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@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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