{Collins} KWM2-A - ? New AGC Problem



Problem solved.  Read thread below.  
Tnx everybody...esp. Dave K6XYZ

73 Torrey N9PY

-----Original Message-----
From: David Harmon [mailto:k6xyz@xxxxxxxxxxxxx] 
Sent: Monday, May 4, 2015 13:57
To: 'Torrey Mitchell'
Subject: RE: {Collins} KWM2-A - ? New AGC Problem

Great.
Glad to help out a bit here and there.
You might post the fix to the list so it will be in the archives.


73

David Harmon
K6XYZ
Sperry, OK

-----Original Message-----
From: Torrey Mitchell [mailto:n9py@xxxxx]
Sent: Monday, May 4, 2015 1:06 PM
To: 'David Harmon'
Subject: RE: {Collins} KWM2-A - ? New AGC Problem

Yup...you're right on...
I must have been sleep-walking with my fingers in the KWM2-A this weekend.
I zero'd the S-meter and everything is cool now.  One crude method I use for
adjustment is to flip back and forth with the KWM-380 or other reliable rig
and try to match the S-meter readings for a couple of solid signals on the
band.  I achieved that...definitely "close enough for government work"!

Tnx again,
Torrey N9PY

-----Original Message-----
From: David Harmon [mailto:k6xyz@xxxxxxxxxxxxx]
Sent: Monday, May 4, 2015 09:53
To: 'Torrey Mitchell'
Subject: RE: {Collins} KWM2-A - ? New AGC Problem

If the meter pins with the calibrator......sompin else wong!

The Collins test equipment setup in the manual is definitely dated.
The sig gen Collins must have been using did not have a 50~ output so they
had to use a pad or impedance matcher.
More recent sig gens have a 50~ output.
Connect the modern sig gen to the antenna jack for this adjustment and adj
the rec sens pot for S-9.
This corresponds pretty closely to the Collins method of .25mv or whatever
it was.
This is the way I have been doing these radios since 94'....however....this
adjustment is after installing SB8A, B, C.

Not all radios can be adjusted to precisely S-9 due to S-Meter ckt
differences....component tolerances, defective components etc.
You can really spend a lot of time trying to get the S-meter to read
correctly....and still not fix it.
I have spent hundreds of hours on this alone and most of the time are
unsuccessful....so I don't try too hard anymore.

Perhaps you adjusted both the rec sens and S-meter calib??

Could also be dirty tube pins.
Remove the 6AZ8's one at a time and using Deoxit or something similar and an
old toothbrush....scrub the tube pins.
Do not mix up the tube placement....the 6AZ8's are alignment sensitive to Vx
placement.

The rig could have had an AVC line failure....something could have failed
which is leaking voltage onto the AVC line causing the 6AZ8 tubes to lose
AVC control.

Back to getting it fixed.....
First.
Remove antenna.
Turn RF Gain fully CW.
Adj the s-meter zero pot for zero on the meter and see if this fixes the
calibrator meter pin problem.
Make the adjustment with the calib off....then turn it on and tune it
in...not zero beat...off to the side for max reading.
If the meter still pins...try cleaning the 6AZ8 pins.
If it still does not fix it...likely there is something else wong.
Don't forget that the newest M2 is almost 45 years old now.


73

David Harmon
K6XYZ
Sperry, OK

-----Original Message-----
From: Torrey Mitchell [mailto:n9py@xxxxx]
Sent: Monday, May 4, 2015 8:24 AM
To: 'David Harmon'
Subject: RE: {Collins} KWM2-A - ? New AGC Problem

Thanks Dave,

I believe I am guilty of messing up the RCVR GAIN setting. Now I know the
"rest of the story"...ie. what the RCVR GAIN pot is for.  Why on earth do
they put it right up front where any yokel (like myself) will play with it
wanting to peak receiver gain.  I have fussed with it since yesterday so
that the S-meter movement is somewhere close to what I expect.  I'm not sure
about the calibrator on 80m approach.  If you mean tuning to the calibrator
frequency...the calibrator is so strong it pins the meter no matter what the
RCVR GAIN setting is.
I do have a calibrated HP signal generator on the shelf here.
Looking at the service manual it says in section 4.6.17 to apply a 25 uV
signal "to the circuit shown in figure 4.3".  Attached is a jpg picture of
figure 4.3.  I don't understand what they are illustrating in 4.3?  Where
does one inject the signal?

73 Torrey N9PY

-----Original Message-----
From: David Harmon [mailto:k6xyz@xxxxxxxxxxxxx]
Sent: Sunday, May 3, 2015 22:54
To: 'Torrey Mitchell'; collins@xxxxxxxxxxxxx
Subject: RE: {Collins} KWM2-A - ? New AGC Problem

Torrey.....
During your adjustments did you mistakenly adjust the 'rcvr gain adj'
located next to the 'alc adj' on the subpanel just under the lid?
If you did...this could cause some strange readings.
You can't readjust this pot without test equipment but if you did move this
pot you can get the gain back in the ballpark by turning on the calibrator
and on any 80mtr segment adj the pot for about 15 over S-9.

I can't tell you how many times I have mistakenly adjusted the wrong pot.
Now...after aligning the radio I put paint on the pot in order to give some
resistance to the screwdriver...ie...if it will not move....sompin must be
wong!


73

David Harmon
K6XYZ
Sperry, OK

-----Original Message-----
From: Collins [mailto:collins-bounces+k6xyz=sbcglobal.net@xxxxxxxxxxxxx] On
Behalf Of Torrey Mitchell
Sent: Sunday, May 3, 2015 3:52 PM
To: collins@xxxxxxxxxxxxx
Subject: {Collins} KWM2-A - ? New AGC Problem

I turned on my KWM2-A this afternoon and immediately was struck by very high
S-meter readings.  The receiver is very "hot" (sensitive).  The no signal
S-meter position today is about S-9+10 and most signals pin the S-meter with
the RF gain set to max.  The 2A is behaving normally otherwise.  It seems
like the AGC action is absent.  I was making some adjustments before going
on the air.  Is it possible that I accidentally turned something to cause
this change?  I swapped out the 516F-2 for a different one and it made no
difference.

 

I am looking for suggestions on how to troubleshoot this problem.

 

Thanks and 73,

 

Torrey N9PY

 

 

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