Faulty Atari 520STe - black screen

[markjw]

picked up a 520STe the other day, powers up but unfortunately only to a black screen. Someone has had it apart before as all the case screws and those holding the FDD are missing.

State of play with it:
1. Power LED is on but no FDD LED on power up
2. TOS ROMs are not original - pair of EPROMS installed in the two sockets instead of ROMS
3. Someone has written RAM 4MB on the bottom case so believe RAM has been upgraded
4. Tried a floppy drive that came from a 520STFM - no activity
5. Removed SIMMs 2 & 4 - no difference
6. Swapped SIMMs 2 & 4 with 1 & 3 - no difference.
7. Checked temperature on the chips - all are cold except the 68000 CPU which gets reasonably hot to the touch in a few seconds.
8. PCB looks very clean - some very minor corrosion on metalwork of the two joystick ports and the RF modulator

Not got access to a meter at the moment so can't check voltages - Any opinions on the next step very much appreciated

Would like to get this working as apart from some bad yellowing it is in good condition (no gouges, scratches or chips in the case and all the keys on the keyboard seem undamaged)

[troed]

Replacing TOS with known good chips is always an easy start. Verify the jumpers are in the correct position for the chips used.

[markjw]

Replacing TOS with known good chips is always an easy start. Verify the jumpers are in the correct position for the chips used.

I'll have to see if I can find someone to program me a set - or can the 2 ROM set from a 520STFM be used?

[mpattonm]

I have spare set of original 1.62 I can give you, provided that you cover postage from Czech rep. I am not sure what language they are, prolly DE, but that should not matter for debugging.

[troed]

No you'll need one for STE. Best is if you can test with someone else's since black screen isn't the typical TOS problem (that's usually a white screen).

[czietz]

No, the ROMs from an STF will not work in an STE.

There is not that much you can do without measuring instruments. You could try to carefully remove and reinsert socketed ICs in case the contacts are bad due to corrosion. You could try replacing the power supply (e.g. by one from the STF, they are compatible). Other than that you'll quickly end up in a situation where a multimeter or even an oscilloscope is required, I'm afraid.

Which kind of monitor are you using? Is it some kind of modern LCD / TV that will show you whether a video signal is present? Iirc, right after power-up you should see a 60 Hz color mode.

[czietz]

PS: As soon as you have access to a multimeter, you should first check, whether the RESET line is stuck low. I've heard of STEs where the reset circuitry (consisting of the push-button, 556 timer and 7406 buffers) was faulty and hence RESET was continuously asserted. This in fact causes a black screen.

[markjw]

No, the ROMs from an STF will not work in an STE.

There is not that much you can do without measuring instruments. You could try to carefully remove and reinsert socketed ICs in case the contacts are bad due to corrosion. You could try replacing the power supply (e.g. by one from the STF, they are compatible). Other than that you'll quickly end up in a situation where a multimeter or even an oscilloscope is required, I'm afraid.

Which kind of monitor are you using? Is it some kind of modern LCD / TV that will show you whether a video signal is present? Iirc, right after power-up you should see a 60 Hz color mode.

reseated both TOS ROMs - also checked H&L matched the sockets and jumpers were correct for the chip type.

Unfortunately the PSU in the STFM is dead (but hopefully only needs a re-cap) - I'll get hold of a meter (can't find mine) and check the output voltages.

Using RF input on TV so no sync info - oddly no loss of signal when reset is pressed, thought RF would lose sync so possible reset circuit is bad

[markjw]

just a quick update now I finally have some free time:

1. Checked the voltages on the 520STE PSU - 5.02V on the 5V line and 12.05 on the 12V line
2. 68000 CPU is getting very hot, very quickly still - reseated it - no change
3. Put a meter on the 68000 RESET PIN - first time was getting 4.93V, but on subsequent attempts got near 0V
4. Put the 520STE PSU into the 520STFM - 520STFM boots perfectly to desktop

So taking the PSU as being good - possible issue with the RESET line (or might not have been getting a good connection with the meter), or possible bad 68000 CPU seeing how hot it is getting.

