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Lever Switch Repairs On HP8552B IF Unit (and others)

My 8552B had been damaged in transit and the SCAN MODE switch arm lever broken off and missing.

A previous post asking for advice on how to fix this switch got no response, so I assume that most of you have easy and cheap access to spare switches and have never needed to fix a broken one. But that's not the case outside the US so I had to “bite the bullet” and have a go myself.

Now I know its likely that other HP gear uses the same switches, or at least the mechanical parts comprising the switch's lever assembly making up the switch lever, spring retainers, actuator rod and switch cam, so although the actual switch bank wafers themselves may be different, at least the above bit is likely to be transferable.

For example my 1822A time base from my 180 CRO has 4 of these switches, all 4 position, which very likely has the same internal mechanics as my 8552B SCAN MODE switch, but again I am not going to disable my still working and excellent 180 which would be blind without its timebase and which is still a rare enough beast to have to find here in West Australia.

So here is a run down on one way to fix it (Applies to VIDEO FILTER and probably SCAN TRIGGER switches too)

The switch arm was broken off just before it entered the support slot on the switch bracket.

To remove the entire switch assembly, then the lever sub assembly.

1. Remove the 4 side screws holding the entire panel assembly to the 8112B box. Swing out the front panel assembly to expose its rear wiring. Now remove the front outer fascia panel You will first have to unsoider 3 of the BNC sockets along the right front except for the CAL OUTPUT BNC, wbich is a special screened prewired thing which unscrews from the front with a round nut and then just pulls free.

Remove all the BNC connectors from the fascia panel as they clamp a seperate mini panel to the fascia with the BNC signal descriptors . You cant get the fascia panel off until you first remove this one.

BEWARE ! After removing the nuts on the “Base Line Clipper” and “Manual Scan” pots and pulling off the plastic 'handles“ on the lever switches, you might think the fascia just pulls straight off . It doesn't! (the three knobs can stay on, as they have large round cutouts on the fascia panel which clear them, but I removed mine to prevent possible damage.) Three of the fascia lamps (“Scanning” and ”+, X“ ) are still firmly attached with plastic clips which also form part of the fascia panels illuminated annunciators !

If you pull the panel straight off YOU WILL BREAK THE WIRES OF THE ATTACHED FILAMENT “WIRE IN” BULBS AND POSSIBLY THE WIRES ON THE NEON TOO !!! (I KNOW this cos thats what I did!!! … and even after looking at the manual, too!!. I had to replace two of them with LEDS, but thats yet another story).

You have to lever the bulbs out of these clips FIRST before you remove this panel

2. Now remove the circlip retaining the switch rotator rod which goes through the slot on both switch wafers (With the switch still attached the panel it makes a stable platform for removing the circlip)

3. Now remove the two screws holding the switch bracket to the front panel proper. You can now swing the switch assy part out now so you can see how the switch wafers are held together. Observing the arrangement of the metal spacers remove the wafer attachment nuts and pull out the bolts which clamp them together.

Note the position of the tiny non metallic washers between the pillars and wafer switchs. Its best to make a sketch of this first. Carefully note the swich rotor positioning of the 2 wafers and the colour coding of the wires which are adjacent each other so there is no doubt as to how it goes back together Add this to your sketch before disassembly.

4. Now you can remove the metal stamping , that is the bracket which contains the spring, the metal cam teeth, the (broken off) operators actuating lever arm and the wafer switch rotator rod. Pull the switch rotator part,which includes the spring and cam up and out so the actuator shaft exits the wafers slots. It may help to remove the spring first.

5. You can now examine the bit you want to fix. In mine the lever switch arm was broken off just before the bend where its angled up to pass through the brackets guide slit. My experience, looking at of the broken off end, told me that it would not weld together well. The surface is too granulated. It would have to be screwed together somehow.

6. WHAT NOT TO DO: I thought I would cut off the bent part with a hacksaw, leaving the “wasted” flat part which the wafers rotator rod is fixed to. I would drill a couple of 10BA tap size holes in it, make up a new metal operators switch arm complete with double bend, coat the screws threads with Loctite and screw the lot into the threaded 10BA holes.

There would just be enough clearance to do it by cutting off the excess bolt lengths and flush filing.

Well, you CANT simply hacksaw it off. This parts made of hardened metal and you will blunt the saw blade with barely a scratch showing. I tried drilling small holes with a new drill in the drill press and that barely makes a dot on the surface.

7. WHAT TO DO: To remove the bent up angled stub ,squeeze itin a vice. It breaks off neatly.

Luckily, I found the metal takes solder easily! I simply silver soldered the new switch arm I had made to the remaining flat stub and it held beuatifully. !! (Tin both surfaces first, of course)

8. MAKING THE REPLACEMENT LEVER: Take the width and length from the other lever switches and shape outa replacement in metal thin enough to go through the panel slots (Note: The outer fascia slot is WIDER in width than the inner front slot. Make sure the thickness of your made up lever arm clears the INNER panel slot width.

Try to follow the “wasted” contour of the original as close as possible when shaping the replacement.

After silver soldering dress the edges with a file to remove any solder squeezed out at the edges.

Its important this lines up with the original 'waist' angle as it affects the mechanical position of the switches at both end positions. Again a file will correct this (no need to worry about filing part of the original away !)

9. Reassemble: You will find it difficult (I did) to get the pillars and tiny washers to go back on the wafers while ensuring the wafer slots align with the rotor rod. Do the “inside” end first as its the hardest to get at.

Check out to see if any of the fine wires have broken off the switch wafer lugs due to your Houdini like finger exercises when reassembling. You will find the sketch you made indispensable if this happens.

I used a black plastic knob off a low end CRO which had similar lever switches (but totally incompatible otherwise)

This didnt fit securely, having too wide a slot,. I shrunk some heat shrink over the shaft end which then secuured it OK.

Watch out for bits dropping down into the panel wiring. They are hard to retrieve.

I can now use my 141T and do the full calibration as HP intended.

Hope all this helps others.


test_equipment/hp_lever_switch_repair.txt · Last modified: 2013/01/08 19:00 (external edit)
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