Mega ST4 - no video output at all

[asapreta]

Hello Everybody!

I just became owner of a Mega ST4 which supposed to be booting to White screen but it turned out to have no video output.
I get a no signal banner in the monitor.

The cable in question is the same I use in my working ST520, in this very same monitor.

Prior to turning on the computer itself did a visual inspection on the board for shorts, tracecuts, etc.

I noticed the reset button is broken so I removed it and installed one while I don't get a new one to fit correctly its place. To eliminate a possible "always resetting state" despite I think reset works in pulses, so even keeping the button pressed there would be just one reset pulse.

And also tested the PSU not connected to the mainboard and I got the correct voltages.

With this complete no video situation, I asked the old owner about anything else he could tell me about the machine and he was brief: when it had blank screen sometimes "a hit or two" on the case solved the problem.

I searched the web and got the information GLUE and MMU are interchangeable with the ones in my 520ST.
And with both swapped between the machines, the 520ST worked fine.

As the two machines have only two EPROMs, tonight I am going to swap them to check if the ST4 ones are good.

I was also thinking in swapping the SHIFTER but not sure it is a good idea.
The ST4 has a C025914 and the 520ST has the C070713. Are they interchangeable?

I downloaded the service manual and there, yesterday, I started my journey into the ST troubleshooting world!
I got a clock present on pin 15.
I can see the reset pulse when the button is pressed.
BERR is TTL high but HALT is always low, it quickly goes up when reset is pressed but stays low then.

If I can determine the TOS are good later today, I read in this forum, lifting the HALT pin and connecting to the RESET one would give some light on the problem.

Should I start this way or any hints for a different approach?
I saw this post and I am about to start reading it, looks like the same issue: viewtopic.php?p=29471#p29471

Thanks in advance,
Cheers

[rubber_jonnie]

First off I'd start by removing all the socketed chips and clean/reinsert them using some contact cleaner. The bang on the side suggests bad contacts hence my suggestion.

BUT be very careful, these boards don't like manual handling, I had vias break on mine (Possibly the post you quoted) just by plugging in an IDE card, so handle with care.

You may also need to consider this: PLCC contact tweaking. but I'd do the clean and reinsert first.

[asapreta]

First off I'd start by removing all the socketed chips and clean/reinsert them using some contact cleaner. The bang on the side suggests bad contacts hence my suggestion.

BUT be very careful, these boards don't like manual handling, I had vias break on mine (Possibly the post you quoted) just by plugging in an IDE card, so handle with care.

You may also need to consider this: PLCC contact tweaking. but I'd do the clean and reinsert first.

Hi rubber_jonnie, thanks for taking your time to reply my post.

I am still reading your crusade into the ST4 fix.
I have already removed, used contact cleaner on the socketed chips and no go.
I will exchange the CPU socket because it isn't really a socked (a bunch of smaller sockets cut to fit the 64 pin IC) and this will cover the check for bad soldering/broken vias there.

The former owner had fitted the EPROMS swapped and told me before the ST4 arrived to me.
So, concerning manual handling, I will, after replacing CPU socked check all traces between CPU, MMU, GLUE and mostly with the EPROMs.

And, I think I read you got the "no signal" also in your monitor, then were able to get a white screen.
I have no signal and the former owner told me it was suppposed to give a white screen.
What have you done, swapped monitors?

Thank you.

[asapreta]

I have just swapped monitors and there is a diference now:

IMG-20241017-164511151-HDR.jpg (258.69 KiB) Viewed 516 times

White border with black center screen

[stephen_usher]

Are you getting any sync signal on the video output at all? Those signals are generated by the glue and it should be outputting something as long as the Glue has power, clock and has been reset.

Check to see if the ROMs are being enabled.

[rubber_jonnie]

I have just swapped monitors and there is a diference now:



White border with black center screen

Hi, can I ask that you please remove the link as it doesn't show on the forum and we don't allow externally hosted images.

Please edit the post to add an attachment and then place it inline, using the cursor to position where you want it in the text.

BTW I have removed you from newly registered so you can post freely now.

Thanks Admin.

[asapreta]

I have just swapped monitors and there is a diference now:

IMG-20241017-164511151-HDR.jpg (258.69 KiB) Viewed 517 times

White border with black center screen

Hi, can I ask that you please remove the link as it doesn't show on the forum and we don't allow externally hosted images.

Please edit the post to add an attachment and then place it inline, using the cursor to position where you want it in the text.

BTW I have removed you from newly registered so you can post freely now.

Thanks Admin.

Hi rubber_jonnie, thanks for the image tip, I was used to add images like that.
And also thanks for authorizing my posts.

About the Mega ST4, I have just checked the EPROMs and both work fine no the 520ST.
Now I will start the battle of continuity.
First CPU x GLUE!

[rubber_jonnie]

Hi rubber_jonnie, thanks for taking your time to reply my post.

No problem

I am still reading your crusade into the ST4 fix.
I have already removed, used contact cleaner on the socketed chips and no go.
I will exchange the CPU socket because it isn't really a socked (a bunch of smaller sockets cut to fit the 64 pin IC) and this will cover the check for bad soldering/broken vias there.

Hopefully it's useful to you

The former owner had fitted the EPROMS swapped and told me before the ST4 arrived to me.
So, concerning manual handling, I will, after replacing CPU socked check all traces between CPU, MMU, GLUE and mostly with the EPROMs.

It's not unusual to have different EPROMs, some of mine do where I've done upgrades, what would help is good quality photos of any work that's been done, and by that I mean high res and properly lit.
The manual handling is a real problem, and the Megas seem a bit more prone to issues, however if you do as you've suggested and check all the pins between the new CPU socket and other main components then that will definitely help.

BTW, just so you're aware the PLCC sockets have a non standard pinout, sou don't pull them thinking you can just buy new, you can't get them easily any more.

And, I think I read you got the "no signal" also in your monitor, then were able to get a white screen.
I have no signal and the former owner told me it was suppposed to give a white screen.
What have you done, swapped monitors?

Thank you.

It's possible, it's been a while so my memory isn't so clear, other than my machine needed an awful lot of work and broke more than once after I'd fixed it!! You'd need to give it a good read through to find out what I did.

Be aware that if the cable you use is monochrome, you may well see a different screen, all black, than with a low/med res cable that could show all white or black with a white border. I prefer to use low/med res for troubleshooting as it is a bit more informative than the all black high res.

EDIT - I just saw your picture after you updated the post, white border and black screen is likely a RAM issue, so MMU or RAM IC's, at least in my experience.

[asapreta]

EDIT - I just saw your picture after you updated the post, white border and black screen is likely a RAM issue, so MMU or RAM IC's, at least in my experience.

I was reluctant to accept it is a RAM issue as it was "too easy" as a Googled a bit and found it was usually the cause.
But as the MMU is good, I tested it on my 520ST, I need now to determine which are the faulty RAM ICs without pulling them all out

I don't have a Test Cartridge, so will try with the scope to determine which ones have different patterns from the others.

Thanks for the help.
Will try to come soon with "good" news.

[stephen_usher]

Well, seeing as the machine isn't able to get anywhere suggests the lowest bank of RAM, so you only need to look at the lowest 16 chips.

Myself I'd use an oscilloscope to look at the data lines so see if any look "strange". Even without the CPU the Shifter will be trying to do memory access.