[R-390] Re. AGC and Stability

[chuck.rippel at cox.net]

Tune through WWV after 1 hour warm up and with the BFO on.  You should 
get a linear note when tuning.  If not, the PTO may be failing.  To 
confirm its the PTO, create a beat note on WWV with the BFO and tap the 
PTO gently with the handle of a screw driver.  If it changes frequency, 
that is the problem.

The last 2 radios I finished both had "noisy" and unstable PTO's.

If its a B+ problem, rot'sa ruck.


Chuck Rippel


On Tue, Jun 26, 2012 at 12:03 PM, r-390-request at mailman.qth.net wrote:


>
>
> Both of my 390as do the same thing.  I have tried to isolate the cause 
> but all I can suspect so far is that with varying AGC the current 
> drawn by the AGC controlled tubes varies, causing the load on the B+ 
> to vary.  The B+ varying could pull one of the oscillators slightly. 
> I know there is a regulator tube in there but it seems like one of the 
> crystal oscillators is not driven with regulated voltage (going from 
> memory here).  Another theory I have is that the AGC applied to one of 
> the mixers, which is also fed from a local osc, could change the 
> operating conditions of the tube, again changing the oscillator 
> slightly as it sees a varying load.  If you are running PSK your best 
> bet is a modern PLL controlled receiver.
> Jim M. N4BE
>
> Sent from my iPad