Millen 92105 Sideband Selector         

text below in bold type is from the Millen website at  http://www.isquare.com/millen/millen-page.htm
 

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   I'm working on a Millen 92105 Sideband Selector, something that's not found often.  First off, if anyone can help me with a schematic, I'd be greatly appreciative.  There is a little info out there on it, but no schematic that I found.  I will paste in, below, some info that I did find, re: history of the unit. 

   "The Millen 92105 is an external replacement IF and audio section dedicated 
for the reception of single-sideband signals. The 92105 was based on a 
design by J.L.A. McLaughlin and appeared in the April 1948 issue of QST 
magazine. This unit uses what is termed a double-oscillator sideband 
inverter. Basically the user would tap the IF signal from their receiver. 
Usually this was done by simply wrapping insulated wire around the plate 
lead of the detector tube. This IF signal would be mixed with either a 
crystal frequency 50Kc above or below the IF frequency of the receiver. 
The 92105 came delivered for a 455kc IF, but simply changing the crystals 
to values +/- 50Kc from your receivers IF frequency would make this unit 
operational on just about any other receivers IF."

   I've re-stuffed the 2x20mf@450v filter cap, put in a 3-wire line cord, and powered it up slowly at reduced voltage,  briefly, after close inspection.  It has signs of life, but low B+ at the tube pins, largely due to leaky micamold capacitors, some of which got warm rather quickly.  The wiring and components are very compact and it'll be a challenge to do a nice neat job of any replacements.

    My quandary is:  what should I use to replace the micamolds?  Normally I don't worry much about using "modern" disc ceramics to replace old BBOD's, brown waxy papers, etc., in most of the boatanchors I work on, but this is a unique piece of eqpt.  Some guys will re-stuff the brown waxy paper jobs to make things look original, but that's not feasible for micamolds, and might not fit back in easily, it's so tight. Those that I can see the markings on are .01's.  I have some small, brownish, .01 discs, should I just use them, at least they're not glaring orange like some are, and will fit nicely in the larger spaces left by the M-M's.

    I presented the above to 2 "boatanchor" email lists.  Most responses said to replace the capacitors with small modern units that wouldn't be too glaring, although obvious.  Several recognized the uniqueness and hoped for staying as close to original as possible, one said leave it alone, just display it as a museum piece, one said to include a small package inside with examples of the removed parts in case someone in the future wished to make a "perfect" restoration.  

   I proceeded with replacements, 6 of the Micamolds were 0.01mf used as bypasses, one was a different configuration, still a Micamold, but was 0.1 mf, and may be of true mica construction.  The others are likely paper, with a plastic covering to make them look like micas.  All the 0.01's had significant leakage, in the order of less than 1/2 megohm resistance at as low as 75vdc, the 0.1 was OK.  I used small, tan, axial lead poly(something) capacitors, that don't seem too invasive.  4 resistors were found to be at least 15% high in value, they were replaced with carbon composition units that match the original types.  2 were of high values, 470k ohms or more, and were in tight places.  They measured 15-20% high, and were paralleled with small 1/4w 5 or 10 megohm resistors that wouldn't be too obvious.  

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    So, how does it work after all this?  It has a crystal controlled oscillator and converts an incoming 455kc IF signal from a receiver to it's own 50kc IF.  The oscillator frequency is switch controlled, using  405kc and a 505kc xtals.  There are 2 slug adjustments on one IF can, no other alignment points.  Unfortunately the 505kc crystal would not oscillate, but the 405 did, and the unit would properly detect the incoming signal on one sideband.  I opened and cleaned the 505kc crystal with Everclear Alcohol, and it worked for about 5 minutes, but would not work again after another rinse.  Present type single sideband suppressed carrier voice signals were not properly detected, as there is no BFO in this unit, to my dismay.  It was made for AM signals only, a "cure" for close by AM signals that produce heterodynes.  QRM was a problem back in the 1940's, as it is nowadays.  

   "After WWII crystal prices were dropping drastically. Military needs 
brought many of crystal manufacturers big business as well as offering new 
opportunity for start-ups. With the war over the competition for sales meant 
sell cheaper or close your doors. Both happened! However, the amateur 
radio market got the benefit of these price reductions and thus a unit 
like the 92105 could be built, bought and used by hams at affordable 
prices.
   In 1948 AM was still king in amateur operations. Yes, SSB was coming of 
age slowly, but mainly for the experienced builder and experimenter. In 
the original QST article the author doesn't really address listing to SSB 
communication, but simply one sideband of AM signals. Any one who operates 
AM today can tell you it doesn't take to many AM QSO's to be going on 
within one of the AM windows to make communications a real nightmare! 
Thus, even in 1948 they were working on making things a little better from 
the communications standpoint by listening to just one of the sidebands. 
An interesting fact that SSB really was addressed on the receiving side 
before the transmission of SSB signals was seeing major popularity in ham 
circles.
   Now back to the 92105. The Millen Sideband Selector contained a 50kc IF as 
a result of the aforementioned crystal offset mixing from the receivers 
IF. Using two tuned IF stages at 50Kc the user was able to get very high 
selectivity needed to receive SSB. After the IF was a low level audio 
amplifier low pass filter stage. The audio output would then be feed back 
into the receivers audio output stage for amplification to drive the 
speaker. The user could also opt for an external audio amplifier and 
speaker system separate from the receiver.
   You don't see many 92105's around these days. After the article appeared 
in QST Millen had their version in print the very next month. In addition, 
several radio resellers were also touting the Millen unit in their ads. 
Why it wasn't more popular I can not tell you. But perhaps like todays 
ham's who enjoy AM for it's rich audio fidelity that reducing the 
bandwidth down to a few Kc's was not of much interest."

   I obtained and installed another 505kc crystal in Feb., 2008.  It works as it should, however proper alignment still needs to be accomplished. To help in that direction I will draw out the schematic, as best as I can, but will just have to leave "boxes" for the three IF cans, and hope I can come up with a good alignment procedure. If anyone happens to have any technical info I'd be obliged if you could forward it to me.

 

 

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this page was started on, Jan. 26, 2008, last revised 2/28/08