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

[Mike2909]

Thanks for the reply I think we may be in the right area now. 

 

The halt pin on the 68000 rests at 0.10v, raising to 0.16v at reset press.

 

556

Neither reset pins change when reset button is pressed. Both remain at 5.12v regardless.

Pin 5 goes high when reset is pressed and pin 6 goes low so they are working ok.

Pins 5 and 9 show zero volts without reset pressed so these too are ok.

 

7405

Pin 1 shows zero volts, which raises to 4.63v at reset press.

Pin 3 shoes zero volts whether reset is pressed or not.

Pin 2 shows 0.10v, which raises to 0.16v when reset pressed.

Pin 4 shows 0.10v, which raises to 0.16v when reset pressed.

 

Am i right in assuming it is the 7405 that has failed? Would it make sense to replace the 556 as well as the 7405? 

 

Thanks for your help

 

Mike

 

[ParanoidLittleMan]

Maybe could just check it with removed 74LS05 . Then should have at least some 4 V at reset pin of CPU. And connect it for short to GND will perform reset, like when pressing reset button while  all chips are OK.  Then will know will machine become alive, so no other error(s) .

[TGB1718]

2 hours ago, Mike2909 said:

Pin 1 shows zero volts, which raises to 4.63v at reset press.

Pin 3 shoes zero volts whether reset is pressed or not.

Pin 2 shows 0.10v, which raises to 0.16v when reset pressed.

Pin 4 shows 0.10v, which raises to 0.16v when reset pressed.

 

Yes, that's your problem, with pin 3 always at ~0V, the system is in a permanent reset state.

As @ParanoidLittleMan says, remove the 7405 and with luck after shorting pin 3 on the socket of the removed 7405

the ST should boot.

 

The 556 is probably ok, the second half only activates during powerup, pressing reset will not change it's status.

 

If you want to verify that half is working, short to 0V on pin 10 should make pin 9 go high, if it does, it's ok, if you release the short, it should go low again

[Mike2909]

OK I have had another mess with the ST. I removed the 7405, voltage at pin 3 was still zero with chip removed, shorted pin 3 but it made no difference at all. 

 

7405

Pin 1 shows zero volts, which raises to 4.63v at reset press.  Pin 1 now permanently stuck high. Reset press has no effect. Turning power on and off pin will occasionally go low but once reset is pressed it stays high.

Pin 3 shoes zero volts whether reset is pressed or not. No change

Pin 2 shows 0.10v, which raises to 0.16v when reset pressed. Now stuck at 0.16v, reset has no effect

Pin 4 shows 0.10v, which raises to 0.16v when reset pressed. Now stuck at 0.16v, reset has no effect

 

556

Neither reset pins change when reset button is pressed. Both remain at 5.12v regardless. No change

Pin 5 goes high when reset is pressed and pin 6 goes low so they are working ok. Pin 5 now stuck high, pin 6 ok

Shorting pin 10 to ground causes pin 9 to float, once short is removed pin nine goes high for a second then goes low

 

Halt and reset pins on 68000 both stay low, reset has no effect

[TGB1718]

5 hours ago, Mike2909 said:

 

With the 7405 removed, was the voltage at pins 2 and 4 of the socket approx. 5V ?

they are both connected to the same pullup resistor and to the HALT pin on the CPU.

 

Sorry, my bad instructions, with the 7405 removed and +5V confirmed on pins 2 and 4, a temporary short to 0V on pin 2 or 4 should reset the cpu.

if this works, then I think the 7405 is duff.

 

If you don't have +5V on those pins, then you need to check the pullup resistor it should be 1K

 

[Mike2909]

With the 7405 removed, was the voltage at pins 2 and 4 of the socket approx. 5V ? No, next to no voltage there at all

 

There is a resistor right next to the 7405 socket, im not sure if this is the correct one but it does measure almost exactly 1k. 

[ParanoidLittleMan]

Seems like CPU failure. Although, there are other chips connected to reset signal on bus - some must pull too much current, and that makes it low.

So, need to remove them to see which one is it.  That pull up resistor is bad is unlikely, but it is easy to check - just must desolder one pin of it to measure it properly with ohmmeter.

[Tillek]

On 6/16/2021 at 3:39 AM, cleoo said:

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

Oh yeah... totally.  The IC's under the power supply... C19... It's blown.

[Tillek]

No, not really... I just couldn't resist.

 

 

Maybe this should split into two topics?  Since we have two machines from different owners with obviously different issues... I'd hate to get things confused.

 

[ParanoidLittleMan]

Confused people will be always confused. Different issues ? Repairing electronic, computers has it's methodology. It is something what people learn over years - in specialized schools, by self. 

The real bad thing is that some think that is enough to describe problem, put here some photo and magic - and will get instructions how to fix it. Folks, this is not movie, this is real life. Things happen much much slower.

In normal times usual way was to bring machine into service. In normal time people would not mocking about other's troubles.

 

[TGB1718]

14 hours ago, Mike2909 said:

There is a resistor right next to the 7405 socket, im not sure if this is the correct one but it does measure almost exactly 1k. 

Unfortunately all the diagrams I have are so poor I can't make out the name of the resistor,

You could continuity check from pins 2 and 4 to the CPU HALT pin.

If that's OK it looks like there's a break on the pullup resistor tracks/solder joints.

 

Need to find out which resistor it is, I'll see if I can find a better diagram.

 

Edit: It could be R75 from a 520ST diagram

 

Edited June 20, 2021 by TGB1718
Update

[cleoo]

Did you fix the white screen problem?

When I was troubleshooting my 520STFM, I ended up breaking the reset switch accidentally.

Prior to that, I had gotten it working. It booted into the GEM desktop. Then I snapped the reset button and when I reinstalled it, the pins might have have remained stuck on reset so when I did boot it. I got the white screen and no green desktop.

I desoldered the reset switch and left it vacant and now it boots onto the desktop. That was my problem, may not be yours though.

[cleoo]

Just got a replacement reset switch from B&C Computervisions. Bruce sent me two. I soldered it into place. For some reason, it just displayed a white screen for a long time. I finally hit the reset button several times and it finally booted up.