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Thread: Commodore PET restoration help needed

  1. #121
    Join Date
    Apr 2020
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    Basel-Stadt, Switzerland
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    Tiny update in the mean time. I know you all said not to buy too much stuff i dont know wether id ever need it but i had a bad case of fear of missing out in regards to those ROM / RAM replacement boards by tynemouth Software. He used to list one shop address, on which he, the seller, was marked as on vacation till Dec 31, 2021, so when he updated the website with a second shop address, I felt pressured into buying it.
    Anyways, the part has arrived but unless you guys think it would be an added value in regards to troubleshooting the device, i would probably stick to leaving it stock until its running properly.

    SkyCaptain

  2. #122
    Join Date
    Feb 2009
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    Southern California, USA
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    The Tynemouth board is very good. I have one in case my PET needs it. But right now you have new tested EPROMs and your RAM appears good so it would be of no benefit to use the Tynemouth at this time. But later if we think for example that your ROM sockets are intermittent, we can try try the Tynemouth to replace all ROM and RAM and see if that makes everything perfect. Right now the issues seem like keyboard, Video memory or screen refresh logic which the Tynemouth will not help.

    I agree with you let's leave the RAM/ROM replacement board alone for the time being. We need to make as few configuration changes as we can while we troubleshoot.

  3. #123
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    Apr 2020
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    Basel-Stadt, Switzerland
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    Hey everyone, i was bored so i put that tynemouth board into the machine. On the basic setting, where the board replaces "all" functions, the machine works almost perfect. It shows BASIC 4.0, it shows the 32KB ram that come with the board. If i set the dip switches to the setting where it takes the original RAM, it shows the 16KB correctly. When I run the diagnostic program that comes with the board, it shows all characters on the screen, and there, the normal letters irradicably switch from caps to small letters.
    Reading the little leaflet provided, it states "IF any of the graphics are wrong, distorted or flashing, this indicates a fault in the video ROM or supporting circuitry." So maybe this is a further pointer

    anyways, im really happy that it does work with the replacement board, but i would be even happier if we could get it to run on its own again

    Thanks!
    SkyCaptain

  4. #124
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    That's good information. What happens when you set switches to run from original ROMs rather than Tynemouth ROM? If you get the 'choppy screen updating' then you may have some bad ROM sockets especially the Editor socket at D8.

    I assume you cleaned the keyboard?

  5. #125
    Join Date
    Apr 2020
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    Basel-Stadt, Switzerland
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    Yes, the keyboard is also fully cleaned and i took great care not to touch the rubber pads on the bottom of each switch to not damage the conductive coating. The board itself i cleaned with alcohol. It now works pretty well and i love the sound it makes. The return key is the only one that needs confident press for it to be registered. Its probably the most used key so theres that.

    I was a bit too quick to say that everything runs fine on the tynemouth board. I have to yet check if the same behaviour exists in the stock configuration.

    I turn on the machine from cold with the tynemouth board taking over everything, RAM & ROM. It starts showing the exact same symptoms. It wont update until a button is pressed, sometimes not at all.
    When it does, and it goes to "ready", the cursor isnt actually solid as i previously mentioned, its just blinking extremely fast. in this state, when i press the space bar it adds a few spaces. you can make it enter just one space by really lightly and quickly hitting the space bar. The return key sometimes still makes everything act up or ever crash.

    It was solid enough to try and type up a quick program, I ultimately failed doing because the computer crashed again after hitting return. But i was a good 10 minutes into using the machine and this was the longest i have had it running. (it ran longer with daver2's diagnostic chip but thats not really the point)
    When i started it for the like 5th time, right after, everything appeared to be fine. It displayed the start screen text, the RAM, the cursor was blinking at a normal rate, and all the keys worked normally. Return key still needs a firm press but other than that its working.
    I was able to type up a some 20 line program and run it without issue.

    I will now try this 10-20 minutes running before actually checking functionality without the tynemouth board and see if that helps.
    are there parts that actually need warming up to work properly when they are old?

    Also something really weird i found and fixed since i had everything around... the power plug. As it was swiss and definitely not factory made, it was probably added by the importer. It was made out of some black glossy material and was cracked. So i replaced it to find out that the importer basically ignored the ground wire and only hooked up the positive and negative wire. It is now properly grounded again.

    SkyCaptain

  6. #126
    Join Date
    Jun 2012
    Location
    UK - Worcester
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    Only use a PET with the safety earth connected. You get to live then if a live to case short circuit occurs within the PET!

    Can you check the activity on the /IRQ pin (4) of the CPU with an oscilloscope (if you have one) and see what the frequency is please.

    Dave

  7. #127
    Join Date
    Apr 2020
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    Basel-Stadt, Switzerland
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    Hi Daver2, yeah i was pretty shocked seeing that ground wire not being attached to anything. It just didnt occur to me to check that cause who would even do that.

    Sadly i do not have an oscilloscope, will a logic probe do?

    SkyCaptain

  8. #128
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    Sky,
    The logic probe should do if you can differentiate between a pulse rate slower than 50 Hz and pulse rates higher than 50 Hz. The interrupt pulse should br a steady 60 Hz.

    Do you have a spare 6520 PIA in case C7 is a problem?
    -dave_m

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