Another difference between the 525D and the 525 is the low/high power
switching.
In the analogue 525 the switch takes the PA out of the system -- the
driver drives to LP filters. In the digital 525 the hi/lo switch
changes the gain on the RF/mixer board so the driver and PA are
always in the RF chain.
So reports from people with 525D for this might not be useful.
I think there might be something going on in the switch or switch
wiring itself (try tapping), the driver (parasitic oscillations?) or
the TR switch on the low pass filter board (the driver not getting
properly isolated and acting as an shunt to ground?).
I'd start by pulling the final connection to the low pass filter
board and see if that makes a difference (obviously it should -- the
problem should go away).
Then (after reconnecting the final to the LPF) try pulling the driver
output on the final board. Does pulling that change anything?
Another idea ... put a constant signal in to the antenna. Flip the
switch. Does the signal drop in amplitude or does the noise come up?
Is this a change in gain or a change in the amount of noise at
constant gain?
Is the driver being turned off on receive? Pull the power to the
final board. Flip the switch hi/lo. Any difference?
Note: I'm not an Argosy expert. Just looking at the (rev A) circuit
diagrams.
On Apr 19, 2007, at 2:48 PM, C. Mark Cring wrote:
> As far as the noise floor goes, I haven't looked to see if the low
> pass
> filter cap in the IF was ever changed (I think my serial is 425?) but
> it is only noisy on the LO power switch position. Only t/s so far I
> have done previously is used deoxit on the switch, tried jumpering
> the
> appropriate switch poles together - no change. I'll tackle that one
> after I get the calibrator going.
--
Kevin Purcell
kevinpurcell@pobox.com
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