Dead Mega ST1

[holger.müller]

Hello,

my Mega only shows White screen. /HALT line is low, even when U12 is removed, so it Looks like /HALT is created by booard itself.
/Reset line is okay.

Diag ROM won't start

Already done:

Check 12V & 5V lines, all ok. PSU recapped
New Reset switch
New U12 74LS07, new reset circuit, GLUE & MMU exchanged to previous good tested one's.
Sockets of GLUE and MMU re-soldered
74F244 & 74LS373 already exchanged
2 Version TOS exchanged to 6 x TOS EPROMs.

Any further ideas, hints etc. ? Currently I am a little bit lost.

[rubber_jonnie]

Sounds like the problems I had with my dead Mega 4, you may want to have a look through that at some of the troubleshooting work done for that: https://www.exxosforum.co.uk/forum/viewt ... =18&t=2163

Whilst there is a LOT in there, there are many suggestions from the excellent techies on this forum, plus I detail what fixed all of the problems I had with mine.

One thing, avoid handling the board too harshly as they are quite susceptible to mechanical failure. I had mine fixed and then I ended up with 2 damaged data lines at the CPU because I plugged something in!!

With /HALT it is important to realise that it can either be the CPU setting it low because there is some garbage making it do that, or it can be brought low by something signalling from the mainboard.

First and foremost, I'm assuming since you have a white screen you're testing in low/med res? Hires isn't good as you don't see anything but a black screen.

It's worthwhile testing the reset circuit behaviour. Using a multimeter see what /RESET does at the CPU. I know you replaced the chip, but the behaviour is important.

It should go low on pressing the button and then on release after a brief period go high again. If you hold reset down you will see it go low and then high on release. Ideally you need a scope or logic probe, but a multimeter will do.

Test /HALT the same way. Hold reset, it should go low, release reset it should go high. A scope really helps here because it's hard to see the transition with a multimeter as it happens so fast you may miss it.

Next remove the GLUE and MMU and clean all the pins and tweak the contacts in the socket: https://www.exxosforum.co.uk/forum/viewt ... hilit=plcc

If you've already done that, then check all the 5v and GNDs are good to all the chips with power on (I know you did this, but worth doing again) and that the clocks are good to all the chips. You can find clock values on the chip pinouts.

What tools do you have? Do you have a scope or logic probe? They are really useful, and logic probes are quite inexpensive these days.

Sorry, yet another question? Do you have a diag cart? These are super helpful, as you can send data out via the serial port and read it on another machine using something like Hyperterm or Teraterm even if the display is blank. Mine has been indispensable.

Also, since you've swapped from 2 to 6 chip ROM set, make sure you've done everything here: https://www.exxosforum.co.uk/forum/viewt ... b2ed5a8a30

[czietz]

You see HALT being driven low by the CPU due to a double bus fault. Unfortunately, this can have a lot of reasons. Search this forum (and other forums) and you'll see a lot of similar errors with different causes.

To get the most obvious thing out of the way: When you say you changed to 6 TOS EPROMs, I assume you also changed the required (solder) jumpers on the mainboard, right? Also, I assume these EPROMs have been tested successfully in another machine, correct? (Unreadable TOS ROMs are one cause of double bus fault.) As rubber_jonnie says, please test with the Atari diagnostic cartridge and a 9600 8N1 serial terminal. If you don't get any output there, either, it's most likely a very fundamental fault, such as a open or shorted data or address line. This cannot be sensibly debugged remotely. If you have a scope or (better) a logic analyzer you can try to narrow down the fault on your own.

EDIT: For example I once successfully fixed a ST by looking at these traces. But -- as you can see -- this requires a logic analyzer.

logic.PNG (75.64 KiB) Viewed 5275 times

[rubber_jonnie]

You see HALT being driven low by the CPU due to a double bus fault. Unfortunately, this can have a lot of reasons. Search this forum (and other forums) and you'll see a lot of similar errors with different causes.

To get the most obvious thing out of the way: When you say you changed to 6 TOS EPROMs, I assume you also changed the required (solder) jumpers on the mainboard, right? Also, I assume these EPROMs have been tested successfully in another machine, correct? (Unreadable TOS ROMs are one cause of double bus fault.) As rubber_jonnie says, please test with the Atari diagnostic cartridge and a 9600 8N1 serial terminal. If you don't get any output there, either, it's most likely a very fundamental fault, such as a open or shorted data or address line. This cannot be sensibly debugged remotely. If you have a scope or (better) a logic analyzer you can try to narrow down the fault on your own.

