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PET 4016 power in from transformer to Jumper 8

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    PET 4016 power in from transformer to Jumper 8

    I'm trying to revive a PET 4016. I've found the voltage regulator schematics, but i cannot find the expected voltages IN to jumper J8 from the transformer.

    connector from transformer to J8:
    1 2 3 4 5 6 7 8 9
    brn red blk red brn blk key blue blue

    2 and 4 go through the filter capacitor.

    what i measure:
    1 to 5 = 18 vac
    8 to 9 = 17 vac
    3 to 2 or 3 to 4 = 0.5 vac

    I think 2/3/4 should provide 5vac?

    any help is appreciated.


    Between pins 2 (positive) and GND (i.e. across C17) you should see about 9 Volts DC (not AC).

    +9V (unregulated) is produced by CR1 and CR2 and then sent back out of connector J8 on pin 4. It then goes to the large external smoothing capacitor and back into the board again (via J where it is fed to the two (2) +5V regulators.

    Although (whilst writing this) something is 'needling' me about pin 4...

    I will give you a definitive answer when I get home and have a look for sure...

    Ah! I see you have also mentioned pins 2 and 4 in your post.

    It may just be that your meter is set to AC rather than DC. You are not reading the transformer output at these points but the rectified output.



      Thanks for the reply.

      Note that I am just trying to verify the transformer voltages and connector positions at this point. Not connected to the pet board yet.


        Originally posted by Dokken View Post
        Thanks for the reply.

        Note that I am just trying to verify the transformer voltages and connector positions at this point. Not connected to the pet board yet.
        Dave was assuming that it's connected to the board.

        Your readings are fine; pin 3 is effectively not connected to either transformer winding. In any case you would not see 5VDC anywhere; if it were connected you would see ~9VDC across the capacitor (1-4 and 5-4 rectified) as Dave pointed out.


          True, I was...



            The rectified DC voltage will be about (1.4*ACV)-Diodes. If half wave the diode is about 0.6V. If center tapped full wave, the Diode is 0.6V. If a full wave bridge ( 4 diodes ) Diode is 1.2V.
            5V usually uses a center tapped full wave, so at the filter cap would be about 8.5V to 9V. This would be a voltage to the center tap of about 6V.


              Originally posted by Dwight Elvey View Post
              ...5V usually uses a center tapped full wave, so at the filter cap would be about 8.5V to 9V. This would be a voltage to the center tap of about 6V.
              That's about right; the no-load AC voltage of the 5VDC is 1/2 of the 18VAC between pins 1 and 5.

              And a correction: pin 3 is ground and connected to the transformer, but not to the DC connections 2 and 4.

              In any case, the readings are correct; time to plug it in and look for smoke


                Thank you so much for your comments. FYI this is an 8032080 board.

                Plugged it in, and i get the this on the display.


                Checking J11 (raw power), schematics say 1/4/6 should be +9vDC. I get + 7.5vDC.

                on J10 (raw power), schematics say 1/2 should be -5vDC. I get -13vDC.

                on J10 (raw power), schematics say 4/5 should be +16vDC. I get +21vDC.

                Thanks for any other test points or ideas on what's going on.


                  J10 1/2 should be -9V unregulated.

                  The voltages are a little high however.

                  Check for the correct dc voltages after each regulator.

                  Note that there are two (2) +5 Volt lines in addition to the +12 Volts and -5 Volts.

                  I would also check the dc voltage on the monitor board after the regulator as well. What monitor board revision have you got as it appears this PET has been upgraded if it has an 8032 main board installed.

                  Do you have access to an oscilloscope to be able to look at the H and V drive signals on J7? I suspect a monitor fault with the display, but checking the drive signals will tell us for sure.

                  The display is indicating a good ‘random’ pattern from what I can see, indicating that the video circuits are OK. But, the CPU appears not to be executing instructions properly - but we will get to that later...



                    Originally posted by daver2 View Post
                    ...What monitor board revision have you got as it appears this PET has been upgraded if it has an 8032 main board installed.
                    I think it's a 4016 'Fat 40' which use a 'universal' 80/40 board.


                      Originally posted by Dokken View Post

                      Plugged it in, and i get the this on the display.

                      Until you get the correct DC voltages, and verify that the horizontal and vertical drive signals are good at the J7 connector, it would be prudent to disconnect the J7 video connector to the video board to avoid any damage to the video board and CRT.

                      The schematics for your computer are here:


                        Good call about disconnecting the monitor Dave. I forgot that one...

                        Would be sensible to post the link configuration for the main board to make sure everything is correct.

                        Where did you get this machine from? Just thinking that the previous owner tried to upgrade the PET and failed or whether the machine was built like this (and worked) in the first instance.



                          The +5 and -5 are +3.8 and -3.4 regulated. any thoughts on where this variance would originate? they're going through different regulators? and the -11 unregulated coming out of bridge rectifier CR2 should be -5. looks odd.

                          voltage schematic with my readings:

                          this pet came third hand but was originally at a local recycler. no idea of the history.

                          I keep the monitor connector disconnected when probing, thanks for that advice. there is a power lead up to the monitor area directly from the transformer, i think. i have not looked in the monitor area yet.

                          I do have a scope and will look at the video connector signals as soon as i can.

                          thank you both so much for your help. i really hope to get this beauty working.
                          Last edited by Dokken; February 19, 2020, 03:45 AM.


                            I suppose the obvious question to the voltage deviation is "does your multimeter work correctly"?

                            Measure the dc voltages with your oscilloscope to get a comparison.

                            >>> thank you both so much for your help. i really hope to get this beauty working.

                            People less able than yourself have managed to resurrect machines in much worse condition than yours. Believe me!



                              As for the +5V outs, many older regulators required more than 2.5V head room. Not getting 9V where it is expected on the raw DC is an issue. It looks like the filter capacitor is missing or failed. One should use a scope to see what things are doing.
                              What kind of current is the load using. Maybe things are just loaded down.
                              Last edited by Dwight Elvey; February 19, 2020, 06:55 AM.