[TMC] PMO-2

Mon Jun 12 15:00:54 EDT 2006

HI Guys,

I have a complete DDR-2 dual diversity receiver.  The thermostat went out on mine in 1966.  Temprature was controlled by a mercury thermometer with two points brought out.  It maintains the PTO within 0.5 degrees Celcius.  The replacenent was no longer a mercury one but a whole assembly ( COSTLY!!) that replaced the original with control circuits was supplied to replace it..  It was a solid state replacement.  The potential of the heater elements going bad is remote to less...  They are beefy..

Good luck in your repair endeavor.  I would look for a replacement thermostat that can controll 110 VAC and use it ,if it has reasonable temprature accuracy range.

Jerry
W3CDE
Atlanta, GA

> 
> From: Roy Morgan <roy.morgan at nist.gov>
> Date: 2006/06/12 Mon AM 11:19:35 EDT
> To: "Ira Curtis" <kch at tie.cl>,  <telegrapher at att.net>
> CC: tmc at mailman.qth.net
> 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 --
> 
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