Help! black screen!!

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By ali6x944

Master (194)

ali6x944's picture

27-12-2018, 18:22

Hi dear MSXers,
I am facing a black screen on my AX-350II MSX2 machine, it used to work fabulously before, last week I was installing a gotek drive when I turned the machine on, then the black screen...
I must note that the last time I booted the machine and confirmed that it is fully functional was in August of 2018, back then I tested the audio out and tried to record form the line out but it was a bit noisy...
now when I turn the machine now video out on the monitor now audio out and no cap lock light, however all voltages are present in the cartridge port, +5,+12,-12, RAM seems to be functioning with only one of the RAM chips having /OE low -probably the lower part of the memory space- on both main RAM and VRAM.
no activity on /INT, CLK on both the Z80 and V9938 is stable and ok, the same is true for /reset.
all chips are getting proper voltages, however I shall disclose that I only tested chips I had pinouts for so the V9938 is basically me following the tracks on the pcb, I would suspect multiple supply voltages for VDP but I don't have the pinout so IDK Question
thanx for all the help with this machine so far,
and I wish you a happy Christmas and a happy new year Smile

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By Jipe

Paragon (1358)

Jipe's picture

27-12-2018, 19:13

Have you micro-switchs on cartriges port ?
see on CX5MII service manual on HansOtten http://msx.hansotten.com/manuals-and-guides/
SW1 and SW2 forbid the power

By Alexey

Guardian (2117)

Alexey's picture

27-12-2018, 21:21

Do you have a RAM expansion cartridge? Try to boot with it in slot 1. If any of the lower RAM is faulty, the machine won't boot.

By Pencioner

Paladin (895)

Pencioner's picture

27-12-2018, 21:28

Jipe wrote:

Have you micro-switchs on cartriges port ?
see on CX5MII service manual on HansOtten http://msx.hansotten.com/manuals-and-guides/
SW1 and SW2 forbid the power

the easy check to see if this is a problem should be booting with two cartridges inserted i suppose, so that it doesn't forbid the power on any of the slots

By ali6x944

Master (194)

ali6x944's picture

29-12-2018, 08:52

unfortunately, I don't have any cartridges or ram expanders...
@Jipe I don't think it is due to the power being forbidden if it was the cartridge port and the chips wouldn't give the correct voltages at least on the 5,12,-12 rails...
@Alexey can I test the lower RAM with an oscilloscope?
@Pencioner pressing both switches were the cartridges would be will just reset the machine temporarily so the pointer starts in the cartridge right? it does not affect power afterward?

By Alexey

Guardian (2117)

Alexey's picture

29-12-2018, 14:36

Try checking the DataOut pins of the RAM chips with an oscloscope. If you see an anomaly, replace the chip. The problem is most likely in the RAM.

By ali6x944

Master (194)

ali6x944's picture

29-12-2018, 17:23

well, there is activity on the D and A Lines, I have noticed that some waveforms are not perfectly square and have a rather weird overshot, however, I suspect this is due to bad probing, I will import the captures from my scope tonight with all RAM pins.
by the way, I have checked several video outputs specifically the RGB and there is no activity on the R, G, and B components
but there was activity on CSYNC but which are probably HSYNC because it would oscillate at ~15.6kHz however I am not sure that CSYNC is HSYNC I'm just speculating.

By Grauw

Enlighted (8031)

Grauw's picture

29-12-2018, 17:43

CSYNC is composite sync; HSYNC and VSYNC combined. However due to the higher frequency you’ll mostly see the former.

By ali6x944

Master (194)

ali6x944's picture

16-01-2019, 04:29

hi sorry for the delay,
I had difficulties with uploading the captures to Google drive, so those may be delayed a bit, however, I got some RAM chips from Jameco and some nice high-quality sockets also, basically I can replace the entire main RAM bank, also there were two chips with corrosion under them, they were an op-amp near the cartridge ports and 74LS368AP flip flop Near the floppy disk interface in the back.
I will try to upload it to Google drive tonight Smile

By Alexey

Guardian (2117)

Alexey's picture

16-01-2019, 15:27

Corrosion under the chip means that the board got wet at some point and was not properly dried. I might want to do a close inspection of the board from both sides.

By ali6x944

Master (194)

ali6x944's picture

16-01-2019, 20:11

I cleaned the corrosion it wasn't as bad as I though all the traces are ok, those are some photos of the socketed replacement main Ram chips:
https://drive.google.com/file/d/1YbvixZGEwjoI0hjZcCJAqFlFNpe...
https://drive.google.com/file/d/1ZwQh0ZUEcMofjPSjvast42waWOf...

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