[HN Gopher] A tricky Commodore PET repair: tracking down 6 1/2 b...
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       A tricky Commodore PET repair: tracking down 6 1/2 bad chips
        
       Author : todsacerdoti
       Score  : 50 points
       Date   : 2025-04-13 16:15 UTC (6 hours ago)
        
 (HTM) web link (www.righto.com)
 (TXT) w3m dump (www.righto.com)
        
       | kens wrote:
       | Author here if anyone has questions.
        
       | cshokie wrote:
       | I don't personally have the time or inclination to do any
       | retrocomputing, but I love seeing the repairs that people do for
       | them. It's interesting in a way I wouldn't have expected. Thank
       | you for sharing.
        
       | jgrahamc wrote:
       | This is wonderful to read. I'm about to embark on getting my
       | Sharp MZ-80K into good shape for the next 40 years. It still
       | works fine but I bet there are a bunch of capacitors just waiting
       | to die, and solder joints that are waiting for the perfect day to
       | fail.
        
         | SoftTalker wrote:
         | If it works, don't mess with it.
        
           | jgrahamc wrote:
           | The problem with not doing some preventative stuff on these
           | old machines is that in the event of a power supply failure
           | (IIRC the MZ-80K has +12V, +5V and -5V) you can get
           | overvoltage and cause damage to the chips.
        
           | cesaref wrote:
           | The problem is that electrolytic capacitors can leak, even if
           | not used. So, you can store a perfectly good computer and
           | retrieve a ruined motherboard a number of years later. SMD
           | capacitors are bad for this, the MZ80K predates all of that
           | though, and I think only has a few through hole capacitors on
           | the mainboard.
           | 
           | The power supply should also be checked, and in this case,
           | it's safety capacitors which are the risk there, since some
           | of the older ones (RIFAs for example) tend to be hygroscopic,
           | and if they crack they end up blowing next time mains is
           | applied across them. I've not had any problems with power
           | supplies killing the machine with an over voltage, but i've
           | heard this is also a risk.
           | 
           | Anyhow, 45 years or so is time for a recap, and it'll do
           | another 30 years before it's time again...
        
             | genter wrote:
             | To add to this, the electrolyte will leak out and damage
             | the circuit board.
        
       | philiplu wrote:
       | That PET is in amazingly good physical shape. I think the
       | keyboard on my 8K PET back in 1978 looked that bright for at most
       | a year or so, before the plastic overlays started fraying.
        
       | morphle wrote:
       | I have one in pristine condition for sale (in Europe, willing to
       | ship worldwide).
        
       | micheljansen wrote:
       | Amazing that they were able to figure this out. I have a PET that
       | also sometimes boots up into a garbled screen similar to the one
       | pictured in the article. I am usually able to get past it with a
       | few hard resets. I don't have access to (or much knowledge of)
       | logic analysers, but this almost makes it look doable to figure
       | out the problem.
        
         | qiqitori wrote:
         | If you want to go that route, you may find it easier to check
         | your logic analyzer dump side-by-side with the debugger in an
         | emulator (e.g. VICE). Break on the first instruction and then
         | go step-by-step. (Just need to make sure you have the same ROM
         | version, but that shouldn't be too hard.)
         | 
         | However, your problem kind of sounds like a power supply
         | problem. So using a logic analyzer will maybe just produce a
         | different result every time. So maybe check the 12V and 5V
         | rails on an oscilloscope while turning on the computer. (Or
         | maybe it's a problem with the reset circuit, etc.)
        
       | dep_b wrote:
       | That keyboard doesn't look like fun to type on. The one on the
       | VIC-20 and C-64 were pretty okay!
        
       | kmoser wrote:
       | > I typed in a simple program to generate an animated graphical
       | pattern, a program I remembered from when I was about 13 [...]
       | 
       | I'm astounded that anyone would be able to remember such a
       | complicated program after several decades. About all I remember
       | from those days is:                 10 PRINT "HELLO";       20
       | GOTO 10;
        
       | userbinator wrote:
       | _How could the computer be operating well for the most part, yet
       | also completely wrong?_
       | 
       | Anyone who has done any overclocking knows about that. When
       | signals or timing are marginal, intermittentness appears. In the
       | PC world, programs like Memtest86+ and Linpack are great at
       | stress-testing to find marginality, but I'm not sure if such
       | software was as common in the 8-bit era.
        
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       (page generated 2025-04-13 23:00 UTC)