Well, I finished wiring up the 47G down converter and started turning up and testing using a gunn source for a signal and using an HT tuned to 912MHz WBFM as the IF. And, nada.....
At first I suspected the 10MHz oscillator as sometimes it seemed a bit flakey but the 100MHz PLL was locked. So then I checked the phase output of the 12GHz brick. It was merrily sweeping through its lock range. Well crap.
I decided to take the 100MHz pll off line and feed the brick with my HP synthesized oscillator set at 100MHz. Still the same issue.
A note about this Brick - it does not have a cavity tuning screw but rather has an automatic sweep feature that engages when lock is lost. I suspect this was used for the brick to quickly jump to a new frequency as the reference changed. Anyway, I had a second brick and tested it at 100MHz - same problem.
So, I started flipping the thumb switches to 99MHz, 98, 97, etc.
At 99MHz, the test brick locked right up and I noticed the phase voltage was around10 volts.
As the reference went lower in frequency, the phase voltage decreased until 92 mhz and
went out of lock. So what I found was that 100 MHz was right at the hairy edge of the
lock limit. 101 MHz would lock fine. OK so the brick is good, the 10 MHz ref is good.
Then I saw the phase voltage drop to 0 and one of the tantalum caps on the regulator
board feeding the mixer multiplier and amp was smoking. I quickly disconnected the voltage and decided that was enough fun for one night. Guess I have to replace the tantalums now. And, figure out
a new reference oscillator to drive the brick and the associated IF.....
