| Hallicrafters HT-40 transmitter |
 |
| I picked up this HT-40 at a hamfest recently for $15.00. This unit was missing the panel meter, the plate tuning and loading capacitors were scraping (shorting) through their tuning range, the crystal/VFO switch was frozen. Also the front panel neon lamps were bad and a tube was missing. I had the correct meter, a 0 to 5 ma edge-reading meter in my junkbox but it was physically smaller than the original. I cut up a facade out of aluminum and painted it dark grey. I mounted the meter and facade on the front panel. The meter now looks like an original. The variable capacitors were repaired by bending the plates until the scrapping stopped. The front panel was removed and the frozen crystal/VFO switch was replaced with a new slide switch. The 12AX7 tube was replaced. The two neon lamps were replaced with two panel mounted LED's. The POWER LED needed voltage to turn on. This voltage was obtained from the filament line , rectified by a 1N4007 diode and 1 uF capacitor. Current was limited by a 330 ohm resistor. See Figure #1 |
 |
Figure #1
| The modulation LED voltage was obtained by the addition of a clamper/monitor circuit. |
 |
Figure #2
| It appeared that the transmitter had not been used for many years so I replaced the 40 uFd filter capacitors with 100 uFd units. Now it was time to smoke test the transmitter. I gradually brought up voltage on the HT-40 with a variac. The filaments came up but nothing happened. There was no high voltage! Could the power transformer be bad? Upon closer examination, I found that one lead of the high voltage secondary wasn't connected. A surge resistor between the secondary and the high voltage diodes was missing. This is probably why the HT-40 was given up for dead. I inserted a 10 ohm 20 watt resistor for the surge resistor and slowly brought up the voltage again. Success! I'm getting grid current. The oscillator is working. Just as I began to look at the output, I heard a loud pop and smoke appeared. One of the high voltage rectifier diodes gave up. I replaced both of them with 1N4007 diodes. Again, I powered up the transmitter. This time I got grid current and power output. The final loaded up to 75 watts out. I switched over to AM and saw 5 watts of unmodulated carrier and 50 watt peaks with modulation.
|
| Click here for a schematic of the modified HT40 |
