Question on composite output and the 520 ST

[cbmeeks]

I have two 520 ST's.  One is a 520 ST (original) and the other says it's an 520 STm.  The case has the holes for the RF modulator but the actual modulator is not in the computer (not sure why).

 

I also have a few 1040 ST's but they are just motherboards (except for one).  One of those 1040 ST's actually has an RF modulator and I was able to confirm it works with a composite cable.  However, this 1040 is very messed up (I accidentally cause some damage on the case about 15 years ago) and there are other issues with it.  But my 520 STm looks amazing besides some yellowed keys.

 

When I tested the 520 STm with the composite cable, I get nothing.  Do I need the actual RF modulator installed for that signal to be routed out?  If so, can I pull it off the 1040ST and use that one?

 

I know it sounds silly to want to use a 520 over an 1040 but I have plans on upgrading the 520 one day.

 

Thanks!

[cbmeeks]

UPDATE

 

I took a chance and removed the RF modulator from the 1040 and put it in the 520 STm.  I get nothing but a black screen.  Before I removed it from the 1040, I did verify that it worked.  The cable worked as well.

 

I'm pretty sure I didn't damage the modulator but I'm not sure how to tell.

Any ideas what could be wrong?

 

The board is a CO70243 REV B.

 

I also verified the CPU is getting 5V and the PSU checks out OK.

 

Thanks

[TZJB]

20 hours ago, cbmeeks said:

UPDATE

 

I took a chance and removed the RF modulator from the 1040 and put it in the 520 STm.  I get nothing but a black screen.  Before I removed it from the 1040, I did verify that it worked.  The cable worked as well.

 

I'm pretty sure I didn't damage the modulator but I'm not sure how to tell.

Any ideas what could be wrong?

 

The board is a CO70243 REV B.

 

I also verified the CPU is getting 5V and the PSU checks out OK.

 

Thanks

 

Composite video connects to J6 pin 2 but only if a modulator is present.

 

520ST_SAMS_Computerfacts_CSCS12_08974.pdf

 

 

SAMS shows the two variants, with the RF modulator pin 10 connecting to J6 pin 2 when installed. Confirm continuity with an ohmmeter. You may need to add the additional components to make the modulator work if they are not already in place.

 

Do you know whether the 520STM was working previously? It may pay to get a better screen in order to view the RGB and mono outputs of the Atari STs that you own.

Edited June 10 by TZJB

[cbmeeks]

I did confirm continuity.  Next, I brought out the oscilloscope and found the following:

 

First, the silk screen on the board reads from right to left (pin 10 on the left, pin 1 on the right).  Going by that, I see that:

 

Pin 4 = 60Hz (VSYNC)

Pin 5 = 15.8kHz (HSYNC)

Pin 6 = 5V (should be 12V)

Pin 7 = 0V  (should be 5V)

Pin 10 = Nothing.  0V, no signal.

 

It would seem the modulator is dead?  I don't see how I would have killed it but perhaps I did.  I'm usually pretty good at removing these components.

 

**UPDATE**

 

I connected two different TV's using pure RF (using an NES RF cable/adapter) and I see a very faint and distorted white screen.  Which makes me think the ST has life but the RF modulator is toast. 

Then again, I couldn't see any other text but that might be because the image is so poor but it was definitely showing some kind of image.

 

 

 

Edited June 10 by cbmeeks

[TZJB]

1 hour ago, cbmeeks said:

I did confirm continuity.  Next, I brought out the oscilloscope and found the following:

 

First, the silk screen on the board reads from right to left (pin 10 on the left, pin 1 on the right).  Going by that, I see that:

 

Pin 4 = 60Hz (VSYNC)

Pin 5 = 15.8kHz (HSYNC)

Pin 6 = 5V (should be 12V)

Pin 7 = 0V  (should be 5V)

Pin 10 = Nothing.  0V, no signal.

 

It would seem the modulator is dead?  I don't see how I would have killed it but perhaps I did.  I'm usually pretty good at removing these components.

 

**UPDATE**

 

I connected two different TV's using pure RF (using an NES RF cable/adapter) and I see a very faint and distorted white screen.  Which makes me think the ST has life but the RF modulator is toast. 

