[R-390] R-390A Noisy Ant. Trim and Raspy Calibrator

[Larry H]

Hi Ken,  On the trimmer - good news about the resistance.  The trimmer is in a sealed area except for a .25" hole in the chassis for the wires to go through, so its probably clean inside.  Getting the pin out of the gear is not easy and you must be very careful if you really need to do that.  I'd first remove the trimmer and can as one unit first.  Then you can inspect inside to see if the rotor is touching the sides or not.  Can you see any physical damage to the can?  Before that you might try exercising the trimmer a lot to see if you can make the scratchiness go away that way.

On the cal,  on using tp e208, I forgot I used my amp probe on that.  But it sounds like you made good progress on it.  If the agc is the problem, the distortion should go away when you turn the rf gain down.  On my 2 r390a's I get about 35-38 db on the cal signal with rf at max.  At max on 1 I do get a little distortion, so I need to work on that, myself.  You could try scoping the IF out for the cal signal and then a sig gen signal to see the difference.  That should tell you if its the cal signal or somewhere else in the rig.

Regards, Larry


On Sunday, July 6, 2014 10:11 AM, Roger Ruszkowski <flowertime01 at wmconnect.com> wrote:
 



Ken,

E208 is on the grid of V201.

The test point is 470K or 33 PF off the cal signal that just happens to be  33 ohms off ground.
That cal tone on the grid of V201 is only about 1 micro volt.

Plus you will only see it if you are in the cal mod of operations.
I am not surprised that you do not see any thing on E208 with the scope.

On E209 you would expect to see micro volts of signal from the RF chain.

The double signal you see could be from either the 17 Mhz first crystal oscillator
or the cal tones. What is the difference in signal at E209 when you are in MGC and CAL.
considering you are switching the cal tones off or on?

I think you are just seeing the first crystal oscillator signal on the grid of V202 at E209
and it looks like nothing as it is being capacitor coupled through a couple different paths
with the 6C4 tube and circuits.

These are tough problems to trouble shoot. The signal levels are just to low for most of our scopes.
And at these low levels there is so much crud about that has no real meaning to the circuit performance.

Go for clean and deoxit.

Get out the little wrench and Philips screw driver. Open every ground lug bolt and clean the connection.
More than one problem has been a 50 plus year old oxide layer in a mechanical connection on a lug.

Roger AI4NI

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