Heathkit AF6C User Manual

HOM rev. new Heathkit Maintenance #1 - Slide Switch Repair

Heathkit of the Month - Extra #1:

by Bob Eckweiler, AF6C

This introduces a new a new feature of the
Heathkit of the month. Occasionally I may
find a repair technique or such to help get a
kit back into working order when parts are
not easily obtainable. These won’t appear
often but will be added when I find they
might be useful to those working on the kits.

MAINTENANCE
Slide Switch Repair
Featuring the HM-102
HF Power Meter

Figure 1: Pot with Integral Slide Switch

Finding a replacement for this pot - switch as­sembly (Fig. 1) would not be easy. The alterna­tive was to take it apart and see what ailed it.
The switch assembly does not come separate
from the pot, but the fiber switch back plate
can be removed by lifting the four metal tabs
that hold it in place. This is not easy to do.

Introduction:

After servicing a friend’s Heathkit HM-102 that
he picked up at a local swap meet, and that ini­tially looked like a basket case, I was surprised
how well it worked. It calibrated easily and
held the calibration. I thought back to the HM­102 that I had acquired recently and was cur­rently using. It was constructed well and ap­peared to be in a lot better physical condition,
but was not nearly as solid in its performance.
It was time I did something about that.

My HM-102 was disassembled and examined
carefully. The soldering was not in question
though I reheated a few joints, more to make
me feel I was doing something than thinking I
was fixing a problem. After cleaning the pots
and switches with a small and careful applica­tion of De-Ox-ID cleaner I check the pots, ro­tary switch and slide switches with an ohmme­ter. The pots seemed fine as did the rotary
switch. But the first slide switch, which was lo­cated on the SWR sensitivity pot, and activated
by pulling the knob out, showed erratic resis­tance. Sometimes it was near zero, sometimes
it was high and sometimes it was beyond the
scale my digital ohmmeter was set on. The
switch had a problem.

With some trepidation I was able to bend the
tabs up carefully and totally disassemble the
switch. Once the tabs were lifted it was obvious
there was at least one spring underneath so ex­tra care was taken not to have pieces shoot eve­rywhere. The switch was then cleaned, lubri­cated and reassembled. The contact surfaces on
this switch were badly tarnished and almost
black, but they cleaned up to shiny silver with a
cotton swab and some 91% alcohol. When I was

Figure 2: Slide Switch from Circuit Board

Copyright 2014, R. Eckweiler & OCARC, Inc. Page 1 of 4

Heathkit Maintenance #1 - Slide Switch Repair HOM rev.new

all done I cussed myself because I should have
documented the process with some pictures.

After reassembly, I tested the HM-102 per­formance and the SWR section worked as solid
as I could ever wish for. Unfortunately, though,
the Power calibration still remained erratic.
That circuit used a second slide switch. This
one was a different style designed for circuit
board mounting and included a vertically ex­tended slider; however the guts of the switch
appeared identical (Figure 2).

Figure 3: Slide Switch Showing the Tabs

Here are step by step instructions for disas­sembling, cleaning and reassembling the
switch:

Items You Will Need:

• Needle nose pliers

• Medium weight wire cutters, Preferably an
old pair

• A 7/8” or 19 mm socket (or something simi­lar) to hold the switch during assembly.

• Cotton Swabs

• Palmolive, Dawn or other dish detergent.

• 91% isopropyl alcohol or other solvent.

• White lithium grease (GC 19-2302)

• De-Ox-Id contact cleaner and lubricant.

Disassembly:

It is assumed you have removed the slide
switch to be repaired. However if the switch is
easily accessible when mounted you may leave
it mounted. When the initial switch was serv­iced the pot assembly remained mounted. This
can make the switch more stable while disas­sembling. Be sure to note the orientation of the
switch terminals prior to disassembly.

damaging the metal frame or fiber plate you
will need to get something under the tab and
lift it slightly. I’ve never had luck using needle
nose pliers, but an old pair of wire cutters can
be used to get under the tab. You don’t want to
mar the metal frame so it is wise to protect the
other cutter blade; I used a thin piece of brass
shim stock. Again just bend the tab up far
enough so you can use needle nose pliers to
complete the bending. Keep pressure on the
fiber plate so it doesn’t come off and allow the
spring and contact to fly off.

Now gently remove the fiber plate, contact and
spring. Also remove the plastic slider. When
you are done you should have five parts as
listed in figure 4.

Cleaning:

Clean the disc contact (figure 5) with the alco­hol. It should clean easily to a silver shine - this

If you examine the slide switch you will see
four tabs that are bent over holding the fiber
plate with the terminals to the metal switch
bracket (See Figure 3). These need to be care­fully bent up. Caution, excessive bending will
cause the tabs to fatigue and break. Without

Page 2 of 4 Copyright 2014, R. Eckweiler & OCARC, Inc.

Figure 4: Disassembled Slide Switch parts.

Left to right: Fiber plate with terminals

and fixed contacts, disc contact, spring,

plastic slider and metal switch frame.