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What did I do to my PDP-8 today.

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  • I have been struggling with the airflow in the RX01 floppy subsystem which uses Calcomp 140 drives, but had a break-through when I read the manual.
    The "Calcomp 140 OEM Manual" clearly states on page 14 that the drive operates with "positive air pressure system". I simply turned around my two external 80 mm fans to blow into each drive rather then suck the air out. I also added a filter in front of each fan so that the air is filtered. I also verified that in this configuration the squirel cage fan on top of the spindle motor still draws air in through its filter. I think I could probably replace the two external 80 mm fans with a cover plate to close up the holes and solely rely on the spindle motor mounted squirel cage fan, but the external fans in this latest configuration at least do no harm and possible provide a little better airflow for the drive electronics.

    With the RX01 saga out of the way I have now reinstalled the LAB-8/e computer and the RX01 floppy drive subsystem into the H960 rack. Here is a photo:

    IMG_20210228_195954118_30.jpg

    Next will be attaching the fascia panels with the new 3D printed panel clips and then the VR14 display which has nasty cataracts and unknown functionality.

    Actually I might build the SBC6120-RBC and Vince's version of the IOB6120 first. I got all the parts and it shouldn't take too long.

    Best regards
    Tom Hunter

    Comment


    • Last night I had a major set back. I run the "maindec-08-dirxa-d-pb" RX01 floppy diagnostics.
      The diagnostics did not find anything wrong with the first drive, but when testing the second drive it reported some obscure error which may have been a seek error.
      I then rerun the diagnostic test just on the second drive and within a few seconds the RX01 circuit breaker tripped.
      I then noticed that the power supply was very warm. The transformer and the power transitor (with the large heat sink) were both at about 50 - 60 degress Celsius.
      By that time the system was running for about 1 hour in a relative cool air conditioned room (my study).
      Since then any floppy access trips the circuit breaker.

      Best regards
      Tom Hunter

      Comment


      • Originally posted by thunter0512 View Post
        Since then any floppy access trips the circuit breaker.
        Nope - just having power on trips the circuit breaker after a few seconds.

        I narrowed the problem down to the power supply.

        Comment


        • Originally posted by thunter0512 View Post
          Last night I had a major set back. I run the "maindec-08-dirxa-d-pb" RX01 floppy diagnostics.
          The diagnostics did not find anything wrong with the first drive, but when testing the second drive it reported some obscure error which may have been a seek error.
          I then rerun the diagnostic test just on the second drive and within a few seconds the RX01 circuit breaker tripped.
          I then noticed that the power supply was very warm. The transformer and the power transitor (with the large heat sink) were both at about 50 - 60 degress Celsius.
          By that time the system was running for about 1 hour in a relative cool air conditioned room (my study).
          Since then any floppy access trips the circuit breaker.

          Best regards
          Tom Hunter
          I have now fixed the power supply. It turned out to be a 2 uF 660 VAC capacitor with a dead-short. Here is the relevant page from the schematic with the cap circled in red:

          RX01 PS_small.png

          It is not clear to me what the stand-alone cap on the otherwise un-connected transformer windings does, but it affects the 9.5 V rail.

          Now the RX01 is full operational again I wonder about the relatively high temperatures I see on the power supply's transformer (45 degrees Celsius) and the main switching transistor's heat sink (70 degrees Celsius) with a room temperature of about 23 degrees Celsius.

          Could those of you with an RX01 please check if their transformer and heat sink are at similar high temperatures after running for 20 minutes?

          Thanks and best regards
          Tom Hunter

          Comment


          • Tom,

            My guess about the isolated transformer secondary and capacitor would be a harmonic filter of some description.

            If the capacitor goes short circuit - this will cause a high current to flow in (what is effectively now) a short circuit transformer winding.

            This large secondary current would be 'reflected' as a higher-than-normal primary current - thus causing the circuit breaker to isolate the powers supply unit (or a fuse to blow).

            Dave

            Comment


            • isn't this a ferroresonant transformer?

              http://www.electroncoil.com/ferrores...ansformers.php

              I found this in many DEC transformers. Like the TC08 power supply or even a pdp8/a. I also had a bad capacitor in one of those.

