Commodore 1084S-D2 repair

Discussion in 'Retro & Arcade Worklogs' started by aXLe, Sep 18, 2016.

  1. aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    I recently acquired a Commodore 1084S-D2 monitor, along with a C64 - both were not working when I received them (C64 is now up and running - see here for worklog). The D2 variant was manufactured by Daewoo for Commodore in the early 90's.

    [​IMG]

    I thought initially that the 1084S was showing some signs of life (de-gauss sounds when switching on) but shortly after setting it up for testing it died completely, and then the power switch failed also - it won't stay locked in anymore, so will have to be repaired/replaced as well (apparently a common issue).

    So the unit was taken apart for some initial diagnostics. Fortunately I was able to find a schematic on-line which has been very helpful so far.

    I started tracing out the power supply section, which consists of a switch-mode power supply to generate 2 main voltages - B+ which is around 108V, and a 22V output that supplies the audio amp (1084S has stereo speakers inbuilt).

    I started by isolating the B+ at the output of the switch mode supply, so as I could attach a load for test and measurement - in this case a 40W light bulb.

    [​IMG]

    The switch-mode supply was dead - no output voltage on the secondary side, though I could measure 330V at the main filter cap (C109) on the primary side (after the rectifier), so there had to be a fault somewhere in the switching power supply section. After a quick poke around with the multimeter, I found an open resistor (R111 - which should be 100k and measured open circuit both in and out of the board).

    [​IMG]

    After replacing that, the supply was back to life and I was able to measure the B+ output at 108V :

    [​IMG]

    So I've put it back together (removed the light bulb and reconnected B+ to where it should be) and retested.

    Nothing - nothing on the screen at all - brightness control has no effect (even without a video input the screen should "bloom" when the brightness and contrast controls are cranked up). Using my home made high voltage probe (worklog here) I was able to measure approx 23.8kV at the high voltage output on the flyback transformer though, which is in spec :

    [​IMG]

    The horizontal output transistor (HOT) and control circuitry must be doing its job, switching the HOT on and off to produce the high voltage output at the flyback, however, there should also be a secondary high voltage output from the flyback which goes off to the board on the neck of the monitor for the focus (around 8kV I think), and it seems that this is missing altogether - I'm measuring 0 volts.

    This is the flyback transformer - the large red wire is the high voltage output (24kV), and the white wire is the focus output that goes off to the board on the neck of the monitor.

    [​IMG]

    So at this stage it's looking like either the flyback has an internal fault with the voltage divider for the focus output, else the connector on the neck board (if that's what you call it!) is faulty. This is where the focus wire runs into the neck connector that plugs into the rear of the picture tube :

    [​IMG]


    I think I'll see if I can take that connector apart so as I can see if there is wiring fault inside - it is possible that the focus output voltage is present, but with a poor (no) connection to the board. It could also be a fault with the pots on the flyback, so I'll have a look at them as well.
     
    Last edited: Feb 13, 2018
  2. adz

    adz Member

    Joined:
    Oct 2, 2003
    Messages:
    1,405
    Location:
    Melbourne
    IIRC the flyback transformer was responsible for 99% of the dead 1084's that passed through the service center I worked at many moons ago, shame I never got a chance to grab all of their Commodore spare parts stock when they were liquidated as I'm sure it all went into a giant skip bin.
     
  3. elvis

    elvis Old school old fool

    Joined:
    Jun 27, 2001
    Messages:
    39,701
    Location:
    Brisbane
    Subscribed!
     
  4. Daft_Munt

    Daft_Munt Member

    Joined:
    Jan 23, 2003
    Messages:
    7,117
    Location:
    Hobart, The Federal Group
    OP your are braver man than me. I would probably fry myself looking inside a CRT.
     
  5. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Yeah - you have to think about everything you are doing - had some fun times with the new probe before I slipped some heatshrink over part of the probe tip - a little arcing action from tip to the rear of the tube - lesson learnt! I will likely shorten the probe tip a little. The probe has been very handy so far though - only finished putting it together this morning!

    I'm using an isolation transformer as well, which makes the whole thing a little safer on the AC mains side of things.

