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test_equipment:hp_lever_switch_repair [2013/01/08 19:00] (current)
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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. |
| | |
| | JOHN BYERS |
| |
