[TenTec] Omni 5, Omni 6 5.0 to 5.5 MHz PLL saga -- for the repair archive, except for the editorial comments

[MadScientist]

As I TRIED to point out on the CCA reflector a year or so ago, generally speaking, alignment is a very poor way to correct radio problems, and I KNEW that. It was my creed!  For my rather strong preference for this TRUE fact over folklore, I was “unsubscribed” from that reflector. Their loss, not mine...

In this case, since light tweaking of the IF adjustments (at a point at which I could easily return to the original settings, the ONLY way I use alignment as a troubleshooting tool) DID improve things a bit so I mistakenly presumed that someone had misadjusted them previously since most of the slugs on this board were seized to the point at which I had to replace many of the cans. That usually does not occur unless someone forced the slugs with the wrong tool.

Gary

> On Sep 6, 2018, at 11:23 AM, Byron Cordes via TenTec <tentec at contesting.com> wrote:
> 
> Could not agree more! Alignment doesn’t fix anything! It might hide the fault but rarely does it cures . 
> The Omni vi is a good radio but boy does it have ailments! Hihi 
> Byron AC9PA 
> 
> Sent from my iPad
> 
>> On Sep 6, 2018, at 10:21 AM, MadScientist <dukeshifi at comcast.net> wrote:
>> 
>> I received an Omni 6+ for repair, no tuning capability. The fault was in the 5.0 to 5.5 MHz PLL, which was out-of-lock. 
>> 
>> I checked the reference, which was OK and dead-on frequency.
>> 
>> I checked the level at the loop IF test point, which was far too low for reliable lock (should be >300 mV). I checked all signals that act to produce this signal voltage level and found no missing signals so I tried the default Collins reflector folklore experts’ method of using alignment to solve problems - bad idea! I got the PLL to work and lock properly, and it worked fine for a while on the bench so I shipped it, but the radio came back with intermittent out-of-lock condition.
>> 
>> When I first powered  the Omni 6+ up on its return, it worked perfectly in every way.
>> 
>> Well, since I knew that the customer was not imagining things, I challenged the radio in many many ways with repeated power up power down operations and got it to fail (loss-of-lock condition in the 5.- to 5.5 MHz synthesizer, with loss of the >300 mV IF test voltage). 
>> 
>> Since I now knew that this radio was intermittent, I began the normal sequence of cooling (freeze mist) and heating (low power heat gun) and got it to fail. However, it failed so many places of directing the heat, and so unpredictably, this process was of little help.
>> 
>> I then began replacing the heavy  hitter items (1723 regulator and electrolytic capacitors around the area of the board showing heat sensitivity). No joy…
>> 
>> Then I decided to simply monitor the loop IF test point, which, as I said,  should be 300 mV or better all across the range of the VFO. Then I found my analytical tool. The voltage at that test point would sit at >300 mV at power up (sometimes) and would, over the course of an hour running, drop to 80 mV, still enough to maintain lock but barely. It turns out this was very reproducible.
>> 
>> I was then able to use freeze mist to locate the part that was causing this drop, even though the radio was still in lock condition. I determined that the dual gate MOSFET, BF988, was failing as it ran a while, resulting in the drop in test voltage. 
>> 
>> I had a few of these on hand so I was able to replace it. Still no joy so I realigned the loop IF again. As it happens, the old FET had sufficient leakage that it pulled the IF alignment off enough to make the test point voltage only about 10 mV at first examination. Realignment with the bad FET in place worked temporarily. Once I replaced the bad FET, the alignment was now so far off that the test point voltage was still stuck at 10 mV, not enough to bring about a lock condition. Alignment of that IF once again brought the level up to >300 mV all across the band. I then power cycled the radio many times with no loss-of-lock. I then ran the radio overnight while monitoring the PLL frequency and the test point voltage continuously overnight. The radio remained in lock and the voltage did not change overnight so I knew I had found the bad FET. 
>> 
>> This would suggest that alignment to fix an electrical problem CAN, as I have always said, cost you a LOT of time (in this case, about 8 hours) and money (shipping both ways). Thus I stand by the notion that one should rule out ALL other possibilities of a root cause of failure before attempting alignment of a radio.
>> 
>> 73
>> 
>> Gary
>> 
>> W0DVN
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