Reset Circuit

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GianDO
Posts: 60
Joined: Sat Oct 10, 2020 11:25 pm

Reset Circuit

Post by GianDO » Sat Dec 12, 2020 8:41 am

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

Thanks
G
A2000 - MC68000 - KS3.1 - WB 3.1 - 2MB ChipRAM - Impact A2000-HC+8 Series II - 8 MB FastRAM - SD2SCSI 16GB - Indivision ECS

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GadgetUK164
Posts: 228
Joined: Fri Jan 04, 2019 2:26 pm

Re: Reset Circuit

Post by GadgetUK164 » 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
My YouTube Channel - www.youtube.com/GadgetUK164

GianDO
Posts: 60
Joined: Sat Oct 10, 2020 11:25 pm

Re: Reset Circuit

Post by GianDO » Sat Dec 12, 2020 12:56 pm

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?
A2000 - MC68000 - KS3.1 - WB 3.1 - 2MB ChipRAM - Impact A2000-HC+8 Series II - 8 MB FastRAM - SD2SCSI 16GB - Indivision ECS

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GadgetUK164
Posts: 228
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Re: Reset Circuit

Post by GadgetUK164 » Sat Dec 12, 2020 8:46 pm

Yes, the reset should go low on power on, then high as it comes out of reset.
My YouTube Channel - www.youtube.com/GadgetUK164

GianDO
Posts: 60
Joined: Sat Oct 10, 2020 11:25 pm

Re: Reset Circuit

Post by GianDO » Sat Dec 12, 2020 11:24 pm

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 ..
A2000 - MC68000 - KS3.1 - WB 3.1 - 2MB ChipRAM - Impact A2000-HC+8 Series II - 8 MB FastRAM - SD2SCSI 16GB - Indivision ECS

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GadgetUK164
Posts: 228
Joined: Fri Jan 04, 2019 2:26 pm

Re: Reset Circuit

Post by GadgetUK164 » 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!
My YouTube Channel - www.youtube.com/GadgetUK164

GianDO
Posts: 60
Joined: Sat Oct 10, 2020 11:25 pm

Re: Reset Circuit

Post by GianDO » Sun Dec 13, 2020 8:52 am

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
A2000 - MC68000 - KS3.1 - WB 3.1 - 2MB ChipRAM - Impact A2000-HC+8 Series II - 8 MB FastRAM - SD2SCSI 16GB - Indivision ECS

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GadgetUK164
Posts: 228
Joined: Fri Jan 04, 2019 2:26 pm

Re: Reset Circuit

Post by GadgetUK164 » Sun Dec 13, 2020 11:21 am

If it doesn't start even though Diag ROM says everything is OK, it's probably going to be something Zorro related.
My YouTube Channel - www.youtube.com/GadgetUK164

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