[Johnson] Viking Desk Modulation additional

[Rodger]

Hi Tom,

Also, you may need to check the screen voltage for the 4-400's and there 
are a couple of different versions of the screen regulator circuit 
depending upon when your Desk was made.  During restoration of mine, I 
added a connection for an external screen current meter which is nice to 
have when tuning the Desk KW for proper operation.

By the way, I note that your ISP is blocking email from my ISP so my 
direct email to you bounces.

73, Rodger WQ9E

Rodger wrote:
> Hi Tom,
>
> This is a curious condition with the modulation being low in both high 
> and low power positions, if it were just in the high position I would 
> suspect the HV rectifiers.  Check the 810 bias and also make sure that 
> you have plenty of audio and RF drive.  I am using a Ranger 1 with my 
> Desk and there is plenty of audio to cause clipping if one isn't careful.
>
> As another thought, with the meter set to the grid position measure 
> the resistance from the 810 grid to ground to make sure there isn't a 
> high resistance connection where the RF Deck plugs into the mainframe 
> as this would create additional operating bias.
>
> 73, Rodger WQ9E
>
> The Tom Elmore wrote:
>> Hello All, I recently inherited a Viking Desk Kilowatt from a SK 
>> friend of mine. I returned it to service after almost a year. It 
>> seems to make output power 275 low power and +700 watts high power. 
>> When looking at the modulation on a scope though it appears to be 
>> only modulating about 80% in either low or high power. I have changed 
>> out the 810's and 4-400 and see little improvement. I do notice that 
>> when I approach 80% modulation on the scope that both positive and 
>> negative peaks appear to curl slightly instead of going flat like I 
>> thought I would see with clipping.  Anyone have any thoughts or 
>> experience on where to look next ?
>>
>>
>>
>>
>> -Tom / KA1NVZ Anchorage, Alaska Johnson mailing list
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>>
>>
>>   
>
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