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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.
+. 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
+. 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)
+. 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.
+.
+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.
+.
+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.
+.
+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.
+.
+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)
+.
+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 !)
+.
+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

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