[Collins] KWM-2 Repair or HELP Needed!

[Gerald]

On Thu, 2005-12-29 at 08:03 -0500, Roger Shultz wrote:
> Normally I can fix any Collins A line radio but in the case of my KWM-2, I
> just don't want to! It has frustrated me to a point where someone with less
> emotion could surely do the job better so I would appreciate references off
> list of suggested resources here in the northeast (NJ) if possible.
> 
> The problem  is a transmit problem that manifests itself in several ways.
> 
> The first problem is that 80 meters particularly seems to be very sensitive
> in terms of getting grid drive.
> 
> I found that work has been done previously in the 80 meter mixer plate area
> but it appears to have been done OK though the trimmer and parallel cap was
> replaced suggesting this problem was around long before I bought it. If I
> tune the exciter tuning very slowly grid drive will literally "pop" up and
> as I go past peak it drops and can't be found until I slowly tune from lower
> in frequency to higher in frequency.

That's a sign of a tuned state oscillating. That's not the
characteristic of a tune stage with drive.
> 
> Additionally, while I have gotten as much as 110 watts out on 80, I can't
> get more than 70 watts on any other band. I have tried new finals, yes, they
> are the 6146W's which are known problems in some radios, they are
> neutralized  and screen and HV voltages are proper. 6146's do not seem to
> change the output though I can;t be sure they are not  weak. Today, power
> dropped down to 50 watts even on 80 meters. I do pull about 230 mils on
> transmit so maybe the problem is in the pi-network output which has not been
> checked.

You have to have proper drive for the PA to work. There are three
neutralization adjustments. Have all three been made by the manual? Are
the neutralization adjustments at something other than the end of range
on the trimmer, either minimum or maximum C?
> 
> Another screwy thing is there is some sort of intermittent that generates
> "static" as I tune the exciter tuning but only when it is lying normally and
> disappears when on its side! I have tightened the coils shields with no
> change. I can tap the chassis and the static appears suggesting a loose
> ground or some connection but because it goes away when flipped over I can't
> find the darn thing!

Find a wooden dowel and use that to move things looking for that loose
ground. Tighten ALL the tube socket mounting bolts as well as all the
terminal strip mounting screws and ground lug mounting screws. Clean the
pressure contacts of the coil shields with DeoxIT from Cramolin. A
poorly grounded shield can lead to driver or predriver oscillation.
> 
> Yet another problem that occurs only at times is that when I bring it back
> from tune or lock key it takes a very long time to come back to receive,
> like it has tons of AGC action going on. There is a SB that sounds like it
> fits and the problem is said to be a screen grid problem in the PA and adds
> a diode to correct it or suggests new finals but it sure is a nasty thing
> when it happens and it is at times accompanied by audio feedback!

That comes from receiver and transmitter being on simultaneously. Have
you checked the bias voltage from the power supply? It can go low due to
a dying selenium rectifier and so not block off the inactive part of the
radio (receiver or transmitter) and also not set the idle current
properly on the PA. A low bias could let the PA draw lots of plate
current without having RF drive or putting out any power.
> 
> It receives quite well even on 80 meters suggesting the problem really is in
> just a few stages. I may well be "dense" and stubborn but for the life of me
> would it have hurt to put in some RF level measurements in the book to
> enable troubleshooting an RF problem! The transmit level chart showing
> signal generator measurements makes no sense to me either!

Its hard to measure receiver signal levels because the probe detunes the
tuned stages and can radiate making stages oscillate. Its more practical
to inject a signal from a low impedance generator looking for a standard
output level, but the gain is affected a great deal by tuning and by AGC
voltage.
> 
> I apologize if this sounds like a tirade (which it is to a degree) hence one
> may understand why I could use some help or if not just an outright 
> admission
> of defeat and let someone else fix this.

Have the black beauties been replaced? Are all the right tubes in the right 
sockets, no 6DC6 replaced by super hot tubes?
> 
> 73, Roger, NJ2R 
> 

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer