[TMC] PMO-2

[Ira Curtis]

Hello Roy,
Somehow I was sure that you will come for the rescue!
First, No one can ask for more elaborate answer backed by an in-depth 
knowledge.
For the benefit of all interested in this tech topic, I would like to take 
the liberty and communicate with you on this net:
First, I am not afraid of opening sealed or closed assy's if they don't 
work. What have you got to lose?
I did open it, and first looked for failed components. I usually start with 
capacitors and that I did. You may be surprised,
But components quality is nothing to write home about. capacitors are NOT 
temp. compensated to my surprise, ant not the best of the line. I have 
replaced all of them with CM's silver micas. The 6C4 osc. tube grid leak had 
gone down in value to 19K from 22K and was replaced. Not a problem here. 
Frequency determining caps, were little lower in values,The only NTC caps,a 
2.5pf parallel with 15pf'  gave total of 12pf, both  replaced just to be on 
the safe side. No evidence of  physical damage to any of the air variable 
caps, and all are turning nice and free.The coil with it's slug turns 
freely, giving total frequency range of 144kcs.Here I  don't understand the 
mechanism. The slug can be  tuned through the panel with a slotted shaft.The 
total axial movement is approx 3/4". problem is that the shaft is connected 
to the slug through a National velvet reduction gear, which doesn't allow 
for ANY axial movement. By turning the slotted shaft you are applying axial 
force which at the end, may damage the slug threads .But again this has 
nothing to do with tracking, or am I wrong here?
Cal. cap gives range of plus/minus 2Kcs.
All gears are back-lash free,
All voltages are within tolerance.(10%)
Both ovens are inoperative due to both thermostats and heaters gone bad. 
This in my opinion will effect stability but not linearity, or am I wrong 
again?
Can't get DeOxit nor Pro-gold. here.
I use light grease, meant for RTTY as gear lubricant.
The corrector system works, however I don't know how to, or where should 
the Max/Min points be.
This is one of the things that got loose. Tightening of the Allen key isn't 
good enough, I thought that by putting it in mid capacity, at  mid 
frequency, i.e 3000.00Mhz, will do it, Wrong. The corrector cap is  being 
moved by a cam, which HAS provision for fine tuning, and can be corrected at 
different points along the cam movements, but without specific instructions 
it would be impossible to tune it, so I have opted not to  touch it.
My search for the thermostats and heating elements, were in the name of 
originality, if can not be found it will be scarified.
Even though I don't have a tactical fleet, (hi hi) I really wanted to have 
everything working just like the day they left the factory.
One mod I did do though, and this was to make a mechanical stops at both 
ends of the cam, in order to prevent over travel of the numerator. Simple 
but effective. I don't know why TMC didn't do it, and opted to put a warning 
notice on the front panel.
After all said and done,still no linear tracking.
Thanks and apologies for the bandwidth....
Ira
CE3AM,,,,,,,,,,4X4SW

----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: "Ira Curtis" <kch at tie.cl>; <telegrapher at att.net>
Cc: <tmc at mailman.qth.net>
Sent: Monday, June 12, 2006 11:19 AM
Subject: Re: [TMC] PMO-2


> 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 --