Apple Cinema display 23" ... detection problem

[Porto]

Hello,

Here I have an Apple Cinema Display 23" I bought. It was sold as non-functional and it was having this quite regular problem that the LED was blinking short-long-short, indicating a problem with the voltage input. Now, I have repaired several of these beauties and those which has this specific problem, a LM1117-3.3v voltage regulator was ALWAYS showing around 4v on the output... so, easy fix.

But not this time.... The LM1117 (800mA max) was outputting 4.059v as expected so I replaced the voltage regulator with a LM1084 (better power ratings = 5A max) as I normally do and the 3.3v is fixed but the monitor itself isn't detected when I connect the DVI plug to my computer. Normally the screen on the primary monitor flickers and there is a secondary monitor detected. With this screen nothing happens so I don't get an image either (Apple monitor only gives an image when it detects a signal on the DVI plug). This detection also works when the Apple monitor isn't connected to power, so only connecting the DVI plug must work.

Testing another identical monitor on the same DVI cable works, so it is definately the monitor itself which is at fault.

someone have a clue what to check?

[stj]

Re: Apple Cinema display 23" ... detection problem

ddi is a function of a special eeprom - something like 24lc21
it gets 5v from the dvi plug and has clock and data also connected.

if your sure the cable is good, find the eeprom and check it's connections to the dvi socket.
if they look good, you may need to replace or reprogram the eeprom - be glad you have a good one to get a dump from!

[Porto]

Re: Apple Cinema display 23" ... detection problem

ddi is a function of a special eeprom - something like 24lc21
it gets 5v from the dvi plug and has clock and data also connected.

if your sure the cable is good, find the eeprom and check it's connections to the dvi socket.
if they look good, you may need to replace or reprogram the eeprom - be glad you have a good one to get a dump from!

There are multiple 8 pin IC's on the logic board so I have to check them for purpose and Vcc voltage / shorts.

I've measured the voltages with the faulty and a working board side by side and found one fault; 2 pins on two different connectors don't have 5v on them, those connector pins are mainly coming from the DVI plug.
A continuity test learned those two pins are originally shorted together so they get their voltage from one source. I'll check where the traces leads to, maybe it's only a diode or resistor which is shorted.

I've taken two pictures of the board. The red marked pins must be 5v with only the DVI plug connected to a computer and the monitor detected.
The pins with an arrow below it are those which are faulty on this board; as said above, they are connected together with a resistance of 0.2Ohm between them. They are connected to pin 22 on the DVI connector, that's the TDMS Clock shield pin. On the faulty board these two pins are connected to ground, where on the working board these two pins are NOT connected to ground... so, I have a short to ground somewhere.



BTW: don't mind the TO-220 voltage regulator, it was there only for testing; it's already been replaced with a LM1084 dpak.

[stj]

Re: Apple Cinema display 23" ... detection problem

it's not about 5v,
the gfx card talks to the eeprom and reads the make, model and supported frequency and resolution tables from it.

the 5v is just to power the eeprom if the monitor is turned off.

btw, i cant see any pictures.
read the part numbers on the 8pin chips.

[Porto]

Re: Apple Cinema display 23" ... detection problem

The pictures are there now.

I've also added some text... seems I have a dead short to ground somewhere.

[Porto]

Re: Apple Cinema display 23" ... detection problem

I have isolated the problem to the Tcon board, well, it's the board directly connected to the LCD panel.

With the interconnect cable disconnected I have still have that short on that board, where the working board does not have this short in place.
On the second picture, the 3th pin from the right is shorted (the one which goes to that left ceramic cap).

This is what I was afraid of, because now there is a very big change the main chip is shorted and I need another Tcon board to fix this issue.


PS. I can't take the board from the other monitor because that one is working flawlessy!

[budm]

Re: Apple Cinema display 23" ... detection problem

It looks like pin 3 counting from the right (is that correct, 3rd pin from the right, right most pin is connected to GND (Left most pin is also connected to GND. Notes those two end pins are not counted as the pin as you can see pin 1 with the White triangle is the pin to the right of the left most pin), 2nd pin is connected to FL6) is connected to the Ferrite bead FL7, the other end of FL7 connected to the VIA.
So what DCV do you suppose to get on pin 3 on the working monitor? may be try lifting pin Fl7 to see what will happen. That pin 3 is small signal line so not much current flowing that traces.
If you can look up the spec sheet for the LCD panel then you may be able to tell the function of each pin.

[stj]

Re: Apple Cinema display 23" ... detection problem

i dont think the t-con has anything to do with identifying the monitor over the cable.
you should go back to looking for an 8pin chip begining with 24
or follow the ddc pins from the dvi connector.

if you plug the monitor into the power, push the power button on it and press the menu button - what happens?

[Porto]

Re: Apple Cinema display 23" ... detection problem

i dont think the t-con has anything to do with identifying the monitor over the cable.
you should go back to looking for an 8pin chip begining with 24
or follow the ddc pins from the dvi connector.

if you plug the monitor into the power, push the power button on it and press the menu button - what happens?

If I disconnect the Tcon board from the logicboard, the monitor gets detected by Windows; also that shorted pin on that 5pin plug (blue wire) is getting its voltage and so is no longer shorted to ground. Only when I reconnect the cable to the Tcon board the short is back and the monitor don't get detected again.

