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

[Flowertime01 at wmconnect.com]

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