Gentner Digital hybrid problem

[TomT]

It's a Hybrid III, I have a book on the II, which looks very similar, but the schematics are very fuzzy.

The unit works, but puts a bad hum on the telephone line, even powered off and unplugged. Doesn't look like there is much directly across the phone line except for a relay, bypass cap, couple of resistors, and what I guess from the fuzzy schema is a zener (the symbol is little more than a blur, the actual devices aren't marked. They are not shorted, however.

And, of course, the hybrid coil.

Anyone run into this?

 

[RememberWHEN]

I can't speak to the specific hybrid but it sounds like you are grounding the phone line. You mention that the hum is evident even with the unit unplugged but if the hybrid is in a rack then the chassis is grounded even with the unit unplugged (assuming grounded rack). Or if the rack isn't grounded then the shield on the output side of the unit, if tied at both ends, could also ground the chassis of the Gentner.

If the hybrid coil has an internal short to the frame of the coil (which is either physically mounted to the Gentner's chassis or the pc board of the Gentner) then that would be the ground path. The 'telephone line side' of the hybrid should be isolated from ground. Check that out.

Hope that helps.

 

[TomT]

Found the problem. There are devices described as "surge suppressors" across each side of incoming telco line to grounds. In circuit no evidence of a short. Indeed, removed, one measures 500K one way/300K the other way, while the second is 400K/200K. Probably breaking down from the line voltage. Anyway, pulling them kills the hum. Already have surge protection on all the lines at the demarc.