    Ok - I removed the neck connector (not sure what it's called) from the neck board and popped open the top cover.

    Solder connection of the focus wire is sound, but I buzzed the connector out, and it doesn't seem to go anywhere! Ie, there may be an internal fault inside the connector - the focus wire goes in, but doesn't seem to come out on any of the pcb connector pins.

    I tested the output from the flyback though - around 6.1kV present which sounds about right (it's adjustable on the flyback transformer as one of the pots, though appears to be glued in position). So that is good news.

    [​IMG]

    So, it is entirely possible that this could be the one remaining issue with the monitor - a faulty board connector. I assume you can buy these things somewhere, but I might see if I can open it up further (there are 4 rivets holding it together) and see if I can fix it at least temporarily so as I can further troubleshoot the monitor.

    I need to hunt down a power switch as well - awkward having to hold the switch in while testing.
     
    Last edited: Feb 13, 2018
  6. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Errrrrm.... that might have been a bad move.

    [​IMG]

    I was expecting the focus high voltage output to come out on one of the PCB pins, but it doesn't - it goes straight to the tube - which makes perfect sense now - doh! There is some over-voltage protection built in - on the left side (the top section) you can see the round disc with the pointy tip in the center - the incoming focus high voltage wire is on the back of that disc, and would arc to the ground loop wire on the lower half of the connector housing if the voltage was too high (when assembled there is a (precisely engineered) gap between the two) - cool :)

    Ok - so my fault is elsewhere, and I've got to work out how to clamp this thing back together!

    Cable ties to the rescue. It's all good - I'm learning :)

    [​IMG]

    The schematic shows a "Screen" output from the flyback which must be the thin red wire that is tied up with the focus and high voltage wires from the flyback - I'll investigate that one next - perhaps the flyback is faulty after all.

    [​IMG]
     
    Last edited: Feb 13, 2018
  7. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Ok - measured the screen voltage out of circuit at approx 362 volts.

    [​IMG]

    In circuit (soldered back in) I'm measuring only 30 odd volts at the pin on the neck connector. There is just a cap to ground there (C513) which I need to check.

    I also checked the low voltage (15V) output from the flyback transformer - checks out ok at 15.5V. I'm fairly confident the flyback transformer is ok but I do need to confirm the focus and screen voltages are where they should be.

    That's enough for today - I'll carry on with this during the week.
     
    Last edited: Feb 13, 2018
  8. badmofo

    badmofo Member

    Joined:
    Oct 10, 2011
    Messages:
    1,106
    Location:
    Australia
    :shock:

    Good luck in there!
     
  9. jeremybh1

    jeremybh1 Member

    Joined:
    Jul 2, 2014
    Messages:
    58
    Very glad to see the level of detail put into my old monitor. I'm sorry that it died further on your bench but then again who else in the world could fix it but you. I have my trusty 1084S here that is basically compulsary and in daily use - it drives my Apple IIgs with a custom cable. It is going just fine (P1) model but I guess a day will come when it too will need some TLC.
    Keep up the good work.
     
  10. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Thanks Jeremy, this is a good chance for me to learn about CRT monitors :) I've learnt quite a bit already and I've really just started on it.

    Forum member xga offered me a power switch, so that should arrive in a day or two (thanks again xga!) and I will install it.

    I hope to spend some time this evening on it - probably checking some of the other smaller voltage regulators on the board (there is a 12v rail and also a 5v rail generated from the 15v flyback output), and generally probing around the board.
     
  11. elvis

    elvis Old school old fool

    Joined:
    Jun 27, 2001
    Messages:
    39,701
    Location:
    Brisbane
    As have I just reading this thread. Very much appreciated. :thumbup:
     
  12. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Ok - had a quick poke around the board - 12V rail is there and looks ok at 11.9V.

    The Screen voltage was bugging me - I previously measured that at around 362V when I desoldered the screen wire from the neck board, and only 30V when refitted. From the internet :

    The screen grid (G2), also known as the cutoff, serves to "push" the electrons so they travel more quickly toward the screen, in much the same way as blowing in one end of a tube will force something out the other end. Although electrons go through the screen grid before the focus grid, it seems that in some systems, there will actually be another anode which serves this same purpose *after* the focus grid, and either or both of these anodes may be referred to as the "accelerating" anodes. The screen grid (or accelerating anode(s)) actually uses a positive voltage to pull the electrons along.

