Modify AL-1200 HV power supply

WARNING – LETHAL VOLTAGES!!! DO NOT ATTEMPT WITHOUT PROPER SAFETY PRECAUTIONS!!! I’m not responsible for what you do with this information!

The Ameritron AL-1200 has a 3,600v power supply. Its filter capacitor bank of eight, 270mF 450vDC runs right at the limit of its design (8 x 450 = 3600). A common modification is to add two additional capacitors, along with the needed bleeder resistors, to increase the filter bank capacity to 4,500v. Keep in mind that the transformer still puts out 3,600v, so this modification essentially allows the components in the filter bank to have some overhead capacity.

W8JI and W7RY both offer a replacement PCB designed for 10 capacitors. W1QJ described the way he modified an AL-1200 for W2RE. I choose to adopt the W1QJ method, and will show how I did it here.

The modification involves adding two capacitors in series with the existing capacitor chain, with a bleeder resistor across the terminals of each capacitor. I’ve done this by constructing a small PCB that I’ve added to the amplifier. Then, move two HV wires, cut a HV trace and add a jumper for the metering, and add a jumper to complete the HV series circuit.

Conceptually, this is the modification. I’ve omitted the additional bleeder resistors on the drawing, but they are required! The existing bleeders are desoldered on one end to allow me to test them. The one on the far left had failed completely open, thus causing the capacitor to “tick”. That’s what has led to this modification. I had replaced all the caps and bleeders only five years ago.
Here’s my first test fit of the additional PCB. Note the markings on the board indicating the bleeder resistors, capacitor polarity, jumper to complete the series circuit, and destination for the 2 HV wires to be moved, and the new HV metering jumper.
The completed modification and reassembled amplifier.

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