Heathkit AF6C User Manual

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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

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.

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.

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

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Heathkit Maintenance #1 - Slide Switch Repair

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).

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.

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

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Figure 3: Slide Switch Showing the Tabs

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

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.

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