[Collins] KWM-2A help (long)

[james.liles]

Good morning Al:
The service bulletins in the January 1978 manual indicate that the 6U8 was 
changed to 6U8A and there are references to that which you speak.  Those 
bulletins are worded a bit like maybe, maybe not logic or use whatever you 
like to make it work.   Kindest regards Jim K9AXN

----- Original Message ----- 
From: "Al Parker" <anchor at ec.rr.com>
To: <geraldj at weather.net>
Cc: <collins at mailman.qth.net>
Sent: Sunday, September 19, 2010 8:43 AM
Subject: Re: [Collins] KWM-2A help (long)


> 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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