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