• 1 Hour Hack: Internal light for Landy

    11/06/2017 at 23:20 0 comments

    i won't lie: the amount of crap that floating around my landrover is shameful at worse. However, something that was flaoting in my passenger footwell got me thinking...

    Yep its one of those crappy pound-land (dollar-store for you yanks) LED cabinate lights from a previously failed attempt of having some internal lighting by using the sticky pads that are fitted to them. The ride in the Landrover is not that great, even on a smooth road its bumpy as hell! so they just kept falling don off the sunvisors.

    However I have no rear view mirror, no point in fitting one as all I'll see is the scaffolding of the rollcage! There are still the screw holes above the windscreen for a mirror, which are M3 thread.

    I took the back off the light and stuck some batteries in to check it worked first, then remove the woefully inadequate adhesive off the back, refitted the back plate and drilled a 3.5mm hole in the back for an M3 screw. This also marked the inside to drill a larger hole for the head of the bolt to fit into, albeit exposing the circuit board inside.

    Hardware in place, I then fitted it to the mounting screws in the landrover above the windscreen, then applied some insulation tape of the head of the screw as not to accidentally short out the traces on the PCB:

    Then it was just a case of refitting the LED light onto it and testing:

    VIOLA! quick tap and light, also not reliant on vehicles battery power, which is sometimes the case when I'm working on the electrical systems.

    Took less than an hour and all out of junk parts!

  • The Thurlby Complex

    08/13/2017 at 19:42 0 comments

    Gather round folks and I will spin a yarn of confusion.

    So I acquired a Thurlby PL310 PSU (least that's what it said on the front) a while ago. It had an issue where the voltage adjustment at the low end (0-3V) was very jumpy:

    Although useable, I decided after  a while that I should really get round to fixing this. A quick Google relieved a few others who had suffered the same problem with a common fix: Replace the old wire wound coarse adjustment pot with a new, better, carbon track one. And whilst your at it replace the fine adjustment one.

    There was a pretty google hit of somebody who had posted pics of their repair that was on a site which had not been maintained/dues paid, but can't seem to get the same hits from the same search terms anymore...

    I also read that the voltage adjustment pots were not wired as you would typically expect in a linear PSU. They appeared to be part of a feedback network and being used a variable resistors/rheostats  rather than potentiometers changing the set point.

    Simplified schematic

    I've a copy of the service manual here if anyone wants it: SINCLAIR THURLBY PL SERIES POWER SUPPLY

    Now if you look at the Thandar range of PL series power supplies you will find that the pots are wired as pots:

    Also IC7-A is an LM324....which I couldn't find:

    No LM324 here! So massive pitfall there! Turns out the Sinclair branded and Thandar branded PSU, SHARE THE SAME MODEL NUMBER!! But why the flipping hell did Thurlby do this!?!

    "I know! New company structure, New PSU design, Lets name it  EXACTLY the same as the old product!!"

    ...yeh well done chaps...could have at least bloody versioned it!

    Anyway, massive model number flaws aside, this was  a distraction from the task at hand, but I though I should make the uninitiated aware.

    The task at hand was to replace the potentiometers:

    Rummage in the spares box and a 50k pot (coarse) and 1k pot (fine) were acquired.

    After fitting them and giving them a little test, the voltage was stable and the fluctuating output had gone completely.

    It should be noted as well that if you're testing this after fitting new pots, and you are resting the front panel PCB near the AC line switch, you will see the same fault you are trying to cure! Seems the Mains interference is large enough to cause the same issue.

  • Upgrading a Kiethley 177 DMM's Input Jacks

    08/03/2017 at 12:47 0 comments

    I recently scored a 177 DMM from a dumpster dive, they aren't a bad meter! 4.5 digit, 2000 count, 20mV range with 10uV resolution & 20uA range with 1nA resolution, for something built in the 70's is not bad!

    However I'm not too keen on the input jacks:

    They are made of some quality metal, but most modern probes now have the shrouded jacks, which don't fit these posts and I don't want to hack up my probes.

    So I found an old 3.5 Blackstar DMM and stole the jacks out of that:

    I then removed the posts off the Keithley:

    The existing holes were too small for the new posts, so I removed the whole front plate off so I could work on it without damaging the DMM internals. This meant removing the zeroing knob on the side of the display. This was done by popping the top off the know and unscrewing the screw a little to free it of the spindle:

    Whilst I had the plate off I thought i might be a good idea the stencil it out onto paper for reference just in case I broke it during drilling and maybe later CAD it out in DXF format so new ones can be laser cut if anyone needed one.

    After drilling the holes out I then fitted the new posts:

    And then fitted the panel and knob back onto the DMM:

    Now I can use my modern probes with it!