Seems my Comanche had an alternator installed long ago. I have been tracking some of my radio issues to alternator noise. I have found the filter required on the drawing but can anyone tell me where the alternator filter should be located on a 62 Comanche 250?

Piper never put an alternator on that early of a Comanche. You will need to look to the STC under which your alternator was installed.

Well I found the original documentation. Seems the alternator was installed "as per the service manual" back in '78. However, the part number listed was for a 12v 70A alternator and regulator. No record of the filter. As I understand it, Comanches came original with 35a generators or the later 85A alternators . The problem is that I don't know if the original field wiring was changed to include the circuit interlock when the alternator breaker is opened. And as far as I can tell, there is no filter that I can find. Any suggestions?

All this (including the engine conversion) was done with a 337 " as per the service manual" back in 1978. Any suggestions?

Welcome to inadequate or inaccurate logbook entries. You can't legally change an engine or install an alternator using the service manual unless there was an FAA field approval involved because the SM is not approved data. The only alternator that Piper installed in Comanches was a 70A in the 400 and the 260. I have attached a copy of the Type Certificate Data Sheet. What engine change was made? That might give you a clue as to the alternator that was installed, and if it was the same one Piper used for the 260's, you can check the parts manual to see if a filter was used. Piper often did use a filter in the alternator output circuit.

If you have had an engine change without a field approval or reference to approved data to make the change, you have a technically unairworthy aircraft. Hopefully there is a field approval stamp and signature in Block 3 of the 337.

It was a field approval.