SECTION 7 - THEORY OF OPERATION
7.1 Power Supply and Control
Refer to Figure 5.
When the ON rocker switch is momentarily depressed (closed), the AC power control circuit is com
pleted through contact 1 of P4, FI, the ON switch, S3 (normally dosed OFF switch), R4, S5, K14, K ll coil, F2,
and contact 2 of P4. K ll latches closed, completing the circuit to the transformer primaries via P4, R7, F4, F3,
R6, and F2. (Depressing OFF opens the control relay circuit, turning off the amplifier.)
(For 120 volt operation the transformer primary windings must be parallel-connected and R4 shorted.)
At tum-on, the discharged high voltage (HV) filter capacitor bank C1-C7 appears as a momentary short
circuit across the output of the HV rectifiers, D1-D4. Primary in-rush current is limited to 12 amperes by
"step-start" resistors R6 and R7. After a period of less than a second, the HV and other DC power supplies
reach nearly full output. Relay K12 closes, shorting out R6 and R7 so that full voltage is applied to the
transformer primary.
In the event of an HV fault, the overcurrent relay and/or fuses F3 and F4 will open, turning off the
amplifier.
The power transformer supplies all necessary voltages for amplifier operation:
1) 13.5 volts AC for tube filaments
2) 7 volts AC for the 5 volt DC supply
3) 50 volts AC for the 28 volt DC supply
4) 1250 or 1785 volts AC for the HV plate supply
The 7 volt winding feeds full wave rectifier D3-D4. The nominal 5 volt output powers control, trans
mit/receive (T/R), and PIN diode switching circuits.
The 50 volt winding feeds full wave rectifier D1-D2. 36V output from this supply provides tube bias
and also is regulated at 28 volts by regulator U1 which powers control, switching relay, and T/R circuitry.
Voltage from the 28V regulator to the coil of step-start relay K12 is appropriately delayed by RC time
constant components associated with Q1 and Q2.
Refer to schematic diagram Figure 6.
Relay K13 selects the 'low " or ''high" tap of the high voltage winding and applies it to HV bridge Dl-
D4. The HV supply negative terminal is floated above chassis ground for purposes of tube biasing and grid
current monitoring. Plate current is monitored by passing transformer HV secondary current through
current transformer T2. The voltage induced in its secondary is proportional to the HV supply current and is
processed for display by the Ip function of the multimeter bargraph. Filter capacitors C1-C6 are a matched
set. Their total capacitance is nominally 50 (iF. Bleeder resistors R1-R14 equalize voltage distribution across
the capacitors.
HV output is applied to the tube anodes and to the PIN diode bias circuitry shown in Figure 7. R16 in
the HV return lead limits fault current in the event of an HV arc or short. The voltage drop across R16 is
applied to the coil of plate overcurrent relay, K14. When K14 activates, its contacts open the coil circuit of K ll
and disconnect primary power from the amplifier. R18 and D6 protect K14 from excessive voltage.
R19-R21 are HV meter multipliers; R22 provides a return path to limit open-circuit voltage.
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