Then again, I couldn't see any other text but that might be because the image is so poor but it was definitely showing some kind of image.

 

 

 

 

Surely the PSU is dead with those voltages?

 

Maybe trace the power lines from the modulator to the source and check the modulator ground is secure and continuous.

 

Also is U41, Y3 and the other supporting components present? If not then the modulator won't work anyway.

 

Can you supply a picture of the motherboard?

[cbmeeks]

I think you may have solved it.

 

Attached is a photo of what my motherboard looked like BEFORE I added the modulator.  It's exactly like the photo from the schematic.

 

As you can see, I am indeed missing U41 and Y3.  What would it take to get all of this working or should I try a different form of video?

 

Unfortunately, a new ST or RGB monitor isn't in the budget right now.  All I have is a 1084S and some composite TV's (I do have S-Video).

 

 

 

[cbmeeks]

I found another schematic showing a much newer revision and it has those missing parts identified.

When I look in my parts, I realized I have almost all of those components.

 

Do you think it would be worth the effort to populate the board with those parts and try to make that REV B a true "M" version?  I've also read that boards without the RF modulator output a composite SYNC signal on pin 2.  Boards with the RF output composite on pin 2.  I'm thinking this board was not original to the case it's in.  If I populate those components, will the two different outputs step on each other?

 

Otherwise, I might try a plan B.  Many years ago I built an S-Video adapter for my Amiga that bought out the RGB signals to an NTSC encoder chip.  I might try and build one for the ST if I can find the appropriate cable.

 

 

[TZJB]

12 hours ago, cbmeeks said:

I found another schematic showing a much newer revision and it has those missing parts identified.

When I look in my parts, I realized I have almost all of those components.

 

Do you think it would be worth the effort to populate the board with those parts and try to make that REV B a true "M" version?  I've also read that boards without the RF modulator output a composite SYNC signal on pin 2.  Boards with the RF output composite on pin 2.  I'm thinking this board was not original to the case it's in.  If I populate those components, will the two different outputs step on each other?

 

Otherwise, I might try a plan B.  Many years ago I built an S-Video adapter for my Amiga that bought out the RGB signals to an NTSC encoder chip.  I might try and build one for the ST if I can find the appropriate cable.

 

 

 

Yes, this details your missing parts. If you add them you will achieve a composite ouput on J6 pin 2 as discussed and a modulated RF output, in addition to the existing RGB and monochrome outputs.

 

You will need to check the board that you harvested the modulator from and probably remove Q10 to remove COMP SYNC.

 

 

However adding the extra parts to enable the modulator seems to me a backward step.

 

The modulator output quality is never that good because it adds another video processing stage.

 

It is rarely used even in the Atari 8-bit computers, where the modulator is often removed to improve picture quality for a monitor.

 

Most people prefer the higher resolution of an Atari ST and a monitor using the Red, Green, Blue colour output.

 

The monochrome output is a higher resolution still and can be preferable to colour depending on the application.

 

Personally I have never used a modulated output and have always used monitors for this reason.

[cbmeeks]

I agree with what you are saying.  RF modulator and composite are not ideal.  However, I only have a Commodore 1084S which has S-Video and composite (and analog/digital RGBI).

 

I really want to build this S-Video adapter like I did for the Amiga.  The Amiga looks great using that adapter and I think I can build another for the ST.  However, I still may populate those components.  If I ever get a better monitor, I can just ignore the composite but at least it will be there.

 

Thanks!

 

[TZJB]

19 hours ago, cbmeeks said:

I agree with what you are saying.  RF modulator and composite are not ideal.  However, I only have a Commodore 1084S which has S-Video and composite (and analog/digital RGBI).

 

I really want to build this S-Video adapter like I did for the Amiga.  The Amiga looks great using that adapter and I think I can build another for the ST.  However, I still may populate those components.  If I ever get a better monitor, I can just ignore the composite but at least it will be there.

 

Thanks!

 

 

If you have said monitor it will work perfectly well with the Atari ST connected to the 9 pin RGB monitor input. This will provide a superior picture to any other choice of input.

