[Johnson] Viking II Problems

[John Lawson]

On Sat, 17 Jun 2006, Rich Soennichsen wrote:

> I have been trying to debug a Viking II, the saga so far:
>
> re-capped

   Which caps?  I know you probably listed them earlier ('lytics and papers 
IIRC) - but did you replace any in the RF section?


> Initial setup on low voltage is normal.


    Does this mean you have verifiable grid drive, as well as normal 
(negative) bias voltage on the grids of the finals?  Ie. the oscillator, 
mults, and buffer all working? Do you know what the resting modulator 
current is?

   Are you testing with a dummy load?  If so, do you know it's 'good'?


>
> Switching on the HV results in high plate current, 200-300 mA.
> This current increases until the fuse blows.
>
> Coupling is set to one and the final to zero.  I tried quickly tuning
> the final but the plate current just shoots up anyway.


   I had (and again have) this problem with my Valiant.  It is caused by 
capacitors in the RF tank that fail under stress can't carry the high 
current loads - my symptoms are the same - it's next to impossible to 
resonate the plate circuit, (the knob settings are way off normal, roo) 
and, even if a dip can be found before it catches fire, the plate current 
immediately begins to 'run away' until the "Chernobyl" alarms start 
flashing.  ;}   (This is why the Johnson engineers used strings of caps in 
seies-parallel - cheaper than one big unit.)


   Does your II do this on all bands?


   Concerning the plate tank and coupling caps:  even if you check them 
'cold', they can still be subject to failure under load.  The only way to 
really check 'em out-of-circuit is to use an ESR bridge that has a high 
enough output frequency.  Best to just pony up for good micas and then 
there's no worries.


Cheers

John KB6SCO DM09fg
Valiant Ranger Elmac AF-67
R390 R390A R388