> According to the schematic, the heater transformer secondary is grounded at
> a chassis point in the PS cabinet. The circuit is completed at pin 1 of the
> tube socket where it too is grounded to the chassis.
>
> A heater feedthrough cap pass-through exists for use when upgrading the 77Dx
> to Sx at C13. Provided than one is willing to abandon the two-hole Sx
> concept, one simply lifts the chassis ground at the heater transformer and
> again at the tube socket.
>
> Then run a suitable AWG size wire from the heater transformer secondary
> terminal strip to feedthrough cap C13 (again, the C13 position exists for
> the second tube in the case of the Sx upgrade). Another wire is then run
> from C13 under the RF deck to pin 1 of the tube socket. Matching the bypass
> cap value already existing on the other side of the heater, a 0.02 uF bypass
> cap is then installed between the tube socket pin 1 and ground.
While you are 'in the area" check for heater voltage at the tube pins with a
good RMS voltmeter.
I have never found a 77DX/SX with the proper heater voltage. I assume the
transformers were wound for 230 volts. My line voltage here is a stiff 242
volts. Seems like it took a .15 ohm resistor in series with the filament choke
to drop the voltage to 4.95 volts. I guess if you used small enough wire to
wind
the choke, you could adjust the voltage that way, if it doesn't get too hot.
Floating and choking the heater circuit is good practice, especially with the
RF
involved with the SX. If a catastrophic failure occurs, the filament winding on
the transformer could cook itself and/or the other windings. Peter Dahl is very
"proud" of his replacement transformers. I wanted total isolation and no RF in
mine. I have seen the results of a "big bang" in an SX that actually welded the
bearings in the blower!
(((73)))
Phil, K5PC
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