exxos's Atari, Amiga & retro forum

Hi,
what is the component corresponding to the NE555 of the A500 on the A2000?

Thanks
G

PST518B - It's connected up to an LM339. Either of those could cause reset problems.

I believe the 339 is used to reset based on the keyboard clock signal (ie. when you press Ctrl+Amiga+amiga) and perhaps has a "power good" type thing going on too where it can influence a reset?

It is confusing coming from the A500!

Edit: The above is for R6, the older boards use different components:-

LM2901 - U20
7438 - U16 (related to _HLT)
7407 - U12

GadgetUK164 wrote: ↑Sat Dec 12, 2020 10:23 am PST518B - It's connected up to an LM339. Either of those could cause reset problems.

I believe the 339 is used to reset based on the keyboard clock signal (ie. when you press Ctrl+Amiga+amiga) and perhaps has a "power good" type thing going on too where it can influence a reset?

It is confusing coming from the A500!

Edit: The above is for R6, the older boards use different components:-

LM2901 - U20
7438 - U16 (related to _HLT)
7407 - U12

I found a Rev6.2 which apparently has no damage, the battery seems to have been removed in time, but when turned on it gives a black screen. Somebody told me that the problem could be in the reset circuit and he thought it was the NE555 which is a chip that provides the reset signal to 68000 and start the A2000, but on the A2000 NE555 is not there, but it should correspond to the LM339.

So what I could do was remove all the socketed chip and try them on a working A2000 motherboard and luckily they all work, I tried them one by one: MC68000, Agnus, Gary, Kickstart, Denise, Paula, the two CIAs and the Buster. Putting them one by one on the working motherboard it turned on without problems. So at least these I would exclude them.
Then I did the test with the tester on pin 18 of 68000 and when I turned it on I immediately detected a peak of 4.95V which remained stable between 4.95 / 4.96V. It's normal?

Yes, the reset should go low on power on, then high as it comes out of reset.

GadgetUK164 wrote: ↑Sat Dec 12, 2020 8:46 pm Yes, the reset should go low on power on, then high as it comes out of reset.

As soon as I turn on the A2000 it is at 0V and after one second from switching on it goes to about 5V and remains like this ..

That's correct! It will go low for a second or so, then high. So it sounds like your reset is OK. It's probably corrosion related or a failed 245 on the buster side. Try running DiagROM!

GadgetUK164 wrote: ↑Sat Dec 12, 2020 11:51 pm That's correct! It will go low for a second or so, then high. So it sounds like your reset is OK. It's probably corrosion related or a failed 245 on the buster side. Try running DiagROM!

Yes, I could try, although I believe DiagROM assumes that some components are functional in order to boot. I don't think it starts regardless ..
I'll try it.

Thanks
G

If it doesn't start even though Diag ROM says everything is OK, it's probably going to be something Zorro related.