Sencore CR70 whining sound - cause?
 

Hello all -

I recently purchased a Sencore CR70 CRT tester via eBay and it arrived today. Whilst in the middle of testing every tube in the vicinity, I've noticed something quite annoying - it emits a loud, high-pitched whine which changes frequency with the adjustment of the filament voltage. The tester itself seems fine, and all CRTs I've tested with it have accurate emissions and shorts readings similar to ones I was given on my old CR168. I also tested the rejuvenation function on a weak 12LP4, and it successfully brought emission readings into the green range. Is there a known cause of this? Am I dealing with something simple like a dying capacitor, etc?

Any advice would be greatly appreciated.

-CJ

I'm no expert on these but I do know they have an overly complicated filament supply regulation scheme. If I recall correctly it's a switched duty cycle that controls the average voltage going to the filament, complete with sense lines I believe. There's something wrong with the sensing circuit on mine because it's output voltage changes as a CRT's filament warms up. If your hearing is real good it's possible you're hearing the pulse width modulation frequency changing.

I know my Sencore CR-70 has always had a squeal from day one. It's not super loud, but I can still hear high frequency squealing at my age of 59! I have been using the CR-70 that I have, since it was brand new out of the box, in around 1992 or '93.