So, to be short, the detection problem is solved but now I have a new problem, a short to ground on the Tcon board, coming from pin 22 (TDMS shield?) on the DVI plug. This has to be 5v.

This monitor only has three buttons: power, brightness up and down. Very minimalistic, so to say.

[Porto]

Re: Apple Cinema display 23" ... detection problem

It looks like pin 3 counting from the right (is that correct, 3rd pin from the right, right most pin is connected to GND (Left most pin is also connected to GND. Notes those two end pins are not counted as the pin as you can see pin 1 with the White triangle is the pin to the right of the left most pin), 2nd pin is connected to FL6) is connected to the Ferrite bead FL7, the other end of FL7 connected to the VIA.
So what DCV do you suppose to get on pin 3 on the working monitor? may be try lifting pin Fl7 to see what will happen. That pin 3 is small signal line so not much current flowing that traces.
If you can look up the spec sheet for the LCD panel then you may be able to tell the function of each pin.

I will try lifting that pin and see what happens.
You are right, the two outer 'pins' are the stands for the connector and also grounded. So, in this case, it's the 2nd pin from the right that is shorted to ground (connected to FL7). There has to be 5v on that pin.

[Porto]

Re: Apple Cinema display 23" ... detection problem

I found the problem with the short to ground!

I followed the trace where that pin 2 was going to and found it led to this little 6 pin IC (see picture). It is unreadable on the picture but it says: 'Z17B'.
In the SMD codebook I found nothing by that code but found an identical looking decoder/demultiplexer IC with the SMD code 'Z19'... it's the 'NC7SZ19P6X' from Fairchild.
I guess this '74LVC1G19' from NXP (Philips) is the same IC.

Vcc is pin 5 and that's also the case with the IC on the board, also pin 1 is connected to pin 23 of the DVI connector and pin 2 is the ground; these two last pins were shorted when I measured the chip out of circuit so it's blown anyway.

I tested with the same IC from another, working, board and the short to ground was gone. I plugged in the DVI plug and the monitor was immediately recognized by Windows. After this I powered the monitor up and I got a clear and fully functional image!

Thanks for all the suggestions / tips, guys!


I hope I am right and it's logical to have such IC in this circuit; I guess it functions as some sort of voltage comparator or something?

Can someone confirm this kind of IC is used for this particular circuit? (waits for voltage coming in from DVI plug and activates the main graphics chip?).

[Porto]

Re: Apple Cinema display 23" ... detection problem

My above assumption (read: very wild guess) was wrong, it is not a decoder/demultiplexer IC.
Luckily I didn't already order it.

After a long search I found this AliExpress store which also did write the SMD code in the advert... because most sellers do not.
Also the pictured SMD chips have the print 'Z17B' so it must be the same ones I need.

http://nl.aliexpress.com/store/produ...048876370.html

It's a Schmitt-trigger, the 'NL27WZ17-D

[budm]

Re: Apple Cinema display 23" ... detection problem

Great job tracing out the board, according to this site, pin 23 is TMDS CLOCK+. Are you sure it is pin 23?

[Porto]

Re: Apple Cinema display 23" ... detection problem

Great job tracing out the board, according to this site, pin 23 is TMDS CLOCK+. Are you sure it is pin 23?

no, my bad, it's pin 22.

[budm]

Re: Apple Cinema display 23" ... detection problem

Pin 22 is the shield which I beliEve is the same as GND.
https://en.wikipedia.org/wiki/Digital_Visual_Interface

[Porto]

Re: Apple Cinema display 23" ... detection problem

Pin 22 is the shield which I beliEve is the same as GND.
https://en.wikipedia.org/wiki/Digital_Visual_Interface

Hmmm, I looked again and again and just now I see what I was doing wrong.

I wasn't really paying attention the whole time.
I looked at the pins on a MALE connector with that one horizontal pin on the right side but all the pinouts are done with a FEMALE connector oriented this way, so a male connector must be held upside down (and counting the pins from the right)....

It's pin 6, DDC Clock.

[Porto]

Re: Apple Cinema display 23" ... detection problem

@Budm,

Can you explain one thing to me.

I have that bad schmitt trigger IC desoldered from the board but being curious I started the monitor up and it powers up normally and gave me the Windows desktop; it's now running for a full hour, no problems whatsoever.

So I guess this IC isn't crucial after all for normal operation and I'm wondering what's it doing there in the first place??! Some sort of failsave and/or a voltage comparator?

[budm]

Re: Apple Cinema display 23" ... detection problem

https://en.wikipedia.org/wiki/Display_Data_Channel
https://www.olimex.com/forum/index.php?topic=2097.0
CLKEDID DDC Clock
DATAEDID DDC data

Extended display identification data (EDID) is a companion standard; it defines a compact binary file format describing the monitor's capabilities and supported graphics modes, stored in a read-only memory (EEPROM) chip programmed by the manufacturer of the monitor.

[stj]

Re: Apple Cinema display 23" ... detection problem

try this on both monitors:

[Porto]

Re: Apple Cinema display 23" ... detection problem

Budm, STJ,

I don't have a problem with the detection of the monitor anymore, everything is working as it should.

I was only wondering why this monitor works 100% despite the fact I removed that Schmitt trigger IC from the Tcon board!?