I run a ten tec centurion modified for AM. I also operate a pair of
3-400Zs modulated by 572Bs. For the modified slopbucket amp I use an
old SB610 monitor scope for the fake trapezoid display. I say fake
because it doesn't have anything to do with the baseband audio but
rather only compares the exciter RF to the RF out of the pair of
3-500Zs and provides a measure of the RF power amp's linearity.
However it has saved my butt a few times when the amp suddenly went
non-linear due to a component on the verge of crapout, such as some
bad hamfest doorkobs in the final tank loading.
You need a visual indicator to tell you something is about to die so
you can kill the rig before real damage such as fire happens. When
that trap suddenly looks like an arrowhead you kill the rig
immediately, sort of like the low oil lamp on a car--get off the road
and kill the engine immediately because you have about 1 quart in
there.
I would not think of operating without these visual indicators and I
use them all the time.
A dedicated oscilloscope for looking at the modulation envelope is a
must, as important or more important than a trapezoid. A real
trapezoid that as the audio modulating voltage on the horizontal plate
and RF carrier sampled on the vertical (but you need an AM transmitter
for this; not a driver / RF linear amp lashup) can provide in one
look, the linearity of the final under modulation, the peak negative
and positive modulation and how asymmetric your modulation is in the
positive direction.
You can find old Tektronix scopes at hamfests, a Tek 453 is great and
they go for 50 bucks. Put the RF sample on one vertical input coming
out of the amp. If you want to get fancy you can modify an old cheap
CB vswr bridge and pick voltage of its diode that drives the meter
movement and feed that to the scope as a signal to trigger on.
>>Zillions of hams have operated in the past and present without using a
>>monitor and have not splattered or over modulated. If you are going to use a
>>commercially available ham AM rig follow the procedure and settings in the
>>manual. Also, have local friends
>>
I could not disagree more with this advice which is very poor in my
opinion. Running an AM rig without these monitor tools is like
driving a car at night with the headlights off.
>>WHO KNOW WHAT THEY ARE DOING and talking about help you in your settings
>>based on their listening to your signal.
>>
That is inviting disaster. You must listen to yourself. Firstly, I
hear crappy sounding audio all the time and in many cases the operator
is not employing an oscilloscope. Secondly, I hear hams give another
operator glowing audio reports (because they want to be nice) when in
fact he sounds awful. It is important that you listen to yourself on
a decent borrowed receiver or one you own so you know how you sound
firsthand.
An old SB610 and scope can be picked up at a hamfest for around $150
for the pair. You'll use them all during your AM operating time. If
they don't work out you can sell them easily at another hamfest.
73
Rob
K5UJ
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