[Boatanchors] Heathkit LMO Linearity

[Jim Liles]

The variable capacitor at the rear of the LMO is used to coordinate the end 
to end frequency points with the dial.  Use the variable cap on the rear to 
align the end points.  Then verify the 100Kc calibrator is right on.  Then 
check the 100Kc points from end to end.  If they are not right on you have 
to knife the main cap to get them there.  Now if the 1Kc points do not align 
within 2Kc, the variable capacitor will need to be knifed again.

While your in the LMO, note the values of the temp comp caps.

Knifing an LMO cap is not for the impatient or slight of heart.

Kindest regards Jim K9AXN

-----Original Message----- 
From: Glen Zook via Boatanchors
Sent: Saturday, November 21, 2015 6:30 PM
To: Mark V Johnson ; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Heathkit LMO Linearity

It is very rare that a tube-type, TRW manufactured, LMO has a problem.  The 
primary source of these problems is that someone has gone inside the LMO and 
"fiddled with" the variable capacitor.
Each tube-type LMO is basically the same.  The tube chosen depended on the 
actual radio in which the LMO was used.  Because of the series-parallel 
arrangement of the tube heaters, some units required a different heater 
current to fully balance the series-parallel arrangement.  As such, a 
certain tube was used.  However, the LMO will work fine with several other 
types of tubes and one should use the type of tube that was originally used 
and not just the tube number listed on the LMO.
The solid-state LMOs used in the SB-102 are a horse of a different color! 
The TRW manufactured solid-state LMOs are extremely linear and are well 
within 1 kHz accuracy.  Unfortunately, those LMOs manufactured by other 
companies vary all over the place.  Those LMOs are, usually, "right on" at 
the 100 kHz points.  Unfortunately, they vary all over the place in between 
the 100 kHz points.  I have found examples where the in between points were 
over 10 kHz away from being correct!  Heath reserved the TRW solid-state 
LMOs for the SB-303 and used the non-TRW LMOs in units like the SB-102 and 
SB-313.
I would use the proper working LMO in the receiver.  Also, good Heath LMOs 
are available, from several sources, at very reasonable prices. Glen, K9STH
Website: http://k9sth.net
      From: Mark V Johnson <mvjohn at sympatico.ca>
To: boatanchors at mailman.qth.net
Sent: Saturday, November 21, 2015 3:35 PM
Subject: [Boatanchors] Heathkit LMO Linearity

Team,



I have an SB-310 receiver with what I believe is the "newer" style LMO in
that it as a 6CB6 tube and a three; vs. four, terminal connection strip on
the back. I have the radio working - after many repairs - but the LMO is not
linear and at the midpoint between each 100khz point, the dial is not
tracking at all. I have the LMO out of the receiver and connected to
external power supplies and I can see with a freq. counter, the LMO is not
linear.



Is there a way to improve / fix this problem? Also, what is the purpose of
the small trimmer capacitor on the rear of the LMO? Was it used at the
factory to set calibration or does it affect the linearity?



I have a replacement LMO I can install - came from an SB-400 - and it works
better, but it is not the LMO the SB-310 came with. It; my possible
replacement LMO, is the older style and uses a 6AU6 and has the 4 terminal
strip on the back.



Suggestions as to how to adjust / fix the linearity?



Many thanks!


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