[Boatanchors] Heathkit LMO Linearity

[Glen Zook]

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!