[Milsurplus] Troubleshooting Tips from the Late Nick Broline, #1

[David Stinson]

I've been asked to write a eulogy for an exceptional man.

While reviewing correspondence with
my late friend Nick Broline,
I've renewed my gratitude for his technical expertise,
his keen intellect and his encouragement.  He was also
a man of deep faith, generosity of spirit and commitment
to service. His counsel on these matters blessed my life
in many ways. Why a man of such intellect
and achievement would extend himself
to lift-up a shade-tree solder-melter like me
I do not know, but I thank The Most High for blessing
me with his friendship.

While reviewing our correspondence, I find many information
and procedures that may help others who still heat-up
their irons and turn a few screws.  My intent is to post t
these as I review them, since I'm certain Nick
would be pleased that his insights might help others.

73 OM DE Dave AB5S

-------------------------------------------
Dave,

I've probably mentioned I've found a number of radios that have low 
overall gain because of low Q in the front end or IF transformers due to 
low-Q silver mica capacitors in the tuned circuit.

The fact is that circuit Q is a very large determinant of the overall 
gain of a receiver. Tubes have a transconductance that is defined by the 
voltages and the tube type, but the coils largely set the gain. However, 
if the Q of the tube's load, such as the Q of an interstage mixer grid 
circuit being low, the gain of the front end is heavily impacted.

Putting it in numbers, if the designed coil Q is 45 and the parallel 
capacitor is lossy, bringing the circuit Q down to, say, 5, the loss of 
gain is 5/45 = .11 of what it should be. That's a 9X loss of gain that 
can't be corrected by all the alignment in the world. Obviously you will 
notice the tuning of that circuit is very broad also. Losses of Q by a 
factor of 2 is not so noticeable at the end of an alignment stick, but 
the overall gain will be low by a factor of two!

I've used the technique in the URL successfully in working on receivers 
such as the PMR-x series who used the Elmenco  SM caps throughout the 
front end. It is also a good reason for the local oscillator to be dead 
in some radios in the high end bands. Often the SM caps that have 
moisture and corrosion inside will also generate an open circuit DC 
voltage on a DVM or VTVM. SM caps should not be batteries!!!

I use a pulse generator that I picked up for $5 at the swap fest, couple 
it to a tuned circuit using a small cap located at the coil, like 33 pfd 
to start with, and my oscilloscope. I'm sure a make-do pulse generator 
can be made using a logic chip as an oscillator and pulse width 
generator, but the pulse generator is far easier.

Hook the probe to the same place the cap is attached, and adjust the 
pulse rate and width to get the best display on the scope of a damped 
sine wave. Once you see the damped wave, you can tweak the cap size, the 
pulse rate and width to get the best display. Adjust the sweep rate so 
you can see the individual cycles in the waveform. Leave the tubes in 
the circuit with the power off of everything. They are open circuits, so 
pose no problem. The capacity of the scope probe and the series cap in 
the pulse generator's output lead may also detune the circuit frequency 
some, but they do not detract from the test.

  On the scope screen, make a note of a convenient point of the 
waveform. It's is easiest if this is nearly full screen amplitude. Now, 
note the point where the amplitude is 1/2 of the original convenient 
point. Count the number of cycles between the two points........call it 
to the nearest 1/2 cycle......and multiply by 4.53, and that is the Q 
for all practical purposes.

I have found the IF transformers to be remarkably high Q, so that's 
where the radio gets most all its gain......or looses it!

If you are looking at the antenna coil, take the load off the input 
terminals.  Or.......you can measure the Q of the RF stage grid coil 
from the antenna input terminals.

Web Site    https://www.giangrandi.ch/electronics/ringdownq/ringdownq.shtml

73

Nick Broline W5FUA

We shall not cease from exploring
And at the end of exploring
Will be to arrive where we started
And know the place for the first time.
     T.S. Eliot--"Little Gidding"




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