exxos's Atari, Amiga & retro forum

Hi all,
new to the forum and finally worked up enough courage to ask some questions. Joined a few weeks ago and have been
reading/studying all that I can about the Atari st (when I can spare the time) and have learnt a fair amount from the wisdom of
the posts and guides here. Atari st was my first computer back in the early 90s which I used for mainly sequencing midi files and the
odd game. Recently recalled having an STE stored away in the garage along with an STFM (ebay purchase that was a bit of a basket case)
and decided to dig them out and see if they still worked. Unfortunately both failed to boot up. Searched online and discovered
this forum, now starts the mission to see if I can revived them.

Started with the FM. Desoldered the reset switch after checking it with DMM, disassembled cleaned and reassembled, rechecked with DMM
and seemed to work ok. Found that contact grease had solidified over the years causing bad contact. Next recapped the PSU and cleaned
main board.Then came the keyboard, previous owner(s) had done a dodgy solder repair to the Mouse/joystick port which I think took out the
keyboard IC chip. Solder pads and traces were destroyed and they attempted to hard-wire it but not very well. Managed to redo repair
and found replacement chip (ebay). Cleaned and recapped floppy drive.
Fm now boots up and works fine..via a SM124 monitor.

The STE has had the same treatment. Cleaned main board, cleaned reset switch, recapped PSU, cleaned recapped floppy, cleaned
reseated socket-ed chips ( only 2 on this model, toss and floppy controller).
Unfortunately STE won't boot up..Get green led but nothing else, black screen. If I disconnect power to floppy drive ( data cable
still connected) the yellow floppy led will light and stay lit.
Tried known good ram, replaced Tos (1.62) with new Tos (2.6) but still won't boot.
Floppy, Keyboard, PSU all work fine in the FM.

Took some voltage readings on main board with DMM but only on the axial caps ( that's the limit of my experience thus far)

Board CA401177 no Rev# 1991

C100 = 5.03V
C102 = 0.01V
C103 = 5.05V
C107 = 12.18V
C202 = -5.68V
C204 = -9.09V
C205 = 0V
C215 = 0V
C216 = 0V
C561 = 0V
C600 = 5.04V

Should some of those caps have a voltage on them? I suspect a fault with the 555 timer chip but don't really know.
Hope that someone here may be able to point me in the right direction as to what to check or do next.
Thanks for any help
Cheers

:welcomewave:

As you mention problems with reset switches, first thing to try is make sure the reset signal is actually working. Use a meter on pin 3 of the 555 chip.. You should see it switching between 5V and 0V as you press the reset switch on the back.

Thanks for the welcome and the reply.. will try that next opportunity I get .. cheers

I'd also have a look at the sockets.

I've run into a few problems with the PLCC sockets that are often used, as over time (30+ years!) the contacts tend to push out of the way, and when you get a bit of tarnish on the chip and socket contacts.

So it's worth removing the chips from their sockets and very gently adjusting the spring contacts in the sockets. They don't need much, but it can help them to reconnect with the chips. If you have any, clean the chip pins and socket connectors with a little IPA.

Double check also that the dot on the chips matches the dot on the socket to make sure you have correct orientation.

While you're at it, clean the contacts on the SIMMs with IPA too and re-seat, taking care to ensure they are correctly locked in, and check the board for anything that could have caused a short. Use a magnifying glass to check the surface mount chips as they have very small pins that are easily bridged.

Also, what speed SIMMs are you using, it may be they are too fast. 70-80ns SIMMs are what I have in my STE's and they seem to work well, but I'm not sure if faster will be happy (I'm sure somebody will correct me if I'm wrong here!)

Also, if you have swapped TOS, check here: http://matsp888.no-ip.org/~mats/Text/Co ... st_ste.htm and make sure you have the W102-W103 jumpers correctly set for the type of ROM you have, and that they are correctly oriented in the sockets.

When it powers on BTW, do you get a floppy 'seek' noise? Have you tried an alternative screen? If you're connecting using SCART, try connecting using just composite, or to a VGA display in HiRes (You may need an adapter to do this). Is it a totally black screen, or a black screen with a white border?

Good luck!

Thanks rubber_jonnie will check all that over the weekend . I'm fairly sure I did get the jumpers for Tos right, the site you linked looks great so I'll check there as well.. thanks again.

You're welcome.

Taken a while to get time to work on this ste as I'm dealing with health problems which seem to take up any spare moments I have.
Managed to do all the above that exxos and rubber_jonnie suggested. Voltage readings on Pin 3 of the 555 chip were strange and intermittent
so decided to remove the reset switch and take another look. The copper slide contact has worn as has the contacts on the switch, so it's beyond it's life expectancy and will have to replace it. This means the one on my FM probably won't be far behind.
For testing purposes I dropped a jumper between the pin holes of 1 and 2 on the board where the reset switch was fitted and took some readings on the 555 chip. On leg 2 of the chip I got +5v but on leg 3 0v. I was expecting to see 5v as it's the output of the 555 chip.
I will have to order another reset switch from the exxos store before I can go any further with testing and restoring this machine. I had planned to get some of the goodies from the exxos store down the track once the unit was up and running but will have to order a reset switch asap.
thanks again for the help and will post again when I'm further along.
Cheers

MagneticFever, I can recommend these from exxos' store:

https://www.exxosforum.co.uk/atari/store2/#0114

I replaced a broken reset switch with one of these and it was a 100% perfect fit.

Thanks rubber_jonnie.
I substituted the reset switch from the FM for the sake of testing as a temporary measure and the results have got me a little confused.
Voltage on pin 8 is 5.03v but voltage on pin 3 is 0v until reset is pressed then it reads 4.57v. I expected to see 5v and 0v when reset was pressed.
I've ordered a new 555 chip in case it's duff but could something downstream be causing this fault?
cheers

Aw, damn shame it's still down.

Quick Q, have you tried to boot it with different RAM configurations i.e. swap around the SIMMs so you try different combos and only two at a time.