Easy solution, cannabilize one from another brand set. should your electro-mechanical skills be in the nil-zilch-nada category. However, using this supplicated info in order to make that unit compatible, should it perchance not be. With most variable caps RF sections capacitance being pretty uniform in working to create the most common 455 Kc IF frequency.
If probably using the reference below, it can typically be fabricated from a heavy picture matte board scrap from a picture framing shop...freebie...FREEBIE !!! and an agreement of the mechanical design of either going with the sequential basket weaving in of alternate turns of hook up wire around the peripheral gapped "teeth" of the design shown in this reference picture.
( Note that some units are effectively incorporating in a single inductive turn such that an external antenna [and possibly ground ] using an isolative capacitor for safety, should one want to use a long wire antenna additionally)
Hi-Tech ref:
http://img251.imageshack.us/img251/9758/learloopydoopyantennnacf9.jpg
What needs to be accomplished is the derivation of the proper value of inductance of that created loop to work in conjunction with the RF variabl capacitor section to produce uniform and optimal reception across the AM band, the capacitiver aspect optimizing performance and tracking at the high end of the AM dial and your to be built loop optimized the low end of the AM band.
You can make a rough estimation of the final inductance (turns) required by loose forming some separate thinner cardboard into an oval approximating the inner diametrical dimensions of the "gap toothed" form profile shown.
You then pass one end of hook up grade winding wire thru a hole in the form of enough length to reach over to the RF tuning condenser /or/ its other terrmination the AVC/gnd bus. Then "overestimate" the winding on of 25 or so turns and secure at points with masking tape / duct tape / modeling clay / refrigeration putty / caulking putty, what have you, to initially-temporarily hold the turns in place. Then the resultant wire gets connected to the other remaining connection option of the two aforementioned.Then you fire up ye olden LEAR (Jets afterburners) and tune to a station in the proximity of 600 Khz. Then you will be taking off individual turns to get the unit in the proper required inductive value. ( And when you take off a turn(s) you want to take that surplus wire and start running it aside as an extended single parallel hairpin loop... so that excess will not be creating an unwanted inductance value aside by being in a coil form ) That units reception performance might be initially so damped that it might require moving over in tuning to find ANY station that will come in. But progressively you will be able to move back to the 600 KC area. You need to have your hands/ fingers off the wire proper and holding away at the cardboard edge when evaluating the enhancement of reception as the inductance is optimized. At one approach point you will hit the point where reception is really shaping up. Also, previously, you should have found that the gripping of the wound loop turns with your palm and fingers and fondling ..(care full there......CAREFULL !!) ...of those clustered turns should have had a marked enhancement of reception. The added body capacitance.... it shifting tuning... and the coupling in of body induced RF ( YOU numbah-one antenna ! ) signals into the coil being the result. Eventualy in paring away of the individual turns, one at a time, a state of matching will evolve such that the hand capacitance test will have a very minumal side effect, with the loops inductive value now reaching an optimum.
As it equally would be on a finely tuned and tracking AM receiver.
So you are now finding that ?XX? turns would be a rough value for the soon to be built new loop..aren't you glad that I didn't tell you 10 turns and then had you haywiring on 1 extra turn at a time !!
With that turns info now in hand you can use either of two common modes of construction of that LOOP. One involves the gap-toothed form where you start winding your coil basketweave style until one turn is completed wherupon the further turns start resting upon the previous wire thickness IF you have an odd number of "teeth". The final result wil have your coil operating in the manner that you had the initial test unit operating..with a final refinement of an overwind of 1 turn and you can use that same moving aside of that single turn and affixing it within the center of the form in order to fine tune in the inductive value to the ultimate degree.
The other option is the showm pictures winding the coil in an ever increasing outwardly spiraled oval. That typically is heavily dependant on holding the wire in check and to form , usually by the use of wax impregnated wire to stick to itself or an adjunct layering of beeswax on the cardboard form to press it into or even the use of tape / or/ double stick to hold the errant wire forming.
'scuse pls....velly solly... if any typos... as this was voice recognition derived, with minumal corrective editing.
Thassit
73's de Edd
73's de Edd
Oohhh Tayyy..
Considering your unit to have been made within the 6614 design mold; and you confirmed the Variable tuning inductors presence.
Check this supplied schema thumbnail for any referencing:
http://img469.imageshack.us/img469/2369/learloopydoopyta0.jpg
I am assuming that ther is nothing present of the old loop, but hopefully snippets of the old wires were left for visual referencing...within the realm of how well 50+ year old wire retains its colorization.
