In article ,
Fred McKenzie wrote:

If anyone is interested in the stability "fix", it involves using a
tantalum electrolytic capacitor (low ESR) in parallel with the 100 UF,
and a 330 Ohm 1 Watt resistor in parallel with the RF Choke.

I found the source of my "fix". It was in a QST "Hints and Kinks"
article by Ron Baxley, N4GB, probably from the early 80s.

I've tried everything I can think of. It appears that a resistor in
parallel with the RF Choke makes NO difference, except the resistor gets
hot! A resistor across the input, suggested by Heath bulletin HA-201-2,
also makes no difference except for a slight reduction in power. These
two resistors may make a difference for a different transistor. The two
I'm working on use a 2N5590, while some kits came with a CTC B12-12.

I removed the C9 100 uF capacitor, and replaced it with a couple more
Tantalum capacitors to agree with the QST article. As far as I can
tell, this makes an improvement but not a cure.

The closest thing to a cure seems to be a revised procedure for tuning
the compression capacitors. Initial settings for capacitors involves
snugging them down and backing each off a specific fraction of a turn.
Tuning then involves adjusting C1 through C5 in sequence, for maximum
output. From the initial settings, peaking C1 involves opening the
capacitor another half turn or so, and gives rise to the oscillation.

My revision was to NOT adjust C1 until after C2 through C5 have been
adjusted. Going through the sequence a second time starting with C1, I
found that all were close to their peak, and the oscillation was not
present.

Listening on a nearby AM radio, the "garbage" previously noted is not
there. But wait! I let the HA-201 cool down, and now there is a "rip"
sound in the AM radio when the handheld is keyed. After a few seconds,
there is no longer a "rip" when the handheld is keyed, after something
has heated up. The only thing I can think of that would cool off and
heat up so quickly, are the 1N4149 diodes used for electronic switching.

Heath discontinued the HA-201 and brought out the HA-201A in the late
70s. It was a major revision that uses PIN diodes for switching.

Fred
K4DII