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Thread: Kaypro 2/84 Troubleshooting

  1. #1
    Join Date
    Apr 2018
    Location
    Los Angeles, CA
    Posts
    158

    Default Kaypro 2/84 Troubleshooting

    Hi All,

    Last weekend I was given a Kaypro 2/84 that didn't work, since the previous owner (also a retro collector) didn't want to mess with troubleshooting or the space it took. Got it home, cleaned it up and started troubleshooting. There were a few physical issues like a broken keyboard connector that i've gotten working temporarily (I think, but I guess we'll see). Initially turning the machine on/off, the power would randomly come on. When it did come on, the power light would come on and the disk drives would spin. There was no display on the screen (if I look closely I can see some light, but very little.. adjusting both the brightness and internal subbrightness made no difference there - the CRT does light up though). I cleaned the machine, resocketed everything and used deoxit on the sockets and power plug. That seemed to solve the random power issues (at least for now). Checking the voltages on the board gave me +12, -12 and +5 voltages. Further troubleshooting noted that the Z80-1 cpu and one of the SIO chips got extremely hot. I swapped the Z80 with my Model 4 and it was indeed bad. I Swapped both of those chips for 'new' chips. As well, it seems the SY6545-1 gets fairly hot, but swapping that made no difference and I assume that's just a hot chip in general. All of the chips now are under 45C with the only ones getting warm being that SY6545 and the Z80 CPU, but still around 45C or less in the hottest area. I tested all the voltages on the board, they all get about 4.9v (except two 12v chips) with the Z80 CPU getting around 4.7V. The ROM in the machine is correct with the 81-292A version (loaded and compared with my eprom burner, at least the first page). It does have a different character rom then is normal for that machine, don't think that matters however at this point. I used IMD to create a 2.2G SSDD disk, but inserting the disk does nothing and it doesn't attempt to read. The floppy drives, while constantly spinning, don't ever turn on the lights (I did clean and oil the drives in the initial cleaning process, as an aside, and physically they look good). The voltage on the drives appear correct as well.

    So, at this point I think i've exhausted basic troubleshooting and looking towards some ideas of what my next steps would be? I do have a scope though my skills with it are pretty limited, so it's usually not my first approach.

    20181011_230859.jpg20181011_230914.jpg20181008_152059.jpg20181007_191933.jpg20181007_215705.jpg20181011_012110.jpg

    Thanks!
    Retro PC's: Apple IIe/II+, Atari 800, Atari 520STFM, Commodore 64, Commodore Amiga 3000, IBM PS/1 2121-B82, TI-99/4A, TRS-80 Model 4 GA

  2. #2

    Default

    My first concern is Vcc at the CPU. 4.7V seems low, that is outside the 5V+/-5% specification. You might need to track down where/why you are getting that voltage drop. With the 81-292a ROM, you should be seeing this in the middle of your screen (and Drive A LED should be ON):

    Code:
                                   *     KAYPRO      *
    
    
                         Please place your diskette into Drive A
    
    I cannot read your diskette.
    So if the screen is blank, some hardware is still not working correctly (could be video, but also could be CPU, etc).

    I also see that the label on the ROM is hand-written, so it is probably not original Kaypro issue - but I assume it is a faithful copy. The label also says "CPM 2.1g" which must be a mistake, as I believe Kaypro only used CP/M 2.2.

  3. #3
    Join Date
    Apr 2018
    Location
    Los Angeles, CA
    Posts
    158

    Default

    Quote Originally Posted by durgadas311 View Post
    My first concern is Vcc at the CPU. 4.7V seems low, that is outside the 5V+/-5% specification. You might need to track down where/why you are getting that voltage drop. With the 81-292a ROM, you should be seeing this in the middle of your screen (and Drive A LED should be ON):

    Code:
                                   *     KAYPRO      *
    
    
                         Please place your diskette into Drive A
    
    I cannot read your diskette.
    So if the screen is blank, some hardware is still not working correctly (could be video, but also could be CPU, etc).

    I also see that the label on the ROM is hand-written, so it is probably not original Kaypro issue - but I assume it is a faithful copy. The label also says "CPM 2.1g" which must be a mistake, as I believe Kaypro only used CP/M 2.2.
    Hah! So the rom was missing the label altogether, i put a new label over it to cover the Eprom part and wrote that on there after I confirmed the rom with eprom programmer. It should say 2.2g, I just didn't write that well, I guess.

    Also noted about what I should be seeing, but it definitely isn't getting far enough into the process to even display anything on the screen. Didn't seem like a CRT issue (at least not at this point) since it doesn't begin to even attempt to try and boot a disk. In any case, any idea where I should be looking for this voltage drop? You're likely onto something because I also noted that the CPU i pulled from the working TRS-80 is actually getting a lower voltage (around 4.1), but it works fine in the Model 4.
    Retro PC's: Apple IIe/II+, Atari 800, Atari 520STFM, Commodore 64, Commodore Amiga 3000, IBM PS/1 2121-B82, TI-99/4A, TRS-80 Model 4 GA

  4. #4

    Default

    Being a Kaypro Guy. My play would be to do a 1+1=1... Try to round up another K with an 84 Mainboard. Starting with a working PS and Monitor helps.

    I'm a component level swapper, I don't have a shop bench and scopes like I did years ago. I was patient and I ended up building a killer K10 with al SSD Floppy and HD. Also replaced the Case fan... virtually silent, and 56MB of storage.

    JA

  5. #5

    Default

    I don't have a lot of experience resurrecting old machines, but my guess would be either damaged traces or shorted capacitors - probably tantalum but possibly electrolytics (ceramics bypass caps are not likely to go bad - unless physically damaged).

    If you don't have it already, maybe download the technical manual from http://www.bitsavers.org/pdf/kaypro/...nual_Sep85.pdf and start working from the power supply to the CPU (it has schematics as well as PCB layout). You could also start measuring Vcc at all the chips and perhaps see a brown-out pattern that might lead to the culprit. I guess also measuring voltage across the caps, too. If the Vcc at the power supply connector is good, seems like there has to be something on board that is dropping the voltage. Clearly, Vcc is pretty low at the CPU. I suppose it could even be something like a bad solder joint at the socket. Sometimes, bad solder joints defy visual inspection. With a sensitive volt meter you should be able to measure for voltage drop between key points, like Vcc near the CPU and the actual pin on the CPU. Also, ground could be the fault as well. You could try measuring Vcc on various chips but keep the ground at the CPU (etc). Different combinations of such.

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