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

[MU 4CX250B]

Ahh, yes, now I remember that circuit. I was under the impression that
blower speed was relatively insensitive to voltage for an induction motor,
with the reduced voltage mostly affecting the back pressure the blower
could accommodate. I remember messing around with reduced AC voltages for
the blower in my homebrew 7877 amp, but never saw much impact on blower
RPM.
73,
Jim w8zr
Sent from my iPhone

On Dec 29, 2022, at 3:07 AM, Paul Christensen <w9ac at arrl.net> wrote:



*>”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