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494AP-How does the Front Panel Amp Cal ie, Blue Shift CAL function work? Mine has a problem


 

I have a problem when using the Blue-SHIFT CAL function, in that, during the routine, when the time comes to adjust either the LOG or AMPL control, the displayed trace near the top of the display is erratic in amplitude. These adjustments cannot be sucessfully made.
I have started out with the cal constants cleared, (Blue SHIFT Pulse Stretcher Menu). The log amp is adjusted properly, the signal levels through out the IF chain seem correct.
I don't understand how or where in the circuitry the cal menu operates. It should be noted that with the cal constants set to disable, the on screen amplitude is correct, and logs properly with 10db, 2db and LIN input steps.
The manuals seem to be silent on how this works.

Thanks,

Bob


 

Bob,
I realize this might not be terribly helpful, but I have encountered a pair of similarities with my plain 494 that I have been waiting to worsen so that I might better trace the problem.
When first turned on, the screen characters, but not the other traces, on my screen exhibit a vertical shake of perhaps 1/16-inch.  This gradually subsides in less than a minute.  But then when reducing the Frequency Span, in switching from 500 kHz to 200 kHz, the screen displays "Phaselock Failure -- 1st LO."
Now by turning the power off and back on again, everything works OK and the analyzer will phaselock properly down to a resolution bandwidth of 30 Hz.
The voltage test points, near the left-front, show no evidence of jitter.  I have not changed the power resistors on the deflection board, as recommended by John Miles, but now have replacements on hand.  It is possible these resistors in the vertical deflection circuit are arcing internally, but eventually heat and heal.
Interestingly, if the Peak/Average control is turned all the way CW so that the horizontal line is off-screen at the top, the screen characters will jitter vertically.  I assume this is an intentional warning feature that is digitally generated, and unrelated to the fault.

Bruce, KG6OJI


John Gord
 

Bruce,
The vertical character jitter is probably due to poor thermal balance in the vertical amplifier. Driving the trace far away from center for a long time (as with a horizontal line off the top of screen) produces a thermal imbalance in one or more of the differential pairs in the amplifier, and they recover with times on the order of milliseconds. Oscilloscopes often have an adjustment for this, but I don't know if the 494 does.
--John Gord

On Wed, Dec 9, 2020 at 01:52 PM, ebrucehunter wrote:


Bob,
I realize this might not be terribly helpful, but I have encountered a pair of
similarities with my plain 494 that I have been waiting to worsen so that I
might better trace the problem.
When first turned on, the screen characters, but not the other traces, on my
screen exhibit a vertical shake of perhaps 1/16-inch.  This gradually
subsides in less than a minute.  But then when reducing the Frequency Span,
in switching from 500 kHz to 200 kHz, the screen displays "Phaselock Failure
-- 1st LO."
Now by turning the power off and back on again, everything works OK and the
analyzer will phaselock properly down to a resolution bandwidth of 30 Hz.
The voltage test points, near the left-front, show no evidence of jitter.  I
have not changed the power resistors on the deflection board, as recommended
by John Miles, but now have replacements on hand.  It is possible these
resistors in the vertical deflection circuit are arcing internally, but
eventually heat and heal.
Interestingly, if the Peak/Average control is turned all the way CW so that
the horizontal line is off-screen at the top, the screen characters will
jitter vertically.  I assume this is an intentional warning feature that is
digitally generated, and unrelated to the fault.

Bruce, KG6OJI


 

Update:

I fixed it, there was not quite enough power from the 110 MHz amp. I upped the gain just slightly and all is well.

Thanks!