[Heath] SB-110 VFO's

[Kenneth G. Gordon]

On 17 Apr 2012 at 8:51, George Babits wrote:

> Digging deeper into the SB-110 and found out that the VFO is not the
> original one with the 6AZ8.  Looks to be solid state and marked on top
> TRW-800668 and 110-48.

That is an excellent LMO (not VFO), probably out of an SB-102.

>  That explained the two loose wires behind the
> panel (B+ and bias for the original VFO) and why the white lead goes
> to ground through a 3.9 ohm resistor (replacing the original tube
> filaments).  So one mystery solved anyway.

Sounds good so far.

> From my reading it looks as if this VFO came from the SB-102 or SB-303
> sets. I have read some unfavorable things about the HeathKit solid
> state VFO's recarding tracking.

Well, my SB-102's LMO (not VFO) is absolutely excellent with regard to 
tracking and stability. Drift is only a few Hertz within only a few minutes after 
being turned on. However, there IS a common "problem" with Heathkit 
LMOs, even those made by TRW. See below.

>  But it seems like all the negative
> information is prefaced by "HeathKit VFO's were not made by TRW.

True.

The "common problem" I refer to above is that the connections between the 
LMO's tuning capacitor's various bits has become intermittant within a very 
few years of their original manufacture, and this leads to instability of the 
"jumping" variety at certain places in the tuning range.

Bob G3OOU has mentioned this here, and has provided his solution to that 
problem. I have tried using very small quantities of DeOxit-5 applied with a 
toothpick to those areas which I thought could help, but it only helps over a 
short period of time before it goes back to being jumpy.

As far as I am concerned, the best method is to solder a piece of braid to the 
rotor and to some place on the stator in such a way that the turning of the 
rotor will not "wind up" the braid and keep the rotor from turning to its limits.

Doing this requires removal and dismantling of the LMO. SOME LMOs are 
held together with screws. Others are spot-welded. Those which are spot-
welded (my SB-102 has one of those) can be opened by very carefully 
drilling directly through the spot-weld with an appropriate sized drill, then you 
can use the hole drilled to install hefty sheet-metal or self-tapping screws to 
hold it back together after you have operated on the LMO.

Only ONE SIDE of the LMO case must be removed. Be sure of which one 
you remove. The correct one will have no other screw-head protruding 
through it. The side you do NOT want to remove has the tuning cap and 
other components fastened to it.

DO NOT touch anything else inside the LMO.

Lastly, it has been my experience that TRACKING has far LESS to do with 
the LMO and MUCH more to do with the heterodyne oscillator crystals which 
have drifted out of spec, sometimes by quite a lot.

Heathkit used different manufacturer's crystals, some of which were pure 
crap, others of which were excellent.

Once you get all the heterodyne oscillator crystals on their correct 
frequencies, tracking, especially from band to band, will be excellent.

Also, although most people, even Heathkit people, are not aware of this, the 
SB/HW transceivers can be easily modified to cover 500 KHz segments 
above and below most ham-bands....except 40 meters...by using an external 
heterodyne oscillator and the appropriate crystal.

This works on exactly the same principle as that used in the Collins KWM-2A 
to cover many complete bands outside the ham bands.

I used my modified SB-100, (I called it my SB-101.5) as my exciter to drive 
my modified BC-610 linear amp (2.6 KW output) to do 'phone patching into 
Vietnam during that war for AFMARS in the 500 KHz segment covering 14.5 
to 15.0 MHz.

Using this external heterodyne oscillator and the appropriate crystal, I could 
cover from 4.0 to 4.5 MHz on 80, 13.5 to 14.0 in addition to 14.5 to 15.0 Mhz 
on 20, and likewise, 500 KHz above and below 15 meters. I never did try this 
on 10 meters, but I see no reason why it wouldn't work there too.

40 meters with the SB/HW transceivers is a special case, as due to the way 
Heathkit connected the DRIVER grid and plate coils in series, the tuned 
circuits of the DRIVER stages on 40 meters just BARELY cover the 300 KHz 
range of 40 meters.

The final amp plate tank circuit is capable of covering almost the entire range 
from 3.5 MHz through 30 MHz, including the WARC bands.

As with nearly all Heathkit transmitters I have ever been familiar with, it has 
trouble loading properly at the LOW end of 80 meters.

Ken W7EKB