Improved audio quality for the Heathkit TX-1 Apache
Actually, much of the so-called “scratchiness” which Apaches are famous for is simply due to misadjustment of the clipper circuits. However if you’d like a little more range in the frequency response, particularly low frequency response, the following suggestions should help. It will not make it sound like a broadcast transmitter, but then at that power level “broadcast quality” audio is not really desirable.
You might start by bypassing the clipping and delete one stage of gain. You’ll find that this is rather easy if you look at the schematic and at the physical arrangement of the components under the chassis. It is easily restored to original condition should the need arise. Of course, you’ll want to check to be sure that the microphone phasing is correct after deleting a stage of gain.
Another thing that you can do to improve the audio on the Apache is to replace the disc ceramic interstage coupling capacitors with 0.05 uf “orange drop” or better quality capacitors. The breakdown voltage of these capacitors should be at least 600 volts. Again, don’t use disc ceramics for audio work.
The above modifications work well with a D-104 microphone using a crystal element, however other high impedance types may work well too.
If you don’t intend to use the Apache on CW you can use the “KEY” jack as a high level audio input. This is a non-destructive change which is simple to do. I feed the driver stage directly through the pot which is physically located behind the “KEY” jack.
The output of my audio proccessing chain is fed to a balanced to unbalanced impedance matching transformer which plugs into the “KEY” jack. When it is not plugged in, the normal “MIKE” audio is active. Again, this is a change which is easily restored to the original configuration.