The good news is that the complete loop was not dependent upon for the total fixed inductive element in your LC pair that forms your RF front end . In the old days when they were small tuned coils, they didn't pick up signal to good so they additionaly used wire connected as an aerial and eventually merely upsized to using loops for that Rf sections coil inductance , thereby, producing some spread out surface area to pick up a better signal also.
Now , in your case the tuned circuit is the L1 variable tuning inductor which is present, BUT basically out of circuit since the loop antenna element is missing.
Ref the schema:
See the very bottom of that coil and it will be readily apparent that there is currently no DC connectivity from where the white wire to brown wire USED to connect through the loop antenna. The front end of your RF stage is now floating.
The next step in a fix would be to find those three wires connection points,if the color coding...or even the presence of those original wires preclude that, gonna' hafta' get out your compass and GPS and track 'em down!.
If you will then get the bottom of that L1 tuning coil connected to the AVC buss line, you will then loose that garble-garble-garble sound and there should now be reception, within the realm of the pickup capabilities of that resultant tuned circuit
pair... with its C20.
The next step on reception enhancement would be the connection of a looooooooooose length of "antenna wire" to an .01 ufd 400-600 VDC coupling/isolation/safety capacitor. The other end of that cap goes to the low Z tap off from the L1 coil that the blue wire is shown to have connected to before.
By now even more stations should be rolling in, what with your deluxe receiver having that frontal RF stage, whereas, the norm on AA5's is no RF stage presence.
I am going to ZUJ...stand by...at this time to see how you are coming....along?
73's de Edd
Tech Reference:
http://img488.imageshack.us/img488/8086/moreloopydoopylearjetxf2.jpg
You can find that AVC buss anywhere in the green channeled routing, For the easiest, I would opt for locating that cold end of the IF transformer, then put one of your ohmmeter probes on that point and take the other and find the other ~0 ohm point up in that tuner cluster near C19 , and that would be where your NEW
white AVC wire would be connnected, then go thru the previous connection procedure.
73's de Edd
.......are we having FUN yet ??????
ADDENDA:
:This might just be a photo of the rear of your radio, but alas the definition / resolution is inadequate on the locating of the missing wires conection point, but at least the detail of the lop shows it t have been on the "gapped tooth" form.
:http://img471.imageshack.us/img471/3184/learchassistopsidexm8.jpg
Edd
( If you will then get the bottom of that L1 tuning coil connected to the AVC buss line, you will then loose that garble-garble-garble sound and there should now be reception, within the realm of the pickup capabilities of that resultant tuned circuit pair... with its companion Cap.)
The next step on reception enhancement would be the connection of a loooose length of "antenna wire" to an .01 ufd 400-600 VDC coupling/isolation/safety capacitor. The other end of that cap goes to the low Z tap off from the L1 coil that the blue wire is shown to have connected to before.
By now even more stations should be rolling in, what with your deluxe receiver having that frontal RF stage, whereas, the norm on AA5's is there being no RF stage.
ZUJ'ing
73's de Edd
Perhaps you can explain the purpose of the tap on L1 and the tap the brown wire makes on the loop. I don't know how many turns of the loop I need to wind, before I connect the brown wire. Another curious thing is the C21 that goes from the brown wire to the external antenna. C20 and the other C21 are part of the tuneing device. My parts list shows only 1 C21.
You are a good Elmer and I appreciate all your help.
73s de Dick, KB7OEI
An actual top view photograph of a Lear-jet chassis Ref 1:
It gives a viewing of the original loop assembly.
http://img471.imageshack.us/img471/3184/learchassistopsidexm8.jpg
Looking at a redrawn Ref 2:
http://img412.imageshack.us/img412/3506/learjetloopydoopytwojpefl4.jpg
Since you said it was most in agreement with your sets wiring and components, with the easier viewing of the 4 meg AGC feed resistor....the other schematic actually was the same, but with it being positionally placed far to the right on that schematics page. Plus, it has the technical drawing of the loop which I have now transposed onto the same page. (Along with an entirely different single winding loop antenna at the top left corner for a further in detail explanation of operation.)
Now IF this were the most common design, there would be a variable tuning condenser working in conjunction with its companion fixed inductance for the frontal RF tuning function of a receiver. The first generation sets were using coils buried within the set with little surface area for direct pickup of the off the air signal. That then meant the pumping in of signal from a co-connected wire antenna for any somewhat decent reception of other than the strongest of signals. It didn't take too long before the idea of a larger separate loop with surface area for greater signal pick up and a tapping into of the main chassis RF coil and a matched impedance coupling in of the signal. Then the smaller, more portable table model sets brought on the aspect of the utilization of a sole larger scaled whole loop antenna serving as the whole RF inductor element of the tuned L/C pair.
