[TMC] PMO-2

[Roy Morgan]

At 11:50 AM 6/11/2006, Ira Curtis wrote:
>Hi Larry,
>Yes I do have the original manual, however, it doesn't say any thing about 
>tracking.  It, to my understanding, deals only with small frequency 
>corrections.

Ira,

Here's the advice I got from a former TMC employee:  "DO NOT open up the 
master oscillator."

Now, that's all very fine, except when the thing does not work right.  I 
have a couple TMC master oscillators here (in a transmitter and in a VOX-5) 
and I think that neither of them tracks right. Sooo.. here are my thoughts 
on the matter:

0) From what you have posted, you do not have a tracking problem, you have 
a component failure problem that must be solved first before you can get 
the tracking to work right.

1) Go ahead and open it up. You'll have a chance of making it run right if 
you do.  If you just leave it alone, it will never work right.

2) It sounds to me like one or more capacitors in the frequency determining 
network are open, leaky, or changed in value.  (An inductor malfunction is 
less likely but possible.) The caps used are a collection of high stability 
fixed caps, temperature compensated units likely selected during 
manufacture to make the oscillator track correctly both mechanically and 
over temperature changes, and the variable caps used for main tuning and 
frequency correction adjustment.  If one or more of those is now 
kaput,  you will have a very challenging time getting it all right. You may 
well be able to get it right enough for you with only a modest effort, 
however.  Note, we are not making broadcasts of classified traffic to the 
US Naval Fleet from headquarters and having to depend on frequency 
stability to support multi-channel MUX RTTY over HF links across the globe.

3) Very likely, there is no alignment/overhaul/re-calibration procedure 
available. You have to figure out the tracking adjustments all out again, 
after you get it running approximately right.  There has been published 
more than one article in the ham literature on temperature compensating VFO 
circuits. These articles would be a good place to start.

4) Some important aspects of compensated, high-stability VFO are:
  - tracking error or linearity
  - short term stability (over minutes or parts of a day)
  - long term stability (over weeks or years)
  - backlash in gearing and other mechanical parts
  - oven temperature control
  - design elements included to allow for manufacturing alignment, aging, 
and testing

Only some of these aspects would be most important to us now.

5) If you are tempted to mess with the split end plates on the capacitor 
rotor(s), don't do it until the thing is working nearly right across the 
whole band.  The bending/twisting of segmented variable capacitor plates, 
called "knifing", is a last stage refinement to improve linearity and 
reduce the need for "corrector adjustment" at various points across the range.

6) When I get to working on my TMC oscillators, here are at lest some of 
the things I'll do:

  - Check power supply B+ for stability, replace regulator tubes as needed
  - Check/ replace oscillator tube (it's a 6AB4, isn't it?)
  - Ensure all moving contacts are restored - variable capacitors, 
including grounding wipers, any switches. Caig De-Oxit and Pro-Gold are the 
preferred treatments. See www.caig.com  Do not even think of using 
alternative products you might have gotten at Rat Shack.
  - Lubricate very carefully all mechanical parts.
  - Check that anti-backlash gearing is working right.  (This may require 
complete dis-assembly of the mechanism for thorough cleaning.)
  - Figure out what the thing is actually doing: Is it more or less linear 
but the end to end frequency span is off?  Is the Corrector system working, 
or working but not right?  Is it ok on some frequencies but non-linear?  If 
the thing is running high in frequency, I'd suspect open capacitors (or 
possibly a shorted coil).
  - Are the ovens working right?  (you can't expect it to run right if the 
ovens aren't heating, or are overheating.)
  - Decide if possible what parts might have failed.  I might remove caps 
from the circuit for testing if needed.  (I am able to measure capacitors 
very accurately here, including loss factors.)  Heating/cooling cycles may 
be needed to find an intermittent cap.
  - Set it right by cap or inductor replacements as needed
  - Then figure out how to align it for dial accuracy and proper frequency 
readout at temperature.

