[R-390] Detector tests followup

[n4buq at knology.net]

On Thu, 21 Sep 2006 08:10:18 -0500, "Cecil Acuff" <chacuff at cableone.net> 
wrote :

> Sounds like you have some sort of intermittent issue.  I typically pull 
> every transformer on the RF deck and clean each pin and socket with 
Deoxit 
> prior to the alignment work.
> 
> The action you noted with the AGC/MGC is completely normal.  Strong 
signals 
> require a reduction in RF gain to prevent overload.  AGC just does it 
> automatically for you...in MGC you have to be the gain control thus 
reducing 
> the front panel mounted RF gain.
> 
> I would go through the Carrier meter zero adjust procedure and the IF 
gain 
> level setting procedure but I recommend the one on Chuck Ripple's page as 
> opposed to the one in the manual.  A version of his may be documented in 
the 
> Y2K manual...not sure....
> 
> Now that the magic switch has been thrown what do the levels look like 
> around the detector stage....is it taking less than 4 v p-p to get proper 
> level going into the noise limiter tube?
> 
> Just curious...
> 
> I don't think you are chasing a ghost because things should not change 
that 
> drastically from day to day....
> 
> One test to keep in mind....the background noise level on any band should 
> always change (increase) when you connect an antenna to the 
radio...that's a 
> good indication that the radio has sufficient sensitivity to overcome 
it's 
> internal noise level which is a good sign that it's working fairly well.  
If 
> you can't tell much difference between antenna connected or disconnected 
> when tuned to a quiet place on the band there is more work to be done.
> 
> Cecil...


I cleaned the RF transformers about a year ago.  The low noise issue was 
evident on all bands so I don't think it was RF transformer issues.  Could 
be, though.

When I tried Chuck's method before, I could not get enough noise with the 
BFO off to get full-scale on the Line Level meter.  That's when I was sure 
something was up.  Last night, I could get plenty of noise with the BFO off 
and antenna disconnected to set the IF Gain per Chuck's method.

Something I have noticed in the last week of testing.  At times, I noticed 
the noise level would just rise a bit on its own and then fade again.  Not 
a really high rise in noise, but I could tell it was there.  Last night, it 
was as if this "noise rise" suddenly got a lot louder and stayed there.  
Must be an intermittent but maybe it went to ON and will stay there...

I'll have to check that voltage level on a good signal to see if it has 
changed very much.  It still bothers me that I don't get the desired output 
for 0.1V on the detector, but I might just have to live with it.

I did notice that the area around V506 got a lot more sensitive to the 
presence of my hand near it.  Not sure if it was because I had the signal 
generator on and it was picking up something from me as an "antenna" or 
what, but it did seem to change there too.  Weird.

I do have an issue with the 0.1 trimmer on the input of the IF deck.  I get 
very little in the way of a peak on that trimmer and the signal level drops 
off quite a bit in 0.1 and 1.0 vs the rest of the bandwith settings.  The 
coil does peak just a bit, but the cap does virtually nothing.  Maybe a bad 
cap or crystal in that can.  I plan to try swapping crystal with my other 
radio to see if that changes anything.  If not, perhaps the fixed cap may 
be bad.  More to do...

Thanks again,

Barry - N4BUQ