Apple 1 replica - getting random characters on screen when powering on

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Joined: Jan 4 2015
Posts: 68

I get the normal alternating '_' and flashing '@' when i turn it on the first time. However, when I turn it off and turn it back on immediately(say about less than a minute) i do get random characters on the screen. Clear screen and reset works fine, all characters are correct when i type. The screen starts up fine if I wait for more than a minute before i power on again. Is this normal?

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speedyG's picture
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Joined: Nov 16 2011
Posts: 2376
Re: Apple 1 replica - getting random characters on screen ...

Hello snazzify,
did you think about that big blue capacitors and their function ?
If you did, then you also know that these capacitors need a bit time till they are discharged....
Specially the biggest one with 5300µF.

While the voltage at that capacitors decreases of course same thing happens at the RAM chips....
and of course there is some small variance at the different chips.....
some "pass away" earlier and some small time later.....
that might result in specific time range to different contents of the bit displayed...
and thereby chnging the contant of a byte.......

after a while more the mainboard is completely discharged and then you have the same conditions
like at the power on at the very beginning....

but meanwhile during the time the mainboard is not completely discharged the state of some chips
is unpredictable...

that´s normaal behaviour....

speedyG

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Offline
Joined: Jan 4 2015
Posts: 68
Re: Apple 1 replica - getting random characters on screen ...

speedyG wrote:

Hello snazzify,
did you think about that big blue capacitors and their function ?
If you did, then you also know that these capacitors need a bit time till they are discharged....
Specially the biggest one with 5300µF.

While the voltage at that capacitors decreases of course same thing happens at the RAM chips....
and of course there is some small variance at the different chips.....
some "pass away" earlier and some small time later.....
that might result in specific time range to different contents of the bit displayed...
and thereby chnging the contant of a byte.......

after a while more the mainboard is completely discharged and then you have the same conditions
like at the power on at the very beginning....

but meanwhile during the time the mainboard is not completely discharged the state of some chips
is unpredictable...

that´s normaal behaviour....

speedyG

Thank you so much! Smile This this clears things up for me.

Offline
Joined: Oct 9 2011
Posts: 1045
Re: Apple 1 replica - getting random characters on screen ...

Perfectly normal.

If you want to shorten the time use a bleeder. You can connect a 1m ohm resistor to pin 6 of the power connector and then to earth ground. This speeds up discharge of the caps on be board.

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Joined: Oct 4 2014
Posts: 64
Re: Apple 1 replica - getting random characters on screen ...

SpeedyG is correct about the capacitors having residual charge after power down.

I suspect the 5300 mfd cap discharges pretty fast after power down and that the 2400 mfd caps hold their charge longer since their load is not as pronounced as the 5300 mfd cap.
The 2400 mfd capacitors provide -5 volts and +12 volts for the DRAMs which could be responsible for the residual storage.

This can be confirmed by leaving a voltmeter connected to the respective capacitors and watching the voltages drop.