[Collins] KWM-2A help (long)

[Al Parker]

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.
	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

On 9/18/2010 8:54 PM, Dr. Gerald N. Johnson wrote:
> Some fundamentals: The oscillator frequency is doubled in the oscillator
> plate for the higher bands. Which can mean lower drive, but there needs
> to be enough.
>
> Finals and driver must be neutralized when changed, else they go unstable.
>
> In my KWM-2 manual the crystal oscillator is a 6EA8, not a 6U8. There
> isn't a lot of difference but that might be part of the low output
> needing revisions in the feedback capacitors for a 6U8. By my tube
> manual, the 6EA8 has 28% greater Gm than the 6U8 pentode. That's
> significant especially in making second harmonics.
>
> Then you need to be sure the signal you measure out is at the 14, not
> the LO injection. Its easy to peak on the wrong one with low gain in the
> mixer.
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>
> On 9/18/2010 6:51 PM, Al Parker wrote:
>> Hi folks,
>> 	I don't know Collins like I know some other stuff, I need some help.
>> 	I brought a nice KWM-2A home from Shelby 2 wks ago, to replace the one
>> that's been flakey (quick change in freq.).  This one came from a fellow
>> I've known for 20+- yrs, he'd used it himself, but put it aside abt 2
>> yrs ago.  Said he'd replaced the finals&   driver then.
>> 	It receives fine, but transmit is a problem.  75&   40m now are OK, 100+
>> watts out, but very little on 20&   above.
>> 	I have done what I think is usual/normal, before any alignment I
>> studied the block diag.&   schematic.  I removed the shield cans&
>> carefully, with a dropper, cleaned the bandswitch wafers with DeOxit.
>> Ckd (&   tried other) tubes in the xmt RF string.  Did a lot of
>> measuring/observing with the scope.
>> 	Here's where I am now, after several days.  I think the problem is
>> in/around the 2nd xmt mixer, V6,&   the xmtr RF amp, V7.  The xtal osc.
>> sends 30v p-p on 7.2mc and 20v p-p on 14.3mc to the 2nd mixer.  I
>> 'think' 20v is enough, but??  (it does go less on higher bands).  I've
>> tried a different 6U8 xtal osc., I know they're flakey, no dif.  The IF
>> input to that mixer doesn't get bandswitched, so it should not be
>> varying by band (I think I ckd).
>> 	Here's the problem evidence, input to V7, xmt RF amp.  3v p-p on 3.9mc,
>> 2v p-p on 7.3 mc, 400mv p-p on 14.3mc.  That's tuned in the mixer's
>> plate, T-3 coil&   slug varied by "Exciter Tuning" ctrl, and switched
>> variables w/parallel fixed capacitors.  All these caps on various bands
>> do peak as expected.
>> 	What am I missing?
>> 	Any help or thoughts greatly appreciated.  I'd like to use this unit on
>> the Vintage Sideband Net tmw at 2PM ET, 14.292-1/2.
>> tnx, 73,
>>
>> Al, W8UT
>> www.boatanchors.org
>> www.hammarlund.info
>>
>> "There is nothing -- absolutely nothing -- half so muchH
>> worth doing as simply messing about in boats"
>> Ratty, to Mole
>>
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