[cbmeeks]

Hey, that's an idea.  I may try that too.  Although I still want to build the S-Video box.  If I use the 9 pin RGB, I won't get audio.  Anyway, I have some options now.  🙂

 

 

[cbmeeks]

Well, still nothing but bad luck.

 

I ran some wires over to the 1084S 9 pin dsub connector following the graphic below.

 

The only thing I get is a rolling signal.  If I pull the composite sync wire out, nothing changes.  It seems like the 1084 just isn't syncing.  If I remove everything and just connect green, composite sync and GND, I see green bars rolling.  Switching green to red shows red bars.  So I think the wiring is good.  Just no sync.

 

I tried this on four different ST's uncluding a 1040 that was confirmed working.  All show the exact same thing.  I also made sure the 1084 was in analog RGB mode.

 

I really don't have the funds for a new "ST approved" monitor at the moment.  It's frustrating because I feel like I have all the parts needed to get a working ST with a color picture to play some games.

The next move will probably be probing all of the sync signals with my scope and see if they are working.  I can't imagine all of them are dead.

 

**UPDATE**

On a whim, I connected my Amiga 600 using the same RGB port on my 1084S and it worked.  So it's definitely not the monitor.  In fact, I cannot believe how good the Amiga looked so now I have to make a cable for the Amiga.  lol

 

So this seems to be an ST issue.  But all four ST's not working??

 

**UPDATE 2**

I got it to work!  Sorta.  I used HSYNC instead of composite sync.  However, I can't stop the image from rolling.  It's rolling really slow but looks AMAZING.

 

 

Edited June 13 by cbmeeks

[cbmeeks]

**UPDATE**

 

SUCCESS!!

 

I was able to find the following pinouts for the 1084S-D  (not D2):

Pin Name Analog Mode Digital Mode

Quote

1 GND Ground

2 GND Ground

3 R Red

4 G Green

5 B Blue

6 I Intensity

7 CSYNS Composite Sync

8 HSYNC

9 VSYNC

 

Then it was just a matter of connecting the wires and I now have a solid display on my 1084S. 

Now on to making a custom cable.

 

Thanks everyone.

[cbmeeks]

Just for fun, I thought I would show my progress.  The 520 ST you see isn't the one I am restoring.  That one seems to not work.  I'm going to pull the RF modulator back off.  There's a good chance it never worked anyway.  Anyway, I am making a custom cable next.

 

Thanks everyone!

[TZJB]

12 hours ago, cbmeeks said:

Just for fun, I thought I would show my progress.  The 520 ST you see isn't the one I am restoring.  That one seems to not work.  I'm going to pull the RF modulator back off.  There's a good chance it never worked anyway.  Anyway, I am making a custom cable next.

 

Thanks everyone!

 

That looks very promising.  Normally HSYNC and VSYNC can be combined with the 120R resistors to make a composite sync which should work with any Atari ST, otherwise you could use the composite sync of an ST without a modulator. I am pretty sure that is what is in my cable to my Philips CM8833 which is very similar monitor.

 

It also has a short stereo phono lead connecting to mono sound pin 1 & GND of the Atari monitor output to connect to both inputs of the stereo audio of the Philips. It's very tight in the plug but it all gets crammed in there.

 

This is a useful site to lookup pinouts https://info-coach.fr/atari/hardware/interfaces.php

 

You just need some disk drives to play with now.

Edited June 14 by TZJB

[cbmeeks]

Well, things took a turn for the worse.  The cable I made no longer works.  Or, both previously working ST's stopped working.  Or, the monitor port stopped working.

I'm beyond frustrated.

 

I have tested the cable a dozen times and there are no shorts, etc.  Thing is...it DID work.  My cable also has an audio cable coming out of the end but it all worked.

The only odd thing I noticed is that on the DIN13 side, the shield is also connected to ground.  I'm not sure how that happened.  But again, it DID work like that.

 

I just don't have the energy to fight this anymore.  lol

 

[TGB1718]

1 hour ago, cbmeeks said:

The only odd thing I noticed is that on the DIN13 side, the shield is also connected to ground.

Shield should be connected to GND.

You said you have an audio lead, with that connected and the ST booted, does pressing any keys make the keyboard click,

remember to give the ST time to fully boot, if it does, then highly likely the ST is ok and it's just a video/lead issue.