Swapped the VCO's again, RX VCO on the TX, and vice versa.
Was stable about 10 minutes until the first time the ID came up, then it unlocked, recovered, and after trying a few TX it fell out of lock for good. RX working 100% with the TX VCO in place.
When it goes unlocked the current drops to about 8uA abruptly and stays there, will not change with tuning the VCO.
I'll look into the steering line test, but at this point, I think the VCO's themselves are not the problem?
Thanks
Tom
W9SRV
On Tuesday, February 16, 2021, 02:50:10 PM CST, Bob M. <wa1mik@...> wrote:
Swap the VCOs again, align both of them, and let it sit for 5-10 minutes to see if you get the same problem.
The 38uA meter reading is not critical as long as it's between about 10 and 45 and moves when you turn the slug. Of course it should also be stable and not move more than about 2uA from cold to hot.
The cap on the interconnect board is on the 5V regulator output that feeds some of the synthesizer circuitry for both the TX and RX sections, supplying only power to some 5V ICs. It's possible the regulator on the IC board is going into oscillation but those are usually pretty good and reliable. It's just a 7805 3-terminal regulator. I would expect an issue here to affect both TX and RX circuits.
You can power a VCO on the bench by supplying about 9V and connecting the steering line to a pot so you can vary that voltage. A higher voltage should cause a higher frequency. Monitor the output signal at the RCA jack. It should be around +10dBm and be nice and stable. That will at least tell you whether it's the VCO or the synthesizer circuitry on the Uniboard. Remember that the RX VCO operates at a slightly different frequency due to the IF.
I recall seeing a "steering line leakage test" procedure in one of the manuals. Basically the steering line should be a very high impedance, well over 1 megohm. If it's too low it will cause problems.
