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Thread: My first attempt at PET Repair. Model 4016. Advice Welcome!

  1. #11
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    Quote Originally Posted by KevinO View Post
    Ok,
    UD6-Pin20 stays low
    UD7-Pin20 stays high
    UB14-Pin7 has no pulses....most of the time. A couple times when I pushed harder with my probe to keep it on the pin, I did get signal. So I'm flexing something that is intermittent.
    The power-up code should only stay in the F Rom for four instructions before jumping to a routine in the E Rom to sound a beep and set up all the I/O chips including the CRT controller which would cause the screen to display the garbage characters in the video RAM for a few seconds before it is cleared.

    So I would have guessed a bad F ROM, but, If you replaced the CPU socket, I would not expect an intermittent. So that worries me. Let's see what the Brain Trust guys say about this data so far.

    Code:
     ; - Power-Up RESET Entry
    FD16 LDX #$FF ; Load X $FF
    FD18 SEI ; Disable Interrupts
    FD19 TXS ; Transfer $FF to Stack Pointer
    FD1A CLD ; Clear Decimal Mode
    FD1B JSR $E000 ; cint Initialize Editor & Screen

  2. #12
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    Just getting back to this after the tornadoes on Saturday. We lost one tree and gained about 100, when the creek overflowed and dumped 10 tons of deadwood in our yard. Chainsaw's gonna be working overtime!

    I just read another 4016 thread where there was no reset pulse. So I checked, and....no reset pulse! Replaced UD15 555, no effect. Then found the .1uf cap C50 had a broken pin and was barely making contact with the board. Someone had been fiddling with this circuit, because there was bad soldering on this cap. Anyway, replaced C50 and now I have reset pulse on IB14 Pin40. Not surprisingly, this also changed the behavior of the lines I was previously asked about:

    UD6-Pin20 has pulses for about 2 seconds after power on, then goes back low.
    UD7-Pin20 goes high for about 2 seconds after power on, then blips low for a fraction, then goes back high.
    UB14 is low for about 2 seconds after power on, then blips high for a fraction, then goes back low.

    Still no chirp and still nothing on the display.
    Last edited by KevinO; April 16th, 2012 at 08:05 PM.

  3. #13
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    Quote Originally Posted by KevinO View Post
    UD6-Pin20 has pulses for about 2 seconds after power on, then goes back low.
    UD7-Pin20 goes high for about 2 seconds after power on, then blips low for a fraction, then goes back high.
    UB14 is low for about 2 seconds after power on, then blips high for a fraction, then goes back low.

    Still no chirp and still nothing on the display.
    It seems like things start out OK, but then the machine is stuck in the F ROM (/chip enable always low). Also there is no instruction fetching after initial flurry. This can happen if an illegal instruction is executed. This implies a bad F ROM or bad connections on the data bus. Are any data lines pulsing after the two seconds? I would guess they are static. Are you prepared to replace the F ROM with a 2532 EPROM? I could burn a replacement chip for you. This may also be the time to consider the new PETVet fix, i.e, bypass all ROM and RAM.
    -Dave

  4. #14
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    Well, if he's going to try a replacement F ROM then he'll have to unsolder it anyway, in which case he could test it in the working 8032 first. Maybe a checksum on the socketed E ROM (UD7) wouldn't hurt either?

    I'm curious: what's the story on UD2? Why is it in a socket and why does it look odd in the picture?

    The PETvet is certainly a good idea but I'm also going to make my standard recommendation of a NOP generator, especially if there's an intermittent; it would keep running instead of halting or looping as it's apparently doing now.

    Got a spare 40-pin socket?

    BTW, what's the problem with the broken CRT? Actual broken glass, or?
    Last edited by MikeS; April 16th, 2012 at 10:47 PM.

  5. #15
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    Quote Originally Posted by MikeS View Post
    Well, if he's going to try a replacement F ROM then he'll have to unsolder it anyway, in which case he could test it in the working 8032 first. Maybe a checksum on the socketed E ROM (UD7) wouldn't hurt either?

    I'm curious: what's the story on UD2? Why is it in a socket and why does it look odd in the picture?

    The PETvet is certainly a good idea but I'm also going to make my standard recommendation of a NOP generator, especially if there's an intermittent; it would keep running instead of halting or looping as it's apparently doing now.

    Got a spare 40-pin socket?

    BTW, what's the problem with the broken CRT? Actual broken glass, or?
    First, Dave- you're right...no pulses on data lines.

    Mike-
    The schematic doesn't identify the ROMS with a letter. Is it safe to assume that ROM F is UD6?. Swapping with the 8032 is a great idea. I will try to do this tonight when I get home from work.