>I believe the problem lies within the correcting capacitor, that doesn't 
>track or isn't synchronized with the main tuning cap.

Do I remember correctly that there is a variable capacitor adjustable from 
the front panel that allows for periodic correction at a 100 kc (or is it 
every 50 kc?) point near the "frequency of interest"?  If that thing has 
gotten a bad rotor to frame wiper or the like, it may be intermittently out 
of the circuit, or only partially making contact.  If so, tiny drop of Caig 
De-Oxit, followed by some Pro-Gold will solve that problem most likely.

It is unlikely that a variable capacitor in there has changed value, unless 
physical damage has occurred due to trauma.  (I have a frequency meter 
which is the successor to the BC-221 series whose main tuning cap has a 
stator mounted with little glass spheres.  A good whack dislodges the 
stator and it's all out of calibration if it works at all. And this is a 
piece of military equipment!  bad design.)

I may be remembering the thing incorrectly, and the "corrector" system 
might be a mechanical adjustment to the main tuning cap, a 
non-changing-until-corrected displacement of the main tuning cap from it's 
nominal position. On the other hand, it might be a separate capacitor in 
parallel with the main tuning cap.  If I remember correctly the TMC master 
oscillators do not use a corrector mechanism that can be set at many points 
throughout the range as the Collins PTO's do.

>And if this is it, although I will finally find how to do it, but would 
>rather shorten the procedure.

If the dial is 140 kc off in places, there is definitely something amiss in 
the frequency determining circuits.  Just "tracking" is not gong to solve 
the problem

>  Does anybody knows of a source to get the heating elements and both the 
> mercury and the thermo couple thermostats for the PMO's?

No, sorry. Other than another PMO. Searching the industrial and electronic 
sources may surface a suitable substitute for the heater elements.  The 
scientific laboratory supply industry may have similar or even the same 
heaters available.  As to thermostats, thermal switches are available, at 
least in crude form, now.  I think "Clixon" or some such is one well known 
brand name.  They no doubt will be different physically than the original, 
but might work just fine.

A moderate amount of design and new parts would get you a proportional, 
thermistor controlled heater controller that might even outperform the 
original as to stability and accuracy.  This might be just the thing for at 
least the inner oven.   The Yellow Springs Instrument company  "YSI" has 
for a long time made thermistors and controllers of astounding performance 
for all sorts of applications, but stand by for a shock when you find out 
prices.
(http://www.ysitemperature.com/ Note:
" In April 2006, YSI Temperature was sold to Measurement Specialties, Inc. 
Please visit ysitemperature.com for information on ordering precision 
temperature components. "

<http://www.ysitemperature.com/about-news-msi.html>YSI Temperature is now 
part of Measurement Specialties.

<http://www.betatherm.com>Betatherm is also now part of Measurement 
Specialties



I don't think the TMC thermostats are thermocouple type, are they? I think 
they are temperature sensitive, bimetallic contact 
open-upon-temperature-rise devices.

>Same goes for the clear pilot lamps lens?

Scrounge at hamfests. All my stuff is now in storage, so I can't even look 
to see if I have one that's just right.  Also, contact Play Things of the 
Past at:
http://www.oldradioparts.com/
A quick tour of his categories did not find the part you need, so an email 
to him may be in order.

Check the AES part number PL-122
"INDICATOR LAMP, JEWEL, CLEAR, REPLACEMENT FOR FENDER
Replacement jewel for Fender amps. Clear. "  $2.25
http://www.tubesandmore.com/

(The thread on the metal cased "jewels" is likely not the same as on the 
plastic-only ones, so know which you have before ordering. Of course AES 
will sell you the whole pilot lamp holder.)


I encourage you to persevere with the TMC oscillator, and to report your 
success.  I'll be digging into mine late this year after I move and get my 
shop re-established.

Roy
Who would like to find any TMC RTTY terminal unit.


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
roy.morgan at nist.gov --