[Johnson] Re: Viking II Problems

[John Lawson]

On Sun, 18 Jun 2006, Rich Soennichsen wrote:

> I believe the problem is in my misunderstanding of the clamper
> circuit.  I turn R30 all the way CW and now I can tune the final
> properly, weee!  There is still a problem with R13 (bleeder) resistor,
> I will replace it.  I am getting 130W carrier out (dummy load) as
> indicated on a power meter.


   Woo-Hoo!!  Progress is Being Made on All Fronts!


However: be careful to soon re-adjust the clamper according to the 
Instructions. A couple of reasons for this are: it will save your finals 
from orange-plate-meltdown in case the drive and/or bias fails, and, if 
the prescribed adjustment can't be made - there is then evidence that a 
Problem still exists in that part of the circuit.



> I tried to tune for phone but I am not seeing any Modulation current at all.
> A least I am making progress now.


   Sometimes it helps to set the Old Girl up on 'er side on the Bench and 
go through the Voltage Table point-by-point with a good DVM or VOM - 
minding the presence of the various high voltages when doing so.  All the 
Johnson docs I've ever seen have a table of points (generally tube socket 
pins) and what the DC or AC voltage ought to be, at a specified set of 
control settings and line voltage.  Be aware that nominal line voltages 
are higher now than in the 40s and 50s, so some of the measured voltages 
will be higher than the Book says - but not a *lot* higher.


   Remember that the Modulator section is just a high-power audio amp, in 
these rigs usually push-pull IIRC (I got my Viking II when I was 9 or 10, 
and traded it for an Elmac AF-67 in Jr. Hi. - back then it was a good 
deal, now... o well) which means you'll have two modulator tubes driving 
the modulator transformer in a configuration like a see-saw, each tube 
conducts on the either the positive or negative portions of the audio 
signal.  First, check the resting bias on them...  also you can feed an 
audio signal from a generator into the Mic input, and then trace that 
signal thru the various sections with your scope. You *DO* have a 
generator and a nice little scope, don't you??  For AM work, any working 
50 or 100 Mhtz swap-meet scope will do fine.  I usually find scopes like 
the Textronix 453 for $20 or so....  and you'll need another, (older) 
scope to monitor your output when you finally get 'on-the-air'...


>


  VY FB OM DE KB6SCO QTH DM09FG
  VALIANT RANGER ELMAC AF67
  R390 R390A R388
  160M 80M AM FONE
  20M FSK RTTY MOD19


  ps: how's that for 'cryptic'...?  ;}