[Collins] KWM-2A help (long)

[Dr. Gerald N. Johnson]

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
>