Breadbin Back from the Brink - finally a C64

Discussion in 'Retro & Arcade Worklogs' started by aXLe, Aug 14, 2016.

  1. aXLe

    aXLe Member

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    I've finally added a C64 to the collection thanks to Jeremy in Canberra - the unit was in trade for some work I recently completed for him.

    It doesn't actually work, but I'm confident it will be up and running again soon :)

    Some pics :

    [​IMG]

    [​IMG]

    [​IMG]

    Under the hood - funky silver cardboard shield :

    [​IMG]


    From this assembly number (250425), I believe this a Rev B machine :

    [​IMG]

    Most chips are soldered down, including CPU, RAM, ROM :

    [​IMG]


    [​IMG]


    The symptoms are no video - black screen - nothing there. I haven't yet started to probe around the board with a scope to see what is going on, but I did check that the main supply voltages are present.

    I thought a good starting point would be to recap the power supply as I'm going to do it anyway at some point. The power supply is linear, using a transformer, diodes, and 5V regulator to provide a 5V regulated supply to the C64, plus an unregulated AC supply. There is just a single filter cap in there - a 16V 4700uF part. I swapped it out for a quality Japanese 25V part :

    [​IMG]

    [​IMG]

    Next up the mainboard - removed from the casing, and with the shield removed from over the VIC chip :

    [​IMG]

    You case see the 3 large axial caps that I'll be replacing on the right hand side of the board. On this revision there are only 3 other smaller caps that I'll also replace.

    The replacements - japanese Nichicon parts :

    [​IMG]

    Installed :

    [​IMG]

    I've also now replaced the other 3 smaller electrolytic caps on the mainboard :

    [​IMG]

    I noticed that while powered up, the Kernel ROM chip was getting quite warm - this is a known point of failure that does result in the same symptom I'm seeing. Still without probing the ROM with the scope, I thought I'm best to remove it and socket it as there are a few ROM upgrades available that I'm sure I'll want to take a look at some time.

    So, out it comes :

    [​IMG]

    A socket is soldered in, and the chip back in place for now :

    [​IMG]

    After some research I found that the original roms are 2364 parts. I attempted a read of the ROM in my BP Microsystems programmer (which is capable of programming MCM68764 pin compatible EPROMS) and it gave me an error - further pointing to the ROM as having an issue.

    Ran out of time tonight, but next up I'm going to make a simple adapter for a 27C128 EPROM and drop it in to see if any improvement :)
     
    Last edited: Oct 30, 2019
  2. partybear

    partybear Member

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    That machine looks like it is in really nice condition cosmetically. My c64 has a dead power supply, but mine does not come apart it is full of resin. It is such a pain that it needs 9v ac I don't own a single wall wart that does that so it isn't just an easy fix.
     
  3. badmofo

    badmofo Member

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    Oh wow I love a C64 revival story, just what my Monday morning needed thankyou!

    That is a physically handsome looking machine and it sounds like you're close to bringing it back to life - I've accumulated a few breadbin boards for spares so let me know if I can help out with any components.

    I needed to replace the PLA in mine and personally I think this is a good idea regardless of whether the original is working or not, because they do get hot and warm everything else up. I also hacked together my own PSU and likewise think that was a worthwhile exercise.

    They were such a capable machine, it's easy to understand why they were so successful.
     
  4. callan

    callan Member

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    Always warms my heart to see a C64 being tended to.

    You may want to look at something like THIS.
    If you've got a warm ROM that's certainly suspicious.

    BTW those power supplies are deadly - they fail with age and temperature, and they fail OVERVOLTAGE, by dropping through upwards of 10V into the 5V Rail.
    RAM is usually the first to go. I'd never trust one now, and use modern drop-in swichmodes instead.

    Callan
     
  5. OP
    OP
    aXLe

    aXLe Member

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    I'm planning to put together one of those adapters tonight and burn an EPROM - I'm hopeful that will fix it, but it certainly could be many others things.

    The PSU voltages are spot on at the moment, but perhaps I should change out the 7805 as well - I did replace the thermal paste.

    I'm a complete noob with C64's - a good learning exercise :)

    An overview of the C64 assembly 250425 PCB :

    [​IMG]

    1. 6526 CIA (Complex Interface Adapter)
    2. 6526 CIA (Complex Interface Adapter)
    3. Basic ROM
    4. Kernal ROM
    5. Character ROM
    6. 6510 MPU (MicroProcessor Unit)
    7. 6581 SID (Sound Interface Device)
    8. RF Modulator
    9. Memory Expansion/Cartridge Port
    10. Power input
    11. Power switch
    12. Joystick ports
    13. 64K RAM
    14. PLA (Programmable Logic Array)
    15. PAL subcarrier crystal (17.734475MHz)
    16. 8701
    17. VIC II (Video Interface Chip II)
    18. +12V regulator
    19. +5V regulator
    20. Bridge rectifier
    21. Power LED header
    22. Keyboard header
    23. User Port
    24. Cassette interface
    25. Serial interface
    26. A/V port - video an audio output plus audio input

    Rest of the chips provide glue logic for ram etc as far as I can tell.
     