Reconsulting the closest reference above, at top left corner, if you were to take the loop shown and visualize NONE of your sets cluster of (C1,L1,T1 and C21) even being existent (most of which, actually isn't !) and only that single coil loop antenna shown above, along with your C20 TRIMMER cap, with it instead, being representive of an air variable tuning condenser. Then, by utilizing the loops two terminals, B would connect in at the RF stage tubes first grid at point B and the loop antennas A terminal would go to the AVC line at A. That then gives an adjustable L/C RF pair to tune in the stations, with the advantage of the L element being constructed in a larger, open surface area for ever so better, off the air, RF signal pickup. To still go back to the idea of transferring in signal from a long wire outdoor antenna, or even a nominally shorter hank of strewn out hookup wire, they started providing the capability of introducing that signal via a single turn or two winding additionally incorporated on the loop.
From the safety aspect of interconnecting any loose external wiring to a potential AC hot chassis, some isolation was provided by a capacitor. (C21 function on your set).
Now, getting completely back to what you have as the real situation in your set, there is the absence of the loop proper, with its wound on (T1) loop element, along with having the ant coupling/isolation capacitor (C21) and RF coupling/DC isolative capacitor (C1) also being amiss from having been also mounted onto that loop infrastructure.
WE are now back to the point where I hoped that you could give me some informational/operational feed back for a more exacting system operational analysis. It seems that if you consult my red X's, they will depict the removal points of the loop and with only those three wires being present, that used to interconnect to the loop. Either by wire color coding, or,if long since faded away, by tracking down the wirings sources, you have identified the three wires. With a consulting of my provided ACTUAL Learjet photo, at the very first of the references, it is quite evident that your loops construction utilized QUITE a few turns in its formation and was coupled into L1 at a tap that was well up from the low end of it in order to acquire a proper and most efficient transfer of RF signal with proper impedance matching. Other sets design might have had a mere 2-4 turns of wire on a form and were coupled in well down towards the "cold" end of L1 with fewer turns on the tap in point.(One that comes to mind is a Truetone D2612..or a Princeton also..of a series...that also used variable inductance tuning, but was stepped up one echelon of performance from yours in the respect of having another variable tuned L/C pair on the plate circuit of the RF stage; where yours uses a R/C coupling via R1 and C3...its Loop coil is on a wooden frame just inside the set with four spaced turns of wire on it.)
Now with your initial testing of the set and having its three wires floating and the resultant "garbled underwater audio sound", plus poor signal pickup, I would ask that you take the "white" wire and the "brown" wire and connect them so that there will then be a completed circuit signal loop again using your L1-C20 tuning elements. Also I am hoping that trimmer C20 has not had its adjustment touched, and will not be, until the very final stages of tuning, if even required then. Now, the question is, after a complete tuning across the AM band, how many stations are now being picked up ? (Hoping that you are NOT located in the Mojave desert..or Kansas... Toto) Then the next procedure would be to take a longer length of wire (18 feet, 7 and 'lebenteen /sixteenths inches ? ) to use as an antenna and then connect its loose end at hand to a .001-.01...totally, not critical...coupling capacitor and tie into the blue wire that terminates into the tap on L1. Then, in doing a band sweep, were the.. stations picked up.. count enhanced somewhat ?
ZUJ'ing for feed back...AND... is either an ANALOG VTVM, Transistorized/FET VOM, or lastly, (at the least), a 20K+Z VOM...type of instrumentation available for future measurement analysis ?
Aside:
Poser..Question of the day...what's wrong with the top left corners loop in Ref 2 ?
73's de Edd
---------------------------------------------------------------------------------------------------------------------------------------
:Thanks to you, I'm beginning to find my way. I have both wires ready to connect to the loop, which I'm ready to make. How many turns do I need in the loop?
:
:Perhaps you can explain the purpose of the tap on L1 and the tap the brown wire makes on the loop. I don't know how many turns of the loop I need to wind, before I connect the brown wire. Another curious thing is the C21 that goes from the brown wire to the external antenna. C20 and the other C21 are part of the tuneing device. My parts list shows only 1 C21.
:
:You are a good Elmer and I appreciate all your help.
:
:73s de Dick, KB7OEI