Atari 1040 STe white screen

[eight088]

Hi everyone,

I recently picked up a 1040ste and it seemed to work perfectly fine when I got it. I have it hooked up to my 1084s monitor with a composite cable. While playing a game the computer crashed, and after power cycling the machine I was only greeted with a white screen. I've tried to do a little bit of trouble shooting by following the Atari STe field service manual, found here (page 26 of the 69.7mb file) and have found the CPU has halted (low signal on the HALT pin). The reset circuit is fine, I don't get any unusual activity on the BERR pin of the CPU (always high), but I do get something that doesn't seem right on the 68901 chip.

In the manual, it says that the 68901 should respond to an MFPCS with DTACK. I monitored these two lines (on the 68901) when I turned on the computer, and there is activity on the DTACK line briefly when first turned on, but the CS line is high the entire time.

Does this seem wrong or do you think I'm going about this the wrong way? This is my first real attempt at using an oscilloscope to troubleshoot a computer, so I am welcome to suggestions and any help I can get. In the screenshot I took (attached), the yellow line is the CS line, and the blue line is the DTACK.

I should also add I haven't been able to test the ROM chips as I don't think I have anything to test them with.

Thanks for any help

 

[eight088]

A little update, not sure what I did but it's working again. Maybe a dry solder joint and reseating the rom chips for the hundredth time is what fixed it. Will have to go over all the solder joints and see if theres anything suss on the rom chip joints.

 

[g4ugm]

Atari STs are notorious for chips working loose in the sockets so I suspect your re-seating did the trick. In the day it was often suggested that you drop the thing onto a hard surface from a height of 2 or 4 inches (7.5-10cm) to re-seat the IC's. It wasn't something I tried in the day, and given the brittle nature of aged plastic its not something I would try now. Same with the solder joints. I wouldn't touch it, I feel you are more likely to cause problems than solve them.

 

[masteries]

Reseating could do the work, but if the power cycle was performed too fast, the data in some registers could not be cleared correctly.

From the experience, if I power cycle my STE too fast (less than a second) it can fall in a screen filled with bombs, till left to the right.

 

[SiriusHardware]

Are there square chips in PLCC sockets? My STe has two such sockets, and one of them was the cause of some bad / intermittent behaviour until I swapped in a new socket for the CPU.

If it starts to misbehave again, you obviously have a scope and hopefully at least one needlepoint probe - next time the symptoms are present go around all of the pins of the CPU and the other device which is in a PLCC socket - can't remember which one it is, look first at the scope 'picture' on the IC pin and then on its associated socket contact, making sure in each case that you are only placing the probe tip first on the IC pin, then on the socket contact - don't bridge or join the IC pin and the socket contact together with the probe point.

For each IC pin and corresponding socket contact you check you should see the same 'picture' on the IC pin and on its associated socket contact. If at any point you see a difference between what's on the IC pin and what's on the socket contact, that is telling you that the IC pin to socket contact connection is open circuit.

ICs in PLCC sockets exert constant pressure on the inside walls of their socket, but there is usually no equal and opposite force being exerted on the outside walls of the socket, so the result is that the walls of the socket gradually bow outwards over time. In some STs and STEs there is a brace fitted over the IC and socket, part of its job is to hold the IC down in the socket, but they often also incorporate down-turned metal side braces which hang down over the outsides of the socket and stop them from being forced outwards.

Problem 1 is that these braces are often either never fitted to the PLCC socketed ICs in STs and STes, or have been removed at some point. And problem 2 is that for some insane reason Atari chose to use PLCC sockets with a non-standard staggered pin pinout, whereas the standard pinout for a PLCC socket has the pins arranged 'straight'. So, if you do diagnose a faulty PLCC socket your problem is only just beginning. There are two ways that I know of to replace a bad PLCC socket in an ST / STe, one is to obtain the correct staggered pinout type from BEST Electronics in the USA if they are still going, the other is to use a special adaptor PCB which rearranges the pinout of a standard PLCC socket to the weird Atari staggered pinout. These were, possibly still are, made and sold by Exxos in the UK.

We have a long running product line at work with a mainboard which originally had 2 PLCC ICs fitted in sockets. After about 3-4 years we started to see that the sockets, originally incorporated to make servicing easier, were in fact causing most of the problems we were getting. (This is how I originally arrived at the contact checking method I mentioned above). Since that time those sockets, which happen to be the surface-mount (not through hole) type, have been treated as remove-on-sight. We take them off the PCB, fortunately the pinout of a PLCC IC is exactly the same as the pinout of the corresponding surface mount PLCC socket, so we can just solder the removed ICs straight to the pads which were originally occupied by the sockets. The ST and STe use through hole sockets, so unfortunately you don't have that option of removing the sockets and soldering the chips directly to the board.

 

[SiriusHardware]

In the day it was often suggested that you drop the thing onto a hard surface from a height of 2 or 4 inches (7.5-10cm) to re-seat the IC's.

I would never, ever suggest doing this to something which contains something as mechanically delicate as a floppy disc drive. Just asking for trouble.

Reseating the ICs (including any mounted in PLCC sockets) is worth trying but only if you do it properly, take the machine apart and remove and insert the ICs one by one.

 

[eight088]

Thanks for your reply Sirius. Unfortunately mine doesn't have PLCC sockets, and instead the ICs are soldered direct to the mainboard. I do have an amiga though that has PLCC sockets so your information will be handy to know to help troubleshoot it if I a problem with it.

Cheers