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Thread: Original Tandy 1000 no boot

  1. #61
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    Quote Originally Posted by falter View Post
    Ok many thanks.

    Just FYI - I've also tried this with a 360K panasonic drive from a Commodore PC10 that does exactly the same thing. I'll have to get a pic of the jumpers but it worked fine as an A drive in other machines, and the Toshiba configured as A has worked in other machines also.
    I have a 1000SX, not the 1000, but I would begin to suspect the floppy disk controller which I believe is U-38 on the SX. Hard to miss as it's a fairly large DIP. As far as switchology goes, check this out if you have not already seen it:

    http://www.thealmightyguru.com/Wiki/...-_Volume_1.pdf

    Tom
    Surely not everyone was Kung-fu fighting

  2. #62
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    Quote Originally Posted by falter View Post
    Ok many thanks.

    Just FYI - I've also tried this with a 360K panasonic drive from a Commodore PC10 that does exactly the same thing. I'll have to get a pic of the jumpers but it worked fine as an A drive in other machines, and the Toshiba configured as A has worked in other machines also.
    That could possibly be something on the controller then. Just to rule out the obvious, have you tried a different cable, and swapping the two Toshiba drives between A and B?

    I'm also looking in the Tandy 1000 refrence manual, on page 493 where the floppy controller schematics are. It looks like DS0*, DS1*, and MTRON* don't come from the floppy controller chip itself (the funky way Tandy did things), but through a flp-flop and some other discrete logic (U71, U83, U84, U98, U107). Might want to check those chips out and see if there's any broken traces, shorts, cold solder joints, etc. Those three signal lines are active low, so a broken connection could force the drive to be on all the time.

    Also, I don't know if it might be related, but there's also a jumper between pin 9 on U85 and pin 34 on the floppy header. No jumper number is shown on the schematics, so it might be permanently bridged instead of having a jumper header installed. If you put your meter on continuity while the machine is off, and go between pin34 and ground, is there continuity? Then with the meter on DC voltage and the machine on, does it show voltage between pin34 and ground?
    Last edited by blackepyon; October 20th, 2020 at 12:10 PM.
    My vintage systems: Tandy 1000 HX, Tandy 1000 RSX, Tandy 1100FD, Tandy 64K CoCo 2, Commodore VIC-20, Hyundai Super16TE (XT clone), and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, and is less likely to lead to a fatal accident.

  3. #63
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    Quote Originally Posted by blackepyon View Post
    Right away I notice that the drive set as DS2 (seems they start at 1 instead of 0) is missing a bunch of jumpers from PJ5. That's probably for the terminator resistor pack (the white chip between the jumper blocks). Like SCSI, the terminator would be enabled on the last drive on the cable...
    I've been eyeballing this myself, and to expand on this here's my thoughts:

    1: Don't have both drives hooked up at once, let's try one at a time. Work up from there.

    2: I concur that those jumpers to the left of the white package are for tying the terminating resistor pack to the bus, so have the last drive on the cable (or the only drive) jumpered.

    3: I'm looking at the Teac FD-55BV-75 drive that came with my Tandy 1000 EX to see if it provides any clue about the other selection options. This is made a little more complicated by the fact that this "-75" seems to be a "Tandy Special" and most of the jumpers are unpopulated; it just has some resistors permanently soldered in a couple spots. The SAMS manual I found for this specific drive seems to gloss over all those permanent shunts. Cross-referencing to a generic FD-55 datasheet I'm going to hesitantly make the following suggestions:

    A: Jumper *both* "LI" and "LD" for the proper operation of the drive LED. It *looks* like the permanent resistor on the TEAC drive corresponds to that setting on a Toshiba drive.

    B: I'm not entirely sure that "HM" is the correct setting, try "HD". Remember, in a Tandy 1000 all drives spin at once while the PC standard is motor control is linked to drive select.
    Last edited by Eudimorphodon; October 20th, 2020 at 12:40 PM.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  4. #64
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    Ok.. I should have just tried this before - I opened up my 1000SX and borrowed the entire drive assembly including cable. Checked to make sure pins were oriented same.. yup. Plugged in and fired up.. same thing. One drive lights up, other doesn't. No effort to boot.

    I then took the cable and drives I had been trying with the 1000, put them in the SX unchanged.. everything boots normally. So this is looking like an issue with the floppy drive controller or something on the 1000.

    I see the SX FDC chip has a different part number than the 1000... the 1000 has an R6765 ... I don't know if any of my other PCs have that for testing purposes.. I have Commodore PCs, Computerland.. but I don't think I've seen that chip on any other machines.. unless there are crossover part numbers..