Any suggestions on a better location than the CPU to check the RESET line?

[dlfrsilver]

as you stated it, let's make it simple : with the STE PSU, the 68000 behaves normally if i read you carefully, right ?

So this means that something on your STFM PSU is making the 68000 going "hot". One of the components or maybe some are causing this problem.

[mpattonm]

Just make sure the sympthom you see (working with one PSU, but not the other, while both read correct voltages) is not rather caused by the fact you have physically manipulated with your computer, while swapping PSUs. Just by lifting it up, shaking it, or turning it upside down you could make some intermittent connection working again.
CPU going hot quickly could indicate your CPU is not receiving clock signal.

[mlynn1974]

What are the states of the following pins?
HALT: pin 17
BERR (Bus error): pin 22
I am thinking that HALT will be low and BERR may be high.

As mpattonm says it would be wise to check the clock signal first.

[markjw]

What are the states of the following pins?
HALT: pin 17
BERR (Bus error): pin 22
I am thinking that HALT will be low and BERR may be high.

As mpattonm says it would be wise to check the clock signal first.

thanks to both of you for the suggestions - need to get my hands on a working scope first

Current state of the two machines I have:

520STFM - original PSU is dead - boots fine with PSU taken from STe (so working on that the STe PSU as being good and rest of STFM OK)
520STe - black screen when powered up using its' PSU (no point using the STFM one as dead) - 5.02V and 12.05V on the output - CPU is hot

Interesting that the CPU would get hot with no clock, I was expecting it to be the opposite.

Guessing these little LCD scopes on eBay (DSO138 etc) would be no good for checking clock etc - if anyone knows different please let me know, else I have either got to get my Hitachi scope working (circa 1980) or get a new scope (are the USB based ones for around £50 any good, eg the Hantek 6022BE). Logic probe would probably be worth me getting as well I would think (again any recommendations on something not expensive would be good)

[troed]

I like my DSLogic LA a lot. The Scope is a bit low bandwidth but otherwise nice.

https://www.dreamsourcelab.com/product/

[markjw]

ok, got myself a logic probe - results are:

BERR - HIGH
HALT - LOW
RESET - LOW

Also CPU replaced with one off eBay - same results.
Any way of checking the clock without a scope?

[mlynn1974]

I would suspect that the ROMs are at fault. I am not familiar with the STs physical ROM chips but I have seen arcade guys on YouTube with hardware to calculate checksums for EPROMs. That would tell you if they are bad or not. If either of them are bad then you should replace them with known working TOS 1.06 or 1.62 ROM Chips as stated.

[markjw]

I would suspect that the ROMs are at fault. I am not familiar with the STs physical ROM chips but I have seen arcade guys on YouTube with hardware to calculate checksums for EPROMs. That would tell you if they are bad or not. If either of them are bad then you should replace them with known working TOS 1.06 or 1.62 ROM Chips as stated.

ROMs in the machine are not original - pair of unlabelled EPROMS so no idea what version is on them. Will see about sourcing a replacement set - looks like there are two options 1.62 or 2.06 available

edit: set of 2.06 TOS ROMs on order - will update when they arrive and I've fitted them - fingers crossed

[markjw]

New TOS 2.06 ROMs arrived and have been fitted - unfortunately no difference to boot - still black screen and CPU RESET/HALT/BERR in the same state.

So looks like I'm going to need a scope to check the clock is running and take a look at the reset circuit to see what it is doing.

Any further suggestions greatly appreciated.

[mpattonm]

...and take a look at the reset circuit to see what it is doing.

