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Omni 6 Phase Locked Loop - Synthesizer Troubleshooting

(this info posted to the Ten-Tec reflector 28Oct2009 by Joe K8MP)

Symptoms: The display reads correctly. It even changes when you turn the VFO. However, the received freq doesn't change. How did I know that? When I changed bands to 160, I could hear an AM broadcast station. Turning the VFO didn't change what I was hearing.

Fixing it: It was suggested that I check the 5 to 5.5 Mhz output from the PLL. I didn't have a freq counter, so I used a Tektronix 456B, which is a 200 Mhz scope. The output showed as 5.8 Mhz and it did not change when cranking the VFO

or hitting the up/down buttons. The 2 inputs to the board (10 Mhz clock and serial freq data) appeared to be good, so I was pretty sure I was on the right track. NOTE: The output also had what seemed like a low voltage level (1 volt p/p). I called Ten Tec about the sig level, but they didn't know what it should be. (More on that later)

Another Ham who had PLL board problems strongly suggested that I re-heat suspect connections on the board. I did this (in fact, I hit every connection on the board) and the radio worked for 2 days. While it was in the working state, I scoped the PLL output and saw it change with the up/down buttons. The voltage level was 1 volt p/p in the working state, so I now knew that was correct.

For the next few days, the radio would work then not work. I tried to "make it fail" with heat and cold, but those didn't seem to make a difference. During this process, I made a few notes and drawings of various waveforms and levels around the board. Unfortunately, 3 weeks have passed and some of those notes don't make any sense to me now. Here are the ones that do:

1) At the junction labeled "Test point" (duh…go figure) on the PLL schematic: When working, and the rig set to 40 meters, it had a 100 mv sine wave riding on a 325 mv DC level. (I forgot to record the freq). When failing, it was nearly at ground potential (less than 15 mv), with no sine wave.

2) At U4, pin 5, with the rig working, I saw 775 mv when on 40 meters. I didn't record it in a failed state.

3) At R11 and R12, with the rig working: You should see a 5 volt level, with negative-going spikes to ground, every 100 usec.

4) U5, pin 6, with the rig working: 133 Mhz signal. I can't remember if it was square or sine.

5) U4, pin 7, with the rig working: 2 v DC

6) U4, pin 1, with the rig working: 3.5 v DC

"Sometimes it's better to be lucky than good"

While beating on components to try make it work, I noticed that a slight tap on L12 could make the rig fail or work. I re-heated the connections to L12, (including L12's case to ground on top of the board) and the radio has been fine since. That was over 2 weeks ago. I even got brave enough to put it all back together several days ago and it's still working. Hopefully, it's been put to bed.

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