Amiga 1200 Suspected DAC - not faulty?!?

[GadgetUK]

Hi,

I recently purchased a 1200 which was advertised as being faulty - displaying incorrect colours when tested with a 'Dizzy' game. The first thing I did on receipt is strip it down to see what state the board was in. Checked the caps - all look fine. Powered the board up and it worked fine with composite... So, since I don't have an RGB cable I thought the nearest test to check the actual RGB output from the port at the back is to use a Modulator from 500/500+, which I did and again that's flaweless.

I've examined the board pretty thoroughly and cannot find anything wrong anywhere. I've even done the tap and press test on components to see if there's a loose joint somewhere but all to no avail. It seems to be 100% Have I just lucked out here or am I missing something with my approach in testing the RGB lines out using modulator? It seems to make sense to me, whilst i've not looked at the schematics for the 1200 I know the MC1377P in the modulator takes RGB input in order to produce a PAL or NTSC signal, so I am guessing that the DAC could not be the problem.

It has been on test for 3 or 4 hours now all OK.

EDIT: I am even wondering if someone has just booted from floppy a 'Dizzy' game that just doesn't work palette wise with a 1200? Anyone know of anything like that? Most games either refuse to load or just show graphical corruptions / crash etc.

[GadgetUK]

Just looking at schematics now, I see AR, AG, AB, which I assume are analogue RGB going out to the D port, which is probably what the modulator from A500 works with. I also see DR, DG, and DB, which I assume are digital counterparts? I guess I need a digital cable or something - is that just a normal VGA cable off EBAY?

EDIT: I am guessing if the digital RGB isnt working then it's going to be U32 (74HCT244), or one of the resistors in series. Cannot see what else if anything could be wrong with this.

EDIT2: I've just thought, I guess I could use my logic analyzer on the 3 output pins - will it just output on those 3 pins all the time?

[jimbob]

I'd guess if a modulator works on the RGB output then you are fine and have lucked out. Digital RGB is generally not used, I think it is only for old 8 colour monitors of some kind.

But, I think there may have been a thread recently where someone had some funky business happening on a 1200 with a modulator. Modulator worked but RGB output black and white only or something. You should try to test the RGB directly. RGB to scart cable into a TV should work and will give you the best quality output without adding an indivision or similar.

[kipper2k]

Hi,

check this thread and see if it helps...

http://eab.abime.net/hardware-pics/7...-solution.html

[TomCrazy]

@GadgetUK
Do you only have these problems with a certain Dizzy game? If so, have you tested what happens when you boot up the same game on another A1200?

Also, have you tested to start a program like Protracker where you have three slidebars for each of the three RGB colors respectively, and then tried to move these slidebars around to see if the colors are changing on screen as they should?

Just looking at schematics now, I see AR, AG, AB, which I assume are analogue RGB going out to the D port

Correct.

which is probably what the modulator from A500 works with

Yep. And composite sync, if I remember it correctly.

I also see DR, DG, and DB, which I assume are digital counterparts?

Do you mean the DR, DG and DB at the Dsub23? Yep, that is RGB digital out. But it's not really the counterpars of the three analog RGB signals.
On the analog RGB signals, you will get a picture where each color has an 8 bit resolution. That is, 256 different intensity levels of each color.
The digital outputs work a bit differently, however. If you take a look at the schematic and examine how they are connected, you will see that these digital outputs will not deliver all the colors that the analog outputs can deliver, because many of the bits that are constructing the picture before the D/A conversion are not included in the digital output at the video port.

Therefore, the picture coming out from the analog RGB-signals is a 24 bit picture (8x3), while the picture coming out from the digital RGB pins is a 4 bit picture (R + G + B + intensity with one bit each = 4 bits).
What this means is that when the three analog RGB signals are working together, the Amiga can display any color from a palette of 16.777.216 colors, according to how many bits are available in hardware for choosing a certain color.
The RGBI (digital) output however is building on the same hardware idea as the CGA standard, and can thus only output a color chosen from a fixed 16 color palette.

Powered the board up and it worked fine with composite...

Then it works fine with RGB also, because the composite video is constructed in the video encoder from the analog RGB signals.

My guess is that you're just trying to run an incompatible game on the A1200, a game that was originally written for the OCS/ECS chipset-based Amigas and is incompatible with AGA-machines. Testing the very same gamedisk on another A1200 can help either verify or disprove this theory.