    Oh, you had to bring up UD2... In my defense, it was before I had a schematic. I saw a chip with two legs cut, and I assumed that it was something someone did while troubleshooting and maybe forgot to resolder the pins. So I put in a socket and replaced the chip. Then I found the schematic and found out pins 10 & 11 were supposed to be cut, and went ahead and put the original back in!

    I assumed the PETvet was for diagnostics. You're saying you can use it as a permanent replacement?
    What's a NOP generator?
    Yes, I've got sockets of all sizes.
    Broken CRT? I don't recall that. What did I say?

    Thanks for your continued help, guys.

    Kevin

  6. #16
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    Quote Originally Posted by MikeS View Post
    The PETvet is certainly a good idea but I'm also going to make my standard recommendation of a NOP generator, especially if there's an intermittent; it would keep running instead of halting or looping as it's apparently doing now.
    Mike,
    Yes, I forgot he has a scope. That would be a good step although if the problem is with the data lines, it may not catch that type of problem. But it would catch all types of address line problems.
    -Dave

  7. #17
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    Quote Originally Posted by KevinO View Post
    ...The schematic doesn't identify the ROMS with a letter. Is it safe to assume that ROM F is UD6?.
    Yes; we sometimes refer to the ROMs by their address, in this case F000-FFFF
    Swapping with the 8032 is a great idea. I will try to do this tonight when I get home from work.
    You don't actually have to swap it if it involves unsoldering the ROM in the good 8032; you can just put it into one of the spare sockets (UD11 or UD12) and sum the bytes in it to make sure it's OK.

    Oh, you had to bring up UD2... In my defense, it was before I had a schematic. I saw a chip with two legs cut, and I assumed that it was something someone did while troubleshooting and maybe forgot to resolder the pins. So I put in a socket and replaced the chip. Then I found the schematic and found out pins 10 & 11 were supposed to be cut, and went ahead and put the original back in!
    Interesting; I'll have to look and see what that was all about.
    I assumed the PETvet was for diagnostics. You're saying you can use it as a permanent replacement?
    Yes, but that's cheating
    What's a NOP generator?
    It's a handy little tool, either a spare 6502 or a 'shim' socket to go between the 6502 and its socket, that has the data lines disconnected from the main board and permanently wired so that the CPU sees a NOP code; it will then perpetually cycle through all the addresses and you can probe all the ROMs, RAMs, buffers etc. with a scope looking for shorts, bad sockets, etc. They're very regular and distinctive signals, each address line half the frequency of the next lower one.

    See here for instructions (in German, but pretty clear if you translate):
    http://home.germany.net/nils.eilers/nopgen.htm

    Broken CRT? I don't recall that. What did I say?
    I thought your first post mentioned "still needs a tube"; did I misunderstand?...

    Good luck!

    Edit @ Dave: you did it again, sneaking in while I'm composing one of my epic replies...

    Granted, the NOP gizmo doesn't do much for a data line issue, but if there's an intermittent somewhere as there seems to be then it could be either address or data (or neither ) Just something to try without spending money or while waiting for the PETvet to arrive
    Last edited by MikeS; April 17th, 2012 at 08:30 AM.

  8. #18
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    Quote Originally Posted by KevinO View Post

    Mike-
    The schematic doesn't identify the ROMS with a letter. Is it safe to assume that ROM F is UD6?.
    Yes, it is the F000-FFFF address area called 'the kernal' of the PET. Unless the working board has UD6 on a socket, maybe you should leave the working board alone for now using the adage "if it isn't broke, don't fix it".

    I assumed the PETvet was for diagnostics. You're saying you can use it as a permanent replacement?
    Yes, it is a quick fix that can be left in.

    EDIT: Mike, you beat me this time! Good explanation of your famous and useful NOP Generator.
    Last edited by dave_m; April 17th, 2012 at 08:36 AM. Reason: edit for Mike

  9. #19
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    Quote Originally Posted by MikeS View Post
    You don't actually have to swap it if it involves unsoldering the ROM in the good 8032; you can just put it into one of the spare sockets (UD11 or UD12) and sum the bytes in it to make sure it's OK.
    To Kevin: It only takes one line of BASIC to run this sum check on a PET as Mike proved to me. You then just compare the answer with Mike's approved list of ROM sum checks.

  10. #20
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    Quote Originally Posted by dave_m View Post
    EDIT: Mike, you beat me this time! Good explanation of your famous and useful NOP Generator.
    Well, I'm certainly not taking credit for it although I am a big fan It was a well-known trick even back in the prehistoric days when you and I (and our PETs) were still young and I used it a lot when servicing my AIM65 systems...

    None of the three PETs that I purchased new actually ever had a problem.

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