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

[Flowertime01 at wmconnect.com]

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>