exxos's Atari, Amiga & retro forum

So you have another good PSU that works in another machine, which means you have a good power source that does not work, and by the behaviour you stated, it seems to be going through repeated on/off cycles.

You may want to have a look at the /RESET line on the CPU, see what that is doing, it might indicate a problem with the reset circuit tripping repeatedly.

Next I'd suggest looking at the MSTE schematic and working in from the power connector, looking at the power good signal and where that is derived from, and if that is also cycling, see if you can determine what's causing that.

What might be an advantage is if you have an ossciloscope, you may be able to catch the timing of events and see if the power good or reset trips before the 5v line drops.

thanks for the reply.

Well I do have a scope. Have not really had the time to learn more than the basic measurements.

I will see what I can figure out from the reset. I think I am more on the idea of some kind of short.. Cap 227 comes out of the power connector 5v. It looks good. I am going to clean up my area today and find my CAPacitor tester. IT is 470UF .. a significant charge.. all the others are none polorized .1uf caps.. dont think they could effect this.. none ground out.

James

Usually a short doesn't cause power flickering on and off. A short usually ends up blowing a fuse if you're lucky. And since the psu seems good, it doesn't sound like that. I'm thinking something in the reset circuit also.

Also a thought....someone correct me if I'm wrong. I know you tested the psu with a hard drive, but I think a psu can still fail under a higher load such as when you are powering the whole system even though it seemed fine with just a hdd only. Do you have another known good psu to try in the megaste?

You can use any old AT-PC PSU too ...

Wonder if a flaky voltage regulator could cause these symptoms?

TheNameOfTheGame wrote: 04 Feb 2021 18:48 Do you have another known good psu to try in the megaste?

He says in the first post he's tried a known good PSU..

Bikerbob wrote: 01 Feb 2021 16:23 So is it Powergood that I am not getting right here? and thats why the PSU is not starting up? I get its a protection, but not sure what I am supposed to be checking here. I notice the reset is not in this part of the circuit, and I assume that could still be one of the issues.

As others mention.. you need to check POWER GOOD line and RESET line..


RESET should be 0V when you turn on the ST (or holding down the reset button) it should go to 5V shortly afterwards.. you can check on a meter...

(can't for the life of my find the reset circuit in the MSTE diagram!)

exxos wrote: 04 Feb 2021 19:09

TheNameOfTheGame wrote: 04 Feb 2021 18:48 Do you have another known good psu to try in the megaste?

He says in the first post he's tried a known good PSU..

Ah, thanks, I missed that.

exxos wrote: 04 Feb 2021 19:19 (can't for the life of my find the reset circuit in the MSTE diagram!)

Search for the power connector in schematics. Trace the POWERGOOD to the LM556. There you have HALT and RESET.