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Hoping to repair my 8250

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    Hoping to repair my 8250

    First off, hello to the group. I've been thoroughly enjoying reading through the threads here. Recently after having been absent from PETs for far too many years, I've recently secured some PETs and a few IEEE drives to play with. Thus far, I've managed to bring both of my PETs back to life, despite any real electronics knowledge. One I determined had a bad bridge rectifier and edit rom, and the other had one bad video ram chip and a defective kernal. So, I was feeling pretty good heading into the 8250 repair... but alas, I'm not sure where to start. I've never worked on one of these before, but that didn't stop me from starting in on it.

    First off, I'll say that my PET I'm using for testing is functioning correctly in that the IEEE port is working fine. The 8032 I'm testing with works fine with my SFD-1001 and my MSD-SD2, so I've ruled out a problem with the PET or cable.

    When I power on the 8250, both drive green lights come on, and the center light comes on red. After a moment, the left drive light goes out, the right drive light stays on and the center light flashes once, alternating between red and green. If I insert a diskette into either drive, when I close the door, the diskette spins briefly. If memory serves, that was correct behaviour for inserting a diskette.

    If I try to do a catalog of the drive, the PET simply hangs. Nothing ever happens beyond that. Reading DS$ results in the same behaviour.

    Now I don't know if this will help, but if I could use my functioning SFD to test chips, that would be ideal, but I don't know if that's possible. Figure it's best to check.

    Looking up the "flash codes" for the 8250, I found that one flash is supposed to represent the (Zero Page 6532 UC1,UE1). So, this is now where it's becoming unclear to me, so perhaps someone could enlighten me here.

    When I look up what a 6532 is, it says that it's a RIOT chip. But then, I thought the RIOT chip was the 6530? I have a little bit of experience with the RIOT in the SFD, as that one was dead for me. So I made up a daughter board based on some schematics found online, and built one of those adapter boards with an eprom on it, found the correct ROM (on the zimmers site), and got that drive going again. So, with the thought in mind that the 6532 doesn't have the EPROM portion in it, I dutifully bought what I thought would work as replacements, those being as pictured.

    s-l1600.jpg

    When I tried these, the chips got boiling BLAZING HOT almost immediately. Needless to say I powered this down immediately. I'm going to take a leap here and say those things are fried, whatever I plugged in there. I put the original chips back in, and the drive returned to it's normal non-functioning behaviour.

    At this point, I thought it would be best to seek some help from anyone kind enough to pity my situation

    So, I guess I'm looking for some perspective and advice as to what I've done, and what I've done wrong, and perhaps where I should go from here. Could I use my SFD to successfully test the 6532's from the 8250? I don't want to blow up my functioning SFD, so better to ask than leap any further. How did I go so wrong with selecting the 6532's that I bought (pictured above).

    Any help or comments would be greatly appreciated.

    I do have some electronics experience (mostly because I can read things on the internet). I'm trying to learn more, but learning this way is a slow process. For test equipment, I do have a scope, but have never used it (figured one day I'd get to the point where I would understand how to use it). The will is there for sure... the skill... not quite there yet.

    Thanks for reading through my troubles.

    Thomas

    #2
    Can you post pictures of the inside of the 8250, include the digital board and also the analog board (thatís the one sitting over the floppy drives).

    Without checking mine, I think there are two 6532 riot chips on the board. Have you tried swapping them to see what happens ? I don't think it will solve anything but interested if there is any change in behaviour.

    the fact one drive light goes out but not the other bothers me as the blink code normally involves all three LEDís

    When you first turn it on and do a print ds$ ... does the PET hang ?

    Andy

    Comment


      #3
      Hi, thanks for the reply. Here are photos of the drive, inside and out:

      sm_IMG_1278.jpgsm_IMG_1279.jpgsm_IMG_1280.jpg

      There are two 6532 chips on the board attached to the lid. I tried swapping them and no difference. Then, I decided to test the two 6532 chips in the 8250, and put them into my SFD. The SFD functioned perfectly fine with the two 6532's out of the 8250. So, I think it's fair to assume that those two RIOT chips are alright.

      With the 6532's back in the 8250, when I power it on it still does the same thing.