              Comment


              • That is most likely a ferro-resonant circuit. The capacitor and winding is tuned to be able to stabilise the output voltage. DEC used them a lot before they converted to switching power supplies. It was very common to have a PCB containing AC cap there. Depending on when the PCB ban happened. I think equipment after mid seventies usually have PCB free AC capacitors. I have replaced all the PCB containing AC capacitors in any running equipment I have. I don't want them to explode.
                www.datormuseum.se

                Comment


                • The transformer is a constant voltage model. It has a quite big air gap. That capacitor saturates the core which is limiting/regulating the output voltage. Without the capacitor the output voltages will be too high.

                  45 degree Celsius is a quite normal temperature for a transformer. But with a too high voltage on the output the heat sinks will be hot as well. So it is important to replace the capacitor with the right capacity.

                  Regards, Roland
                  WTB: Case for Altair 8800 ...... Rolands Github projects

                  Comment


                  • Thatís one of the references I was trying to find.

                    I suspect the voltage will fall not rise if the capacitor fails open circuit though.

                    Dave

                    Comment


                    • I picked up some hex Omnibus boards that had the gold fingers cut off as a source for hard to find ICs. I was a little disappointed to find that a pair of boards was an FPP8-A. I really would have liked to add them to my PDP-8/a.
                      Member of the Rhode Island Computer Museum
                      http://www.ricomputermuseum.org

                      Comment


                      • Originally posted by m_thompson View Post
                        I picked up some hex Omnibus boards that had the gold fingers cut off as a source for hard to find ICs. I was a little disappointed to find that a pair of boards was an FPP8-A. I really would have liked to add them to my PDP-8/a.
                        Bummer! Sounds like a great resource though. The FPP8-A docs do a good job of documenting the contents of the programmable parts, but there are a lot of them, so making them machine readable (that is, typing them back in) is a big job. Maybe it would be possible to image the parts in your damaged one? Or has this been done already somewhere?

                        (The 128K MMU could use similar imaging, if someone is sitting on a damaged one.)

                        Vince

                        Comment


                        • Originally posted by m_thompson View Post
                          I picked up some hex Omnibus boards that had the gold fingers cut off as a source for hard to find ICs. I was a little disappointed to find that a pair of boards was an FPP8-A. I really would have liked to add them to my PDP-8/a.
                          Too bad there is not some way to replace those fingers to make those cards work again.

                          Comment


                          • Originally posted by tradde View Post
                            Too bad there is not some way to replace those fingers to make those cards work again.
                            I could cut the fingers of a RK8-E and use those.
                            Member of the Rhode Island Computer Museum
                            http://www.ricomputermuseum.org

                            Comment


                            • Originally posted by m_thompson View Post
                              I could cut the fingers of a RK8-E and use those.
                              No! I was just thinking some type of 3d printed wrap around could be made to have the new
                              fingers (not sure where to get those) attached and then have wires on the top end to be soldered
                              to the lands on top. Probably not possible but just an idea.

                              Comment


                              • I am hanging out with Vince in his shrine to the PDP-8. Every time I visit I see something I have never seen before.

                                We scanned in some of my Straight 8 diagnostic documents that we have not found anywhere else on the internet.

                                We booted Serial Disk using the new boot loader I wrote that is based on the help loader on "real hardware".

                                We worked on the issue where Vince's 32k CMOS memory board does not work reliably when equipped with the Alliance 62256 part but does work with the Cypress version. I believe we have identified the issue. Now to come up with a fix for it. This is Vince's story to tell but I will say that replacing the pair of 74ALS245 with 74LS245 chips (E1 and E2) makes it behave a little better. This is not a fix so don't order any chips just yet. With the Alliance memory chips and the 74ALS245 parts you are lucky if it runs well enough to boot OS/8. With the 74LS245 we can (usually) boot OS/8 and run memory tests for a while. But it does give errors and eventually crash.

                                50% of the time it works every time!
                                Doug Ingraham
                                2nd owner of Straight 8 SN1173
                                5 other PDP-8's including an 8/i and a DECSet 8000
                                SOL-20

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