1040 STE boot problem after installing Exxos PSU

[Magnus]

Hi,

I recently bought a new Exxos PSU for my 1040 STE.

After installing it, the computer won’t begin booting. I only get at black screen, no floppy disk activity. However, if I press the reset switch, it starts booting normally and reaches the desktop.

If I hold the reset button while powering on and then release the reset button, everything boots perfectly.

I checked the voltages and I get 5.16V and 12.24V with the new PSU under load in the machine, so to me that looks correct.

My questions are:

- Is this harmful to the computer?

-Why this behaviour? With the old PSU this never happend.

- Is there something I can do to make it boot without holding the reset button? I read about “reset delay circuit” and adding a larger cap. Is this the thing to do?

- Should I replace the reset button? I read on the forum that it can give all sorts of problem.

I would really appreciate any help!

/Magnus

[exxos]

Its a common problem with caps going bad in the reset circuit. You need to replace them, maybe even increase the value.

The new exxos PSUs have a current limited "soft start" on power up, its to limit the inrush current to about 0.5amps rather than power up with the full 5amps etc. Unfortunately the softstart shows up as symptoms you mention when the reset circuit has gone bad and no longer gives the correct reset time.

I have never seen this happen on any of my machines. But I do hear reports of such issues now and then. These machines are 30 years old now. Any hardware modification can cause odd side effects. They all end up with their own quirks.

[Magnus]

Thank you for your very quick reply Exxos!

I’ll replace the caps in the reset circuit. Do you know where I can find them on the motherboard or where I can read more about replacing them?

/Magnus

[exxos]

Likely C102.. its sort of near the front left side of the machine... just solder another 22uF across it, see if it cures it.

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[Magnus]

Thank you again Exxos! I wish all customer support was this good!

[Magnus]

I just wanted to mention that in the end, my problem wasn’t solved by just changing C102 to a 47uf cap, but I also needed to change C100 to a 47uf cap. Now, everything boots perfectly!

[exxos]

I just wanted to mention that in the end, my problem wasn’t solved by just changing C102 to a 47uf cap, but I also needed to change C100 to a 47uf cap. Now, everything boots perfectly!

Thanks for letting us know! I found some odd things with the reset circuit today in fact. It does seem like C100 needs to be a higher value than C102 (the diagram has it the other way around!)