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

[Dan Merz]

Roy,  much thanks for the comments.  I concluded from your comments that the
multiple peaks that might occur with T402 adjusted with too much inductance
are due to occurrence  of various harmonics as the trimmer on 31 is swept.
The goal is to have only the right harmonic there and low enough inductance
that the others can't be produced,  and only one (at least according to the
manual procedure).  I'll go with that interpretation and see what falls out
when I start alignment.  The VTVM measurement in the manual is made on the
grid of the mixer tube - the oscillator signal is injected to the cathode.
The Engineering Report on the 390 says about 10 to 15% of the injection
shows up on the grid so I guessed that's what was being measured at the test
point specified.  However,  in rethinking this,  the VTVM measurement is a
dc measurement of negative voltage on the grid so probably reflects how the
bias on the grid peaks as the various harmonics are produced by the trimmer
sweep and not really an integration of the rf voltage that I had in mind.
I've been measuring at the cathode with the scope so far and using the 10x
probe on the scope.  I don't have a vacuum tube VTVM,  but do have a 1
gigohm input impedance "electrometer" that I built for looking at grid
voltages.  It should work fine as long as we're talking about dc voltages.
More later,   Dan.  

-----Original Message-----
From: Roy Morgan [mailto:roy.morgan at nist.gov] 
Sent: Monday, May 02, 2005 12:17 PM
To: Dan Merz; R-390 at mailman.qth.net
Subject: Re: [R-390] R390 progress 6AK5/6AJ5

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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