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Author Topic: KWM2 Exciter tuning - strange behavior - Help!  (Read 5045 times)
W5SU
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Posts: 22




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« on: December 25, 2012, 10:06:26 AM »

This is a little hard to describe, but here goes...

I've got a winged emblem KWM2 that exhibits strange behavior in the exciter tuning control but only in the 3.5 MHz portion of its "eyebrow".  On 80M in receive when I try to peak a signal or background noise, I get oscillations that sound like "pip...pip...pip...pip...pip..." while moving the exciter tuning knob back and forth.  If I peak on one of the "pips" it leaves me with an oscillation on that frequency, and broadcast band images across the band.  But if I peak the noise and try to stay between one of the "pips", it receives OK with the exception that there is still a strong oscillation almost like a calibrator signal at a couple places on the band. 

This only happens on 80M, however even on 40M if I crank the exciter tuning to the left (in the 3.5 MHz part of the eyebrow) I can pick up weak "pips".

Tuning the transmitter is unaffected even on 80M and it provides full output. 

I tried rocking the 80M trimmers and coils back and forth then to their original positions, but made no difference.

Can someone shed some light on what may be going on?
(Am I going to get in trouble by also posting this on Mods and Repairs forum?)

Many thanks and '73!

Carl - W5SU
Dallas TX
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AC5UP
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Posts: 3900




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« Reply #1 on: December 25, 2012, 10:34:30 AM »

Since you're describing an issue unique to the bottom of the tuning condenser range I'm going to speculate you have an oxidized set of contact fingers on the rotor shaft. This could be as simple as the variable condenser equivalent of a scratchy volume control.

Remove the top cover and any shielding over the condenser. Look for brass or phosphor-bronze wiper contacts between the tuning shaft and frame, treat any you find with DeOxit, CRC 2-26, or even 3 in 1 oil. Chances are at least one of them has gone intermittent. Exercise the condenser to clean the contact surfaces, wipe out any excess oil with a Q-Tip, then reassemble and test as needed.

While you're in there give the hairy eyeball to the tuning condenser assembly for cracked solder joints, loose ground screws, or anything else that might cause it to go flaky. A drop of oil on the condenser end bearings could also be a good idea.
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W5SU
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Posts: 22




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« Reply #2 on: December 25, 2012, 11:38:43 AM »

Thanks for taking the time to respond.

What you propose makes sense, except the shaft doesn't go to a condenser.  It's a long shaft from the front to back of the radio that is belt-driven to a bar that goes up and down, connected to 5 different tuning slugs - one for each band.

Appreciate your comments.
Carl
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AC5UP
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Posts: 3900




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« Reply #3 on: December 25, 2012, 12:25:08 PM »

I'm so used to seeing variable condensers for the load & plate adjustments it never occurred to me this one might be permeability tuned... And I own both an R-390 and R-390A so I know The Collins Way of doing that...

In any case, if it's intermittent and affected by shaft rotation I'd look at anything that used to be a path to ground which has oxidized, the solder connections to the coil windings, and the grounding to the coil shields. Since this is most pronounced on 80 you should give that coil your full attention and might consider a little tap & nudge with a plastic stick pen to see if it's affected by mechanical stress.  ( ? )

If so, you've narrowed it down to the neighborhood the issue calls home............  Tongue
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KE3WD
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Posts: 5689




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« Reply #4 on: December 25, 2012, 02:52:18 PM »

The PTO permeability tuned oscillator was the big shnitz about the KWM back in the day. 

That is what blew all the other contenders out of the water at the time. 

Until that company you love to hate came out with the same, available pre-assembled and pre-calibrated as a rather more affordable kit that had the look and feel of a KWM. 

Heathkit.

As for the OP and the KWM with pip-pip problem, the Collins lovers have their own meeting place on the internet where there is a rather vast plethora of knowledge on just about any and all issues Collins. 

http://groups.yahoo.com/group/collins_radios/

There's the hotlink, sign up and search first to see if your problem may already be covered and if not, ask.  There is an incredible knowledge base available there. 


73
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WD8AJY
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Posts: 60




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« Reply #5 on: December 25, 2012, 03:51:14 PM »

Are all of the metal covers on the coils if they are try losing the nuts on the covers and re tighten the nuts. Corrosion on the covers can cause this problem.

The following statement is from the CCA web site.
 
Poor grounding of the RF coil shield cans may cause oscillations, particularly on 80 meters. This can be remedied by loosening the two nuts holding the shield cans and re tightening them. 73

Bob WD8ajy.
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W5SU
Member

Posts: 22




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« Reply #6 on: December 26, 2012, 06:34:50 PM »

Bob -

Well there's NO WAY something as annoying and seemingly complicated as that obnoxious oscillation could be fixed by simply tightening some shield cover nuts.  Yeah, I understand the logic behind it, but all that stuff that's supposed to make a difference like cleaning switch contacts, pots, re-tightening circuit board fasteners, etc. just never works for me.

Well I don't know where you came up with it but GUESS what!  Your suggestion worked!  After taking the radio out of the cabinet and figuring out where those nuts were then tightening them, I put it all back together and in amazement was able to peak signals wihout the pip...pip...pip.

Many thanks, '73, and Happy New Year

Carl - W5SU
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K1DA
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Posts: 514




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« Reply #7 on: April 11, 2013, 09:23:40 AM »

Ahhhh, the Heathkit "LMO" was NOT tuned by an iinductor. 
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KG6YV
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Posts: 514




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« Reply #8 on: April 16, 2013, 01:55:47 PM »

THe Heathkit LMO used a tuning capacitor with a 2 bazillion to 1 reduction gearing.  They only used about a fifth of the total available capacitor tuning and picked the most linear portion of its rotation.  I know because
I took one apart and saw the ingenious mechanical design.  The one I had was built for Heathkit by TRW and had a TRW sticker on it.
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KE3WD
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Posts: 5689




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« Reply #9 on: April 16, 2013, 02:01:35 PM »

All I said was that heath came along with one that "had the look and feel" of the KWM2. 

And that is all. 


73
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W2RKJ
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Posts: 55




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« Reply #10 on: April 17, 2013, 04:28:14 AM »

I learned that lesson a while ago, wait until you can't get the loading right after trying and trying. You go about troubleshooting and can't find anything. The last thing you would think of is spraying the band switch because it's so simple. Once that's cleaned-----You're up and running again. Amazing these radios.
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KE3WD
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« Reply #11 on: April 17, 2013, 08:04:59 AM »

"Cleanliness is next to Godliness."   --Poor Richard
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K1DA
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Posts: 514




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« Reply #12 on: April 17, 2013, 09:07:12 AM »

You said "Until that company you love to hate came out WITH THE SAME".....
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KE3WD
Member

Posts: 5689




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« Reply #13 on: April 17, 2013, 04:30:52 PM »

You said "Until that company you love to hate came out WITH THE SAME".....

And indeed it was the same level of reproducibility as to the dial location and tuning, just not quite the same method of implementing it. 

And, due to the ingenious way the cap was incorporated, it was very affordable, offering that level of near drift-free operation to a rather large amount of the amateur community who would not have otherwise been able to afford it.  That was big news at the time.  I lived through it.  A lot of those rather ubiquitous and annoying Swan drift-o-matics fell by the wayside by the end of the first Christmas, man.  Almost as revolutionary as the Phase Lock Loop when it first appeared, but I guess you had to be there then. 


73
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