[R-390] Christmas Wish List (part 2)

[Craig C. Heaton]

Rodger KC6TRU,

I see said the blind man! Your following explanation is just for the RF
section. More in coming days, thanks.

Putting this into terms that us non-radio background old grayed hair farts
can understand helps. Using equipment on hand, I will add these comments.

For lack of a TS585; the first time setting up this test, I'll use two
meters to measure voltage and current across the 600 ohm load resistor.
(P=IV)Write down those values somewhere not to be lost, for future
reference, and should only have to dial in the voltage next time (oh my). In
other words the first target is 0.4 watts across the 600 ohm load.

To make things just a little easier, more than one way to skin a R390/A,
I'll use a db chart for power, voltage, and current ratios from an old ARRL
handbook. This way, just have to measure voltage across the 600 ohm resistor
and I trust the two meters owned.

After setting switches, dials, sig-gen, etc. and arriving at 0.4 watts; turn
off modulation. Then back down the local gain to one volt AC across the 600
ohm load resistor. Next step turn on modulation and read voltage; a smidgen
more than 3VAC is 10db, 10VAC is 20db, and 30VAC is close to 29.5db. Or
where 20 x log(V2/V1)=db

I had to do the math to understand the chart, suffering from CRS. If this is
a valid method of squeezing the proverbial last bit from the RF section,
I've got work to do, tubes to find, and so forth.

Going to guess the IF follows in similar fashion, it is fine in that event??
That part of the radio is near 28db. But will check everything again after
all comments and votes are in.

wd8kdg
Craig



-----Original Message-----
From: r-390-bounces at mailman.qth.net
[mailto:r-390-bounces at mailman.qth.net]On Behalf Of
Flowertime01 at wmconnect.com
Sent: Wednesday, December 07, 2005 10:34 PM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] Christmas Wish List (part 2)


Craig WD8KDG

Part 2 and more to follow (tomorrow)
This about the RF signal to noise test.
Tomorrow will cover those tube swaps and IF deck test.

Thank you Barry Al, and Pete for the Y2K manual. It is still the best book
an
R390 owner or an A owner can have next to his receiver. Just my 2 cents.

OK its Christmas. I owe Craig. Hopefully someone will put this in the
frequent question file. You all copy this into you personal files. Healthy
comment is
welcome but when, Barry, Barry, Barry or Barry start to complain the horse
is
beginning to smell we drop this thread.

A short dissertation of what happens when a sig-gen, such as an URM-25() is
used without consideration of impedance matching, RF leakage, etc.

Now this is a very dead horse in the archives. Mostly no one has an isolated
shielded environment where impedance matching makes a "measurable
difference".
 Les and other fellows provided some very real detail on this the last time
we flogged this poor horse to death. Craig, from your point of view, do a
Force
mind trick here and just ignore it. Hey it worked for every one in service
for years. It was just the way it was done. You start getting scientific
here
and the fun just falls right out of it. But you are right it is a relevant
subject. Just saying ignore it is a flippant response. But the truth is not
provided in the TM. Some Fellows have thought about it and there are real
answers in
the archives. From a practical point of view in any ham shack you just
ignore
it because the leakage exceeds any gain from cable matching and or impedance
matching. At the real levels in use, the open bench work area, and the
leakage,
the effort to shield things and do matching exceeds return on investment.
Accept that thousands of technicians working on thousands of receivers for
half a
century did not even venture down this path and produced good results every
day. There is an explanation for why this has worked. It is scientific. It
stands up to good logical investigation. I do not have the exact data here
to pass
on in this mail. Hopefully someone will dig it out of the past mail and post

it again. Its Christmas and we can put it on the wish list.

Addition three: Same thing in the above paragraph to the RF section all the
while hoping for the 20db difference between modulated signal to
un-modulated
signal. Of course ignoring impedance matching between the sig-gen/receiver,
RF
leakage, plus the antenna to be used with the receiver will vary your
results
in real life.

Exact calibration level and signal generator level is not required for this
test. Exact output level is not required for this test. If you hang a 600
ohm ½
watt resistor (1 watt preferred, 560 ohm is OK) on the line output of the
receiver you can use the line level meter for your output measuring device.
The
meter of military choice was exactly a TS585 test set. This is a milliwatt /
DB
meter with range switch and internal load resistors. One load resistor is
600
ohms and about 10-watt (de-rated because its in the case to 5 watts). Any AC
voltmeter with a DB scale will work. Better meters and finer granularity of
DB
values just aid in getting the job done. Beyond the scope of this Christmas
gift is the fact that with some math, just a plain AC volt meter can be used
to
meter the local or line output of the receiver for this test.

