tachs going bad ? other problem?

[crossy]

I have a little problem here i just want a few opinions to back me up. the tach in my 75. The 75 has the stock HEI in it. Some of you 'long timers' here on this forum have referred to me as the tach man, but I'm not. I'm Just a bit more knowegable than most about interchangabilty more than anything. Anyway i had a few good tachs laying here and i even sent 3-4 out a while back and had them 'rebuilt'. The guy was not ACTUAL testing them on an engine , but more of what i would call a simulated test with electronic scope and other things i don't understand. he was very knowedgable as that was his line of work in the service. well i had an 85 truck here back in 09 ,so i took ALL my tachs outside and tested them on the 85 with an old style HEI(non-ESC) and I also had a BUNCH of aftermarket tachs also including two expensive ones to compare readings. The tachs he did for me were on the money. I marked the back of everything I tested and so far TWO of tachs that tested good back in 09 have gone haywire in this 75 of mine. the one started going absolutely crazy , now the second one is reading real high all the time and gets worse as my RPM goes up. I know for a few years they used something referred to as a FILTER and it was supposed to be about radio interference suppresion. It looks like an old condenser and mounted to the firewall on the main tach feed wire. I can't verify that it was a filter,only that most tach specialist say you can run w/o it and that IT can be the cause of the tach going bad. These early tachs that have no holes for warning lights in the face plate are VERY hard to find and as far as i know no one is making a new circuit board for them yet . let me know if you have any educated guesses here short of I seem to have s--t for luck cuz I already know that. lol