      Power on sequence:
      - both drive lights turn green, center light is red
      - the the left drive light goes off, right drive light stays on
      - center light flashes red - green - red - green (endlessly)

      When I power on the drive and my PET, if I do a ? DS$ it hangs the computer. If I try to do a catalog, it hangs the computer.

      Hmmmmm...

      Thomas

      Comment


        #4
        I seem to have a restriction on my ID here on the site which is really slowing down my replies as they have to be approved by the moderator.

        In case my last post has somehow been lost forever, here are the photos of the board again:

        sm_IMG_1280.jpgsm_IMG_1279.jpgsm_IMG_1278.jpg

        Now, on the to progress (not much sadly) report.

        I've made some progress since yesterday, but sadly still basically the same behaviour.

        I've tested the following chips:

        UH3 : 6502 - Found it was defective and replaced it with a known good part
        UE1 : 6532 - Tested ok
        UC1 : 6532 - Tested ok
        UN1 : 6502 - Tested ok
        UM3 : 6522 - Tested ok

        I furthermore used my 6530 adapter board that I have in my SFD (replacing the rom of course with the (6530-47 RIOT DOS 2.7 Micropolis)) to replace the RRIOT 901885-04.

        I also replaced all 8 of the 2114 ram chips with sockets and new confirmed good chips.

        The only improvement that was noted in ALL of that testing was when I replaced the bad UH3:6502 processor. When I did that, the light flashing behaviour continued the same, however, now when I power on the drive, the drive 0 runs continuously.

        Thomas

        Comment


          #5
          So no progress today on this, but did spend much of the day reading what I could find on the internet regarding 8250 problems. One suggestion that I read somewhere was that perhaps one or more of the "glue logic" chips may be the culprit. I guess it's easy enough to test that. Just have to pull the board again and start removing chips and adding sockets and replacing chips. Barring any other thoughts I will likely start looking at that. I'm sure there must be better more intelligent ways of fixing a drive.

          Also read something about checking for AC ripple no greater than 10mv, but I am not sure how to test that (yes, I have an oscilloscope, but have never used it... don't know where to start with that). I see more reading in my future.

          I also read that perhaps the ROM(s) have gone bad on the drive. I have yet to find where I could download them, and what eprom I'd need to replace them with. From my reading today it didn't come off as something that was all that straight forward... or I was just getting cross-eye'd from hours of random searching. Would be interesting to be able to read the ROMs that are in the drive, and compare them to copies of good ones. Something I will have to continue to investigate. That would at least tell me if the ROMS are bad or not. Just have to make sure I don't blow them up trying to read them. Would kind of defeat the purpose.

          I did create a short video that I put up on youtube in case you're at all interested in seeing the behavior of the drive. I've been told that what it's doing isn't the usual flash code. I've never actually seen a normal error code out of one of these drives.



          Haven't noticed the drive running continuously lately when powered on. Why that started happening and no longer does is beyond me. Hopefully more stuff isn't going wrong with the drive. And, one other thing worth mentioning, now, when I insert a diskette into one of the drives, that drive briefly spins, which I think is normal. So I'm going to take that as a good sign!

          Thomas

          Comment


            #6
            Do you have an eprom programmer ?

            If I look at this article, page 9, bottom line there is reference to a flashing error light being done to a faulty GCR chip.

            https://www.vintagecomputer.net/comm...DriveNotes.pdf

            The image can be found here to check you rom against

            http://www.zimmers.net/anonftp/pub/c...050/index.html

            A 2716 eprom should be ok I think as a replacement.

            Worth a try at least.

            Check the other two Roms are ok on your eprom programmer using the images found in the above link. Have you tried reseating them?

            Drive 1 led going out, drive 0 staying on and the error light flashing red/green is very odd and not come across that..... hence try above.

            One other thing, when you power up both the PET and drive, after the error lights starts flashing type

            print ds$ and report back what it does
            Last edited by AndyG; May 1, 2020, 12:08 AM.

            Comment


              #7
              Hi,

              Yes, I have a MiniPro USB programmer that I've used quite a bit in making C64 cartridges, so I'm familiar with the software, selecting chips and programming. Also have a UV eraser for when things go wrong...

