720 vac

be soldered between pins 5 (cathode) and 2 (anode) and between (cathode) and 7 Refer to Fig. 1.

The two 5U4 rectifier tubes, power transformer, and 6DQ5 final amplifier tube are all located in a small corner of the transmitter chassis resulting in quite a bit of heat being generated at this point. Replacing the 5U4s with solid state rectifiers would

lower this generated heat significantly, but the decreased voltage drop of the solid state rectifiers could cause problems if not properly addressed. In this case, I found that the 5U4s have about a 100-volt drop,

I started with V15, the high-voltage rectifier. The high-voltage supply uses a choke input filter. A solid state rectifier was built on an octal tube plug. The details are shown in Figs, 2 and 3. The transformer high-voltage secondary is rated at 720 V ac rms, In order to keep from exceeding the diode breakdown voltage, the diode stack must be rated at the peak-to-peak transformer voltage of 2.8x720 = 2016 volts. Three 1 N4007s (1000 V piv) would be sufficient, I used four so as to have two identical rectifier boards.

Fig. 5, V76 socket 68 Seventy-three, March 1980

Equalizing resistors (470k, W-Watt) and transient protection capacitors (0,001 uF, 1000 V) were also used. The 5-volt filament winding was disconnected and pins 2 and 8 of the tube socket were connected together with #18 bus wire

After plugging in the solid state rectifier and applying power, I found that the high-voltage dc was now 710 volts instead of the original 610 volts. I connected up the newly freed-up 5-volt filament winding in series with the 117-V ac power transformer primary phased so as to oppose the input voltage. See Fig, 4. I suggest that when you do this, tack the wires together for the first try since Murphy's Law decrees that you will first phase the voltages to add. When this had been properly done (two tries were necessary), the high-voltage dc was found to have dropped to 670 volts. I left it at this, and now load the transmitter to 150 mA instead of the original 160

mA so as to keep the final plate power input approximately the same as it was originally, A second benefit of this change was to slightly decrease the filament voltages on all of the tubes, which should lead to increased tube lifetime. The filament voltages had been running slightly high due to the fact that the transmitter was designed for 115-V ac nominal input and the line voltage here in Piano, Texas, is between 120 and

The low-voltage supply was attacked next, l his utilizes a capacitive input filter to give a nominal 275-V dc key-down voltage. I soldered two 1N4007 diodes in series for each leg of the fuil-wave rectifier directly on the tube socket of V16, See Fig. 5. Power was applied, and a quick check of the dc voltage showed that it had risen to 360 V dc (no 5U4 rectifier drop)! Going to a choke input filter reduced this to 250 V dc (a reduction of \fl, as was expected). This is slightly less than the original 275 volts. This voltage is not critical, however, and proper operation of the transmitter was stil! evident- l he choke input filter was made by tying both sections of C15, the dual 40-uF filter capacitor, in parallel and connecting choke CH2 between the rectifier output and C15. See Fig. 6. This completes from vie pin 2

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