G7400 black screen

[Spacemonster]

Recently I acquired a G7400/00. The on/off button was broken and it doesn't give a picture.
Got a lot of tips from this great forum. I have now replaced the on/off switch, it works again. Fortunately the local electronics store was still able to supply an exact replacement.

I haven't been able to figure out what's wrong with the image yet. When the G7400 is switched on, it only shows a black image. No sound, very occasionally the image jumps a bit when you press one of the keys. But no further action, tried this with different working cartridges. I also cleaned the cartridge connector in the console.
In the meantime I have replaced the TEA1002 PAL encoder chip (still managed to find one somewhere), but unfortunately no result. The power supply supplies 5 and 12 Volts.

Does anyone have any idea what it could be and what else I can try? Unfortunately I have no other parts to try.

Gerko

[Xuul]

Try to replace IC652 (Video processor)

Also check the 5V, 12V and GND on the PSU PCB for cold solder joints.
All G7400's I've disassembled so far had these bad joints as seen in the picture.
The ones on the mainboard were always good, though.



Cheers,
Xuul

[Spacemonster]

Good tips, thanks Xuul. I have replaced the IC652 without result. Also tried IC652 from the G7400 in a G7000, this works fine, so chip is okay.
The solder tips on the PSU and Systemboard are fine, I soldered them again just to be sure.

[Xuul]

You're welcome.

Are you sure the RF-Modulator is ok?
You could build a composite AV mod for the G7400, it's very easy and you just need a couple of cheap components:
G7400 Composite AV Mod

Also, there are 2 service manuals, one for G7000 Rev. 20/35/53 (12 pages) and one for G7000 Rev. 08/21/22/36/37/49/54 (19 pages).
Both can be used for troubleshooting the G7400, but the 19 page version is more extensive with a better troubleshooting guide, especially if you encounter issues like no picture, distorted picture, noisy picture etc. If you can't find it on the web, I can send you an eMail with the pdf file. Just drop me a PM.

Cheers,
Xuul

[Spacemonster]

I am not sure the RF-Modulator is okay. Unfortunately I do not have an other one for testing as I understand the RF-Modulator from the G7000 is different. Therefore your tip to do the Composite Video Mod is worth taking the effort. Thanks!
p.s. I send you a PM for the service manuals

[Xuul]

Check your inbox 

[Spacemonster]

I did the G7400 Composite AV Mod, unfortunately with the same result, stil a black screen when I turn on the G7400 with a cartridge and reset. So the original RF-module should be OK. I will have look in the servicemanuals.

[Xuul]

Could be a faulty IC651 (CPU).
Have a look into the service manual, so you can troubleshoot step-by-step.