              That's a great find on the bottom of page 9. That's sounding remarkably similar to what's happening on my drive. Awesome, thanks.

              From the GCR rom perspective, it appears that it's a 6316 chip, which my programmer doesn't have a profile to read. Been looking for a datasheet for it, but no luck. I take it that an adapter is going to be necessary to not only read it on the programmer but also one to write the code to a replacement chip and to interface that chip to the 8250. I've been searching today to see if I can find some schematics but no luck. Any thoughts on that?

              I think it's a great idea to do the other ROMs as well, but figured I'd tackle this one first since your discovery on page 9 of that document is a bit of a beacon of hope for me.

              As is, when I power the drive on, connected to my PET, if I do a "print ds$" the PET hangs as soon as I hit return.

              Thomas

              Comment


                #8
                Originally posted by COREi64 View Post
                Hi,

                From the GCR rom perspective, it appears that it's a 6316 chip, which my programmer doesn't have a profile to read. Been looking for a datasheet for it, but no luck. I take it that an adapter is going to be necessary to not only read it on the programmer but also one to write the code to a replacement chip and to interface that chip to the 8250. I've been searching today to see if I can find some schematics but no luck. Any thoughts on that?
                Maybe yes, maybe no; try reading it as a 2716.

                it's a great idea to do the other ROMs as well, but figured I'd tackle this one first since your discovery on page 9 of that document is a bit of a beacon of hope for me.
                Most of the Commodore ROMs have EPROM equivalents (2716, 2532, 68764/6 etc.) but in some places (including the 8050/8250) they use a high (positive) chip select; those will need an adapter.

                Comment


                  #9
                  The fact that print ds$ hangs and doesn’t report back with the DOS version tells me the digital board has not booted up properly.

                  The PET cannot communicate with the 8250 floppy drive properly and since you have tested the PET works fine with other IEEE devices, we can rule the PET as having a fault.

                  I would check the Roms.... they are 2364’s but as Mike said they use a high CS.

                  I assume all the power rails are correct.
                  Last edited by AndyG; May 1, 2020, 11:44 PM.

                  Comment


                    #10
                    Thank-you for the feedback.

                    Yes, positive that the PET is working fine, as my SFD and MSD/SD2 works without issue, using the same IEEE cable. Definitely the 8250.

                    Looks like the next step then is to check the ROMS to see if they're any good. Reading them / comparing them to known good copies and figuring out adapters will be my next step.

                    Unfortunately I think you're speaking at a bit higher level that I'm able to understand at this time, but I'm tryin'.

                    I've ordered some 2716's and 2732's from two sources, so will have to wait for those eproms to arrive. Hopefully those could be utilized. Still pretty fogging on this.

                    So, first off, I want to start by reading the 901467-01 GCR rom for starters. Earlier it was mentioned that maybe yes, maybe no, that it could be read as a 2716. Well, that's a bit ambiguous, as to whether it would work or not, and I don't want to blow up the chip (even if it may be bad). So I still need to determine how to properly read that chip.

                    Then, what I think is being said is that these ROMs should be considered like a 2364 (pin-out wise presumably), and to use something like a 2716 or 2732 in it's place, an adapter would be needed with some sort of logic involved to get a high chip select. Anyone have a schematic on hand that I could utilize to accomplish that?

                    And you mention if the power rails are correct? I'll double check the voltages based on the information that I've obtained so far unless you mean something else...

                    Thomas

                    Comment


                      #11
                      I have built a 2364 adapter for my eprom programmer as per this webpage... I have been able to read 2364 masked ROMs using the 2764 device configuration.

                      http://mess.redump.net/dumping/2364_mask_roms

                      I haven’t built it to invert the CS line mind you though the guy does indicate how this can be done by wiring in a 74ls04 hex inverter ..... hope this will direct you to successfully constructing an adapter
                      Last edited by AndyG; May 3, 2020, 03:21 AM.

                      Comment


                        #12
                        I think you are safe reading the 2316 eprom using the 2716 configuration on your programmer.

