Problems with ST VIDEO DAC BUFFER PCB 0253 and ATARI 520ST

[sportingt]

Hello,

I tried the next step and had removed the three capacitors.
No effect. Problem is still there. (mono works fine, color not)
I noticed, that the DAC goes warm. approximate 38 degrees. Not very high, but warmer than other chips.
I tried to measure RGB output signals with a scope, but i can't get a stable out signal from the DAC.
2,6V at 16kHz, I don't now what is normal.
Incoming BLANK signal from the Glue seem stable at the DAC.

In the picture in the original instruction I can see, that the transistors Q4 and Q5 are missing.
Is it necessary, that additional parts eg transistors Q3,Q4,Q5 must be removed also?
Or only three resistors R27?

First I thought it will be a 15 min mod, but it seems to be more complicated.

[exxos]

As said before, I don't know the effect of leaving the RGB drivers in place. You might be causing a fault leaving them in.

I also assume you correctly traced back the RGB signals from the monitor port to work out where to solder the RGB wires to the DAC ?

Indeed it should be a simple install. Assuming soldering is good on the DAC and you correctly wire RGB wires. You should be good.

If you been following the numbering off my image it is likely different than your board. So you can't use those numbers you have to trace the RGB wires back. Also numbering on schematics may not match your board either.

All the install does is to disconnect the old RGB transistors which drive the monitor port and you feed the DAC outputs directly to the monitor port instead. I don't know of any other reason why it wouldn't work.

MONO goes direct from the shifter to the monitor port. It has nothing to do with the DAC board.

I'd suggest disconnecting the RGB wires and scoping them as maybe your shorting them out some how.

I also assume your using a standard RGB scart cable ?

[krs]

I must have somehow missed the part where you're supposed to remove the transistors when I installed mine because I did not do that either. I only removed the resistors. Just posting to say that it has worked just fine like that for me anyway at least, although my DAC chip gets a lot warmer than 38 degrees. Maybe I'll remove them too then if it's recommended to do so.

[exxos]

I must have somehow missed the part where you're supposed to remove the transistors when I installed mine because I did not do that either. I only removed the resistors. Just posting to say that it has worked just fine like that for me anyway at least, although my DAC chip gets a lot warmer than 38 degrees. Maybe I'll remove them too then if it's recommended to do so.

My board didn't work with the RGB drivers left in. I plugged my DAC board in simpley try the ram connector and all I got was a blank screen ( was booting the diagnostic cart) . So the shifter as driving the RGB transistors and the DAC didn't like it for some reason. It lead me to grabing my cam to record my install as had no choice at that point.

What motherboard did you install on ?

The DAC will get warm. It's driving into 75R loads which is like 200mA or about 1 watt. It doesn't sound like much but it's a fair bit of heat to dissipate. Plus whatever internal losses are on the IC. So more like 2 watts I suppose.

[krs]

My board didn't work with the RGB drivers left in. I plugged my DAC board in simpley try the ram connector and all I got was a blank screen ( was booting the diagnostic cart) . So the shifter as driving the RGB transistors and the DAC didn't like it for some reason. It lead me to grabing my cam to record my install as had no choice at that point.

What motherboard did you install on ?

The DAC will get warm. It's driving into 75R loads which is like 200mA or about 1 watt. It doesn't sound like much but it's a fair bit of heat to dissipate. Plus whatever internal losses are on the IC. So more like 2 watts I suppose.

Hmm interesting. I have a 070859-001 "REV." (yes it doesn't have any number/letter after REV.) board with SMT GLUE/MMU.

Aye I was told by Icky in the thread for the Video DAC board that they do get quite hot so I think it's normal. I put a little heatsink on mine. Have had it running for hours just fine.

[sportingt]

Hello, last Update.

what happened until now.
Step 1 - removed 3 resistors 27Ohm and feed new RGB signals
Step 2 - Removed 3 capacitors C117, C118, C119
Step 2.1 - Bring the signals directly from the DAC Board to the connector
Step 3 - disabled the whole RGB driver section by removing the 3 diodes CR4,CR5,CR6

Step 3 give a change, but it it is even worse.
(change from white screen with light yellow shadowy display, to black screen with light green shadowy display)

My conclusion is that the DAC-Chip must be defective or had shorts that I can not found.

Next step may be removing the transistors, but section ist complete trunciated yet? (maybe place diades in place and remove then transistors)
May be there are Ideas, whow I can test the new DAC, if its working correctly?

DAC:
See +5V is present, see CLK from BLANK-Signal is present, 9 Signals (RGB) from the Shifter (Leg 21 -Leg29) are present.
And something coming out of the RGB out pins.

Last Change.jpg

[exxos]

The Only thing you can do is send your board to me to try in my test machine while I have it out. If it needs a new DAC IC I will change it, but I bought 800 which were all factory sealed new, so you would be the first to have a actual faulty chip.

Step 3 - disabled the whole RGB driver section by removing the 3 diodes CR4,CR5,CR6

I really don't understand why you are not following the instructions and doing other things instead. When people go "off-script" your basically doing experiments which may or may not work. It just ends up complicating things unnecessarily.

There are like 20+ variations of the ST circuit. We only have a small number of them. We know the numbering is different, parts locations are different, there's all sorts of variations on parts used and various changes of the circuits. You really can't assume the schematic you are looking at is correct, complete, or matches your motherboard in any way. Its basing variations on the install instructions based on assumptions that what you are doing seems correct.

Leaving the transistors in place not only drives 75R load to the DAC directly, but you also have 100R to gnd via the transistors. If at some point you had that connected and even driving the monitor port via DAC and RGB drivers at the same time, you either short out the DAC outputs as transistors will switch slower than the DAC outputs and be at different voltage levels etc, or because 100R in parallel with the normal 75R load which then doubles the current thought the DAC outputs and could also damage it.

[sportingt]

Hello,
final update.

I solder in Diodes again. Again white Screen with shadowy outlines. (situation as before)
Remove the transistors Q3, Q4, Q5 seperatly. No effect!

I removed the DAC-Board and put in place all previously removed components.

All works fine again, with the typically ST low resolution quality restrictions.

In the moment I have no time for further test.
Thank you for your hints.

[sportingt]

"The Only thing you can do is send your board to me to try in my test machine while I have it out. "

... Is it possible to send it back for further test?
If you give me delivery address I will send it back to you!

It will be very interesting, if the board is ok or not.

Thank you for support.

[exxos]

Possibly after the post recovers after Xmas. But may take me a while to get around to it now as that machine isn't connected up anymore as I'm working on some STE stuff currently.