| Hallicrafters HT-40 Audio Modifications |
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| The Hallicrafters HT-40 is a controlled carrier AM transmitter running about 50 watts PEP output. The speech amp and modulator sections are almost identical to the audio sections of the Heathkit DX-60 transmitter. The fidelity of the modulation sounded a bit restricted to me, so I swept the speech amp section to see the response. I used pin 6 of V4 as the test point. I found that the 300 Hz response was down by three db and 3000 Hz response was down by three db. See figure #1 |
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Figure #1
| I made these changes to the modulator to improve the frequency response. Removed: R13 1 meg R12 4.7k C30 .001uF Changed value: Component old value to new value C28 .001 uF to .01 uF C31 .001 uF to .005 uF C33 .001 uF to .01 uF C37 .005 uF to .001 uF The audio response is now 100 - 9000 Hz. See figure #1 |
| Click here for a schematic of the modified HT40. |
