[1000mp] MkV and AM power?

[Tom Rauch]

> I've never used my MkV in AM mode but thought I'd try it into a Dummy
> and see how it behaved.  When I tried to set the carrier to 50W I
> found that anything more than about 1/8 turn of the power knob sent it
> up 200W and the ALC shows about 1/2 scale with no audio going into the
> mike.  I could set it to 50W by holding the PTT and turning the power
> knob but it is extremely sensitive and difficult to set.  Once set, it
> seems to behave normally and sounds good in the monitor.  The power
> control in CW, SSB, and FM modes works normally.  Anybody have an idea
> of what's going on?

The proper way to control power level in a transceiver that generates 
AM at low level stages and amplifies it through multiple linear 
stages is through a power control system that limits power in an open 
loop.

ALC for SSB and CW generally uses an RF power output detector that 
detects peaks, and feeds the detected peak envelope voltage into a 
comparator. When you set the power control on the rig, you are 
setting the threshold voltage of the comparator. This system also 
works fine for RTTY and FM. The result is a smooth control feel, 
because it is a closed loop system with heavy negative feedback. The 
power simply follows the voltage applied to he comparator. (This also 
could be a VERY good way to generate "click-free" CW, if we keyed the 
ALC system and wave shaped the comparator keying voltage with a 
lowpass filter.)
  
If you used the SSB system on AM, the carrier power would go up 
without modulation and down with modulation! It would prevent normal 
100% modulated AM peaks from reaching four times the carrier power.
(That is why the AM is crummy on many rigs, like most ICOMs.)

If you see the power control is very "touchy", it almost certainly 
indicates Yaesu wisely has opened the ALC loop up and just applied 
a fixed bias to control gain. Just be sure you NEVER allow it to go 
into ALC on AM peaks or you will have negative carrier shift. One way 
to adjust for reasonably clean AM would be to go back and forth 
between carrier (drive) and mic gain and adjust both so the carrier 
is about 1/4 of peak envelope power with the power set low enough you 
never see ALC action. 

They may not do CW properly, but they apparently have a good grip on 
doing AM!

73, Tom W8JI
W8JI@contesting.com