Atari 520 STFM white screen on power on. Doesnt boot to desktop

[Mike2909]

Hi. My name is Mike and I have just picked up a faulty 520 STfm that I need a little help with. Upon power on, all that is shown is a whitish grey screen. There is no activity from the floppy drive whatsoever and the LED does not come on. There is a faint low pitched beep coming from the tv speaker while it sits at the white screen however but this seems to stop after a while. During my initial fault finding I did come across a bad RAM chip at U5 (presumably added by a previous owner) which i replaced but it didnt change anything. Here is what I have tried so far without success

 

1. Checked PSU. No bad caps or anything. Voltages were 12.03v and 5.01v. All the VCC pins on the chips I tested showed between 4.9 and 5.0v

2. Reseated every socketed chip, cleaned the chip legs, cleaned the sockets themselves.

3. Reflowed anything that looked even remotely suspicious. 

4. Nothing on the board gets hot. The 68000 gets a little warm as does the big chip on the board linked to the floppy drive (sorry not sure what its called)

5. Tried booting with all the socketed RAM removed but no change.

 

So at the moment I am out of ideas. I do have a logic probe but only have very basic knowledge at the minute of how to use it. If anyone has any ideas or suggestions 

they would be much appreciated. 

 

Thanks

 

Mike

Edited June 13, 2021 by Mike2909
mistake

[cleoo]

Hi. I just picked up a 520STfm also and it’s hooked up to a SC1224 monitor.
Turned on the monitor first. Then turned on the 520STfm.
I get this:


I haven’t done any of the steps the OP did.

[ParanoidLittleMan]

"Checked PSU. No bad caps or anything. Voltages were 12.03v and 5.01v. All the VCC pins on the chips I tested showed between 4.9 and 5.0v "

Weak cap is actually bad cap. How you checked them ? Voltages are very good, but are they well filtered ?

Anyway, this is most likely some component failure. And to fix machine need to replace main parts in order.

Logic probe can help in testing diverse lines, but that's very complex and time consuming. Other ST is what really helps. Because STs are our friends ?

 

[+SoulBuster]

13 hours ago, Mike2909 said:

Hi. My name is Mike and I have just picked up a faulty 520 STfm that I need a little help with. Upon power on, all that is shown is a whitish grey screen. There is no activity from the floppy drive whatsoever and the LED does not come on. There is a faint low pitched beep coming from the tv speaker while it sits at the white screen however but this seems to stop after a while. During my initial fault finding I did come across a bad RAM chip at U5 (presumably added by a previous owner) which i replaced but it didnt change anything. Here is what I have tried so far without success

 

1. Checked PSU. No bad caps or anything. Voltages were 12.03v and 5.01v. All the VCC pins on the chips I tested showed between 4.9 and 5.0v

2. Reseated every socketed chip, cleaned the chip legs, cleaned the sockets themselves.

3. Reflowed anything that looked even remotely suspicious. 

4. Nothing on the board gets hot. The 68000 gets a little warm as does the big chip on the board linked to the floppy drive (sorry not sure what its called)

5. Tried booting with all the socketed RAM removed but no change.

 

So at the moment I am out of ideas. I do have a logic probe but only have very basic knowledge at the minute of how to use it. If anyone has any ideas or suggestions 

they would be much appreciated. 

 

Thanks

 

Mike

I had an STFM that came with a memory upgrade that was flaky on boot.  Sometime it boots, sometimes it get white screen, sometimes checkered, sometimes black.  I re-flowed ALL memory upgrade solder points and it was cured.  Go over all the solder points with a magnifying glass to make sure.

 

Peter is right, having another working ST around can help by swapping known working parts to test.

[Tillek]

20 hours ago, cleoo said:

Hi. I just picked up a 520STfm also and it’s hooked up to a SC1224 monitor.
Turned on the monitor first. Then turned on the 520STfm.
I get this:


I haven’t done any of the steps the OP did.

