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Posts posted by techknight

  1. Hard to say, it will all determine where its at in ROM. the CPU is attempting to execute code. But its dying off somewhere. Probably trying to initialize a hardware peripheral and keeps running into the address error or bus error trap handler exception. 


    if you remove the ROM, it should attempt to walk the bus. so monitor the address lines and make sure. It should treat it as an OR.I instruction so itll just keep going. However if not, then see below:


    You can also make a nop-inserter ROM Simm, (basically hardwiring $4E71 on the data bus, but driving with only pull-up resistors to prevent bus contention when others are selected) and watch the address lines like a counter. they should all be div/2 from each other. you would check these at all peripherals. 


    Also watch these lines with an oscilloscope. you need to make sure each data bit and address lines are able to drive to the full bus voltage without contention issues. 

  2. Check the traces between the ROM and back to the ADC/SWIM/SCSI. As well as check the ASC between the GLU. 


    Also, Check D-line to D-line for shorts, as well as A-line to A-line for shorts. as well as to both VCC and GND


    If that fails, grab an oscilloscope and probe the data lines for activity, and more importantly, bus contention. Also if the SWIM goes bad, it will hold up the boot process. Same with the ASC if it cant be communicated to properly.


    SCC wont matter, it would just freeze system 7 before it reaches the desktop. 


    Last but not least, check the interrupt lines on the VIA as well as all 3 IPL lines into the CPU to make sure they arnt being held in a stuck interrupt state. 

  3. On 2/12/2020 at 1:54 PM, redruM69 said:

    It's worth noting that although the Toshiba 74HC166D is a CMOS part, it appears to be TTL tolerant.  It worked fine as a replacement in my case.


    Not in all cases. See below:


    On 10/10/2019 at 3:27 PM, redruM69 said:

    ISSUE RESOLVED!  Techknight, you may want to add this to your notes.

    It appears that Texas Instruments SN74HC166DRG4 is NOT a valid replacement for the the 166 shift register @ UE8.  It causes the discussed artifacts.
    I swapped it out again, but this time with a Toshiba 74HC166D, and the issue completely resolved!


    Guys, I can finally put this thread to rest. Should be pinned because its something I overlooked in the beginning and this is one of those times when TTL vs CMOS is hyper critical. 


    In some cases, I have used these parts interchangeably for years and never had an issue, but this is the one case where it became an issue. 


    I removed the HC166 that was on the PCB from a previous attempted repair, and replaced it with the correct LS166 and the issue is now RESOLVED. It is fixed. Works perfect. So thanks for the idea, I thought I had already looked and checked for that beforehand, but guess not. haha


    This is the one time where TTL vs CMOS had gotten me, and its super important information for others to know. 


    So finally after all these years, I can bring this thread to a close. 

  4. I actually use the ATXmega128A1U in our product. 


    Sadly this chip even though powerful, is super sensitive to voltage glitches and flash erasure. 


    Mine has a bootloader so I can do software updates, Even with tons of filtering, and the BOD enabled continuously with the thresholds set it, every great once in awhile i find a device that is stuck in a bootloader reset cycle and flash memory erased. Its weird. Only reason why the bootloader remains is I have the fuses set to make it read/write locked. 

  5. you just need to tack a couple capacitors. 0.1uf ceramic/film, and 1uf electrolytic. across the VCC/GND lines directly at the chip. Also, a 5.6V Zener diode with the stripe side of the diode body soldered on the VCC pin, and other lead to ground. 


    Then, put a 10 ohm resistor between the wire coming in from the switch, and the chip's VCC where all the above parts are attached. this will serve as a dump resistor if something were to overvolt or short causing the zener diode to clamp. You could go with the metal film non flammable type here. Maybe fusible type so it pops open if failure. 

  6. Wouldnt it have just been easier/safer to put a switch in the sweep supply at pin10 on P3?


    You will blow out that 74LS IC eventually doing it the way you are doing it. 


    At minimum you need a Zener diode, a resistor, and capacitor filter network now at the IC. Just C23 alone isnt good enough anymore now that you have wires running half way to China and back, along with EMI generated by the switch, which is now closely coupled with the 12V line. All kinds of bad here.


    But hey, at least you tried. :-)

  7. looks like its nearly burned, so yea i would say that IC is shot. 


    But that is the least of your concern. The bigger concern is the chain-reaction collateral damage potential that happened when the IC blew out. How many other parts went with it? 

  8. Unless the thing has 50,000 hours on it and the CRT has gotten weak. its rare, but I have seen a couple computer monitors this has happened to. 


    IBM PS/2 monitors being one of them, I have an IBM PS/2 monitor that the CRT has been worn into the ground. Has burn-in of course, but the emissions from the electron gun are so weak, turning up the brightness has severe color bleeding and blooming. Thats how you know the CRT is done for. 


    Surprisingly, I found a NOS replacement. Havent changed it out yet though. 

  9. Ohh ok, thanks for clearing that up.


    These particular monitors are KNOWN for capacitor issues, so that is likely the cause. Sometimes the flyback housing can crack and start developing arcing points, but youll smell the corona it gives off if this is happening. 


    More than likely though, you have bad caps, especially the two or three near the horizontal drive stage. 

  10. The trinitron CRTs are pretty much all the same. 


    that CRT, (if its the same size) can fit in some of the other Apple Trinitron monitors of the same size.


    I know some of those displays were actually 13" but the CRT may still be the same. 

  11. Your video signal is kinda weak. Might need to look into that. Check the CRT Neck board for a shorted video driver transistor, and/or check the logic IC on the analog board where the video is gated through. 


    Also probe the video signal on the logic board as well. 


    it seems as if the signal for the video is being damped for some reason. 

  12. I would almost need to see a picture of what your describing. 


    CRT Monitors are very complex little devices and it can be many different things to cause those symptoms. 


    Could be capacitors, could be the video amplifier IC, could even be the flyback itself. 

  13. Im worried. Changing the 9V batter should not have changed anything with the system turning on or off. 


    All that battery is for, is when you slide the battery cover off, it switches into the circuit to keep the memory maintained. Until you slide the battery cover back on. 


    the 9V battery servs no other purpose. 


    So if changing the 9V battery made a difference, that tells me your battery latch switch is either bad, or the tab is broke off your battery cover. 


    The portable wont last long in this manner. 

  14. Good thing about using an ARM Cortex, you can get speed improvements of course, and then since the firmware is written in C, it should be fairly easy to port over without doing too much work, but you would have to replace the ASM routines though. 


    Regardless, this is a really nice project :-)


    Honestly, with a littel bit more cobbling, you can probably replace the ethernet PHY with an ESP8266.. and then use some of the WiFi driver development done by ants, combine the two, and you have one really badass product, with a wifi icon in the menu bar to boot!


    Then, if its miniature enough, it can be put inside vintage powerbooks, replace the internal HDD, and gain wifi support at the same time? HELL yes!