    Last edited: Oct 30, 2019
  6. the3coopers

    the3coopers Member

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  7. OP
    OP
    aXLe

    aXLe Member

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    I'm not sure about the serial number sticker - I don't think it's been replaced, but certainly damaged. Perhaps someone was looking for screws? It was like that when I received it.

    I didn't have much time last night but I did make a ROM adapter for a 27C256 EPROM :

    [​IMG]

    [​IMG]

    I've pulled the A13 and A14 address lines to ground with 10k resistors at this stage, but will later wire a switch so as I can select 1 of 4 ROM images by applying +5V to the respective address pins.

    So I burned a ROM with the standard Kernel at $0000 and JiffyDOS at $2000 - with the current pull-down resistors in place, the standard Kernel should be selected.

    [​IMG]

    I ran out of time, but a very brief test showed no improvement. I need to double check my adapter, but it's time to get out the scope and get serious. I've also been testing using only the RF output - I did buy a plug for the AV port so will make a composite cable up, but the scope will show me whether a video signal is present anyways.
     
    Last edited: Oct 30, 2019
  8. badmofo

    badmofo Member

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  9. OP
    OP
    aXLe

    aXLe Member

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    Cool - thanks - I'll have a read through that for sure. C64's are new to me, so I'm sure this will be very helpful and will hopefully speed up the troubleshooting :)
     
  10. OP
    OP
    aXLe

    aXLe Member

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    Had a poke around the Kernel ROM tonight - the chip select line looks all wrong to me :

    [​IMG]

    Scope is at 1 volt per division and we are talking 5V logic - for logic purposes the chip select is essentially clamped to 1V = low.

    The chip select is an active low signal, therefore the chip is always selected - the data bus is a mess - with the ROM data clobbering everything else.

    The chip select for the Kernel ROM is directly connected to the PLA chip (Programmable Logic Array), which is responsible for memory mapping. Thus, it would appear, that my PLA is faulty, and that a replacement is in order. PLA failure is apparently very common - the chips run very hot at the best of times (this one is scalding hot).

    Fortunately, forum member badmofo has offered me a replacement chip (thank you!), so I'll prepare the board for its arrival by removing the original, and fitting a socket for the replacement.

    I also had a quick look at the video signals with the scope - the makings of a composite video signal are there, just no video data on the signal. Hopefully the replacement PLA will change that.

    *fingers crossed*
     
    Last edited: Oct 30, 2019
  11. OP
    OP
    aXLe

    aXLe Member

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    While waiting for the PLA to arrive, I removed the existing one and installed a socket.

    [​IMG]

    I also took the opportunity to tidy up my workbench a bit, with a rework of the shelving, (and have since added more LED strip lighting again from the when the photo was taken). I built that bench 6 years ago now - worklog here

    [​IMG]



    A huge thank-you to badmofo - he sent me not only an aftermarket PLA chip based on a Xilinx CPLD, but also a complete NTSC motherboard that I can use to trouble shoot mine. Thank you again!

    The CPLD based PLA runs cool, and draws about one fifth the current of the stock one.

    [​IMG]

    The motherboard is a slightly earlier version, but has most of the chips on there that I can swap if required, plus it is extremely handy just to be able to view the signals from the working board on the scope and compare.

    [​IMG]

    The PLA installed in my board :

    [​IMG]

    The replacement PLA fixed some issues, but I still have a black screen :( On the plus side the control signals (chips select etc) now look as expected, with proper logic transitions from low to high, and the data bus is clean.

    Tonight I started probing the various clocks and discovered another issue. as As I understand it, the VIC chips supplies a 1MHz clock to the CPU on pin 1 (phi 0 input), and the CPU buffers it and outputs it on pin 39 (phi 2 output). I can see a 1 MHz clock from the VIC on pin 1, but I'm not getting any output on pin 39 at all on my cpu - zero volts out.

    I need to investigate further and double check to see if there is anything that would prevent the CPU from outputting the clock, or whether any other components could be pulling the line down to ground, but it could be that my CPU is bad.

    I may have to socket the CPU on both boards and swap them to test but I'd prefer to be sure before I desolder the chips. I feel I am making some progress in any case, and should have a fair amount of time this weekend to further troubleshoot the board :D
     
    Last edited: Oct 30, 2019
  12. partybear

    partybear Member

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    Man I am so jealous of your workbench setup that is gorgeous.
     
  13. OP
    OP
    aXLe

    aXLe Member

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    After a quick study of the schematics and also the 6510 MPU datasheet I was convinced that my MPU (MicroProcessor Unit) was stuffed. So, the MPU was removed and a socket installed :

    [​IMG]

    The MPU was then removed from the board badmofo sent me :

    [​IMG]

    I will install a socket on that board later so it is usable again. The donor cpu was then installed into the socket :

    [​IMG]

    And fingers were crossed as I fired it up and started probing with the scope.

