[Boatanchors] Morrow Modulation Question

[Jeff Anderson]

Hi Carl,

Regarding the 6146 screen voltage -- no screen resistors are jumpered 
(per the manual), and I've measured the overall resistance and it's in 
spec (around 56K ohms).  Screen current (on the Morrow meter) measures 
about 6 mA during xmit.  The manual only states that this should be less 
than 8 mA, so this seems OK.

The 6AU5s' bias has been set to 40 mA (and I've verified that the meter 
is calibrated correctly).

The mod. xfrmr looks like it's the original.

Unfortunately, I don't have the parts here for the shunt feed circuit.  
I'll have to use other tests.

Modulation percentage is measured with a scope (Tek, with cursors) as 
well as with an LP-100 power meter (see Aug '07 QST Product Review).  
I'm used to seeing peak power be about 4x carrier power (when I use my 
Johnson Ranger), but I only get about 2x with the Morrow (which makes 
sense:  40% modulation should give a peak power of 2x (3 dB)).

By the way -- testing has been done both using a mic (and my voice) as 
well as with a signal generator driving the rig's mic input.

Thanks again for your (and everyone else's!) help.

- Jeff, k6jca

http://k6jca.blogspot.com/



Carl wrote:
> OK, thats good info Jeff. Thats the same procedure I use to setup and 
> evaluate my high power modulators.
>
> I'll assume that you have set the correct 6146 screen voltage as per 
> the manual and are using regulated screen voltage on the 6AU5's. And 
> the 6AU5 regulated bias set for 40ma idle current. Im just scanning 
> thru the manual here.
>
> I also have to assume that the modulation xfmr is an original and not 
> some unknown impedance replacement.
>
> Another step to evaluate the xfmr would be to isolate the mod xfmr 
> secondary from any DC. You do this by shunt feeding thru a cap and 
> choke. I dont know if you have the parts to do this plus that rig is 
> sure packed tight. I thought my AF-67 was packed!
>
> Since this is a test only and we are not looking at the audio bandpass 
> the values arent very critical. Any 2-10 uF 1kV oil filled (no 
> electrolytics here) cap and a 10H or greater choke that can handle 
> 100ma is fine.  Connect the cap from the bottom end of the xfmr 
> secondary to ground. Seperate the modulator and RF B+ and feed the RF 
> side into the bottom of the choke. The output of the choke goes to 
> both the top end of the xfmr secondary and off to the 6146. The xfmr 
> has DC applied to the secondary but no current flows.
>
> I dont suspect the xfmr at this point but rather something that is 
> affecting a DC voltage.
>
> Another thought as I reread this thread. How are you measuring the 
> modulation percentage?
>
> Carl
> KM1H