[R-390] T207 T401

[Flowertime01 at wmconnect.com]

Carrol,

Sorry to be off line and long in coming back to you.
This is long but I hope it provides some insight.
It is still not an exact step by step cook book.

The Second Osc alignment is about as clear as Mississippi River Water after a 
summer rain. After reading the TM a few times the subject clears up. It looks 
like the Beach Surf in Okinawa after a Typhoon just misses the Island. Once 
you the do the alignment a few times the subject looks like the crystal water 
and coral bottoms as viewed from the high beach cliffs of Okinawa and you 
wonder what the fuss was all about.

The TM says align the second crystal oscillator caps from the Cal tones and 
using the carrier meter as the output indicating level. That got some writer 
off the hook back in 52 for the R390 TM and was copied into the R390/A TM. See 
TM 11-5820-358-35 Paragraph 74 (page 114 in the 8 Dec 1961 printing) The 
process works. And you can spread peanut butter on bread with a pop sickle stick.

But I digress and you asked me about T401 and the trimmer caps in the Second 
Oscillator in the R390/A receiver.

The TM says there is no adjustment for 0 to 7. But we know 0 - 7 lays over 
17- 24. The TM says start adjusting the caps for maximum carrier level output on 
each Mhz band from 8 to 31. Mud in your eye. Do this from 31 down to 8.

There is one obscure clause of a sentence in the R390 TM that suggest that 
T401 (numbered something else in the R390) be adjusted for maximum output at 
31Mhz.

These two items, set T401 to max at 31Mhz and set C31 to max at 31Mhz, imply 
these two adjustments will peak the output of the second crystal oscillator 
for maximum receiver performance across all frequencies in the range of the 
receiver. Another fantisy that passed muster with the TM editors. Hey it reads 
good in Jargon with no sarcasm showing.

In real life do the following:
Turn the BFO off because BFO on masks the real signal level.
Set the receiver to MGC to defeat the AGC which will cause output meter 
variations.
Hang a DC volt meter off the diode load as you choice of output indicator as 
this is the most sensitive easily accessible point to meter the receiver 
output.
Inject RF into the antenna input and use a level that gives a diode load 
reading in the range of -5 to - 10 volts. The RF may or may not be modulated but 
un modulated RF is mostly preferred for adjusting and testing.

The Receiver has a range of 31,000+ - 500 = 31,500 hertz. There is one 
transformer (T401) and 24 caps that need to adjusted to optimize the second crystal 
oscillators output such that all frequencies at the receivers antenna input 
have optimum output at the headphone jack. Optimum output is left undefined as 
an exercise for the operator to complete.

The first crystal osc mixes RF input under 7,000+ Mhz with a frequency near 
17Mhz and passes it on to the second mixer. Between 17 Mhz and 24 Mhz the 
second crystal osc uses a different crystal to mix that RF input under 7,000+ Mhz  
to a range of 3.455 to 2.455Mhz. The third mixer then mixes the VFO with this 
signal to produce a signal at 455Khz. On a good day this signal is centered 
into a crystal in the if deck with a band pass near 455Khz. Anywhere in the 
frequency range you can grab the zero adjust the and slide the VFO around a bit to 
peak the transfer.

The nonlinear VFO and its band spread will also impact the mixing frequency. 
One end of the VFO or the other may add or subtract from any given second 
mixer crystal error to increase the or decrease the receiver output.

So what good alignment? When do you quit?

Because the 17Mhz crystal is not exactly 17,000,000.000 you may find the 
optimum cap setting for 18Mhz is not the same set point as for maximum 1Mhz.

Like wise because some crystals at 17 -24 are not exact, the optimum setting 
for one of them may not be the same for the double conversion and the single 
conversion. If the 17Mhz crystal is off and the second mixer crystal is off the 
differences may add to make things poor, subtract to make things OK, cancel 
to make things good, do none of the above just to add reality.

Some of the second mixer crystals are used at more than one harmonic. You 
have a different cap to peak each of these harmonics. The output level at the 
harmonics may not be equal in amplitude or adjustable to equal amplitude while 
each frequency has a nice double peak on it's respective trimmer cap.

