OJ’s projects…usually breaking stuff.

[olivier.jan]

Pretty busy Atari day today...

1. I removed all the memory chips of the 520ST, and replaced it with proper sockets. Using soldering gun, hot air and braid. No trace or pad damaged :)

2. Tried to replace the chip with known good ones extracted from a Mega ST 2, just to realize the chips are differents (41256 in the ST - 16 pins and 421000 - 18 pins in the Mega ST) :(

3. Used hot hair to unsolder the original piggy-backed chips from one another and braid to clean every pin :headbang:

4. Built a quick and dirty DRAM tester using an Arduino Nano, 2 LEDs and socket...

5. Tested chips, tested chips, tested chips...

6. Re-installed 16 "good" chips in the newly installed socket and booted to ... a black screen again.

So, same as before, using the drag cart, the machine boots and I can access the menu from a serial console, but the screen stays black. I tested everything again, it now shows only 512k instead of 1M obviously, but apart from that all tests are fine.

Still, I have identified some new symptoms:

1. Using an Exxos "ST2VGA": Same behaviour as usual (Diag cart accessible via Serial, black screen, Sync is OK and changes depending on MONO/Colour and 50/60hz)

2. Using an UBE Switch: The machine keeps rebooting. On the serial terminal, I keep seeing :

I6 Testing Bus Error handling
No (I6) error



I6 Testing Bus Error handling
No (I6) error



I6 Testing Bus Error handling
No (I6) error

3. Connecting directly using a SCART cable: Black picture, Upscaler says it's using CSYNC and Sync is jumping all around the place.

So, maybe the ST2VGA is "hiding" the actual issue by forcing a proper sync signal...Will do some further tests tomorrow or next week.

I forgot to mention I also took some time to build an H5 while testing the DRAM chips... First boot gave me a pink screen full of garbage, after swapping parts it revealed a damaged shifter. Now I have a very nice 60hz white screen but nothing else. I actually managed to get 50Hz once or twice and even some bombs... Some further troubleshooting next week... I need to check as I have an old damaged Reset switch as the one provided with the H5 has 6 pins instead of 3...

(And yes, I reversed the amp IC which was upside down, installed the Flashy Clock link and all jumpers)

[olivier.jan]

Big progress on the 520 ST today !
I finally managed to get a picture:

IMG_2318.jpeg

I tested a lot yesterday, realized I had some signal on pin 2 (composite) of the video connector but the signal is very noisy and that might cause issues when using a SCART cable, so I reverted to RGBHV using a VGA cable but instead of using UBE switch or Exxos ST2VGA I decided to build one myself:

IMG_2319.jpeg

And this works in Mono or in Colour:

IMG_2320.jpeg

The only strange thing is the aspect ratio in colour mode which seems off to the right, I tried two different scan converters (OSSC and Medusa), both provides the same picture.

So, thi ST is kind of working, I still don’t understand what is off compared to other STs and prevents it from working with ST2VGA or UBE switch, but it’s still progress…

@exxos , are the ST2VGA schematics available somewhere to try to identify the difference with my simple cabling ?

[exxos]

I think the UBE schmatics are out there somewhere. Loads of people copied it...

Are you taking into account they need 12v ? Some ST and STF machines don't have the 12v feed to the RGB port (via 10k or 12k) .It powers the digital switching ic.

The next gen ST2VGA adapters won't need the 12v . Prototypes have been built for several months, but too many projects ongoing as always... I need to finish the case as well, 99% done..

[olivier.jan]

Your message gave me hope, I didn’t have any 12v on pin 8 of the video connector, so I added a12k resistor between this pin and pin 14 of U14 but it had no effect, still the same symptoms : with ST2VGA, I have sync but a black screen, with my homemade connection, it works, with UBE Switch the machine crashes.
One funny thing, with a standard SCART cable, the OSSC gets very briefly some sync, then looses it and gets another sync which is much higher and so on with sync signals being higher each time.

I guess something is off, I compared the clock signals with a working ST and the difference is minimal (32.0488 vs 32.0852).

I have a modern external PSU on order for the ST, i will check again when it arrives.

[olivier.jan]

Some more investigation today, but still no results.

