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TRS-80 Model 4P: No monitor and disk activity on power on

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    TRS-80 Model 4P: No monitor and disk activity on power on

    I recently acquired a TRS-80 Model 4P in rather rough exterior condition, but after some cleaning with an air compressor the internals were in surprisingly good shape. However, when I try to power it on the tube does not light up and the disk drives do not seek. Here is a picture showing how the unit is indeed on but the screen is completely dark:

    https://i.imgur.com/JRW3aHi.jpg

    It was sold to me this way and I had assumed it was due to a loose internal connection, but even after I opened the unit up, checked every connection I could find, and firmly reseated all the socketed chips on the motherboard, it still does not give a raster or seek the disk drives. I'm not overly familiar with the hardware of the TRS-80 (I have a regular Model 4 that is also faulty, but that one at least displays a raster) so I don't really know where to start in diagnosing this.

    #2
    I am no 4P expert but I can suggest grabbing a copy of both the service manual technical manuals for the 4P and start looking at the bootup procedure and where to start looking for issues, personally I would disconnect the floppy drive cable (flat white one) and see if it will boot.

    service manual for 4p https://drive.google.com/file/d/1aKU...ew?usp=sharing
    Technical manual for 4P https://drive.google.com/file/d/1gpX...ew?usp=sharing
    Texas Tandy Blog: https://texastandy.blogspot.com/, My Discord " Tandy and TRS-80 Owners" (https://discord.gg/VCDPG8j)
    Gaming: i7-8700K, 16GB, 1TB SSD, RTX 2060, Dual 27" LG IPS Monitors.
    Vintage: TRS-80 MODEL 4, 128K RAM, Internal: 360K FDD, Gotek USB FDD & FreHD HDD Combo, External: 360K & 720K FDD, RS232-WIFI MODEM, DWP-210

    Comment


      #3
      Originally posted by Patrick Bureau View Post
      I am no 4P expert but I can suggest grabbing a copy of both the service manual technical manuals for the 4P and start looking at the bootup procedure and where to start looking for issues, personally I would disconnect the floppy drive cable (flat white one) and see if it will boot.

      service manual for 4p https://drive.google.com/file/d/1aKU...ew?usp=sharing
      Technical manual for 4P https://drive.google.com/file/d/1gpX...ew?usp=sharing
      These manuals are a huge help, thank you! This poor 4P was not stored in a very nice environment given the rusty screws and severe yellowing so I'm hoping I can give it a new life.

      Comment


        #4
        I ended up finding a severely blown fuse on the main power board so replacing that is certainly a good start when it comes to diagnosing this computer.

        Comment


          #5
          look a blown fuse indicates a short, where that isa the non-obvious answer, does it blow as soon as you turn o0n power? does it blow if nothing is connected to it.

          did you inspect all the wirring, contacts and a possible sign of a rsistor or cap that are damaged. ?



          MyAARPMedicare

          Comment


            #6
            Originally posted by Sanders443 View Post
            look a blown fuse indicates a short, where that isa the non-obvious answer, does it blow as soon as you turn o0n power? does it blow if nothing is connected to it.

            did you inspect all the wirring, contacts and a possible sign of a rsistor or cap that are damaged. ?



            MyAARPMedicare
            I haven't found any obvious signs of damage aside from some corrosion and a crusty capacitor on the motherboard, so I don't have a clue what blew the fuse on the power board. It was already blown when I received the computer, it seems.

            Comment


              #7
              Well when I put in a new fuse and hit the switch it blew instantly, so there's definitely a short somewhere. The problem is that I have no idea where, and no idea of even where to start. Electrical work is not something I'm experienced with.

              Comment


                #8
                Blowing the big fuse on the power supply would indicate something is wrong on the power supply itself. Have you checked for shorts between the two legs of A/C? Line filter caps are supposed to fail open, but if one fails shut (as it did on an IBM project I worked on 3 months ago) the main fuse pops immediately.

                Comment


                  #9
                  Second on the filter caps, usually they blow in spectacular (and smelly) fasion to protect the fuse. Probably the first thing you will want to replace.

                  Comment


                    #10
                    Have you tried disconnecting everything from the power supply except for the line power? If it still pops a fuse, you know for sure the issue is on the power supply itself.

                    Comment


                      #11
                      They're an easy replacement, lookup Rifa capacitor explosion (they're notorious) to get an idea of what they look like. You can test the system with them removed. Don't tell anyone, but I pulled the two off of my 4p a year ago and haven't gotten around to replacing them.

                      Comment


                        #12
                        Originally posted by TheDrip View Post
                        Blowing the big fuse on the power supply would indicate something is wrong on the power supply itself. Have you checked for shorts between the two legs of A/C? Line filter caps are supposed to fail open, but if one fails shut (as it did on an IBM project I worked on 3 months ago) the main fuse pops immediately.
                        Hmm, the big 0.22uF Rifa cap does look a bit worse for wear, removing that is probably a good start.

                        https://i.imgur.com/fq3E25q.jpg
                        Last edited by itsvince725; April 16, 2021, 05:06 PM.

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