[sdiy] Do Tempcos go bad?

[Nicholas Keller]

Strange question, probably, and I imagine all parts can age and fail, but are tempcos generally formidable?  

The reason I ask: as an SRM owner, a friend in another state asked me to help troubleshoot his Cat SRM which is behaving in such a way that each VCO is drifting independently (I apologize for the remote third-party help request).  He's had the PSU looked at about a year ago or so and was told that it was stable and that the keyboard CV is also reliable. 

I had heard that the Cats used poor quality trimmers and that often these were the cause of jumpy and unstable pitch. However, a tech in his area who's services he cannot afford has suggested that the tempcos are probably the culprits.  Can anyone throw in some opinion or experience so I can pass on some advice to him?  Swapping out tempcos is something he can probably handle himself, although I've warned him that post-repair calibration could be tricky without good test equipment. Thanks for any suggestions. 

 Below are links to some pics showing the location of the tempcos "as shown" in the schematics and "as installed" on the VCO board, one is across the AD821 and two (both labeled R150) are across the CA3046.  Keep in mind that the schematics for the SRM do not always match the layout diagram, and neither of those match the actual PCB :(  I suspect the "extra" R150 is actually across the AD821 and that the "real" R150 and R83 are across the 3046. 

https://imgur.com/a/3sRfG
https://imgur.com/a/9Wsl9

Thanks 

Nick
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