    The rest of that article can be read here - I found it a handy refresher on how CRT's work! Essentially, without a healthy screen voltage , I wouldn't likely see a raster.

    That voltage could have only been dropped across 2 components - the monitor screen grid itself (which would have been an internal fault in the picture tube), or the capacitor to ground (more likely).

    To test, I initially removed the neck board from the tube, powered it up and took a reading - 30V. Aha - it had to be a faulty capacitor with a near short to ground!

    So, I removed the capacitor in question - a 10nF 1000V part :

    [​IMG]

    I didn't have another, so left it out and fired it up. I measured the Screen voltage at 222V both on and off the tube (I'd been dicking around with the pots on the flyback so the voltage was less than previously measured). Much more healthy!

    So I cranked the brightness up and checked for a raster.... its a bit hard to make out in the photo but YES! We now have a raster!

    [​IMG]

    Alrighty, good progress, but there could still be many issues. Time to test with a video signal though and troubleshoot from there. What better test source than the C64 :)

    Plugged it in - nothing. Wait - there are a bunch of switches on the back of the monitor for the composite input - click! Woohoo!

    [​IMG]

    Success :D I'm very pleased that I built that high voltage probe - I would have struggled to test anything properly without it - testing the flyback was what led me to the Screen voltage issue and there's no way I would have been testing the flyback without the probe.

    I'm looking forward to receiving the replacement power switch from xga and then I'll tweak the picture for the best image I can get - I will now of course order and install the replacement cap for the neck board, but I may also start re-capping the rest of the board (there are a lot of caps on there!). I'll be sure to post pics of the new switch going in, as I may have to modify it slightly to fit it to this monitor.

    Another retro item saved from the scrap heap though - thanks to Jeremy for not throwing it out!
     
    Last edited: Feb 13, 2018
  13. lostintranslation

    lostintranslation Member

    Joined:
    Sep 6, 2002
    Messages:
    839
    Location:
    left of the middle
    :thumbup:

    Awesome work! Loved reading this!
     
  14. nimmers

    nimmers Member

    Joined:
    Dec 20, 2005
    Messages:
    1,238
    Location:
    Sydney
    Wow awesome read!
     
  15. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Thanks - just imagine how many monitors like this have been thrown out because of a 50c part (I know it's normally the labour cost that is the killer, but still).
     
  16. DonutKing

    DonutKing Member

    Joined:
    Mar 21, 2004
    Messages:
    1,298
    Location:
    Tweed/Gold Coast
    Awesome thread!


    Quick question - does your 1084S make any audible noise? Mine makes a high pitched whine, louder when there's no signal going to it. I put it down to the 15khz horizontal sync frequency.
    I've heard this could be a vibrating component but buggered if I could find it.
     
  17. elvis

    elvis Old school old fool

    Joined:
    Jun 27, 2001
    Messages:
    39,701
    Location:
    Brisbane
    Yup, a very sad thought. Both for the people who want to use these things still, and for the environmental/waste impact.

    Even just educationally, fixing old electronics is something not enough people attempt (even on safer, low voltage stuff).
     
  18. Alby1976

    Alby1976 Member

    Joined:
    Mar 1, 2002
    Messages:
    1,662
    Location:
    South Oz
    Great read. Remember that blue screen well, but didn't have a flash S monitor, just the 34cm telly.
    Excellent work.
     
  19. OP
    OP
    aXLe

    aXLe Member

    Joined:
    Jun 27, 2001
    Messages:
    1,207
    Location:
    Brisbane
    Yes - there is definitely a high frequency sound present - I'm hoping that may be cured if I re-cap the board. I'll start with the PSU section and work out from there.

    I will also now be able to make up an RGB cable for the C64 - initial testing was via composite :)
     
  20. xga

    xga Member

    Joined:
    May 4, 2005
    Messages:
    32
    C64 doesn't support RGB, but it does support S-Video.
     

Share This Page

Advertisement: