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What did I do to my PDP-8 today.

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    Originally posted by m_thompson View Post
    We made LTspice simulation files for the H724 power supply so we could understand how it works.
    I've never seen a H724 in a PDP-12? Is your H724 LTspice sim file available for download?
    WTB: DEC PDP8-E rack header panels, AD8E, A008, A231, A232
    TA60, TM8-E, PC04, PRS01, BC80J, BC80M cable (RL8A cable)

    Comment


      Originally posted by kb811 View Post

      We have the h724a (230V, 47-63Hz, 5A) version. I don't remember seeing anything about jumpers - I will check the schematics.
      After studying the schematics the only difference I can see in the H724A is that different taps are used on the two transformers and different outlets are supplied. Ok, the parts list also shows a different circuit breaker which makes sense. In the H724 only half the primary is used. I do need to check the output of the secondary but no harm will occur if someone changed it. It just wont have enough voltage to provide to the output regulators. If your US version blew the primary you could rewire it to use the other half and continue computing.

      The resistor that fell off the A2 board is a 2 watt 82 ohm which is part of the power ok signal generation. It is part of a pullup of a 5.1 volt Zener circuit that generates a separate 5 v reference for comparison purposes. If this fell out there would be no power ok signal and the machine would become an earth anchor.

      I am thinking the issue was thermal cycling. There is a slight discoloration of the PCB underneath the resistor and the foils it attached to are debonded right at the lead connection point.

      The +15 volt fuse is popped. There might be more wrong here than I think. The transistors in the +15 section seem to transist and the diodes seem to still diodes and nothing else is burned so

      The jumper on J1 is missing. The jumper on J2 is wrong, probably it should be on J1 and the J2 jumper is missing. J1 and J2 are the interlock jumpers. The J1 connector block is missing one of its mounting ears so it wont stay in the backplate. For now it doesn't matter but I will need to see if I can find a matching housing. Are those style of connector still being made?

      J4 which is the rear omnibus power connector is slightly damaged. The clip that holds the mating connector in place is broken off on one side.

      One fan grill screw is missing and one is loose and I can't get it to tighten up. Since the fans are different I am betting that one failed and got replaced, poorly. I don't know if they even spin up.

      I have all the caps reformed now and no issues. I thought I might explain my statement about not using variacs. The reason is that with linear supplies you often cant reform the output caps by providing low voltage AC to the input. This does work with the input filter caps so it is not entirely bad. Lets take the case of the H724 here. If the +15 volt supply doesn't come up then the LM723 that controls the 5V supply won't wake up. And if the +5 volts supply doesn't wake up then the LM723 that controls the -15 won't wake up. What can happen is that until you have pretty significant voltage on the AC side the control logic is not awake and you don't necessarily get adequate leakage through to reform the output filter caps. Once the threshold voltage is reached then stuff wakes up and tries to regulate which causes the significant inrush you were trying to avoid in the first place. Better to disconnect one side of each cap and reform them one at a time with a CC/CV bench supply set to low current limit.

      I think that is enough fun for today. I will jumper the interlocks, borrow the spare key from the Straight 8 and see if turning it on with no load lets any smoke out. I will report on the results next time.
      Doug Ingraham
      2nd owner of Straight 8 SN1173
      5 other PDP-8's including an 8/i and a DECSet 8000
      SOL-20

      Comment


        Originally posted by DougIngraham View Post
        I think that is enough fun for today
        You were working hard on that whilst I was updating my original post
        It sounds like the H724 you have acquired has had a rough life.

        One of the fans on the H724A I have may also have been replaced at some point (but it is still contemporary with the original) - it is held in with nuts and bolts rather than the original screws. They're both somewhat over-noisy and one has a fair bit of rattle. Otherwise, appearance wise, this H724A looks to be in very good shape. It had been powered up just before I got hold of it, so I did have it powered up for a few minutes - all the voltages looked correct.

        I am also going to check and reform the capacitators individually.

        One thing I noted when I had it powered up is there is a (not unexpected) fair amount of earth leakage; but it did have a tendency to take things over budget for the house RCBO to which sockets and so on are connected (but at least it meant I have tracked down another "offender" in the house recently)!.

        Tom sent me the following link about that and the H724/H724A: https://avitech.com.au/?page_id=3585

        I have had some of the PCB's referenced on that page made (JLCPCB minimum order count was 5). I haven't tested/test fitted one yet, but it means I have a couple of spares and they're "letter" sized (theoretically cheap to post)...




        Last edited by kb811; November 14, 2021, 02:42 AM.

        Comment


          Originally posted by kb811 View Post
          You were working hard on that whilst I was updating my original post
          It sounds like the H724 you have acquired has had a rough life.
          I was adding to that post all day long.

          When I first looked at it I didn't see all the little things wrong and thought it had lead a fairly sheltered life. It was pretty clean inside. The one place that was dirty was the top side of the A1 control board. It had a black powder sticking to it. Water+detergent followed by an alcohol rinse removed it easily. I don't know what it was and I don't see it anywhere else. I am sure it will show up somewhere else in the machine.

          Thanks for the link to Malcom's avitech page. A while back I spent a few hours reading it. I remembered reading that particular page once I started re-reading it. Funny how memory works.
          Doug Ingraham
          2nd owner of Straight 8 SN1173
          5 other PDP-8's including an 8/i and a DECSet 8000
          SOL-20

          Comment


            Well.... after a nice warm October, November has turned cooler and we have had our first snow. So I have migrated from the garage to the basement. Some cleaning and re organization was needed and I discovered that some mice had also migrated to the basement. Hopefully I found and sealed the egress point. They must have been recent arrivals since there was no damage, just dirt. Looking at my DEC note book shows that I have not used the PDP8E for more than a year, even though I spent a lot of time in the basement during the covid isolation. I was working on ( and still need more time) programming a Raspberry PI to convert my Friden code (and voltages) to ASCII. I want to be able to use the Friden with my CP/M and PDP8 machines. I have the gotten the print function to work, and still need more for the keyboard function. The biggest problem is the high voltages the Friden uses and the fact that the Friden keys do not have all the ASCII keys. Some of them can be program using two ASCII. For example a ! is a period, backspace, and apostrophe. Yet some are can not and need to use a substitute.

            That being what it is, I turned on the PDP8E and no smoke came out, a good sign. I saw Roland's Star Trek sign on and Kyle's serial disk logged on. All is well with the world and God is in his heaven.

            Over the summer I came in possession of two UART cards for the PDP8E. A M865, which I think is a 110 baud current loop board and a M8650YA. My first project was to set up the M8650YA to work with my CIT-101 monitor at 9600 baud. Should be an easy task. I changed the jumpers for the address (03/04) and for the baud rate. I also replaced the 6.8uF capacitors. Then optimistically I plugged the board into the PDP8E and connected the CIT-101 and..... nothing happened. Surprise. So..... out came the schematics and the documents. My plan was to see if the baud rate and address was correct, but after starting the PDP8E, data bits 0 and 1 appeared to be stuck on. Rats, maybe all is not so well with the world. Turns out the problem was on the front panel board. The data bits were stuck on regardless of how the Indicator Selector Switch was set. A DEC 380 popped. My supply of them is getting low and will have to get some more soon, but don't remember where I purchased them.

            Now I'm back to playing with the M8650YA. Let you know what happens, Mike
            Last edited by Mike_Z; November 17, 2021, 07:51 AM.

            Comment


              Had a chance to do some testing today. I checked the transmit flag flip flop, using 6040, 6042. 5000. This set and reset the flip flop. I could see a 3.5 uSec high followed by 200nSec low. This tells me that the flag was working and that the transmit address 04 was working. I checked the baud frequency and found 153.8 Kc after dividing. This is just about correct, I calc'ed 153.6Kc. Then I noticed that the delay line seemed a little loose. An ohm check showed it may be open. I have another board and I measured about 200 ohms 1 to 2 and 2 to 3. Removed the delay line and the #3 wire was broken. I dug into the plastic case and soldered a small wire to the stub, but still it appeared open. Maybe #1 and #2 are also broken. I'll try and dig into the case and add some new wires. The schematic says this is a 30 nSec delay. What is in this thing? If I can not repair it what can be used as a replacement? Thanks Mike

              Comment


                After looking at the H724 power supply some more I decided I needed to pull the heatsinks mounted to the fans so I could get a better look at how the fans are mounted and locate the missing nuts and bolts that might be rattling around inside. DEC did not make this easy and I am not certain I understand how they assembled it originally. Now that I have access to the fans I am going to find suitable replacements and replace both of them. One of the fans (the remaining original I think) makes a ticking sound when I give it a spin. The other did not use aerodynamic principals when the blades were designed and I am certain they will be noisy. I don't want to have to do this a second time. I will need to take some photos at some point because I don't think I can adequately describe how this is put together.

                I have looked for a procedure and come up empty. There were 4 screws (or are they bolts?) missing. Three of these were inside the power supply. There is also a missing standoff that I know was in there and still is somewhere. Probably trapped in some wires. The three missing bolts inside were probably not replaced when the fan was changed. One of the bolts inside used a flat washed rather than the inside lock washer used everywhere else. With that in mind I think I need to look for a lock washer as well.

                It is stuff like this that makes you want to slap someone silly.



                Doug Ingraham
                2nd owner of Straight 8 SN1173
                5 other PDP-8's including an 8/i and a DECSet 8000
                SOL-20

                Comment


                  "this is a 30 nSec delay. What is in this thing? If I can not repair it what can be used as a replacement?"

                  From the 8E era (early 1970's), it's most likely a simple L-C circuit (series inductors, parallel capacitors). If you can't repair it, maybe you can salvage a replacement part from a 'scrap' module? If you're willing to experiment, maybe you can adapt a modern silicon delay line as a replacement part?
                  WTB: DEC PDP8-E rack header panels, AD8E, A008, A231, A232
                  TA60, TM8-E, PC04, PRS01, BC80J, BC80M cable (RL8A cable)

                  Comment


                    Doug, when I first obtained my PDP8E, it needed IMG_0279.jpg a complete cleaning. It lived in a machine shop for many years and was full of dirty machine oil, coolant, dirt and metal chips. The heat sinks and fans came out as a unit (see picture). I have a few more pictures that I took while repairing the unit. I can post a few more if you like. Almost 1/2 of the pass transistors were bad, not to mention all the cleaning needed.

                    FTC, I opened up the delay line more and can see inside. There is an inductor made of very fine wire. The other leads were not broken at the case, but must be broken where the fine wire connects to the 'large' wire. I'm going to try and remove the inductor and repair the wires. It's already bad, can't make it worse. Also I found that Roland Huisman has a modern replacement. It is listed on GitHub. It was posted a couple years ago. I would like to make one of these. I have never worked with a circuit board making company. Maybe someone has done this already and has a few CB's that they would part with? Or, I can order some boards and share the extras? I will have to learn how to deal with a company. Looking around I found OshPark, but am unsure of what I need to send to them. Will need to investigate some more. Thanks for the help, Mike
                    You do not have permission to view this gallery.
                    This gallery has 1 photos.

                    Comment


                      Originally posted by Mike_Z View Post
                      [FONT=Consolas]... A DEC 380 popped. My supply of them is getting low and will have to get some more soon...
                      Someone in France has a couple that look legit. The guys at UTSource claim to have over 33,000 in stock?, but their datasheet is bogus.
                      WTB: DEC PDP8-E rack header panels, AD8E, A008, A231, A232
                      TA60, TM8-E, PC04, PRS01, BC80J, BC80M cable (RL8A cable)

                      Comment


                        Well..... I removed the inductor from the plastic case. It was a little difficult, The way it was made was the problem. Apparently (this is only an inductor) the inductor was wound on what looks like a resistor core, yet I can not measure any resistance. Once it was wound it must have been placed in the case and epoxied over. The epoxy held the larger wires and the core, so when I removed it the connection broke. AND I can not determine how the inductor was wound. I assume the #1 and #3 terminals, at the ends are the ends of the inductor.. But I can not determine how the center connection #2, which is grounded on the schematic, was connected. Anyway this part is toast. I'm investigating Roland's replacement part and how to get a CB made. Thanks for the help, Mike.

                        Comment


                          Thanks Mike. Mine is not nearly that dirty.

                          If I understand correctly, you just removed the 8 screws on the outside of the case, the ones that also hold the fan grills in place and were able to remove the fans and heat sink assembly as a unit? I can see that might work if all the screws on the heat sink side of the fans were in place. In my case there were three missing and that caused a bit of confusion on my part.

                          Thanksgiving road trip starts tomorrow, no 8 hardware work until I get back. Some interesting boards on EBay I may have to bid on.
                          Doug Ingraham
                          2nd owner of Straight 8 SN1173
                          5 other PDP-8's including an 8/i and a DECSet 8000
                          SOL-20

                          Comment


                            Doug that sounds correct. I can not remember having that much trouble, unless your puzzle starts out with some pieces missing. Have a good Thanksgiving. Are you still driving that electric rocket? Good luck bidding. Mike

                            I ordered a bunch of Roland's M8650 delay circuit boards from Osh Park. Probably more than I will ever need. When I receive them, if anyone would like one let me know and I'll gladly mail you one. Thanks Mike

                            Comment


                              Back from my trip. Spent some time trying to figure out the best fans to order. They are 115 volt AC 50/60 hz and the size is pretty easy. 119 mm square (4.69 inches) with mounting holes spaced 105 mm (4.13 inches) and a thickness of 38 mm (1.5 inches). The problem is how many CFM do I need? Neither of these fan manufacturers exist so it isn't possible to look them up on a datasheet. Also, I don't trust that they were originals. What I decided to do was go with the watts consumed as an upper limit. Both draw in through the side with the label like they are supposed to but one turns clockwise and the other turns counter clockwise. DEC specifies just one part number for both fans. One fan has 7 blades and the other has just 6. Pretty much all the replacement fans are 5 blades. The fans both claim to be around 20 watts. Currently made fans that consume 20 watts do over 100 CFM but are louder than I would like. And as many of you know, these are not quiet machines because of the fans. A few years back I replaced the four fans in one of my 8/a's with the quietest fans I could find. They are 52 CFM at 25 db consuming 8 watts. This was plenty of air for cooling the Omnibus boards but not really enough to cool the power supply section. After several hours of operation the corner of the machine with the transformer is almost uncomfortably warm. But it is quiet enough that it doesn't bother me sitting right next to it. This is kind of what I am looking for here. Probably around 15 watts each will be a good compromise. If it isn't enough the thermal breaker will open and I can increase it. These fans are between 15 and 30 dollars each for the ones I am considering so not terribly expensive all things considered.

                              Mike, I believe my problem is due to the less than excellent technician who last replaced the fan(s). The original assembly was probably easy because the wires to the fans had not been cut too short. And yes I still have the 2013 Tesla. Its my beater now since it has over 142000 miles (170000 km). Still the best car I have ever owned.

                              I guess I never took a delay line apart but I always assumed that it was just a coil of wire the length needed to give the required delay with a driver and receiver. A 30ns delay would be about 20 feet of wire since the speed of light through a copper conductor is about 2/3 that of light in a vacuum. It would most likely be even shorter than that because there would be some delay in the driver and receiver electronics. You would want the least inductance possible so probably just wound over an empty bobbin.
                              Doug Ingraham
                              2nd owner of Straight 8 SN1173
                              5 other PDP-8's including an 8/i and a DECSet 8000
                              SOL-20

                              Comment


                                Hi Doug,

                                I've also stumbled into the question about which fans to use in several machines. I could not find any information in the DEC documentation about this. I agree on using the same power usage and same blade count. Or just find the data sheet from the original fan. But what if the fan's already have been replaced in the past? In my low model PDP11/10 were two different type of fans. 3 and 5 blade and also one with less power. I replaced both with the higher power an 5 blades. For my 11/20 machines I had old non original fans... These made so much noise that I decided to replace all of the fans. Now it is comfortable to sit next to these machines and run are quite cool.

                                Maybe it is a good idea to make an inventory for these old DEC machines and which original FAN's were used. Try to find the original specs so one can search for a good replacement.

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

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