Amstrad GX4000 - Video Problem but audio in Headphones

[josepprada]

Hello guys,

I'm in the process of restoring, or bring back to live a GX4000, when I bought her, she has power (green light) but nothing on the screen. And the controllers with the wire's completely cut.

I was aware of the double input voltage, so I never put a power supply on the 5v in (but I can assure that someone did it before).

First time I plugged in a 9v 1A PS in the 11v in, green light appears, but some smell of burn appears in a few seconds, so I powered down immediately, and began reading in depth the forum to find the issue.

Reading about burned smell, I saw that the if the RAM was bad, it heats a lot, so I put my finger on them and it was hot as hell! So I decided to replace it.

But nothing happens, no video, no audio.

I began doing measures in the Power Board, and I find that the Voltage regulator IC181 7805 had 9v in both pins (in and out). I replace it, but same happens, and find a video in Youtube about change the Diode D183. I replace it, and now the IC181 output pins is 5V!

So, I put a multigame cartdrige (like C4CPC )  that I bought power on, but no video at all. So reading the forum I find several references to no video, but audio in the MiniJack. So I connected headphones in the jack and EUREKA! There is sound!!! I connected a repaired controller and can hear changes in the menus or in the game, so the GX4000 process is working!!

Now let's go to my problem now, I have no video on RF or on SCART. On RF sometimes I see snowed images of the games if I move the RF antenna cable. So I have a problem on the video part.

My first test was to measure the other voltage regulator IC17 that I understand that powers the video section, and the measures are weird. I suppose to have 9v in and 5v out, but I get 5v in, 3v out. I did not replace the IC17 because if it was a problem with it, I'll see 9v in, so maybe the problem is somewhere else.

Can you help me find we're the problem is? What test can I do?

I only have a multimeter to do the readings, I do not have an oscilloscope or advance tools.

Thank you in advance!!

[McArti0]

Try check IC15 pin 15,16,17 RGB after DAC.

[josepprada]

Hello MrArti0,



I check IC15, these values are not fixed:

15 --> 0.05v - 0.10v
16 --> 0.35v - 0.40v
17 --> 0.10v - 0.15v

Thanks for the help.

[McArti0]

I check IC15, these values are not fixed:

choose the game with the whitest scene and the blackest scene and check again how high and low the voltages on these pins go.

[josepprada]

Ok, I'll try during this week and post it back the results.

Thanks!

[McArti0]

Check whether transistors Q3,4,5 between the base and the emitter have approximately 0.7 V
Connect nsync to the sound line. You should hear a 50hz buzz in your headphones. Try connecting the LUM signal straight to the TV video input.

[josepprada]

Hello MrArti0,

First, I cannot test the whitest game and blackest because I do not know what games I'm playing only by sound, so it's a little difficult test.

Although, today I manage to get audio from RF , and a snowy screen. I can only identify Robocop 2.

About the second answer: Q3, Q4 and Q5 with a game on, give me a measure of -3.70v -2.70v  -3.70 , so it seems weird .

I did not understand the last sentence, sorry I'm not used to work on electronic, I'm kind of a noob.

"Connect nsync to the sound line. You should hear a 50hz buzz in your headphones. Try connecting the LUM signal straight to the TV video input. "

Thanks for the help!

[josepprada]

Hello MrArti0,

Finally it's working!! I orderder a 78L05 voltage regultator, and change it today, and voila! Works perfectly!

Now I have to rewire the controllers because some buttons do not work propery, but the main issue is solve.

Thank you for your help!

Here is a Screenshoot of she working

https://imgur.com/a/Eww5bwl

[andycadley]

Woohoo. And welcome to the GX owners club.

[josepprada]

Thank you Andy!

I'm proud to join the club!