[R-390] WA3KEY's Motorola R-390

[wb5uom at hughes.net]

Said very well.

David/WB5UOM


----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, February 13, 2010 4:07 PM
Subject: Re: [R-390] WA3KEY's Motorola R-390


> Tom,
>
> TM come in different flavors with new publication dates. New TM's
reflected
> the new changes in the receivers.
>
> My R390 TM is dated 9 March 1962.
>
> Any real change would have a modification. So maybe some resistor changed
> value or cap changed value to provide some more gain or better filtering.
You
> would need to compare schematics from different editions and study for
> parts value changes.
>
> Another source of change would be a different number of turns on a
> transformer some where to change the impedance match or resulting filter
band pass.
> These parts would still have the same part number but vary from production
> run to production run. The change could not be great as the parts still
> needed to interchange from production run to production run. A bunch of
changed
> parts could not accumulate enough change to puts receivers out of minimum
> performance.
>
> As a production manager you could not just specify a new transformer. You
> were required to specify parts to published standards. If a manufacture
run
> in a few extra turn to ensure parts meet performance test, we would never
> know. But it would not be a published change. In time we would find some
runs
> of receivers were better than others.
>
> We know that while gear trains are all the same gears, Some models are
just
> a lot smoother than other models and no amount of cleaning and lube will
> bring to poor gears up to par. We can imagine the same thing happens with
> other parts. Caps may be picked to get the better limit. Resistors may be
picked
> to get the better results. Ground points changed from production run to
> production run. This makes changes in total circuit performance.
>
> I think most of it was Motorola tried to better select the big black
> plastic caps for better value from the beginning. The parts were specified
the
> same. Motorola just tried to get better production parts. A little
attention to
> detail in manufacturing can go a long ways.
>
> As part of the R390 school most of us got to build a 5 tube receiver from
a
> kit. The same kit for every one. Some never worked. Some of us built nice
> receivers that got many more stations at night. The only difference was in
> the parts placement and solder jobs. There was a point to this building
> exercise beyond basic solder skills. The same attention in production can
make
> some changes in receiver performance.
>
> That MPF stuff slopped all over inside receivers to prevent mold growth
was
> not supposed to effect receiver performance. I do not believe this to be
> exactly true.
>
> Just some ides. I know from years of maintaining receivers on the bench,
> that all receivers were not equal. No amount of PM could bring some of
them up
> beyond a minimum level of performance. While any receiver would make 20 :
1
> not all could get up to 30:1 no mater what you did. The fact that some
> receivers could get to 30:1 while other would not does indicate that there
were
> differences. I never though it was special parts in any one production
run.
> I have always considered it parts placement, solder joints and the
selection
> of the original parts. Just better resistors, capacitors and wire coating.
> More or less stray capacitance and many very small points that added up
over
> time.
>
> The Motorola builders likely did get it more right in more places than
> other builders did. I think Motorola did more in line testing and could
see
> where parts placement did effect performance of circuits. Motorola was
more
> performance orientated. I think Motorola could see where a batch of caps
or
> resistors made a difference in production lots. I think Motorola could and
did
> cherry pick parts during fabrication.
>
> No real magic or real parts changes. After you select 50 or so caps along
> the way just a little better than average and in the end you have a better
> total sum of parts. Lay out a tube stage with a few PF of less stray
> capacitance and stage works better than that stage in other production
runs.  little
> here and a little there and soon you have a reputation above the rest.
>
> Roger.</HTML>
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