[GadgetUK]

Thanks for the info Jimbob, Kipper2K and TomCrazy! I haven't been able to fault the machine at all - it was sold as claiming to show incorrect colours and the seller said he tested it with a Dizzy game. I cannot find anything wrong with it at all, so it must have been an original game of the 500 which only works on OCS or something.

I've inspected the board again this morning and it looks like new, ie. there's absolutely no cap leakage or anything like that around any of the video circuitry. There's always the chance the seller tested it on a dodgy RGB cable I guess, or they could have a faulty display etc.

[Loedown]

Originally Posted by GadgetUK

Thanks for the info Jimbob, Kipper2K and TomCrazy! I haven't been able to fault the machine at all - it was sold as claiming to show incorrect colours and the seller said he tested it with a Dizzy game. I cannot find anything wrong with it at all, so it must have been an original game of the 500 which only works on OCS or something.

I've inspected the board again this morning and it looks like new, ie. there's absolutely no cap leakage or anything like that around any of the video circuitry. There's always the chance the seller tested it on a dodgy RGB cable I guess, or they could have a faulty display etc.

Dizzy game was probably Spin Dizzy.

[TomCrazy]

@GadgetUK
It seems to me that several sellers on ebay are selling items as "not working" or "untested", when it in fact is working without any problems at all. I guess some people are selling things on ebay that they don't know much about, and don't have the knowledge and/or the time to test it properly before selling it. Lack of knowledge can also make someone believe that there is a hardware fault when there in fact isn't.

I guess you were lucky this time to get an A1200 that was working better than the seller stated.

[GadgetUK]

Originally Posted by TomCrazy

@GadgetUK
It seems to me that several sellers on ebay are selling items as "not working" or "untested", when it in fact is working without any problems at all. I guess some people are selling things on ebay that they don't know much about, and don't have the knowledge and/or the time to test it properly before selling it. Lack of knowledge can also make someone believe that there is a hardware fault when there in fact isn't.

I guess you were lucky this time to get an A1200 that was working better than the seller stated.

Yes, I think you are correct. I've lucked out this time, there appears to be nothing wrong with this 1200. Damn, I am never going to get to replace a PLCC chip on one of these old machines lol.

I recently bought 2 Dreamcasts supposedly with 'no video', expecting to replace the DAC on both and suprise suprise neither of them had any faults what so ever. Now the same thing with this 1200, I had again expected a DAC fault and it's just perfect...

[Turran]

Ebay...

"Not working" = Probably works. Just lazy seller.
"Not tested, sold as seen" = Dead for sure.
"Works fine" = Works fine, except for the parallel, serial, external floppy and/or joystick output.

[GadgetUK]

Originally Posted by Turran

Ebay...

"Not working" = Probably works. Just lazy seller.
"Not tested, sold as seen" = Dead for sure.
"Works fine" = Works fine, except for the parallel, serial, external floppy and/or joystick output.

Hahaha!!! That is so true, especially the "Not tested, sold as seen", 99 times out of 100 that means, "I've tested it and its knackered so in order to maximize its value I will pretend I don't know how to test it".

[TomCrazy]

Originally Posted by GadgetUK

Yes, I think you are correct. I've lucked out this time, there appears to be nothing wrong with this 1200. Damn, I am never going to get to replace a PLCC chip on one of these old machines lol.

Just out of curiousity, how come you have such high desire to replace a PLCC chip on an Amiga motherboard that you are going through the trouble of actually finding a faulty board on ebay in order to have a justified reason to perform such a procedure? Have you recently aquired some new cool soldering tools that you would like to try using, perhaps?

[demolition]

Maybe you just never knew that you were actually colorblind in some way, which is why you can't see the problems?

[GadgetUK]

Originally Posted by TomCrazy

Just out of curiousity, how come you have such high desire to replace a PLCC chip on an Amiga motherboard that you are going through the trouble of actually finding a faulty board on ebay in order to have a justified reason to perform such a procedure? Have you recently aquired some new cool soldering tools that you would like to try using, perhaps?

Hehe, tbh I am just looking for a semi challenging repair involving a PLCC chip. I only really got back into electronics mid last year and haven't done anything SMT related for many years, and infact when I last worked on Amigas and STs as part of my job, SMT tech was only just becoming popular.

Originally Posted by demolition

Maybe you just never knew that you were actually colorblind in some way, which is why you can't see the problems?

Hahaha!!