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Thread: Model 100 No power up

  1. #11
    Join Date
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    The Tandy 100 has a cap issue. All of my miniature radials (they aren't SMD's but they are still a problem people overlook) had leaked and started to corrode under the masking below them and wouldn't do much else until they were all replaced.

    Also replace the backup Ni-Cd with a 5v 1F supercap. Your time to replace the main batteries and retain the contents of the memory is reduced from several days to a few hours but the supercap will not leak anywhere as near as the battery.
    = Excellent space heater

  2. #12
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    Yeah I think I'm going to just replace most of the caps. I did pop out the 470uf and 4.7uf at C83 and C82 respectively (the service manual says to check there first if no power). They do seem to test okay out of circuit with my DMM, but the bottom of C82 has some definite corrosion signs going on.

    I'm trying to understand the circuit as best I can. I have +6V to pin 2 of the power switch if the switch is off - as soon as I switch it on, this drops to +0.85V. I had a lightbulb moment and noticed that what I presume is the ground wire (a green wire that sticks up and then goes under to the foil like piece under the motherboard) had some corrosion at both ends, I replaced that, and briefly while messing about physically bending the board or leads on C83 was able to get 5.6V there, and 2V at points like VEE, etc. But that stopped happening and now doesn't work at all no matter how much I flex or play around. I can see if the lithium batt is switched off, no voltage at all comes up when you try to power on. So it's detecting that properly.

    One thing that confuses me is the DC jack on this is a weird shape - it isn't round.. it has one side that is round and the other side is square. There are three points of contact - the usual center pin and outside, but also a third wire in the middle that I think might contact the outer ring of the adapter? Anyway, the outside contact is too far away for any circular DC power connector to make contact. It's strange.

    Anyway, i'll keep following this schematic - I've hooked up the 6V DC directly to the battery inputs and bypassed the jack for now.

  3. #13
    Join Date
    Dec 2005
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    The jack includes a switch to disconnect the AA primary (non-rechargeable) batteries when the adapter's plugged in.

    Service and Tech ref manuals here:
    http://www.club100.org/library/libdoc.html

  4. #14

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    Quote Originally Posted by falter View Post
    Yeah I think I'm going to just replace most of the caps. I did pop out the 470uf and 4.7uf at C83 and C82 respectively (the service manual says to check there first if no power). They do seem to test okay out of circuit with my DMM, but the bottom of C82 has some definite corrosion signs going on.
    Any electrolytic capacitor showing corrosion is a throw away.
    What is the voltage at T3 base?
    Dwight

  5. #15
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    Seems to be +0.83v

  6. #16

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    Quote Originally Posted by falter View Post
    Seems to be +0.83v
    Ops I meant T13. This is the regulation feedback to the primary side of the switcher. T21 and T22 form a oscillator with the primary coil. It T13 is turned on it would stop the oscillation. T20 could also stop the oscillation but I don't know what PC3 or VP are. These look like they could also shut down the oscillator.
    Dwight

  7. #17
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    Quote Originally Posted by Dwight Elvey View Post
    Ops I meant T13.
    That's what I thought
    I don't know what PC3 or VP are. These look like they could also shut down the oscillator.
    Dwight
    Indeed; VP is the main battery voltage; PCS (not PC3) is the low power shutdown signal from the CPU through the PIA (pin 33)

    A good writeup in the service manual pp. 4-32 & 33

  8. #18

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    Since you saw corrosion on c82, you might want to check the trace that runs underneath it. On mine, that cap had leaked and destroyed this trace. This caused the display to stop working until I patched the trace on the back side of the circuit board.

    http://lists.bitchin100.com/private....ly/048322.html

  9. #19

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    Ive seen a lot of videos where cap 100 goes bad. It was the issue with mine and seems to have also burned out OT2. I shoehorned a murata 782100/55JVC and it works now but is dim.

  10. #20
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    Okay so I finally got some parts including a new 3.6 NiMH battery and got those all installed, but same issue. So I checked the base of T13.. I don't know if I'm testing it right -- I've positive probe from DMM on base and negative on ground, and it gets -0.6V with power on. When I switch power to OFF, it shoots up to -2.6V.

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