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Thread: First PET post of October. 4032 with dead RAM subsystem

  1. #1

    Default First PET post of October. 4032 with dead RAM subsystem

    Hi folks,

    Long time no post, life and non-PET projects got in the way. I'm slimming down my PET collection because nobody needs 12 and I'm moving house, and was just going to list them on e*ay as faulty/untested.

    Then I realised my repairing conscience wouldn't let me do that* and have already fixed up an 8096 (dead Editor ROM) and discovered to my surprise a spare 3032 worked fine without me doing anything to it. The next one is a 4032 with 40 col 8032080 board, and after replacing the white CPU socket I fitted my Tynemouth Software ROM/RAM board and slowly eliminated faulty items until it's now just providing the lower 16K RAM to boot, so I have a RAM fault.

    Half of the RAM chips are socketed already so I've tested those in my 4116 RAM tester and also cleaned up and refitted R10 and R11 which had been badly butchered by whoever worked on this last. Both CAS0 and CAS1 measure 27ohms back to those resistors from each chip so they're still good despite how they look. The power is OK at 12.05V, -5.06V and 4.88V and there's continuity along both banks though I've just realised I haven't tested back to and beyond UE8/9/10 for the address bus. There's a task for tomorrow.

    I've watched CAS/RAS/WE/DI/DO with a logic analyser and they all seem stable with no dead spots.

    I feel lazy asking for help at this point but I bow to the superior board experience of people here. I put DaveR's excellent PETTEST V4 onto a 2516 and it's showing 'b' for 99% of the memory locations but crucially not all of them. Just for the hells of it I enabled the lower 16K of the ROM/RAM board just to watch PETTEST complete

    Any tips for where to look next before I start checking the LS244s for dead gates?

    Cheers!

    Witchy



    *apart from an 8032 that's missing half a dozen chips and has a few lifted traces plus cracked CRT neck, hate to think what the previous owner did to THAT one
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
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  2. #2
    Join Date
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    Default

    Thanks for the vote!

    Any chance of posting a photograph/video of your G/B screen? With a knowledge of what is stored where and the results it may be possible to work out whether you have a data fault or an address fault and where they are. Since the first tests only check pages 0 and 1 all address faults can't be detected at this time though.

    Is the G/B display static or do some of the results change as the test loops?

    One possibility is the address multiplexer chips as well as the data buffers.

    Another section to add to the documentation...

    Dave

  3. #3

    Default

    You could "cross" R10 and R11 (lift one end of both and swap their end's connections to the pads) so you could identify if you have a good 16K bank or both are faulty. If you have a good bank, then you can boot off of it and write a simple RAM test in basic to identify the bad bit(s).
    I've done more or less this here:
    https://www.youtube.com/watch?v=H5_mTETwWJk

  4. #4

    Default

    If you look at how R10 and R11 were when I first looked at the board at the weekend (first pic) I'd guess someone already did that and couldn't be bothered to put things back together properly. This machine did work when I got it back in the early 00's but obviously that was a long time ago in terms of silicon not dying in the meanwhile.

    Dave, yes the display changes while the test loops but only for areas that initially show as 'g' which as you can see isn't a whole bunch. Middle pic is the tidied up R10/11 and new Kernal.

    IMG_4645.jpgIMG_4662.jpgIMG_4670.jpg
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
    facebook.com/binarydinosaurs

  5. #5

    Default

    Another possibility you have is to run a diagnostic image in place of the kernel ROM if you can burn your own 2532.
    Frank

  6. #6

    Default

    I'm already running Dave's PETTEST V4, that's what's produced the third pic in my previous post showing bad RAM everywhere, pretty much I'm now going through the schematic to look at likely culprits for producing this fault while at the same time thinking that the first PET I fixed back in 2015 had this exact problem and it was down to a 74LS00 or similar. I also did a breakdown image of the universal board showing which chip did what and to whom, need to dig that out too.
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
    facebook.com/binarydinosaurs

  7. #7

    Default

    Quote Originally Posted by Witchy View Post
    I'm already running Dave's PETTEST V4, that's what's produced the third pic in my previous post showing bad RAM everywhere, pretty much I'm now going through the schematic to look at likely culprits for producing this fault while at the same time thinking that the first PET I fixed back in 2015 had this exact problem and it was down to a 74LS00 or similar. I also did a breakdown image of the universal board showing which chip did what and to whom, need to dig that out too.
    on a "FAT40" universal board, I've been luckier with this diagnostic:
    https://drive.google.com/file/d/1bPR...ew?usp=sharing

    It goes in the kernel position.
    YMMV

    Frank IZ8DWF

  8. #8
    Join Date
    Jun 2012
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    Witchy,

    Thanks for uncovering a bug in V4! I will fix it at the weekend...

    The '.' should change to a character that depends upon the value that was actually in the memory cell that was read if the value read incorrectly. It does actually do this... but then overwrites the value with the '.' in error !

    Dave

  9. #9
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    Jun 2012
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    My apologies for that...

    I have uploaded a fix to my google drive.

    It runs under VICE - but I haven't had time to test the fix with a duff memory location though - I have to persuaded VICE to do that and I need to remember how to do it...

    Dave

  10. #10

    Default

    Well that was a good evening's work thanks to both you and Frank! PET is alive again and running without the ROM/RAM board for the first time

    Only on 16K though, Frank, is that the official CBM diagnostic for a 40xx? It mentions 'removing the clip' so I assume it is.

    Dave, I'll download the new V4 tomorrow and run it up, given that 4x 4116 had failed in bank 0 I'll be expecting a similar number in bank 1!

    Night all.
    www.binarydinosaurs.co.uk - UK home computer history
    Where RIFA capacitors come to die
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