I guess you have already answered your own question:

ok, got myself a logic probe - results are:
BERR - HIGH
HALT - LOW
RESET - LOW

Both RESET and HALT are "active low" signals, meaning your CPU is stuck in reset condition. Have you checked reset switch circuity, as czietz suggested (http://www.atari-forum.com/posting.php?mode=quote&f=15&t=34368&p=356632#pr353396)? Reset pushbutton is very common point of failure.
Once you have that rulled out, I would start removing all socketed chips sharing XRESET line (consult schematic i.e. here: http://www.gossuin.be/images/schemasata ... 600dpi.pdf) one by one to see which one is holding the line down. If it comes down to CPU and its getting VCC and clock all right, your CPU is most likely faulty. That would actually be the best case scenario, as unlike Atari custom silicon, they are dirt cheap and easy to get.

EDIT: I would order replacement CPU in any case, as replacing your stock CPU with HC variant for 2.2USD is always a good idea. Even if the original turns out not to be the root cause of your problem.
https://www.aliexpress.com/item/MC68HC0 ... 86143.html

[markjw]

...and take a look at the reset circuit to see what it is doing.

I guess you have already answered your own question:

ok, got myself a logic probe - results are:
BERR - HIGH
HALT - LOW
RESET - LOW

Both RESET and HALT are "active low" signals, meaning your CPU is stuck in reset condition. Have you checked reset switch circuity, as czietz suggested (http://www.atari-forum.com/posting.php?mode=quote&f=15&t=34368&p=356632#pr353396)? Reset pushbutton is very common point of failure.
Once you have that rulled out, I would start removing all socketed chips sharing XRESET line (consult schematic i.e. here: http://www.gossuin.be/images/schemasata ... 600dpi.pdf) one by one to see which one is holding the line down. If it comes down to CPU and its getting VCC and clock all right, your CPU is most likely faulty. That would actually be the best case scenario, as unlike Atari custom silicon, they are dirt cheap and easy to get.

EDIT: I would order replacement CPU in any case, as replacing your stock CPU with HC variant for 2.2USD is always a good idea. Even if the original turns out not to be the root cause of your problem.
https://www.aliexpress.com/item/MC68HC0 ... 86143.html

thanks for the info - CPU has been swapped with no luck - going to take a closer look at the reset circuit, think I've worked out how it works from a schematic I've found - should normally have 0v on the 555 output, going to 5v when the Reset button is pressed which feeds two NOT gates on U210 (a 7406? - diagram is a bit blurred) which should give me RESET high normal operating, going low when the Reset button is press.

(this is a learning experience for me - not touched electronics for years!)

EDIT - interesting, 555 @ U104 is getting hot (hadn't noticed that before) - appears to be no output on PIN3 (OUTPUT) and outputs on U210 are low. When the Reset button is pressed I get HIGH on the 555 OUTPUT

[mpattonm]

EDIT - interesting, 555 @ U104 is getting hot (hadn't noticed that before) - appears to be no output on PIN3 (OUTPUT) and outputs on U210 are low. When the Reset button is pressed I get HIGH on the 555 OUTPUT

Carefully lift pins 2 and 4 of U210 from PCB and measure the signal levels on them, while the are still disconnected from PCB. Both outputs are normally HIGH and should (only) go LOW when reset switch is pressed.

[markjw]

ok getting somewhere now!!!

Reset circuit is BAD - output on the 555 is floating when the reset button is not pressed so with no input the two gates on U210 are stuck LOW

Tied pin 1 of U210 to ground after powering the system up - immediately get HIGH on the CPU RESET PIN and video output. Get the Atari logo and it runs the memory test (4MB) - then get the desktop after 10 seconds or so

So now just need to figure out what has failed in the reset circuit - 555 getting hot is suspicious and there is a electrolytic cap at C102 which could be on the way out given its age

[markjw]

well finally good news - got the 555 U104 chip replaced and did a quick power up test - appears to be coming out of reset and attempting to boot from the floppy drive.

Next steps, de-yellow the case (it is more brown than gray) and either re-cap or replace the power supply