On 3/01/14 11:35 AM, Magnus Danielson wrote:
On 24/02/14 19:14, Paul Berger wrote:
A couple further observations, to make life easier when adjusting C245
you can unplug the top card and move it aside, the CVXO will lock
without it, but you will not have the 10MHz to monitor, but you can
monitor the CVXO output instead. On the 6 pin connector you will see a
1 near the outer long edge of the second card, the CVXO output appears
on pin 5 of this connector and on mine, it locks at 50.25505808 MHz
according to my 5335A. I also noted that when the 5680A is well warmed
up it sweeps through a much smaller range.
In that case it would be good if the "mid" voltage would be
established, so that once lock have been achieved, further trimming
can be done until the VCXO steering is in the middle of the range.
That would make sure there is plenty of margin to either end, rather
than sitting on the edge and just barely lock.
Cheers,
Magnus
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After some careful looking around, I determined that this C245 that we
are adjusting is part of an LC tank in one leg of the crystal. The one
that I am having trouble with definitely has a fault in the VCXO
oscillator it seems to want to start up up at about 50% of the desired
frequency with a very small noisey signal, but if I touch almost
anything around the crystal it snap into the right frequency range, with
a nice strong, clean sine wave and locks rather quickly. I am finding
it hard to trace out the circuit as this little card has components on
both sides of it, and to make matters worse seems to be multi layer as
well. I have set it aside for now but will go back to it later.
I thought it was a little odd that these do not have a block of
insulation over the bulb, does this matter? The other thing that is
different about the two that I just got, from my first one is, these
seem to be using an AD9830A synthesizer chip to generate the 10MHz
output signal. There is a PIC beside which likely controls it and the
PIC is connected to a RS-232 level converter chip however the external
side of the RS232 chip is currently not connected to anything, but that
can be quickly remedied.
Paul.
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