exxos's Atari, Amiga & retro forum

It's a bit odd, not consistent.

Sometimes when turning on the ~Reset line out of the TL7705 goes straight to high and stays there and it won't boot

Other times on switch on it will go high then low and the board will reset and boot. the reset switch has no effect pressing it the line does not go high and low again.

And on other times on switch on the line goes high and on press of reset button will go low and back to high. Screen also displays reset flicker.

Odd... I think the circuit is from the mega st... Might be worth a look?

Yes - Have been comparing. I noticed the pull up on the reset switch is 1K on MEGA ST and 10K on STF. Have tried both still the same.

Also the caps are different 10uF for the electrolytic where as STF is 47uF.

I will test this circuit out on breadboard for behaviour as I have a spare TL7705. That way I can play around with it easily.

OK - prototype version

This is the exact match of the Mega ST reset circuit from the schematic


Power it up and this is what I am getting on LA. You can see that the inverted reset out is going high and low sporadically.


Second is with selecting reset several times which is the yellow wire to GND.

Interestingly was also looking at other schematics around the reset circuit and ventured into the MegaSTE as well. It looks like Atari went towards a 556 away from the TL7705 on the Mega.

If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.

Actually, noticed that a second cap is connected across C3 on my Mega, so maybe this is the problem?

Second cap is a 470uf. The other Mega doesn't have one but is also more unreliable, so might try that on the other one too.

Forgottenmyname wrote: 10 Nov 2018 19:51 If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.

20181110_194921.jpg

Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?

Icky wrote: 10 Nov 2018 19:58

Forgottenmyname wrote: 10 Nov 2018 19:51 If it helps, here's a photo of my Mega reset circuit, in case something is a different value to the schematic.

20181110_194921.jpg

Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?

Must of seen the same thing at the same time :D

Icky wrote: 10 Nov 2018 19:59

Icky wrote: 10 Nov 2018 19:58

Thanks Forgottenmyname - I notice your C3 is two caps in parallel. What value are they?

Must of seen the same thing at the same time :D

470 and original is the 10