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Thread: Commodore 16 won't boot - what next after replacing the TED

  1. #31

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    Quote Originally Posted by Zippy Zapp View Post
    It sure seems like I have seen and heard about far more dead CPUs then TEDs in these machines. It would be nice if there were drop in replacements for both of these chips. The TED systems are somewhat unique.
    I have a very small sample, but out of 3 systems, I have 3 dead CPUs and 0 dead TEDs (tested borrowing a working CPU from a friend).

    Frank

  2. #32
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    I would concur with KC9UDX and iz8dwf, in that it's most likely a RAM device or addressing fault. Piggybacking RAM only gives you another data point, it's not a definitive test, so don't rule out the RAM being at fault just yet. If you have the ability to burn an EPROM, there is diagnostic tools such as Diag264 which can help diagnose a RAM fault most specifically.

    The screenshot tells you a lot. The fact that you have the right border and screen colour, and that you don't have random junk on the screen tells us that the CPU has reset correctly, started executing the kernal initialisation routines, cleared the screen and TED. In turn, TED is rendering the display correctly. Where it's likely crashed is when it starts to use the stack page between $0100 and $01FF, as the values pulled back off the stack will most likely have the same fault.

  3. #33
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    Quote Originally Posted by crock View Post
    I would concur with KC9UDX and iz8dwf, in that it's most likely a RAM device or addressing fault. Piggybacking RAM only gives you another data point, it's not a definitive test, so don't rule out the RAM being at fault just yet. If you have the ability to burn an EPROM, there is diagnostic tools such as Diag264 which can help diagnose a RAM fault most specifically.

    The screenshot tells you a lot. The fact that you have the right border and screen colour, and that you don't have random junk on the screen tells us that the CPU has reset correctly, started executing the kernal initialisation routines, cleared the screen and TED. In turn, TED is rendering the display correctly. Where it's likely crashed is when it starts to use the stack page between $0100 and $01FF, as the values pulled back off the stack will most likely have the same fault.
    OK perhaps it isn't quite as irretrievable as I thought. Unfortunately I don't have the ability to burn an EPROM, so I'm still stuck at replacing things blind. Fortunately once you move onto the generic chips though it's all very cheap and if the replacement TED isn't needed I can re-sell it to claw some money back.

    I've now ordered a couple of sockets and will fit the new RAM. Failing that, moving on to the support circuitry, as far as I can see there are 8 chips that are in some way directly connected to the RAM, I would guess that the below 3 are the most likely to have failed?:

    U9 - 7406N. Multiplexer
    U14 - 74LS139 - Demultiplexer
    U11 - 74LS127AN - Buffer

    But I've also got:
    U10 - MC1455P1. Is this just a 555 timer or do I need that specific chip?
    U7 & U8 - 72LS257AN - 2-to-1 data selector
    U15 - 74LS175LN - Flip Flop
    U12 - 74LS02 - NOR Gate

    It wouldn't be hugely expensive to do all of them, still less than I paid for the replacement TED. Any ideas on where to start would be most appreciated!

  4. #34
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    U7 and U8, the 74LS257's multiplexers are most likely.

    U10 is the 555, and as it's reseting correctly, probably OK.
    U9 (7406) inverts the reset pulse, as well as driving the output ports of the serial port, so don't worry about that yet.
    U14 (74LS139), U15 (74LS175), and U12 (74LS02) are for latching the right ROM bank if cartridges or funtion ROM's are installed, and driving the chip selects for kernal and basic ROM's, so not RAM relevant.

  5. #35
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    TBH I've had this thing sat on the bench for long enough. Found some 74LS257AN's for peanuts so ordered them (it was actually cheap to buy 5 than 2...go figure) and sockets for those too so they'll be on hand if swapping the RAM doesn't work. Then at that point I think it might be time to give up on this board.

    Still not sure what I'll actually do with the C16 if I ever get it working. It was never something I particularly lusted after, I just wanted to flesh out my 8 bit collection. My intention was to get a VIC20 but when I saw the C16 for cheap I thought I should jump on that due to them being much rarer. I do have the matching tape deck so assuming that works I could try a bit of programming on it, for the novelty of programming a Commodore machine that has graphics and sound commands!
    Last edited by cwathen; August 25th, 2018 at 08:40 AM.

  6. #36
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    Ahem...

    C16 working.jpg

    The first RAM chip I replaced fixed it! Shame I've now ended up with a spare TED, spare RAM and 5 x spare 74LS257 chips. I might just throw the TED back on ebay. Happy to have it working!
    Last edited by cwathen; September 1st, 2018 at 06:53 PM.

  7. #37
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    Very nice!

    If I were you, rather than eBay the TED, I'd wait for a broken C16 at a low price. If you get lucky and it's not the CPU, you'd stand to make a fine profit.

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