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Microdrive units with weak (not dead) ULAs...

SpiranthoSpirantho
edited March 2012 in Hardware
Hi everybody,

I've got a Microdrive unit here. It didn't work. It does now (actually, it was the ribbon had broken and the ground had lost contact, so the drive just kept spinning as the ULA never told it to stop).

Anyway, this Microdrive is working. However, not fully. When I format a cartridge with a normal unit, I get about 90K-100K. With this unit I get 60K. Reading back a formatted cartridge loses data too.

I looked at the voltage on pin 1A (the data signal) and on the bad unit, the voltage is 1.3V. On my good unit, it's 2.2V.
I also have another ULA that exhibits similar behaviour - it works, but loses data.

Is there any way we can get this poor ULA back up to speed? Anything we can twiddle? I know we can change the gain in the circuit by altering the resistors on pin 14/15 but I believe that's the signal before going into the ULA. It's the output that's weak.

Looking at the output on pin 1A under an oscilloscope shows it looks identical on both units, interestingly.

Any suggestions? I don't want to write off two ULAs if I can help it!

Thanks!
Post edited by Spirantho on

Comments

  • WinstonWinston
    edited March 2012
    Just guessing here, I don't have a microdrive unit - but a small "cockroach board" with a CMOS buffer should do the job. If it needs to be small you can use a 74HCT1G125 (1 gate tri-state TTL compatible CMOS buffer, just wire the enable pin so that it's permanently enabled if it should never go into high-Z mode).

    If the 2.2v shouldn't be exceeded on this output (what does it go to?) then you can cook up a small amplifier with transistors instead.
  • SpiranthoSpirantho
    edited March 2012
    I imagine the 2.2V is because it's actually flip-flopping between 0 and (probably) 5V, as it's a square-wave signal. I'm wondering if something as simple as a transistor might do the trick operating as a switch...
  • WinstonWinston
    edited March 2012
    It depends why. If it's a problem strictly with voltage (i.e. the output pin can put out enough current, but whatever damage it has taken has resulted in doing so with less voltage), you'll need at least two transistors. A single transistor as a switch (a transistor and a resistor) works as an inverter, and you'll get the opposite signal of the input. So you have to have to have another transistor+resistor amplifier afterwards to make a buffer. Or you can use four MOSFETs connected up so they form a buffer, and no resistors (the MOSFET circuit is generally simpler). Effectively the equivalent of what you would try to do with the 74HCT1G125 IC (or quarter of a 74HCT125)

    If the low voltage is just an artifact of the output not being able to put out enough current (and therefore, the voltage sagging), you can use an emitter follower type amplifier which only requires a single transistor. An emitter follower will not give you any voltage gain, but it will present a high impedance input to the dodgy output of the ULA, and the ULA might be able to drive the input of an emitter follwer to the right voltage, and you're sorted.

    My guess is that the output just can't put out enough current, so an emitter follower would work, but you'd have to try it of course.

    See http://en.wikipedia.org/wiki/Common_collector for details of an emitter follower. The basic circuit is very simple.
  • SpiranthoSpirantho
    edited March 2012
    So could it be as simple as a PNP transistor and a resistor? What sort of value resistor? I can debug software and hardware, but I'm not so hot on designing circuits myself...

    Thanks for your help!
  • WinstonWinston
    edited March 2012
    Well, typically an NPN transistor is used in these circuits, as in the first circuit diagram shown on the wikipedia page. It's a guess what the resistor should be, it depends mostly on the input impedance of the following circuit. I'd just experiement and start out with something like 300 ohms, and see what the output waveform and voltages are looking like on the scope.
  • spodulaspodula
    edited March 2012
    May be worth replacing the 7805 and any associated capacitors first and seeing if that makes a difference.
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