I don't know if I've ever seen a black screen like that.

 

Do you get any floppy activity?

[Tillek]

Actually, that's not entirely accurate... my SST Mega does this.. but in that case, the screen is all black when the monitor is on, not black with white frame.

[cleoo]

No activity from the FD. Light does not come on.

[Tillek]

Sorry... also forgot to ask... Any key clicks?

 

[cleoo]

The keys don’t click. Nor do I hear any sound through the monitor. (Does it have speakers?) 1st time ST owner.

[Mike2909]

Thanks to everyone for the replies. Sadly I dont have access to another ST to be able to swap chips back and forth. I did order an Exxos Diag card and null modem cable which has arrived this morning so hopefully i am going to give that a try tonight or tomorrow. Fingers crossed it is going to tell me whats wrong or a least point me in the general direction.

 

Mike

[zzip]

On 6/13/2021 at 5:58 PM, Mike2909 said:

Hi. My name is Mike and I have just picked up a faulty 520 STfm that I need a little help with. Upon power on, all that is shown is a whitish grey screen. There is no activity from the floppy drive whatsoever and the LED does not come on. There is a faint low pitched beep coming from the tv speaker while it sits at the white screen however but this seems to stop after a while. During my initial fault finding I did come across a bad RAM chip at U5 (presumably added by a previous owner) which i replaced but it didnt change anything. Here is what I have tried so far without success

 

1. Checked PSU. No bad caps or anything. Voltages were 12.03v and 5.01v. All the VCC pins on the chips I tested showed between 4.9 and 5.0v

2. Reseated every socketed chip, cleaned the chip legs, cleaned the sockets themselves.

3. Reflowed anything that looked even remotely suspicious. 

4. Nothing on the board gets hot. The 68000 gets a little warm as does the big chip on the board linked to the floppy drive (sorry not sure what its called)

5. Tried booting with all the socketed RAM removed but no change.

 

So at the moment I am out of ideas. I do have a logic probe but only have very basic knowledge at the minute of how to use it. If anyone has any ideas or suggestions 

they would be much appreciated. 

 

Thanks

 

Mike

Have you tried with the floppy unplugged completely?   I've seen boot problems when they aren't hooked up correctly

[Mike2909]

2 hours ago, zzip said:

Have you tried with the floppy unplugged completely?   I've seen boot problems when they aren't hooked up correctly

Hi mate. Yeah i have tried booting with it completely disconnected, machine shows no change in behavior.

[snarkdluG]

reset working?

[Fivetwenty]

On 6/14/2021 at 5:12 AM, cleoo said:

Hi. I just picked up a 520STfm also and it’s hooked up to a SC1224 monitor.
Turned on the monitor first. Then turned on the 520STfm.
I get this:


I haven’t done any of the steps the OP did.

Hi

I have seen similar symptons when I made a memory upgrade to my 520 stfm. It has memory under power supply (16 chips) and I installed sockets for another 16 chips+ some resistors. Something went wrong and I got black screen with white borders after boot. I resoldered all the socket pins and boom...It was working again. Maybe there was a shortcircuit or some pin(s) not properly soldered I don`t know. Maybe you have cracked solder joint somewhere in the motherboard?

[cleoo]

Interesting that you said that. The previous owner said that he did a memory upgrade. (Did I just buy this from you? lol)

[Mike2909]

2 hours ago, snarkdluG said:

reset working?

Doesnt appear to have any affect at all.

[Mike2909]

Now im probably barking up the wrong tree with this one but could a non functioning floppy drive stop the machine from booting? The reason i ask is because the drive belt on mine is broken

and was in about 5 chunks of sticky rubber inside the case. I was wondering if because the floppy drive isnt rotating due to the broken belt, is the computer picking this up when it tries to check the drive 

during boot and halting the boot process? Probably not but you have got to ask havent you lol

 

Mike 

[Fivetwenty]

2 hours ago, cleoo said:

Interesting that you said that. The previous owner said that he did a memory upgrade. (Did I just buy this from you? lol)

Haha, no I still have that ST. I like this motherboard revision which  it has because it has smd glue an mmu chips so no contact issues compared with those revisions with socketed ones. I have also one of those versions and sometimes it is more or less  unstable but works fine  a while after reseating chips. 30 years, sockets and corrosion is not so good combination I have noticed ,lol.

