[Amps] What to do about 'Neutral' in HB amp?

[Paul Christensen]

>”I agree it looks like there is a 50v winding it goes through and then takes off someplace.”  

 

The 50V active lead winding goes off-page on terminal block J5, then comes back to one side of the 240V line. The 50V primary winding is wound together with part of the main HV primary winding and functions only to buck the blower as current demand increases.

 

Here’s a further description from the 77Dx manual:

 

“Cooling Blower B1 is provided with reduced operating voltage of approximately 75V by the use of a 50V transformer winding, “bucking connected”; resistor R4 permits adjustment of blower speed over a substantial range (an advantage of using the 50V bucking winding rather than simply operating the blower from a 75V transformer winding is that blower speed tends to increase slightly under extreme heavy load conditions, rather than slowing as transformer voltage drops slightly)”

 

>”The whole AC wiring would never pass UL listing today.”

 

I’m not so sure.  The amp’s internal wiring is fine.  The unacceptable bridge between chassis and blower for the neutral occurs on the cord side of the Cinch-Jones power connector.  Clip the jumper, run 4-wire service, and the amp owner doesn’t even need to take the covers off the amp to isolate the chassis from neutral.  Kinda’ neat really, although I’m not sure if it was designed with that thought in mind or it occurred by “engineering accident.”

 

Paul, W9AC