The thought is that crystal output is highest at low frequencies and drops 
off as frequency increases. If T401 is peaked at the highest frequency, it 
response will drop off as frequency decreases. The slope of T401's output  plotted 
against the slope of crystal loss across frequency is considered to yield a 
near flat mixer output across the frequency span.

Thus the TM read to peak T401 at the highest frequency and adjust each cap in 
the second crystal osc deck from 8 to 31 Mhz. The procedure details using the 
Cal tone, BFO and carrier level meter. Me know this process is not the most 
sensitive. 

We would like to think that peaking the trimmer caps only changes the 
impedance match to yield a better power transfer of the oscillator output and that 
the cap adjustment does not vary the frequency of the crystals. We would like to 
think every crystal is spot on exact to within under 100 hertz and stable as 
a rock.

What we find is one or more crystals have an output level below the curve.
If it is off frequency, and in the range of the zero adjust, then zero adjust 
the VFO and peak the cap for the megahertz band and move on. This is just an 
off frequency crystal but still in specification. If the zero adjust it peaked 
and the output is low then the oscillator output is weak for that band. Try 
cleaning the crystal contacts, the tube contacts and the cap. But the first 
easy quick fix in a clean receiver where you know corrosion is not the problem 
(1968 - 1975 era) is to adjust T401. 

You slide the low frequency slope of T401 down. This lets more of the crystal 
output from the weak crystal through the circuit to bring the weak band up to 
par. You hope the top frequencies do not go so far over the hump they fall 
under par.

You find the low spot (a dip / a weak output crystal) in what would be the 
curve of the crystal output levels. Then move the cutoff slope of T401 by 
adjustment so that when the two functions (crystal outputs / T401 cutoff slope) 
cross, the output performance level of the second osc is of acceptable performance.

Start at 31 Mhz and adjust T401 and cap 31 for maximum output. Use a RF 
signal generator and DC volt meter on the diode load for best indication of 
adjustments.

Continue down the bands in frequency to 8 Mhz. Adjust each cap but do not 
reset T401 while doing these adjustments.

Mark the 17 - 24 cap setting (pencil on the deck in line with the screw 
driver slot) and continue down in frequency. 

Reset the caps for best output on the 7 - 0 bands.

Look at the cap settings when you complete this process.
Are the caps still peaked at the same alignment point?
Are the 17 - 24 caps not all offset the same way (17Mhz crystal off 
frequency)?
Are the 17- 24 caps set above and below (the second crystals osc crystals off 
frequency)?

Now you have a choice.
Peak the caps for the low band (0-8) or the high band (17-24) or balance 
between the bands.

If in the process of aligning the whole receiver you find one or some of the 
1Mhz bands to be low you can now think about readjusting T401 to bring the 
bands up to par.

First consider if you have done all the other adjustments on the receiver.
Working over 17- 31 Z206 and T206 will yield more improvement than trying to 
optimize T401 and C20 to bring up a weak 20Mhz band.

Second consider if you have a clean machine.
Clean contacts under, caps, transformers, tubes, crystals and connectors go 
further than peaking adjustments.

After you adjust T401 to bring up some low band you should then go back and 
check all the other caps. What ever the last setting for T401, peak all the 
caps without ever touching T401 again.

The TM implies that setting T401 is not an exact most critical adjustment. 
The TM further implies that just close with a cal tone and carrier meter is good 
enough. But after 50 years, consideration and attention to detail can get 
more out of these adjustments than just a good receiver. 

Put some time into your receiver working through these adjustments to come to 
an understanding of how your particular receiver balances out. In the end you 
will have a better receiver to listen too. 

There are many other stages in the receiver that can compensate for the 
elected less than exact test book adjustment of the second crystal osc deck. 

Once you understand what bands of the receiver you want to optimize for your 
use, how the many adjustments interact and the limits of the exact parts in 
your exact receiver, you find a pattern of adjustments that optimizes the 
receiver for your enjoyment.

I hope this helps.

Roger L. Ruszkowski AI4NI   </HTML>