very old (probably 2000/2001) version of this kit - PMK134’1

It’s worked flawlessly for years, but has recently started to play up - the whistle sound breaks up, and the steam sound is now more like the busy tone of a landline telephone.

I’ve checked the soldering and can’t see any obvious dry joints. I’ve also removed & reinserted the LM324N chip in case that wasn’t seated properly. The battery still seems to be okay - it’s testing at about 7.5v.

Can anyone offer suggestions on how to fix this?

Since you have had this kit for so long 17/18 years my first guess would be that the electrolytic caps are drying out.
You might try to replace them.
Sometimes (not always) they will bulge on the top.

The caps had crossed my mind, but there wasn’t any visible deterioration so I dismissed the idea - suppose it’s worth trying to swap them out - worst case, I’ll just get another kit and be done with it. Thanks for the suggestion.

No problem,
I hope it works out for you.