    Phase 0 (phi 0) clock present on MPU pin 1? Check!

    [​IMG]

    Ok, do we now have the phase 2 clock (phi 2) which was missing before on MPU pin 39? Yes!

    [​IMG]

    This is looking promising - time to check the video signal. Earlier when I first started on the board I said that I could see the makings of a video signal, but with no video data. See the reference image below :

    [​IMG]

    I was seeing the front porch, hsync, breezeway, burst and back porch, but with no video data beyond that, resulting in a black screen (the signal is produced by the VIC II chip so I was pretty confident the VIC II was working). And now?

    [​IMG]

    Very promising indeed! Video data is there! Time to connect it to a monitor - in this case a projector for testing. Cobbled together :

    [​IMG]

    Power on - Woohoo!! It's alive :D

    [​IMG]

    [​IMG]

    So on this board, the original fault (black screen) was caused by a bad PLA plus a bad MPU.

    At this point I was going to test the SID chip (the audio chip) by entering a simple basic program to generate test tones. That's when I found that the '4' and '8' keys are not responding. My first thought was that one of the 6526 CIA (Complex Interface Adapter) chips (the one responsible for the keyboard matrix) may have had a bad input. I googled the keyboard matrix :

    [​IMG]

    Whilst the '4' and the '8' keys are on the same keyboard matrix row, if that row input on the CIA had been dead then none of the other keys on the row would have worked, but I'm sure I had used the '2' key. Thus the problem is most likely a dirty key switch, or an issue with the keyboard PCB. This is the next job to tackle but I'm very happy to have the C64 running again. Thanks again to badmofo for the donor!

    Oh, and by the way the warm ROM chip was a red herring - I'm not sure why my EPROM reader failed to read it (I will look into that) but the ROM on the board right now is the original one. The heating was due to the bus conflicts as a result of the malfunctioning chip select on the PLA - the ROM is fine :) I'm yet to test the adapter and dual ROM image that I made - its on the To Do list once the keyboard is fixed.
     
    Last edited: Oct 30, 2019
  14. OP
    OP
    aXLe

    aXLe Member

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    Removed the keyboard PCB by unscrewing all the small screws that retain it, and desoldering the solid wires to the caps lock key (the square white key that is visible through a cutout to the right of the PCB in the image below) :

    [​IMG]

    The key switches simply press a conductive rubber pad across pads on the PCB to complete the circuit. PCB is made by Mitsumi :

    [​IMG]

    PCB was a little dirty....

    [​IMG]

    .... plus I found this mess :

    [​IMG]

    I desoldered the two corroded wires, cleaned and repaired the tracks and finally cleaned the whole PCB plus the key contacts with isopropyl alcohol and put it back together.

    Keyboard is now working 100% :D

    I reassembled the C64 - shown here with my next project - a 1084S-D2 monitor (monitor doesn't work - seems to have high voltage but nothing else).

    [​IMG]

    Once the monitor is back up and running (I will create a separate worklog for that one), I'll experiment some more with the C64 ROM's and buy a flash drive adapter for it so I can play some games etc.

    For now though, this project is complete - the C64 is fully operational (as far as I can tell at least), and another retro computer has been saved from landfill :)
     
    Last edited: Oct 30, 2019
  15. badmofo

    badmofo Member

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    Amazing work aXLe, what a thrill to see it come to life again :cool:

    Fixing the keyboard should be a breeze with your mad sk1llz - I've found that an IC puller is a good way to pop the keys off without breaking the stems and losing the springs.

    EDIT: and BAM you've done it already!
     
    Last edited: Aug 28, 2016
  16. DonutKing

    DonutKing Member

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    Great thread :thumbup:

    I'm looking forward to your monitor repair. I have the same one here and it's working, although I hear that the flyback transformer is a common failure - and replacements are hard to come by.
     
  17. callan

    callan Member

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    Makes me happy :)

    Callan
     
  18. psyolent

    psyolent Member

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    i think i had a moment when i saw that blue C64 screen. good work mate.
     
  19. Kosti

    Kosti Member

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    This is fantastic revival of some classic piece of history, well done..Makes me want to pull down my C64 and plug it in..

    I admit your desoldering skills are awesome and getting sockets in there is a great idea

    Well done, am going to look at some of your other projects for inspiration!

    BTW I love your work bench, I wish I had room for such a great setup

    PEACE
    kosti
     
  20. OP
    OP
    aXLe

    aXLe Member

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    Photobucket images replaced :)

    Some time ago I picked up an Ultimate II+ cartridge for the machine. This emulates a floppy disk drive, allowing you to boot disk images that are stored on a USB stick, plus it does a few other cool things as well, such as allowing you to create physical disks from the USB disk image file.
    [​IMG]
     
    Last edited: Nov 15, 2019
    Phido, adz, slavewone and 1 other person like this.

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