Author Topic: [GX4000]No display, no sound  (Read 185 times)

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Offline Querubin

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[GX4000]No display, no sound
« on: 13:05, 16 May 18 »
I'm here with another cheap GX4000 faulty board I'd like to repair with your help.

With a cart inserted it doesn't boot. When first powered on you can see a white screen. If you power off and on it again black screen is seen.
No sound is listened from display speakers or headphones. All this happens with 5v, 11v, scart or din connection.

Things I checked.
Both 7805 have right voltages.
Z80 is right.
Cart slot is clean and no shorts between it and ASIC or Z80.
RAM are new, with slots and without shorts between them and ASIC.
No warm on any ICs.

Maybe someone could tell me the route to follow now and the measurements it should be on what component.

Thanks in advance.
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Offline Bryce

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Re: [GX4000]No display, no sound
« Reply #1 on: 13:13, 16 May 18 »
Check that the ASIC is getting 5V. Then check it's getting a clock signal (Pin 2), then check that the output clocks are working (Pins 16, 125 and 141) then check the video out pins (pins 107 to 115).

Bryce.

Offline Querubin

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Re: [GX4000]No display, no sound
« Reply #2 on: 13:49, 16 May 18 »
Thanks!
How do I check these? Volts? How many?
« Last Edit: 14:12, 16 May 18 by Querubin »
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Offline Bryce

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Re: [GX4000]No display, no sound
« Reply #3 on: 14:24, 16 May 18 »
No, you'll need an oscilloscope.

Bryce.

Offline Querubin

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Re: [GX4000]No display, no sound
« Reply #4 on: 14:57, 16 May 18 »
OK, waiting for a logic analyzer then.
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Offline Bryce

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Re: [GX4000]No display, no sound
« Reply #5 on: 15:01, 16 May 18 »
A logic analyser isn't really the correct instrument to use. It will tell you if a signal exists at all, but the shape of the signal is also important.
Also, the next step (testing the output of the DAC) definitely can't be done with an LA.

Bryce.

Offline Querubin

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Re: [GX4000]No display, no sound
« Reply #6 on: 15:08, 16 May 18 »
I don't know the cost of a simple oscilloscope if it will be a good idea or better no to try to repair it.


Any cheap and reliable one?
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Offline Bryce

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Re: [GX4000]No display, no sound
« Reply #7 on: 15:43, 16 May 18 »
Price is relative. The questions you have to ask is: How much will I use it? What will I use it for? How much can I afford?

If you intend doing serious electronics in the future, then what type of electronics and what requirements will the scope have to cover?

Scopes start at under €100 and go up to several 100K. The questions above should help you to at least know in what price range you should be looking.

Cheap: https://www.ebay.de/itm/Hantek-6022BE-2-Kanale-PC-USB-Digitaler-Oszilloskop-20MHz-Storage-Oscilloscope/163050430370
Not so cheap:
https://www.amazon.de/Gowe-Digital-Oszilloskop-1-GHz-140-MPTS-180000wfms/dp/B00NBQ4DPS/ref=sr_1_2

(I'm not recommending either of these scopes)

Bryce.

Offline Querubin

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Re: [GX4000]No display, no sound
« Reply #8 on: 15:50, 16 May 18 »
Thanks,
for the use I will make possibly the best option is not to buy one of them and look for some mate with one and maybe wanted to repair the board.
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