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White "fungus" around a ni-cad

Discussion in 'Electronic Repair' started by N_Cook, Mar 9, 2012.

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  1. N_Cook

    N_Cook Guest

    What exactly is it and what , if any, damage does it do to anything
    adjascent/ same box.
    Thurlby DSA524, digital scope adaptor From 1987 . Nickel dendrites like Tin
    whiskers ?
    Went to use it for the first time in 5 years and no response to any input,
    was ok previous use. A vaguely staircase output on an analogue scope , but
    thats about all. Composite and trigger out work as do some anunciator
    readouts. So presumably ROM, RAM, PS, Clock are ok. So is it realistic to
    suspect the A-D a Ferranti ZN449. ? One of those "better than nothing"
    service manuals is out there but thats about all
    This happens to be right next to one of those pcb soldered oval shape blue
    cased nicads of that era, before EEPROMs came in.
     
  2. N_Cook

    N_Cook Guest


    Pineapples remove fingerprints also, but we eat them

    I can't see how KHO creates tendrils or matting or whatever the 3D tracery
    structure is
     
  3. N_Cook

    N_Cook Guest

    Relevant looking signal (alternate time slices varies with varying DC at ch
    1 and ch2)gets to the analogue data-in of the ZN449 , enable, clock etc ok
    but invariant data out on the 8 data lines.

    I've some SMD ZN449 , any tips for converting to 18 pin conventional DIL?
    Start with a turned pin socket in the pcb to receive another one to carry
    the SMD, yes but any other tips ?
     
  4. N_Cook

    N_Cook Guest

    Will have to deal with the conductive contamination on the pcb around
    the nicad, down to 300K over the polyester of the pcb, soaked in
    capilliary-wise?. Removing 4x 74HC as well as the ADC
     
  5. IanM

    IanM Guest

    Conc. aqueous potassium hydroxide (KOH) attacks E glass
    (alumino-borosilicate glass fibre) leaving an insoluble white amorphous
    deposit of potassium silicates and aluminium hydroxide. This deposit
    will be conductive if contaminated with residual KOH as KOH is both
    ionic and hygroscopic and will be extremely difficult to remove as it
    has probably etched its way below the board surface. After aggressive
    cleaning, neutralisation, thorough washing and oven drying, you will
    still probably need to grind away the affected PCB surface until the
    track to track resistance remains acceptably high after 24 hours
    exposure to normal atmospheric humidity, then wash, re-dry and rebuild
    the ground away areas with epoxy.

    KOH also attacks a number of common plastics to a greater or lesser
    degree. Polyester is likely to be moderately affected, Epoxy is
    resistant. If you are very lucky, it may simply have made the resist on
    the PCB surface conductive and not attacked the underlying fibreglass
    significantly.
     
  6. N_Cook

    N_Cook Guest


    Maybe I'll be lucky. Removed 5 ICs and the nicad from that area. Washed
    locally with meths and while still wet dremmel and rotary wire brushed over
    that area . Now I cannot find any conduction to/between any isolated pads or
    the plastic/glass less than the 30Meg of my DVM. Made good the IC holes
    ready to receive 5 turned pin sockets

    ps
    I also have a > 30K filter on my (valid) email inbox. It is me who decides
    who clogs up my inbox /HD
     
  7. IanM

    IanM Guest

    Sounds like it was just surface contamination or leaky resist. I would
    wet scrub it with deionised water with some IPA as a wetting agent using
    a abrasive pencil type glassfibre brush to be certain the dremel wire
    brush hasn't left any metal contamination that may give you trouble with
    metal salts later, Rinse well with dry alcohol and dry thoroughly
    with forced air circulation at an elevated temperature. Finally coat
    the damaged area with a good brand of solder-through PCB lacquer before
    refitting any components.

    If I think there is any chance of future battery leakage I usually fit
    the new battery through a piece of polyethylene, silicone or ptfe sheet
    between it and the board to limit the damage next time.
     
  8. N_Cook

    N_Cook Guest



    Well I suppose that is progress. Back to exactly the same malfunction as
    before , but with the 5 original ICs in sockets now. Varying activity on the
    analogue side of the ADC and no variation at any of the 8 output digital
    lines . Will try a SM ZN449 , which strangely is wider package than standard
    DIL. so looks like it will have to be a skewed adaption to fit .
    Using the original nicad at the moment, slung underneath. I like the idea of
    a silicone bund. I think , if I get this working, I will replace with a
    new one , stout-wired upside down, right side of the board , over a
    silicone sweet or cupcake mould
     
  9. N_Cook

    N_Cook Guest

    My tip for a SM IC to DIL adaption.
    3/8 inch wooden dowel , cut a small flat to receive the SM IC neatly.
    0.2mm tinned copper wire, free end looped around first pin , around the
    dowel with a bias so the wire lays tight against the up-side of the opposite
    pin and wind biased spiral fashion over the top of the IC and around the
    dowel. Finish with a final loop around the last pin. Clamp free end of dowel
    in a vice and solder each pin to its wire. With a razor cut the wires over
    the top of the IC close to the pins and cut along the centre back of the
    dowel. straighten wires. Some 1mm PTFE spaghetti tubing , small length
    pushed over every other wire and glued to the IC body. Fan out the wires and
    solder to the pins of a turned pin socket. Continuity check for contacts and
    bridges.
     
  10. N_Cook

    N_Cook Guest


    I was not aware of them but can they adapt from wide .45 inch SM down to .3
    inch DIL for this purpose and 18 pins ? And for one offs,? thats more for
    production environment at 40 squid a pop .
    My free "adaptors" are fully adaptable to any contingency, just don't look
    so pretty, and probably a bit higher profile due to use of a socket.
     
  11. N_Cook

    N_Cook Guest


    Come to think of it, my method would look neater without the intermediary
    socket. Larger dowel and longer cut 0.2mm wires, so can pass through the
    original board. Just that over the years I've got in the habit of socketting
    any replacement IC I've desoldered, there's always been the height available
    with the stuff I deal with .
     
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