So the signal generator does not mater, the output meter does not mater; the
cabling does not mater. What counts is a relative difference in output meter
reading when the signal generator modulation is switched on and off. All
this
gets you is a relative merit value of your receiver on any given day. It is
not
calibrated and it will not travel across the internet in mail as my receiver
is better than your receiver because we have no clue as to the wholesomeness
of any of the receivers, equipment or people involved in the comparison of
the
two events reported to have been conducted on planet earth in one or more of
its current dimensions or incarnations. This silly little test does work to
determine if the last, tweak, tube swap, change, adjustment, fiddle, nudge
or
whatever was an improvement.

Why does this test need the un-modulated signal? Because with no input to
the
receiver, the front-end stages do not produce an output of the first stages
noise into the next stage and thus yield a noise level at the output. So one
test-state is with a continuous signal activating all the stages and
providing
an output that reflects all of the receiver noise.

Why does this test need the modulated signal? This provides the test with a
second different state that can be compared to the first state. Someone
please
jump in here with some good real explanation of the signal to noise test. We
were asked quite politely as a Christmas wish. I'm begging here not
beginning
here. (See past post). Actually the modulated signal is richer in content
and
more of the receiver noise mixes with the modulated signal to produce a
greater
output level. Greater output level is not by its self, good. Observe that as
we make changes to the receiver and inject the same modulated and un-
modulated signal the difference between the two test states increases (good)
or
decreases (bad) and the relative output power may go up or down
(indifferent). You
may change one tube then measure; less noise less power (OK), less noise,
more
power (good), more noise less power (very bad) or more noise more power
(bad).
More or less power is not the true grail. As long as there is the required ½
watt (OK 0.4-watt). The exact input level is not critical the exact output
level is not critical. Notice that the absolute noise of the signal
generator is
not an aspect of this test. As long as the modulated signal from the
generator
is not so microphonic that every thump on the bench pegs the output meter.
Some is OK as long as you let the setup rest while you are trying to
evaluate
the output meter reading. Again moving targets should be avoided.

Accept that any change you make to the receiver that lets you reduce the
signal generator output is good. Accept that any change you make to the
receiver
that produces a larger output meter reading between modulated and un-
modulated
signal is good. The method of coupling the signal generator into the antenna
input is not critical because the leakage of the equipment on the bench
often
exceeds shielding provided by the test setup. Here impedance match can be
ignored for much the same reason. On the output you do want to provide a
600-ohm
load. 600 Is the "manufactures recommendation" 550 - 800 is likely OK. A
wattage rating large enough to not smoke and change the resistance value
during the
conduct of the test is sort of nice. I hate working with moving targets. The
output meter scale is not critical. It need not even be a DB scale. The
right
scales and easy to read numbers just makes the project more fun. More
difference in range between the two test states is good. Having the value in
DB across
a 600-ohm load just takes the math out of the problem. Hang a 600-ohm
resistor
across the line output and use the line meter to find the DB ranges on your
AC voltmeter. The better resolution of the AC meter will help you judge if
small differences are better or not.

The military required 10:1 ratio in these two test states. The military
required 0.4 watt output for 4 micro-volts of input. Collins engineers did a
very
good job on the design. We find that with good lab grade calibrated test
equipment in very controlled test setups, the receivers will do 20:1 any day
of the
week even after half a century. Back when (68 -75) I saw receivers do 30:1.
Using just military calibrated test equipment, miss matched cabling, no
extra
shielding, just setting on the bench, one side of the balanced antenna input
grounded, long ground straps from receiver to a bench ground that when
forever to
the station ground, and a TS585 for a load resistor and output meter. That
sorry test setup was used every day by every one in service. The test got
good
receivers up to the best we could get out of them. We shoot for a 20:1
ratio.
If you were not getting it easy, you went looking for a few good tubes to
install. Tweaking will bring the whole power level up and it will help the
ratio.
But you cannot tweak a receiver up to 20:1 if the tubes are not up to it.
Tweak
20:1 on some poor tubes and you can have 25:1 just by swapping in some
better
tubes.

