Schneider 6128 - Yet Another Dark Green Screen (YAGDS)

[FruityFrank]

Follow up to my Post in the YABS thread.

quicky: no intention to hijack this thread but since im fighting with a similar problem: got my spare 6128 and it shows no sign of life (apart from pwr led). I only checked for hotspots on the board so far and the floppy controller is the most toasty one by a good margin. That normal? or should i try to desolder it? would the cpc start up with a missing fdc?

FDC is clearly warm. CRTC too. Write what tools you have and what you know about electronics.

Simple multimeter and an old analog oscilloscope. I know the 5V won't hurt me  ... so just basic knowledge.

The FDC does tend to get quite warm, so I wouldn't automatically suspect it. If you have a spare CPU and GA, then I would try swapping them first. The CPC won't boot without the FDC, so don't touch it until you have confirmed that everything else is ok (voltages, clock signal, reset signal etc).
You probably should start a new thread for this repair though.

I hooked up my old scope and probed some random pins on random IC. I can see that there is live in there. But on the some pins the squarewaves don't look very square'ish. Will try to make photo ....

[Bryce]

Square waves aren't going to look square unless you have a scope with a high bandwidth. What frequency does your scope cover?

Have you checked the power pins of any IC to check that the 5V rail really is at 5V?

Bryce.

[McArti0]

some random pins on random IC

schem for not random pins and not random IC

https://www.cpcwiki.eu/index.php/File:CPC6128_Schematic.png

Check voltage on pin11 Z80 ~5V

check Clock - pin6 Z80 we need 4MHz

and pin29 for check live Z80

and ask who lives near you.

[FruityFrank]

Powered the CPC with a bench supply: it shows exactly 1A, I think thats fine.

z80 pin11 shows only around 4.6V 

z80 pin6:  
I think thats 4Mhz 



z80 pin 29:

Thanks so much for your help!

[FruityFrank]

just for fun: powered on for ~2h

[Bryce]

What power supply are you using? Many chips in the CPC don't work under 4.75V. Nothing on the board looks like it's getting too hot.
The "not so square" square waves look fine though. If you have another CPU, I'd try swapping that out first.

Bryce.

[McArti0]

Z80 working
GA 40010 A bit too.
But M1 pin29 lo 1,5us? ? ? Its permanent interrupt?

Show z80 pin16  should be 300hz peak low

[FruityFrank]

What power supply are you using? Many chips in the CPC don't work under 4.75V. Nothing on the board looks like it's getting too hot.
The "not so square" square waves look fine though. If you have another CPU, I'd try swapping that out first.

Bryce.

Its one of these cheap ali bench power supplies, 0-30V and up to 5A. I measured again on the z80 +5V pin and it shows 4.8V now. I guess there is some oxidation layer on all the pins that I have to grind through with the probe.

Concering dirty squarewaves:

thats from pin 10 (d6) of the z80, other datapins look similar. Didn't know my z80 has three-state-logic

I watched Noels video and the signals there looked bad as well, but not that bad imho ....

[FruityFrank]

Z80 working
GA 40010 A bit too.
But M1 pin29 lo 1,5us? ? ? Its permanent interrupt?

Show z80 pin16  should be 300hz peak low

pin 16 on the z80 is high all the time 

[McArti0]

to test pin39, pin40 CRTC - its Hsync(15.6kHz or 4-8kHz), Vsync (50Hz-or 4Hz).

[FruityFrank]

to test pin39, pin40 CRTC - its Hsync(15.6kHz or 4-8kHz), Vsync (50Hz-or 4Hz).

Theres an impulse with ~3,3ms on pin 40 but nothing on pin 39

[McArti0]

Pin21 crtc 2Mhz
Pin4 crtc 1MHz?

[FruityFrank]

exactly half the freq: 1 and 0.5MHz

[Bryce]

to test pin39, pin40 CRTC - its Hsync(15.6kHz or 4-8kHz), Vsync (50Hz-or 4Hz).

Theres an impulse with ~3,3ms on pin 40 but nothing on pin 39

That sounds like the CRTC hasn't been set up. Does holding the DEL key down give a beep? I'm not convinced the CRTC is the problem. I'm tending towards the CPU at the moment.

Bryce.

[FruityFrank]

currently recapping it out of desperation ... many voltage levels appear to be somewhat low'ish. Osci shot above, pin 4 of crtc: clean signal but only 4V, as if something is pulling down all signals.

[McArti0]

40025 ROM Is pin22 realy Lo?

ps.

Pin21 crtc 2Mhz  1MHz
Pin4 crtc 1MHz?  0,5MHz

Ok CRTC work a bit.

[McArti0]

Z80 working
GA 40010 A bit too.
But M1 pin29 lo 1,5us? ? ? Its permanent interrupt?

Show z80 pin16  should be 300hz peak low

Could You Add to M1 second channel and probe to IORQ pin21 Z80?

[FruityFrank]

Took a shortcut and installed the z80 from my main CPC ... works! 
Now I need to source a new z80 ....

Thanks for all the hints and help!

 

[Bryce]

Took a shortcut and installed the z80 from my main CPC ... works!
Now I need to source a new z80 ....

Thanks for all the hints and help!

 

That's not a shortcut, that's just eliminating the most probable sources of the problem that are the easiest to confirm ie: diagnosis.

Congrats on getting it working.

Bryce.

[FruityFrank]

... in order to celebrate I launched the batman forever demo and guess what happened

[FruityFrank]

no, worse:

[McArti0]

but it works