[Johnson] Grid current

[John Lawson]

On Wed, 21 Dec 2005, Chuck Hurley wrote:

> John,
>
> Loads on 160/80 with uncontrollable plate current.
> Has grid current at low end of 80 mtr band but disappears as frequency is 
> increased
> No broken wires at coil.

   This do:

   Lay out the schematic.  (I took my Valiant book, removed the staples, 
punched the pages and put it in a 3-ring notebook.)

   At this point I'd be tempted to remove five 6146s and two 866As and lay 
them aside, just for safety. Don't mix up the finals and the modulators, 
though.

   Put the rig on the bench on it's side and fire it up in 'zero' mode. 
Switch bands to 160 or 80M - 160 is probably best. See if you can eke a 
little grid drive out of it by careful tuning of the exciter knob with the 
'drive' control up to '10'.

   With your scope, look at the grid of the buffer for signal coming from 
the VFO - if it's there, and if you have a freq counter, see what the 
frequency is - should be the basic VFO freq. You can also use the scope to 
check rough freq.  Look on the plate of the buffer - should be more sig 
than the grid.  Look on the grid of the multiplier - if OK then the check 
the output tank of the mult - should be 'straight through' in 160M band 
and twice that in 80M - now go to the grids of the finals, check there - 
should be the same, and should vary as you adjust the 'drive' control.

   Any failure will point to the place you just were.

   I'd be very surprised if the fault lies with the VFO itself - this 
sounds like "tired old capacitor" syndrome.


   IF all is well in the Exciter path - I'd lift one end of the final tank 
capacitor stack for 160/80 (C47? C49?? can't remember, I'm at work, book 
is at home)  anyway check them for any leakage at all - they are under a 
great deal of stress - I had to replace mine twice, learned a great deal 
about RF transmitter design in the process....  ;}


    I (and others) am/are Standing By.....    lemme know.


Cheers

John  KB6SCO