My last post on this subject explained how to tune up for AM when using an
SSB radio in the AM mode.
If you are using an AM transmitter like a Johnson Ranger or other such
vintage rigs as your exciter with your linear amp you have to use a little
different approach to tune the amp properly.
The linear amplifier still needs to be tuned for the maximum PEP.
The only way that I know how to do this by applying full modulation to the
ranger or other such transmitter while tuning the amplifier.
You could do that by modulating the transmitter to 100% with a single tone.
That will give maximum PEP. Then tune the amp for maximum output and you
should be good. Trying to tune the amp with just the carrier is not going to
get you properly tuned. As discussed in the last post the amp needs to be
tuned at the PEP level not at the carrier level.
Instead of a single tone you could do a long ahhh into the mike and tune on
that. A little more difficult than the tone though.
Be careful with modulating your old transmitter with a tone as you can
easily stress the modulation transformer if you overdrive it and /or its
final is not tuned correctly.
Keep the time short that you apply the full tone modulation as that amount
of audio continuously is hard on everything in them. They are designed for
voice service.
In order to not over modulate your transmitter a scope is almost essential
when operating AM. You do not want to over modulate on negative peaks at any
time. With a tone for modulation you can easily see 100% as the carrier
starts to get pinched off. There will be a brighter spot on the waveform at
the zero level on the scope as you hit 100% negative. You don't want to flat
top on the positive peaks either.
The ranger is pretty good but some transmitters will flat top on positive
peaks before you get to 100% modulation.
The Johnson Viking is one of those. They have the modulation transformer
impedance such that the modulator goes into clipping before 100% modulation
is achieved to provide some high level clipping. This happens when the
transmitter is tuned for full output.
Watch your scope! Negative clipping (100% negative modulation) is easily
seen while positive peak clipping (flat topping) is much harder to see on
voice.
A good way of establishing the proper amount of drive from your old AM
transmitter to the amp is to first use your SSB transmitter to tune up the
amp, then drop the carrier level to 1/4 power and measure the transmitter
power out to see how much drive power is needed for your amp on AM.
It is then a good idea to measure the output power of your AM transmitter
and match the amp drive level with it to what you had with the SSB
transmitter at 1/4 power. If the AM transmitter can not be reduced by
reasonable loading adjustment then you may need an attenuator between it and
the amp.
You don't want to load the AM transmitter too lightly as the modulator
impedance will not properly match the final in it and the screen voltage
will not be proper anymore either. It will then be very easy to have your
modulation transformer arc or something in the final arc.
Once you establish the proper carrier level needed for the amp and your AM
transmitter can do that then you can apply the modulation tone and tune the
amp for max PEP and you will have a clean signal if you don't over modulate
the transmitter.
Again, your scope pattern should double in size if you are getting full 100%
modulation on positive peaks.
Coupling a scope to your amp is easy. Often all you need is to have the
probe laying next to the feed line or maybe a little piece of wire attached
to the end of the probe and you will get enough pick up to be seen. No need
to have a direct connection. Just have a stable place for the probe to lie
so that the pattern doesn't change every time you move something.
Did I mention always use a scope!
73
Gary K4FMX
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