[Collins] KWM-2A help (long)

[james.liles]

Good evening Gerald:
The Collins manuals and doc are not on BAMA.  Can you point me to a location 
where all levels are available?  Kindest regards Jim K9AXN

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: <anchor at ec.rr.com>
Cc: <collins at mailman.qth.net>
Sent: Sunday, September 19, 2010 1:27 PM
Subject: Re: [Collins] KWM-2A help (long)


> I'm working from the last Rockwell manual, stored on line at CCA.org. In
> the extensive list of revisions, it hints some vintages used the 6U8 for
> the crystal oscillator, then maybe 6U8A.
>
> Now I've also found my yellow book with a 1959 version of the schematic.
> I sure do wish someone would have compared schematic to radio and to
> production drawings. This vintage schematic shows V12A calibrator is 1/2
> a 6U8A, but V12B, cathode follower is a 1/2 a 6U8.
>
> The bandswitch circuits are all wrong. They show a rotor tab that
> contacts only one pole at a time and I'm sure the tab is wider
> contacting two so those open poles aren't really open. Then in the 1959
> schematic of the crystal oscillator on 20m it shows an 8-50 (C65) by
> itself and then a 47 fixed (C66) with the wires crossed on an adjacent
> switch pole. THEY SHOULD BE SHOWN CONNECTED with a dot. The last
> schematic was redrawn and neatened making it worse. I'm pretty sure the
> 8-50 won't tune down to 3 MHz because for 3.5 with the slug way deeper
> in the coil it took a 240 fixed in parallel with the 8-50.
>
> The transmit second mixer is kind of balanced for the LO from LO signal
> going to the grid of one triode and the cathode of the other. The 3 MHz
> signal is neutralized by the capacitive unbalance of the two capacitors
> inside L6 and the 6 pf capacitor C26 from mixer output to the bottom of
> that coil. If C26 is missing there would be more 3 MHz on the output.
>
> If L5 has been replaced with a choke having a series resonance in the 14
> MHz range it would kill off the 14 MHz output.
>
> These 8-50s have been known to go open. Its where the shaft hits the y
> shaped conductor on the back. DeoxIT and exercise is a solution,
> providing they will still turn. Sometimes they lock up and there is no
> modern replacement that will stand the RF voltage and current.
>
> We don't know the vintage of your M-2, but with 6U8 crystal oscillator
> its older rather than newer. Can't go by the serial number, they were
> issued in random order (by decree of Art Collins), but you can get an
> idea by the date code on the crystals and the mechanical filter and
> maybe the electrolytics if they can be decoded. Early M-2 used a metal
> cased (Y) filter, later used the plastic cased (FA) filter with a
> different mounting under the chassis.
>
> On 9/19/2010 8:43 AM, Al Parker wrote:
>> Thanks Jerry & Dave,
>> I'll try to comment on Jerry's & add my observations:
>> 1. Yes, the 14mc range is doubled in the osc. plate, it uses a ~8.7xx mc
>> xtal doubled to ~17.4 to mix with the ~3.1xx mc variable IF signal to
>> get ~14.3xx. I'm getting abt 20v p-p out of the osc. on 14 vs abt 30 on
>> 7mc, so it is less. Is that too much less? I'm not sure, because---
>> 2. At the plate(s) of the 2nd mixer I see the mixed signal, but the LF,
>> ~3mc, signal is by far the dominant one, the 14mc (or 7mc) is much less,
>> appears as a modulation freq. on the LF signal. I didn't record the
>> amplitude of that signal, but I'd guess that the HF component was less
>> than 1/4th of the LF. (I'll see if I can get a look at it with the spec.
>> anal.) At the following RF amp grid, the tuned ckt has eliminated the LF
>> component, and just leaves the wanted final freq. But it's only ~400mv
>> for 14mc, vs 2v at 14mc. That tuned ckt seems to be peaking properly,
>> but I will ck to see if there's another peak in the cap's range (not the
>> mirror peak). I'll do that this AM. That may be the solution, I sure
>> hope so, but then it's mis-tuned on 15 & 10m also. Those trimmers have
>> an 8-50 range, so it's just possible on 14 & 21mc to peak both the
>> wanted & the 3mc IF signals.
>
> Not likely to tune to 3, but maybe to tune to 20.5 MHz.
>
> When a mixer isn't balanced or neutralized, the gain at the input
> frequencies to the output is always higher than to the converted output.
>
>> I did ck the final & driver neutralization, it's not perfect, but
>> they're stable.
>> I have 2, from different manuals, schematics that show the 6U8 osc.
>> tube. I wish they'd date coded the schematics.
>> many thanks for the insight, I'll let you all know what the outcome is.
>> 73,
>>
>> Al, W8UT
>> www.boatanchors.org
>> www.hammarlund.info
>>
>> "There is nothing -- absolutely nothing -- half so much
>> worth doing as simply messing about in boats"
>> Ratty, to Mole
>>
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
> ______________________________________________________________
> Collins mailing list
> Home: http://mailman.qth.net/mailman/listinfo/collins
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Collins at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html