EDIT: For example I once successfully fixed a ST by looking at these traces. But -- as you can see -- this requires a logic analyzer.
logic.PNG

Good point @czietz about the LA's, they are quite cheap these days and can be very helpful for this sort of thing.

[holger.müller]

1st of all, Thanks!

I test with Low/Mid Res, with Mono I get a black screen.

With Diag-Rom and X-cable (9600 8N1) nothing will be showed on 2nd machine.

/Reset line already checked with multimeter (toogling hi/lo by Pressing switch).
/HALT remains on low

I got an 2 x scopes, I will check Crystal etc I just did a quick and dirty measurement with multimeter so see if there is a voltage..

I just erased, splitted and burnt & compared the new EPROMs, so I assume they will work.
Jumpers (W2,W3,W4) were set accordingly

Removed blitter fix also and fixed cutted pcb line under the 68000.

PC-Speed was installed, so I am going to check the data lines and resolder 68000. Due to mechanical movement of PC-Speed perhaps….

Next steps:
Tweak and clean the MMU & GLUE PLCC again
Replace the 74LS11 for /OE TOS / ERPOMs

[czietz]

Note: When I said "diagnostic cartridge" I was not referring to DiagROM but to a proper diagnostic cartridge such as this: https://www.exxosforum.co.uk/atari/store2/#0165.

With the scope, have a look at the activity on ROM2. On a 6-ROM machine, this is pin 20 of the ROMs in location "Lo-0" or "Hi-0". Are there only a few pulses after reset, like on the trace I posted above? Or is there longer activity on ROM2?

[rubber_jonnie]

Note: When I said "diagnostic cartridge" I was not referring to DiagROM but to a proper diagnostic cartridge such as this: https://www.exxosforum.co.uk/atari/store2/#0165.

With the scope, have a look at the activity on ROM2. On a 6-ROM machine, this is pin 20 of the ROMs in location "Lo-0" or "Hi-0". Are there only a few pulses after reset, like on the trace I posted above? Or is there longer activity on ROM2?

Completely agree, it needs to be a diag cart with the Atari field service diagnostic ROM like exxos sells. That way you can get the Atari diagnostics over serial and run them, or at least pick up error messages via the terminal.

If you had a PC speed fitted, was it soldered or in a socket, as I said, they are susceptible to mechanical faults (As you possibly suspected), so I'd also consider buzzing out all the lines from the CPU to make sure they do actually go where they are supposed to.

You can scope out things like the address and data lines at the MMU and GLUE, but what @czietz suggested at pin 20 of the ROMs is one of the places I looked at, and I compared to a working ST, so if you have one you can compare against that is also helpful. I've also attached some pinouts you may find useful and some notes taken from the H4 troubleshooting section that are relevant.

One of the problems I came up against with my Mega 4 was a bad GND on one ROM chip. It was a cold joint and wasn't immediately obvious until I buzzed out the GND connections. Also check for shorts 5v to GND, and between pins.

[holger.müller]

Thanks again

PC Speed was in a soldered socket on top of the cpu.
CPU resoldered.
With your Docs, I will buzz the lines.

Changing 74LS11 and bending / cleaning PLCC socket and GLUE & MMU wasn't successfull either.

Swapped EPROM from me working 1040, also no success.

Do you have a hint for a cheap logic analyzer ? My scope shows some short peaks on PIN20 /CE of the EPROMs when resetting

[troed]

Do you have a hint for a cheap logic analyzer ? My scope shows some short peaks on PIN20 /CE of the EPROMs when resetting

I'm happy with my DSLogic (Plus I believe it is).

https://www.dreamsourcelab.com/product/dslogic-series/

/Troed

[rubber_jonnie]

Thanks again

PC Speed was in a soldered socket on top of the cpu.
CPU resoldered.
With your Docs, I will buzz the lines.

Changing 74LS11 and bending / cleaning PLCC socket and GLUE & MMU wasn't successfull either.

Swapped EPROM from me working 1040, also no success.

Do you have a hint for a cheap logic analyzer ? My scope shows some short peaks on PIN20 /CE of the EPROMs when resetting

You may want to look here: https://www.exxosforum.co.uk/forum/viewt ... c+analyser