Yahama CX5M black screen (MSX 1 computer)

By karlh13

Supporter (2)

karlh13's picture

28-04-2020, 17:59

Hi and apolgies for the essay. I'm investigating an MSX for a friend and have put together all of the info that I can from this splendid site, and have gone through all of the steps below. Thought it may help others who are new to the MSX.

When turned on the computer displays a black screen. The power light is lit. Not sure of the backstory as testing it for a friend, although I have a feeling that it hasn't been used for a long time.

Pressing the Caps key, toggles the light on and off.
Q: Does this just prove that power is getting to the keyboard, or does it prove that any chips are working?

Pressing CTRL and G does not cause a beep.

Have tried two different RF leads, two different televisions. Have also tried to connect via yellow/red/white RCA connections. All shows consistent behaviour.

Power supply
The output pins on the power supply are too close to get a good reading with a multimeter. So measured the voltages on the cartridge port instead. They seem to indicate that the correct power is being received.

Opening the case
The next part of the plan was to open the case to check if there is any obvious physical damage to the board, or if any of the capacitors looks like they were bulging. The board looked like it was in good condition albeit a bit dusty. No obvious damage.

Checking for 'chip creep'. After many years socketed chips can start to lose good contacts with their sockets. In this case there were no socketed chips, so this was a dead end.

Checking whether any of the chips were getting warm. Left the machine of for five minutes and touching the chips with fingers, none of them felt particularly hot.

Schematics
This model of the MSX being a music based is probably fairly rare (at least in the uk), luckily the service manual is available from http://www.msxarchive.nl/pub/msx/docs/service_manuals/yamaha... .

Checking the voltages on the chips
VRAM is NEC D416C
Using pin 16 as the ground; pin 1 measures -5.26V (-5V), pin 8 measures +11.68V (+12V) and pin 9 measures +4.92V (+5V). All the VRAM read about the same voltages, there are some awkward wires in the way, so didn't do a couple of the readings.

RAM is Mitibushi MB81416-12
Using pin 1 as the ground; pin 9 measures +4.81V (5V). All RAM chips are about the same value.

Video chip is TMS9918A
Using pin 12 as the ground; pin 33 measures 4.87V (5V).

Processor is Z80
Using pin 9 as the ground; pin 11 measures 4.84V (5V).

Measuring the clock signal
So this is the limit of my current knowledge, and I may be doing this bit entirely wrong. Have no oscilloscope, but do have a logic probe. Attached the logic probe to the case for the ground lead, and the 5V leg of the voltage regulator for the signal.

Using the service manual (Pg28), it appears that the clock signal (CLK) goes to the video chip first and then exits this chip and connects to the Z80. Assumption (correct?) that there should be a detectable pulse where the clock signal enters the video chip, and also where it exits the video chip, and finally where it enters the Z80. Another assumption is that the logic probe will be able to detect a pulse signal.

Measuring pin 37 on the video chip (clock signal enters chip from origin) with the logic probes shows the High and Low signals flashing, so I think it's measuring a pulse here.

Measuring pin 38 on the video chip (clock signal exits video chip), this pin appears to be stuck on High.

Curiously measuring the pin 6 on the Z80 also gives a High and Low flashing light, that I think is a also a pulse signal.

Any ideas of what to try next?

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By Alexey

Guardian (3120)

Alexey's picture

28-04-2020, 20:10

Without an oscilloscope further checking would not make sense.

By Wlcracks

Champion (438)

Wlcracks's picture

29-04-2020, 17:06

For me the |INT of the z80 (pin 16) is the heartbeat of the MSX. I don't know the Yamaha's special roms, but if the bios and basic starts the |INT should puls 50 or 60 Hz. If the interrupt works you can check CAPS lock led and the ticking of the keys from the normal MSX sound output.

For repairing with a probe this guy has amazing tips (a lot are z80 based boards)

OneCircuit

By karlh13

Supporter (2)

karlh13's picture

29-04-2020, 18:41

Thanks for the replies, and the useful link everyone. A few more things to try Smile

By kirikl

Supporter (1)

kirikl's picture

08-06-2021, 02:24

Bump!
Anyone has any idea? My friend upgraded the chip for me. Did clean job, I checked.
And then once upon a time while I was playing that new FM thing, computer hung... after that I had blank screen. Once it was blue and then weird things happened from time to time , for example caps lock (with click) could lit when module is on and power...
I want to investigate in PSU, maybe due the age it needs maintenance.
Anyway... Any ideas is appreciated.
Thanks!

By Grauw

Ascended (9904)

Grauw's picture

08-06-2021, 11:02

karlh13 wrote:

Pressing the Caps key, toggles the light on and off.
Q: Does this just prove that power is getting to the keyboard, or does it prove that any chips are working?

Since the caps lock light is controlled by the IRQ, it proves that the Z80 is working and the BIOS ROM as well. Also it’s not in a halted state (DI / HALT), and interrupts are being generated.

You should also be hearing key clicks?

However if CTRL-G does not work it suggests that it does not end up in BASIC (ROM). Or a sound problem of course.

By Alexey

Guardian (3120)

Alexey's picture

08-06-2021, 17:05

Check the temperature and outputs of RAM chips (1416). If some chip is hotter than others or its output is missing/low, replace the chip.