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Cbm Pet 2001 restoration

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    Originally posted by daver2 View Post
    Which capacitors did you change?

    Dave
    Daver, i changed C751 and C652 because they were a little swollen and a yellowed...

    the screen is enhanced by turning the ring behind the yoke...now must i repair Pet logic board
    because i have always full random charachters on screen...

    Can i use your last pettester version on ud8 socket?
    Thanks again


    Comment


      Originally posted by ilpaninaro View Post
      Update:

      I re-changed yoke with bad crt s' yoke, i turned yoke s' ring mentioned by Dwight and i changed two capacitors....Screen now seems to be better...only little moved on the left...
      This means the two Yokes you had were not the same, in terms of the amount of width they gave, as you had said they were. Probably the yoke that gives the smaller width image came from a TV set that used a higher than 12V power supply, and this is why it gave the reduced width. So the original Pet yoke is the better one to use.

      Comment


        The PETTESTE2KV04 EPROM goes into the Editor socket ($E000)
        If 8032 PET then UD7 socket
        If 2001-N then D8 socket

        Comment


          Update:

          I tryied Pettester but doesn't works....i can see only random chars screen.... if i touch some ic ram they are very hot...

          Comment


            If the ICs are hot this could indicate that the voltage rails are too high. Please measure and report all of the voltage rails.

            Have you performed the basic checks before using my PETTESTER?

            Check the /RESET, /NMI, /IRQ, SYNC and PHI2 pins of the CPU and see what is going on. I have posted these steps before on numerous VCFED Commodore PET repair threads.

            After that, the Kernel ROM (or address decoding) could be dead. You need to use a NOP generator to proceed.

            Have you considered purchasing a ROMulator (or similar) to help diagnose your problems?

            Dave

            Comment


              Originally posted by daver2 View Post
              If the ICs are hot this could indicate that the voltage rails are too high. Please measure and report all of the voltage rails.

              Have you performed the basic checks before using my PETTESTER?

              Check the /RESET, /NMI, /IRQ, SYNC and PHI2 pins of the CPU and see what is going on. I have posted these steps before on numerous VCFED Commodore PET repair threads.

              After that, the Kernel ROM (or address decoding) could be dead. You need to use a NOP generator to proceed.

              Have you considered purchasing a ROMulator (or similar) to help diagnose your problems?

              Dave

              I checked voltages on ic rams and i have these measures in every 4108 ram ic:

              Pin 1=-4,98 V
              Pin 8=16 V
              Pin 9= 5 V

              It's ok?
              Thannks

              Comment


                No, not OK.

                Pin 8 should be +12V relative to pin 16 (0V/GND). Not +16V.

                Check the voltage across either C25 or C62 (the DC input voltage to the +12V regulator VR5) and across C24 (the DC voltage from the +12V regulator VR5).

                Dave

                Comment


                  Originally posted by daver2 View Post
                  No, not OK.

                  Pin 8 should be +12V relative to pin 16 (0V/GND). Not +16V.

                  Check the voltage across either C25 or C62 (the DC input voltage to the +12V regulator VR5) and across C24 (the DC voltage from the +12V regulator VR5).

                  Dave
                  Voltage across C25 and C62 it's 21V

                  Comment


                    Also, make sure you are using a circuit ground and not just what you think is a convenient ground. They may be different than you'd think, for a number of reasons.
                    Dwight

                    Comment


                      I should mention, for TO3 type regulators, it is not uncommon to see the case not well connected to the circuit through the screws holding it. This is especially true for case where moisture has caused other issues on boards.
                      Connectors also fail.
                      Dwight

                      Comment


                        >>> Voltage across C25 and C62 it's 21V

                        That seems high to me. It should be round about -9V (and negative to boot). Why are you reading a positive voltage?

                        What about the voltage across C24 I asked for? This is the more important reading !

                        Dave

                        Comment


                          Originally posted by daver2 View Post
                          >>> Voltage across C25 and C62 it's 21V

                          That seems high to me. It should be round about -9V (and negative to boot). Why are you reading a positive voltage?

                          What about the voltage across C24 I asked for? This is the more important reading !

                          Dave

                          Sorry Dave,

                          C24= -16,2 V
                          C25 and C62= -21V

                          Comment


                            Sorry, my bad. I was following the wrong signal on the schematic. Note to self, don't try and do two (2) things at the same time...

                            Please ignore my previous posts !

                            Your measured voltages should be positive and not negative. The voltage that is incorrect (across C24) is 16V - it should be 12V. This looks to me as though VR5 is short circuited (or otherwise faulty). As Dwight has already mentioned, if the ground connection to VR5 has been compromised - the voltage regulator will fail to regulate correctly.

                            The input voltage is still high though (+21V) when compared to what it should be (+16V ish). However, the data sheet for the 7812 voltage regulator indicates that the device should handle this voltage (although it will run warm/hot).

                            Can you measure the AC voltage between pins 8 and 9 of connector J8 (from the transformer to the main board) please.

                            It is just possible that with the 12V rail being too high that the DRAM has been destroyed. Brace yourself for this possibility.

                            Dave

                            Comment


                              Also, check out diode CR12 (1N4001) across the voltage regulator VR5. If it has gone short circuit it could be shorting out the voltage regulator!

                              Dave

                              Comment


                                Originally posted by daver2 View Post
                                Also, check out diode CR12 (1N4001) across the voltage regulator VR5. If it has gone short circuit it could be shorting out the voltage regulator!

                                Dave


                                OK, 17 V AC across pin 8 and 9, tomorrow i ll desold CR12 to test it...
                                Where is VR5?? I see only VR6 on board...
                                Thanks so much!

                                Comment

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