Faulty video-output: white flashing picture

[jacobsson]

Hi.
I have a mainboard that functions properly except for the video output, I hear sound and the controller board works.

The video-output gives a flashing white screen, almost like an old dirty NES cartridge, any ideas where to begin troubleshooting?

I have soldering skills and tools.

[aykiri1]

can be problem video dac chip

[jacobsson]

(15.05.2017 13:25)aykiri1 писал(а):  can be problem video dac chip

You're probably right. Is there any good troubleshooting guide for measure test point voltages with a multimeter or so? Unfortunately I don't own an oscilloscope.

[aykiri1]

is your chip sega marked code 315-6145 or 315-6258 ?

[jacobsson]

(16.05.2017 10:41)aykiri1 писал(а):  is your chip sega marked code 315-6145 or 315-6258 ?

Let me check tonight, I'll come back to you!

Also good to know, since I haven't used this mainboard earlier I've already checked for screw marks around the video port and I could't find any (in case the former owner used a longer screw to install the GDROM).

(I guess the last resort would be to heatgun the mainboard with solder flux on the DAC pins and video output port.)

[aykiri1]

usually similar problem from video dac.
can you check audio ? audio could work

[jacobsson]

(16.05.2017 14:04)aykiri1 писал(а):  usually similar problem from video dac.
can you check audio ? audio could work

Audio works fine, I can hear the boot up sound and if I wait some time I can hear the cursor sound when pressing the D-pad.

[aykiri1]

change that chip (if you can find it)
315-6145 (sega code) using usually sega va0 cards
315-6258 (sega code) using usually sega va1,2 cards
BU1426KS (IC custom code)

all codes equivalent

so i don't know where you can get that chips. so difficult

[Gillbert]

Most of times is really the DAC chip that is faulty, as aykiri1 have told.

But, as a last resort, replace CE203 and CE402, these capacitors are responsible for +Vcc filter of video circuit stages.

[aykiri1]

hi Gillbert

Thx for good info. and how can find that DAC ? any web site or etc ?
i have 8 DC for same issue

[Gillbert]

8 DC's with that same issue? That's really too bad. Anyway, it's worth to check the caps I mentioned, I already found two cases where the problem were the capacitors.

Unfortunately is only possible to find that DAC chip on spare DC motherboards.

[aykiri1]

that dc used iside arcade cabinet with jamma
i will replace and check again.
thx

[jacobsson]

Wow thanks for all the good info guys!
@aykiri1 8 faulty units with the same issue? Daaamn!

I didn't have the time to dig up my mainboard yesternight but I have a bunch of stuff to test, mainly changing those capacitors: CE203 & CE402. You don't happen to have the values for those at hand? (I'm at work thinking I might be able to pick those up the way home)

[jacobsson]

(17.05.2017 00:35)Gillbert писал(а):  8 DC's with that same issue? That's really too bad. Anyway, it's worth to check the caps I mentioned, I already found two cases where the problem were the capacitors.

Unfortunately is only possible to find that DAC chip on spare DC motherboards.

I found the shematics here
VIDEO DAC
CE402: 47uF 10v

GPU AND VRAM
CE203: 100uF 10v

Sounds about right?

[aykiri1]

CE402 is 47uF 16v
CE203 is correct

[jacobsson]

(17.05.2017 15:34)aykiri1 писал(а):  CE402 is 47uF 16v
CE203 is correct

Thanks! I've ordered them now, I'll come back to you within a month hopefully =)

[aykiri1]

that caps are tantalum but, you can use electrolytic for temporary test only