                        Comment


                          #13
                          Originally posted by AndyG View Post
                          I have built a 2364 adapter for my eprom programmer as per this webpage... I have been able to read 2364 masked ROMs using the 2764 device configuration.

                          http://mess.redump.net/dumping/2364_mask_roms

                          I haven’t built it to invert the CS line mind you though the guy does indicate how this can be done by wiring in a 74ls04 hex inverter ..... hope this will direct you to successfully constructing an adapter
                          Actually was able for read Consistently the 2364 ROMs from a 8050 by connecting CS to VCC on the 2364 (had a switched 24 adapter so tried it) and leaving pin 20 CE on the 2764 part of the adapter disconnected (ie not connected to the 2364). Rom dumps where identical to that found on Zimmers.net so we’re good reads.
                          Last edited by AndyG; May 3, 2020, 07:43 AM.

                          Comment


                            #14
                            Thanks very much for the link and information on reading the ROMS.

                            Some Success!!!

                            I had a revelation late last night when I was looking for the ROMs for the 8250 to download to my PC. On a site, the description of the GCR ROM file said that it was an SFD-1001 file. I quickly opened up my SFD, and discovered that not only is the GCR rom (901467-01) identical to the one in the 8250, but so are the two DOS roms (901887-01, 901888-01). Given that I have a working SFD, I figured, this would be my chance to rule out these roms.

                            http://www.zimmers.net/anonftp/pub/c...001/index.html

                            The plan was to try the 8250 ROMS in my SFD, and see if any of them caused the SFD to fail. I first tried the GCR rom, and no difference, SFD was fine. So, the GCR rom is good. Then I tried the 901887-01 and the same result, SFD was good. BUT... the 901888-01 rom was an entirely different story. When I put that in my SFD, the error light started flashing and the drive light stayed on, exactly like the 8250.

                            So, I put all of the original 8250 ROMS back into the machine except the 901888-01. In that position, I put the known good one from the SFD. And the error went away! The joy was short lived unfortunately, but this is what I see now when doing a ? DS$ after powering it up.

                            sm_IMG_2218.jpgsm_IMG_2219.jpg

                            Was thrilled to get to this point. So, what I did next was take a couple of SFD formatted diskettes, and tried to catalog them in the 8250. What I quickly discovered was, when I did a catalog, the drive lights did not come on when it was trying to read each disk, and each drive reports an error. I did a video so you can see the behavior now.



                            Please excuse the dirt in the drives, I have not yet gotten to cleaning it as up to this point I couldn't even get the drive to boot up. Now that the digital board is booting as you can see, another problem has reared it's head.

                            My next step is going to be to clean the drives, so I'll take them out and clean all the dirt off them, lube up the points that look like they could use a little work, then re-assemble. I suspect however that this isn't going to fix the problem. I believe that when accessing the drives, the drive light should come on, on each respective drive.... so I think there is still a problem.

                            Have any of you ever encountered this new problem? Where, when you try to access the drive, that the drive light doesn't come on? I'm suspecting that that's part of the problem that the drive isn't reading disks...

                            So to recap, so far I've found the following chips to have been bad in the drive.

                            UH3 -6502 - BAD/replaced
                            901888-01 - Disk Operating System (DOS) ROM high. - BAD/replaced

                            While I was testing, I also tested to see if my RRIOT replacement had actually worked, and I'm happy to say that that works as well, even though the factory RRIOT chip is good, it was fun to confirm that I did that right.

                            I will still need to make a new rom for this drive, so I can put my SFD-1001 back into service, so that's something I still will need to address.

                            Has anyone encountered this new problem I'm seeing before in their travels?

                            Thomas

                            Comment


                              #15
                              Originally posted by AndyG View Post
                              I think you are safe reading the 2316 eprom using the 2716 configuration on your programmer.
                              Yeah, sorry for the ambiguity; I meant you'd either be able to read it or not depending on how chip select was programmed but AFAIK the 2K chips like the 2316/6316 all used 'normal' low-going CS so you should be fine.

                              Interesting that tying CE to Vcc works; I always thought it might but never tried it.

                              Here's a good selection of ROM adapters, including one that handles high CEs:

                              http://store.go4retro.com/adapters-1/

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