First, I added 12v to Pin 8 of the video connector, using 1.2k ohm resistor (not a 12k ohm as suggested by Exxos as we need to provide at least 10mA).
This doesn't bring any more picture, but at least the ST is not rebooting when connected to UBE Switch, that's progress...

So far, same situation as before:
- Known good SCART cable (works with every other ST/STF/MegaST/STE/MegaSTE I have): Nothing at all.

- Exxos ST2VGA or UBE Switch:
- I get sync signals
- Black screen
- Machine is actually working, with Diag cart inserted I can see the Sync changing when pressing V in Colour (Switch NTSC/PAL)

- Home made adapter : I get a picture, even though it's shifted to the right in color (see previous posts).

So I used the scope to try to identify some issue, configured the adapter as Mono to only have one video signal to scope : There is signal !!!
Here are a few scope screen shots (Labels for VSYNC and HSYNC are inverted):

1. Trigger on HSYNC

SDS00003.png

2. Trigger on MONO

SDS00004.png

3. Trigger on VSYNC

SDS00007.png

If someone spots something wrong, I'm interested.
Signals are taken from GLUE 37&38 for Sync and from R42 (last resistor before the connector) for Mono.

[exxos]

Looking back it seemed you used cables straight into the RGB connector on your home-made adapter ? Possibly they are just too wide and damaged the socket?

With my adapter, you should be able to trace the MONO signal ( adapter must be set for MONO also) and you should get MONO output on the RGB VGA connector.

Check you have MONO output on the ST RGB connector, then plug in ST2VGA, and check on the PCB that you still have MONO..

[olivier.jan]

Just tried the following:

1. Connect my crude adapter made only of jumper cables, confirmed I had a picture on the screen and scoped the Mono signal.

2. Connect ST2VGA in Mono mode, confirmed I had only sync and a black screen and scoped the pin 11 of the ST co connector: There is a signal.

So the connector doesn't seem to be damaged, but... Here are the two signals I get:

1. With my home made VGA adapter:

SDS00009.png

2. With STGA:

SDS00008.png

So in one case I have 0-0.7v which is in specs for VGA but with the ST2VGA I have 0-2v which is not at all in specs...
And this has nothing to do with the ST2VGA which is working perfectly fine with all my other STs.

[exxos]

Mono from the shifter is 5v and buffered via a transistor which also switches from 5v.

0.7v output doesn't make any sense unless the mono buffer circuit has failed in some way. Or you have a heavy termination resistor on it or something dragging the voltage down.

[olivier.jan]

Ok, so som clear progress tonight: I finally have a picture using ST2VGA and UBESwitch.
A bit ashamed of not finding this one earlier : the other day I brought 12v to Pin 8 of the Video connector through a 1.2k resistor but I had forgotten to cut the connection of pin 8 to GND...
This is jut below the connector, so I had to remove the connector first in order to cut the trace.

Still, it's still not perfect:
- Mono is fine
- Colour still exhibits a shift of the display to the right of the screen (see previous post) and the picture quality is pretty bad.
- Still no luck with a standard SCART cable, not even a sync signal.
- If I use a Medusa (from Lotharek) as a scan converter instead of OSSC, the Mono display is unstable. I can see the sync signal changing a little all the time. Always around 71hz but to stable.

I'll try to check various settings of the OSSC and Medusa this week.
At least there is a picture now, which will make further troubleshooting easier.

[olivier.jan]

Finally getting somewhere…
Now that I can get a picture using ST2VGA, I tried to understand the remaining issues:
- No video when using a standard SCART cable
- Colour mode picture badly shifted to the right

Swapping the GLUE with another one fixed the shifting issue, picture is now perfectly centered.
For the SCART cable issue, I tested the cable with an STFM and it worked perfectly fine. But when I tested with a second ST, I managed to get a picture but it took a while, with sync initially coming and going before stabilizing.

The only common part between the 2 STs is the external PSU, which honestly doesn’t seems to be in great shape with ~600mv variation on the 5v.
Waiting for a replacement PSU, ordered one from Centurion nearly a month ago but nos shipped yet :(
I might try a bench PSU tomorrow to check if it’s better, but I won’t get the 12v