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Re: MSF TX VCO issue

 

Hi Bob-

Ok, I'm diving into new territory in troubleshooting here and approaching the edge of my comfort zone, but I will do my best to keep up and learn. Please forgive my ignorance, I appreciate your patience!

I did clean the obvious spots caps leaked out, there was litte to no damage that I can tell. I still have the cap on the interconnect board to tackle, but I don't know that is bad either.

I Put the VCO's in their correct positions. fired up the station, all working, peaked up each VCO to 38uA (I know you said it wasnt super critical, but I wanted a good starting point).

Checked the output of both VCO's on the service monitor. I should probably mention, the station is programmed for 441.975 TX and 446.975 RX.

I dont have a frequency counter available and the counter in the analyzer only goes to 500kHz, so I just used the scan function on the monitor, hope that is a valid way to test.

When working and everything is locked:
The TX VCO has a +2.5dBm output on 443.71
The RX VCO has a +2.5dBm output on 437.23

Waited until the TX lock went out (easy to hear the carrier on my local Icom drops when it loses lock), again took about 10 minutes. I can seem to make it happen faster if I key the station a few times.

When it lost its lock I didnt change anything, I just looked at the TX VCO output into the service monitor again, and it was still +2.5dBm on 443.71, no change.

So...I guessing that probably means the VCO is ok? I'm still familirizing myself with how the MSF works, is the next step to look into the steering lines? Or other suggestions?


Thanks!
Tom
W9SRV



On Wednesday, February 17, 2021, 11:29:42 AM CST, Bob M. <wa1mik@...> wrote:


The two caps under the shield are in the RX IF circuit and probably aren't worth the effort. The leaking caps definitely needed to be replaced. Did you thoroughly clean the board of all signs of leakage on both sides? It doesn't take much, but 5-10 minutes sure seems like a heat-related issue.

The fact that the meter reading goes real low means the VCO is running at too low a frequency and usually needs the tuning core to be turned clockwise. But since it does this with both VCOs, your problem sure seems to be on the Uniboard. One of those filter/connector assemblies is used primarily for the TX synthesizer; another one serves mainly the RX synthesizer.

Can you connect a freq counter to the output of the TX VCO and see what it's doing when it fails? That may help diagnose the problem.

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