[R-390] RF subchassis separation

[Ken Harpur]

Hi Mark,

There is a youtube video by Gregory Charvat   http://www.youtube.com/watch?v=OxSzw5d6tCM
He has the same problem and the fix was simply a bad tube V-207 the 6AK5. I have also fixed some issues in my radios that were simply a bad tube and replacement was the cure. 
Also you might want to look at C-327 the 100pf capacitor across T-207.  In one of my radios most of the micas that had B+ on them were bad so C-327 could certainly be the culprit. I'd do the easy things first and try a new 6AK5 V-207.

Regards,
Ken


On 15/03/2014, at 3:22 am, Mark Richards <mark.richards at massmicro.com> wrote:

> Roy,
> 
> That was an excellent go/no go test.
> 
> Both crystals responded and therefore I can only assume they are oscillating.
> 
> I've some tube extenders, so will probe further...
> 
> Thanks for the suggestions.
> 
> 
> On 03/13/2014 15:29, Roy Morgan wrote:
>> 
>> Before you order expensive crystals, make this simple test:
>> 
>> Hook the suspect crystal between the output of your signal generator and the RF probe on your VTVM.  Set a tenth of a volt output and low voltage range on the VTVM, and tune around the crystal frequency with the generator.  Slowly.  You’ll see a peak if the crystal is working.
> Got to get me one of these.  Fell from need for one when I got into digital 30 years ago.  Time for a nice hamfest/flea market.
>> 
>> You can make an RF detector with a diode, resistor and a cap if your meter doesn’t have an RF probe.
> Ended up simply watching the output of the crystal on a scope.  Fine for these purposes.
>> 
>> For the 200 kc crystal, you need a signal generator that goes that low.  The URM-25 is ideal.  *Some* audio generators go that high.
> Used the URM25.
> 
>> 
>> All of this is extra justification to have TWO (or more) R-390A’s, or at least spare small crystal ovens equipped with the right crystals.
>> 
> 
> /K1MGY
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