[R-390] R390 progress 6AK5/6AJ5

[Roy Morgan]

At 01:39 PM 5/2/2005, Dan Merz wrote:
>Hi,  I decided to get into the non-a 1st/2nd oscill crystal box ...
>The task was easier than I thought because Collins must have anticipated my
>skill.

Dan,

Collins anticipated the skill and time available with the military folks 
who kept these things running.  I'm confident that you are now far ahead of 
some of the repair people of back then!

>.... I decided to tweak T402 the output transformer in the plate circuit 
>...in the manual that T402 is adjusted only once on band 31 and the 
>procedure is somewhat cryptic.

Hah.. not the first time we have found cryptic alignment instructions (the 
HQ-170A manual has some, too.)

>It is adjusted so that the trimmer for band 31 can produce only one peak on
>a VTVM over the trimmers whole range.  I interpret this to mean that the
>inductance of T402 is reduced to the point so that only one set of overtone
>harmonics are produced using the trimmer,

Generally, the term "overtone" is used with crystals that are oscillating 
at a multiple of what would be their normal, fundamental, frequency. Some 
crystals are made to operate this way, where the frequency of oscillation 
is three or five times what it would be in a normal circuit.

My guess is that T402 is being resonated with the trimmer on band 31 so 
that the inductance of it will be right for the other bands, which all have 
separate trimmers. Note that most of the trimmers have a fixed capacitor in 
parallel. This sets the range of capacitance that will resonate the primary 
of T402 to the desired frequency.

>... My question is: when using a VTVM to monitor
>this adjustment,  does production of an overtone produce a peak in reading?

We could say it this way: tuning of the T402 and band 31 trimmer is done so 
the crystal's second harmonic is peaked by noticing the DC voltage at the 
(mixer ?) grid.

>... The waveform in seeking out overtones,  as the trimmer is 
>adjusted,  goes from complete waves where the average pp would be high 
>through regions where the peaks are not reached then to the next overtone 
>where the peaks are again completely formed.  Am I interpreting this correctly?

I think what you are seeing with your scope is the result of tuning the 
circuit to the fundamental or to the harmonic of the crystal frequency.

Very likely the range of a trimmer is not enough to tune both the 
fundamental and twice the crystal's frequency.  Or, if it is, then the band 
31 procedure is intended to set the T402 inductance to make sure you can 
get only the fundamental or the double of it as you tune the trimmer for 
each band.  Note that the majority of the crystals are used on two bands 
(at least one is not.)

The manual may mention this, but if you find after setting T402 and the 
band 31 trimmer, you cannot get a good peak on some other band(s), you may 
be able to tune T402 differently and still get the peak on band 31, but 
then be able to get the needed peak on the other band(s).  Is there a 
picture of the trimmer positions that correspond to max and min capacitance 
(there is in the Collins S-line manuals.)  If so, think through where the 
band 31 trimmer setting is in it's range, and the resulting T-402 
inductance, and what that might mean to the alignment on another band when 
you get there.

A couple of notes:
  1) make sure as you measure the DC voltage at an oscillator or mixer grid 
that you have a high impedance probe - that is the probe of an old time 
VTVM that has a one meg resistor in it, or if you are using a DMM, put a 
100 K to 1 meg resistor on the end of the probe. This is all to prevent 
loading the RF in the circuit or adding extra capacitance.
  2) If you are using a scope on a high impedance circuit or on a tuned 
circuit, make sure you have a 10:1 probe for the same reason.

>I'm  embarking on doing the complete alignment of this oscillator,

It sounds like a fine idea!

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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