[TGB1718]

57 minutes ago, Mike2909 said:

Now im probably barking up the wrong tree with this one but could a non functioning floppy drive stop the machine from booting? The reason i ask is because the drive belt on mine is broken

and was in about 5 chunks of sticky rubber inside the case. I was wondering if because the floppy drive isnt rotating due to the broken belt, is the computer picking this up when it tries to check the drive 

during boot and halting the boot process? Probably not but you have got to ask havent you lol

 

Mike 

Shouldn't stop it booting to the desktop, may take a little longer to get there, but when it does, you may or may not have floppy icons.

The only way a faulty floppy might stop it booting is if it had some sort of fault/short on the floppy interface/cable.

[cleoo]

reset working?

I press on the reset and the monitor goes entirely white and then comes back with the black box.

[cleoo]

Hi
I have seen similar symptons when I made a memory upgrade to my 520 stfm. It has memory under power supply (16 chips) and I installed sockets for another 16 chips+ some resistors. Something went wrong and I got black screen with white borders after boot. I resoldered all the socket pins and boom...It was working again. Maybe there was a shortcircuit or some pin(s) not properly soldered I don`t know. Maybe you have cracked solder joint somewhere in the motherboard?

I will have to look closely and see if there are any broken solder joints. I’ll report when I get a chance.

Hopefully, Mike and I can each get answers to fix our 520STfm.

Thank you for all the help.

[cleoo]

Here’s the motherboard. Can you see what the problem is?

[Fivetwenty]

3 hours ago, cleoo said:

Here’s the motherboard. Can you see what the problem is?

Oh well...You have earlier motherboard revision yhan mine totally different layout with 1 meg ram directly soldered to the mobo. Schematics and service manual can be found here: https://www.gossuin.be/index.php/520-et-1040-stx

if it helps troubleshooting? Those 2 plcc chips could be removed,pins/contacts cleaned and chips reseated if it helps. Those sockets are quite fragile so plcc extractor would be handy...and try same procedure also for those 6 rom chips under psu...power supply caps seems ok but it doesn`t tell much they can be "well done"  anyway with high esr and low capacity. There should be about +5v on red wires and about +12 on blue one black is gnd?

One possibility would be if you have an another st to swap those socketed chips one by one to see if it fixes the problem. Main cpu seems to be soldered so it is difficult to try out that, PITA to desolder it. Fortunately, those are very common and quite cheap to buy replacement if needed.

Edited June 16, 2021 by Fivetwenty

[Mike2909]

Ok, I have been having a play around with the multimeter and logic probe on the ST today and have found a couple of things. 

 

Firstly the voltage on the reset pins of the 556 chip remain at 5.12v when the reset button is pressed. Should they drop to zero when the reset is pushed?

Secondly, the voltage at the reset pin on the 68000 shows 2.33v and doesnt change when reset is pushed. It shows no activity at all on the logic probe.

 

Anbody have any ideas please 

 

Mike

[TGB1718]

One half of the 556 is for the reset button and the reset pin should go to 0V when pressed, the other half

provides the power on reset, both outputs of the 556 are buffered by the 7405 whose outputs from pins 2 and 4

are connected together and then to the halt pin on the 68000.

 

When you press the reset button, pin 6 should go low and pin 5 should go high.

when not pressed, you should have ~0V on the output pins (5 and 9)

check for the same levels on the 7405 pins 1 and 3 correspondingly , pins 2 and 4 on the 7405 should be high.