[HBR] LWM-3

[Walt Hutchens]

Kees said:

> The unit was built by a Dallas ham who had access to a large USAF
> electronics dump while in the service and those PTO's are out of
> some period bomber which he found. He was going to use one in the
> Transceiver and the other two are spares. They are all Collins, of
> course. What parts are missing I don't know. It will be a labor of
> love to bring it to life, but what a rare piece of homebrewing history.

I have Kees' LWM-3 in house now, and what a beautiful piece of work it
is. It's probably 80% finished and the workmanship is about 3.9 on a
4.0 scale.

It appears completely wired except with regard to the non-installed
linear oscillator, missing mechanical filter, and minor odds and ends.

I have what is probably a suitable mechanical filter: It's one of the
small ones from an SB-34 transceiver: Intended for use with
transistors but I think I can fudge that.

The PTO's that came with are Collins 70H-13 units from the ARC-58
transceiver.  They tune 2-4 Mcs in 10 revolutions while the LWM-3
wants a range of 200 kcs that's within that range -- I don't have the
numbers in front of me right now.  So that could be fudged by using
just one revolution or by changing the tuning range -- not as brutal
an operation as it might seem.

Because the PTO drive should be direct (for zero backlash) and 20
kcs/knob revolution is about right for SSB I lean toward changing the
tuning range. Add a coil in parallel and another in series with the
existing coil, just as when padding a tuning cap to a desired range.

There's a knob and shaft to tune the PTO on the panel but no gearing
or other mechanism. There's a mechanism for the permeability tuned RF
amp/driver but it does not work well and will likely be replaced: I
can either use a modern belt drive -- that stuff is excellent and
reasonable in price now due to computer printers and other office
machines -- or substitute a permeability tuning assembly from a scrap
FT-101. I lean toward the latter since the Q of those coils is
outstanding.

It needs a power supply but that's not complicated -- I have a KWM-2
p/s that probably would do it for starters. The connectors on the
LWM-3 are the same as for the KWM-2 so at worst I might have to change
a few connections on the LWM-3 receptacle and do something about a
lower plate voltage for the PTO (two 5749 subminiature pentodes) if
the KWM-2 p/s doesn't provide.

None of the crystals are there but that's a solvable problem.  No top
or bottom metalwork -- another small project AND far in the future.

Some tubes are there, some are missing -- I probably have what's
needed. Usually when doing a project like this you find more as you go
but this has the look of being at least partly checked out.

This will be a winter project: I have far too much outdoor work to get
done to tackle anything substantial right now. However, I'll start now
on the PTO re-tuning and drive. The KWM-2 dial drive was designed for
low cost but beginning where this rig is I can probably go first
class.

The original LWM-3 uses a command transmitter tuning cap so that's not
applicable. The chassis in this unit was cut for the PTO and there.s
not room for the tuning cap. I'll try to drive the PTO directly from
the knob and use a toothed belt drive for the dial: 10:1 reduction
with the PTO retuned to cover just over 200 kcs.

One of the three PTOs has a badly bent shaft -- no surprise since AF
scrap yards typically get gear removed from busted up airplanes. I
will start with that one, see if I can get it working, and then tackle
the retuning and mechanical drive issues.

I'll hope that by winter the obvious separate projects -- PTO,
mechanical filter, power supply, tubes, crystals, etc. -- are done so
I can move on to getting it to work at the point where I can start to
concentrate more.

You cannot believe the nice workmanship. The constructor made
fiberglass-epoxy terminal boards and much of the wiring is on them.
The point-to-point components and cabling are professional grade, the
wire is Teflon insulated. The relays are military sealed units,
ditto the audio output transformer. Assembly is with 4-40 machine
screws tapped into aluminum plates that were home cut/bent but very
nicely -- nothing beat up with a hammer to make it work. The rear
panel was taken from a piece of Collins gear complete with the Collins
logo decal on it.

Thanks again, Kees!

Walt
KJ4KV