exxos's Atari, Amiga & retro forum

Stephen_User,
Let me guess..I have to remove the motherboard and flip it over to see the revision of my motherboard? Just my luck. Well looks like I need to do that anyway to reseat all of the removable chips. Everything will pretty much be removed except for the motherboard.

On my Mega STE, once I removed the PSU, I could see the Rev #.

Well, it does NOT have the daughter board. But it DOES have the 1.44 MB floppy drive.

Thank you.

Randy

Which side are the ROMs and does it have SMD or socketed custom chips?

You can use this for identification: https://www.atariancomputing.com/blog/t ... on-guide-4

However, if you're going to be replacing caps and/or the reset switch you'll need it out of the case anyway.

stephen_usher,

I did not remove my TT motherboard, but did remove what I thought was the important items so I could determine what my TT motherboard revision is likely to be. This appears to be my TT motherboard:
This is photo of the bottom right corner of my board:
Also, my TT is the German model.

So, while the co-processor is mounted in a PLCC socket, the 68030 is a SMD chip.
I am still trying to locate the Reset Circuit. Still looking.

I hope the two photos appear properly in this post. Please let me know if they do not and I will try to post them again.

Thank you for suggestions and guidance. I appreciate it very much.

Randy

According to the schematic C108 is the cap which is used by the 556 timer to run the reset. On your board it's in the bottom left of the board, near the keyboard socket and right next to the 556 timer chip.

exxos wrote: 05 Sep 2023 13:13 It looks like the reset circuit has 2x 10uF so you could just solder another 10uf on top of those and see if it makes any difference or not.

Actually, the same capacitor set appeared in two places there, the electrolytic being C108 in both circuits somehow. I guess by sharing the capacitors it guarantees that both timers run for the exact same amount of time.

stephen_usher wrote: 06 Sep 2023 21:59 Actually, the same capacitor set appeared in two places there, the electrolytic being C108 in both circuits somehow. I guess by sharing the capacitors it guarantees that both timers run for the exact same amount of time.

I don't think they are the same caps because they have different pull-up resistors. Difficult to tell with the blurry numbers for everything.

S'pose it could be C106 and C108 as they're both next to the 556 dual timer.

stephen_usher,

You are correct. the cap on the left is C106 (10uF 50V), the cap on the right is C108 (10uF 50V). This is the Reset circuit. I will need to order two of the 10uF 50V Radial Electrolytic Capacitors, as well as a new Reset Switch, just to be safe. May as well do it all as long as the motherboard is removed. Unlike other STs,the capacitors in the TT Reset circuit are Radial capacitors, instead of Axial capacitors.

The question is should I increase the value of the of the capacitors to maybe 22uF?

Thank you again for all of your help. The Exxos Forum is great and the members are happy to assist anyone with questions.

Regards,
Randy

The original 10uF values work perfectly in thousands mainboards, so there is no reason to install larger ones. The fault must be have other reasons. The Powergood line from the powersupply is important for example...

Changing the capacitors in the timing circuit run by a 555 or 556 timer needs calculation. Just blindly changing it is inadvisable.