Exactly what were we getting? Who knows. But it was every thing those
receivers were capable of. We could determine if every change we did was
making the
receiver better or worse. You just had to know your limitations. How dead
could
you beat that receiver before the horse-meat began to smell so bad you got
banded from the mess hall at mid meal time. There is just no exact absolute
benchmark in this test. But getting a 30:1 ratio was an all day job with a
supply
room that had all the tubes I wanted at no cost to me and no restocking
charges.

At what RPM does your Mustang idle? Who knows. But you know when you get it
down low and smooth enough so it fells right and does not stall at the
stoplight. Where does your Mustang red line? Who knows. At what ever you
need to dust
that thing in the other lane.

So for the RF test, This is the full receiver from end to end. Specification
is 10:1 signal to noise at 4 UV in and a 0.4-watt out across a 600-ohm load.

Wonderful life is a 20:1 ratio and about 4 UV in and 400-Milliwatt
(0.4-watt)
output. Never mind the impedance match or shielding. The exact audio
modulation frequency is not critical. The URM-25 had 1000 and 400 Hz. We
used 400 HZ
just to save our ears.

Run the signal generator into one side of the balanced antenna input.
Ground the other side of the input
IF bandwidth switch set to 2KC.
Antenna trim to max.
KC and MC to peak the signal pass.
AF gain to Max
RF gain to Max.
Function to Manual
AGC not being used.
Limiter off.
BFO off.

What did I miss? Like it says in the TM. Someone quote the Y2K paragraph
just
to drag this horse a few more yards.

Paragraph 93 Sensitive Test in the TM details this test. (Almost no one has
a
copy)  The procedure calls for a ratio of 10:1 in milliwatt on the TS585. We
would get a 20:1 ratio with this test set up. You can fudge a 30:1 DB ratio
if
you work at it.

To do a by the book 10:1 we would set the AN/URM-25 for 4.0 microvolts.  Set
AF and RF gain to max. Adjust the IF gain for 0.4 watts with the 30 % 400
hertz signal. This is also 26DB on the TS585 meter (16+ 10). Back the local
gain
off some. Switch the meter down one step. Back the local gain off unit the
TS585 reads 10 milliwatt (10DB). Turn the modulation off and switch the
meter
range down until you get a meter reading.  To pass the test you needed to
switch
the meter down one step. This was down 10 DB and from the 10-milliwatt range
to
the 1-milliwatt range. The meter had to read less than 1-milliwatt (1DB) to
pass this test.

For the 20:1 20 DB test we would set the AN/URM-25 for 4.0 microvolts.  Set
AF and RF gain to max. Adjust the IF gain for 0.4 watts with the 30 % 400
hertz
signal. This is also 26DB on the TS585 meter (16+ 10). Back the local gain
off some unit the TS585 reads 100 milliwatt (20DB).  Turn the modulation off
and
switch the meter range down until you get a meter reading.  To pass the test
you needed to switch the meter down two steps. This was down 10 DB and from
the 100-milliwatt range to the 10-milliwatt range. Plus down 10DB from the
10-milliwatt range to the 1-milliwatt range. The meter had to read less than
1-milliwatt 1(DB) to pass this test.

For the 30 DB test we would set the AN/URM-25 for 4.0 microvolts.  Set AF
and
RF gain to max. Adjust the IF gain for 0.5 watts with the 30 % 400 hertz
signal. This is also 27DB on the TS585 meter (17+ 10). Turn the modulation
off and
switch the meter range down until you get a meter reading.  To pass the test
you needed to switch the meter down two steps and watch the meter very
close.
This was down 10 DB and from the 100-milliwatt range to the 10-milliwatt
range. Plus down 10DB from the 10-milliwatt range to the 1-milliwatt range.
The
meter had to read less than 1-milliwatt (1DB) to pass this test. If you had
a
good clean reading at 1-milliwatt 1 DB you were at a 27:1 ratio. If the
meter
would peak on noise at less than the number 9 mark on the meter you were
down
30:1. This was of course a judgment call and you had to work at getting a
receiver to do it.

I have read this a couple times and I think I have this all correct. We can
rewrite it and re-post it until someone doth protest to much.

Roger KC6TRU

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