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@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@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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