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Thread: Model 2 drive questions

  1. #11

  2. #12
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    Quote Originally Posted by pski View Post
    Try disconnecting the 50 pin cable from the floppy drive and see what you get.
    if you havenít already, disconnect the floppy drive cable from the fdc card. iíve seen these cables go bad and cause all sorts of problems.

  3. #13

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    I did try that and it’s the same. I see there are a lot of little single pins sticking up on various cards. I assume these must be test points so I’ll look in the tech ref manual and see if they tell me what and how to check at them. I have a digital scope but not much experience with it. I really appreciate all the advice. I’ll keep poking at it until the next retro meet up which appears to be in May. Hopefully I digo it out by then

  4. #14
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    u18, a 74ls04, is an inverter that feeds the drive rdy signal to the fdc. use your scope and check both sides of the inverter—if you see the same logic level then that may be the problem. RDY will be high at the fdc with a disk in the drive and the door closed. no drive or no diskette it should be low.

    i think there are other problems... other signals from the drive are probably involved. you can check the schematic to see what else u18 inverts. there’s a resistor pack that pulls that and other signals high. it could be bad, doubt it, but worth checking.

    make sure your 5v is in spec.

  5. #15
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    correcting my previous post... the 'U' number depends upon which version of the FDC card you have... so the best thing to do is get the FDC schematic that matches your card. there are other differences between the board revisions, too. apologies for the confusion.

    regardless, the RDY signal is found on pin 32 of the 1791 FDC chip--test point TP-5. look at that with your scope and see if it is high with no drive attached. if it is, then you can work backwards until you find where the problem is. the signal should be low at the FDC and TP-5 with no drive attached.

    the RDY signal is pulled high when it enters the FDC card with a resistor pack. this signal is then inverted before it connects to the FDC.

    if it is high at pin 32, then the FDC will attempt to read the boot sector of the floppy. with no drive attached it will return a boot error--and i'm thinking that's probably going to be a "RS" error.

    i guess the FDC chip could be bad or something else, but this is where i'd start if it were mine.

  6. #16

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    5AF2C591-5B4E-4433-93ED-4EB25B7D76DC.jpeg9631E1B0-936B-463D-A115-84D66B8F374E.jpeg5879EC4C-C20C-4936-AD1C-771B4FD448CE.jpeg

    Thank you very much for the suggestions Shank. So I looked at the board and U18 is the inverter you stated so I checked with the logic probe since I had it out. I was checking opposite pins and I donít think thatís correct based on the chip data I found. Looks like they invert to the pin next to them so pin one inverts to pin top, etc. So Iíll check that again tomorrow to see what I get. I wanted to ask about the test points. Am I right that the pin in the attached pic is the test point? I checked it with the card up on one of the extenders I got from Mav and with the drive cable disconnected I was getting low like it should be
    Last edited by TXNathan; February 18th, 2020 at 06:41 PM.

  7. #17
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    glad you have the extender!

    i’m not home and my phone isn’t rendering your photos properly so i can’t answer tonight. sorry about that, but i’ll look tomorrow, figure out what board rev you have and get back to you.

    right, devices in DIP packages can have all sorts of pin configurations. you have to look them up if you don’t have readable schematics. what schematics do you have? there should be a long number beginning with “8” in the ID block at the lower right. let me know what it is.

    if pin 32 at the FDC is low as it should be... i need to think about this.

    also, “The TTL Cookbook” is a good source of chip info. it was written about the same time as your machine was made and is available free in ebook format at the internet archive. paperback versions are available from amazon, believe it or not..

  8. #18
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    OK.. you have the Model II early style FDC. I'd been looking at a later version schematic. I've attached a schematic section of the FDC circuit--there is no test point on this board for this measurement, but you can see the inverter, chip number, and pin numbers. This is from the "Model II Technical Reference Manual."

    If you haven't already, a very good source of info is "Notes and Jumpers" put out by RS Technical Support. It's in 2 large files--one with the images and the other contains the notes. Grab the Technical Bulletins for the Model II also if you haven't already. Neither of these are likely to solve your problem, but they offer good info and the bulletins should be checked and perhaps done once your machine is working. N&J is great for determining the board revisions you have and their differences. I do suggest checking your board jumpers against N&J just in case someone moved one through the years. Wouldn't hurt to pull them off and push them back on just to make sure they are making good connection.

    I've also attached 2 lesson plans from the RS technical training program. The formatting is a bit strange... I don't have the overheads that went with these, but they should give you a good overview of the FDC board to compliment that given in the Tech Ref Manual. The lab procedures might help you narrow down what's going on.

    Looking back at your first screen shot with the boot error RS--was all that tearing or noise on the screen visible to you, or is that just a camera artifact? If you see all that video noise, then i suspect you may have a really noisy power supply. Pski told you about needing good 5 volts, but you need a good, clean, 5 volts. You can check that with your scope. If you have a much higher voltage on the 5 volt line at the power supply then on the card, try disconnecting and cleaning the pins in the power supply Molex connectors--at the power supply and at the card cage. Before going further with your FDC repair, you must have a clean 5 volt line within spec. Fix that first if you need to.
    Attached Images Attached Images
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  9. #19

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    Yes, the noise is visible to me. I noticed a few times when I was moving things it would go away so I assume its a bad ground or a loose connection. I measured the 5 volts as mentioned earlier at 4.602 at the highest spot right out of the supply which is below the minimum acceptable level of 4.9 I think. This is with my Fluke bench meter so I believe it to be accurate. I am still learning how to use my scope so I am freaked out about measuring the power supply because everywhere I read about that they give you a hundred ways to blow up your scope but no sure fire ways not to. I feel like I should be able to connect to the chassis ground a poke away but I don't want to fry the scope so I haven't tried that yet. Also, I replaced every electrolytic cap and all the RIFAS on the PS. The voltage is the same so I do not know how to "fix" that anymore than I have. I am open to suggestions. I have considered purchasing an aftermarket Jameco like Pete suggests but I wanted to see if the machine could be made to work before I put too much more into it. If that's a must at this point and the PS repair is out of my league then I may go that route.

    I appreciate the continued patience and advice, keep it coming.

  10. #20
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    if you’re not experienced with your scope then i don’t blame you for not wanting to risk it—and other than giving you more info it probably wouldn’t help to know. i’m not aware of any way to adjust the voltages on these supplies—when these machines were being actively used and the supply was out of spec, the supply was replaced. easy then; not so easy now. i’ve never troubleshot one. i can tell you that these machines were flakey with out of spec or noisy supplies. flakey enough to cause a boot RS error?

    there’s no way to know if replacing your current supply will fix the boot problem and i don’t know how wiring in a separate 5v supply would work. those astec supplies have to be loaded to work properly. if you run the supply with the 5v on it unloaded you may cause more problems including blowing it—i don’t know. maybe someone else has tried that already and can offer you advice about that.

    if you haven’t already, i suggest pulling each card and doing a close visual inspection looking for obviously damaged components, components folded over, rusted jumpers that aren’t making connection... next, closely inspect the power cables paying close attention to the Molex connectors and making sure it’s all tight. your video noise is worrysome—see if you can track that down and fix it—that noise is probably running all through your system. everything on that system is interrelated—i think pete mentioned your problem might be on another card—very possible. that’s about all i have to suggest at this point.

    hauling it up to Dallas may be your best bet to get it working whenever the next vintage gathering is happening. someone else may have better advice for you.

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