  5. #65
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    Yeah, that's definitely something on the motherboard.
    My vintage systems: Tandy 1000 HX, Tandy 1000 RSX, Tandy 1100FD, Tandy 64K CoCo 2, Commodore VIC-20, Hyundai Super16TE (XT clone), and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, and is less likely to lead to a fatal accident.

  6. #66
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    I'm pretty sure Tandy used a bog-standard NEC 765 up through at least the start of the TL series, so it shouldn't be too hard to find a replacement, if that's the problem. The drive select and motor-on signals don't come from the FDC, though, so if the drive motor is spinning as soon as you power it on then there's a fair chance that's not the problem.

    According to the schematic for the original 1000 (if this is a 1000A life gets harder because apparently the schematic to that is unobtanium) the motor and drive selects come from a 74LS273 latch at U71, and the raw signals off those latches get massaged by an or gate and a couple inverters and, possibly important, go through a 7416 inverter (motor) and 74LS38 nand (drive selects) before coming out on the connector. Those last two parts are open collectors. I'd suggest chasing down that chain before assuming it's the FDC.

    (* This might be where I'd worry about the difference between a 1000 and a 1000A. The drive select and motor-on signals come from a tandy proprietary keyboard controller ASIC on the SX/EX/HX generation machines. I've gathered from a previous thread that apparently the 1000A uses a similar-but-not-the-same-part ASIC as the later models instead of the discrete parts for CPU clock generation, I don't know if the 1000A might also have integrated other parts.)
    Last edited by Eudimorphodon; October 21st, 2020 at 09:06 AM.
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  7. #67
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    Quote Originally Posted by Eudimorphodon View Post
    The drive select and motor-on signals don't come from the FDC, though, so if the drive motor is spinning as soon as you power it on then there's a fair chance that's not the problem.
    '273 is a flip-flop, '573 is a latch... But I was looking at the schematics yesterday, and that's also what I'm thinking at this point. If there was already one broken trace on the motherboard, there's a possibility of another break somewhere between the flip-flop and the header, so I'd test the traces first with a multimeter if you can't see any more breaks. Even if it's not immediately visible, there could be a cold solder joint.

    To make it easy, print out page 493 of the technical reference manual (the floppy controller schematics) and just colour in the traces with a pen after you've tested them point to point.

    Failing that, get a desolder pump or desolder gun and pull the logic chips out for testing in an TL866 programmer if you have one (it's a pretty handy feature). 74xx logic ICs are easy to get.
    My vintage systems: Tandy 1000 HX, Tandy 1000 RSX, Tandy 1100FD, Tandy 64K CoCo 2, Commodore VIC-20, Hyundai Super16TE (XT clone), and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, and is less likely to lead to a fatal accident.

  8. #68
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    Quote Originally Posted by blackepyon View Post
    '273 is a flip-flop, '573 is a latch...
    I'm not actually sure the distinction matters here. Both can serve as a static memory register, which is what the '273 is doing here. (Although I did technically say the wrong thing, I'll certainly own up to it. I meant to say "register" but "latch" came out because 573s seem to be more common in this kind of role.)

    Anyway, that's neither here nor there. Definitely check all that wiring out. I was wondering if it might be more likely one of the open-collector buffers, but since *both* the LED is on and the motor is running the register is a decent place to start.
    Last edited by Eudimorphodon; October 21st, 2020 at 10:40 AM.
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  9. #69
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    Okay made a few interesting discoveries.

    Warning: this is coming from a still inexperienced technician - but I cannot find a proper clock signal at U71 pin 11. So I start tracing backwards, and end up at U58 - an OR gate which seems to have clean clock signals coming in at pin 4 and 5 but the output doesn't look healthy.. very faint , unstable signal. So I'm guessing maybe the gate on that IC might be dead.

    I noticed by accident when messing with my logic probe and Rigol probes if I accidentally connected pins 9 and10 of U51, the drive stopped spinning and light went out, and only came on as if it were trying to read the disk - but wouldn't successfully read it (head didn't move). I could also, if I temporary shorted 9 and 10 on U51, see clock signal at pin 11 of U71 (273).
    Last edited by falter; October 21st, 2020 at 09:13 PM.

  10. #70
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    Sounds like we might have a winner. You got a spare 74LS32 handy you could pop in?
    My vintage systems: Tandy 1000 HX, Tandy 1000 RSX, Tandy 1100FD, Tandy 64K CoCo 2, Commodore VIC-20, Hyundai Super16TE (XT clone), and some random Pentium in a Hewitt Rand chassis...

    Some people keep a classic car in their garage. Some people keep vintage computers. The latter hobby is cheaper, and is less likely to lead to a fatal accident.

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