CPC 6128 not working, hangs after a few seconds

DrDoom · 13:20, 10 July 22

Ok, I see this is the CRTC: UM6845:

pin 40 (VS) gives me this:

but pin 39 (HS) nothing as you can see :

Bryce · 14:05, 10 July 22

Then there's a very high chance that it has failed.

Bryce.

DrDoom · 21:36, 13 July 22

Now I see that my oscilloscope has a 200kHz bandwidth only,

so I will need a better one at least 8MHz? Could you suggest me one just for this task?

Bryce · 09:46, 14 July 22

How much are you prepared to spend?

Bryce.

DrDoom · 10:15, 14 July 22

About 200€

Bryce · 11:57, 14 July 22

You don't get much scope for €200, but this one might be a step up from what you have: https://www.amazon.de/Hanmatek-Oszilloskop-HO52-Oscilloscope-Multimeter/dp/B08VRDG94N

Bryce.

DrDoom · 10:09, 23 July 22

Ok, now I've got an FNIRSI 1014d "100MHz" oscilloscope for 200€! and got some measurements:
Crystal oscillator: 16MHz

Z80 pin 6 (clock): 4MHz

Gatearray pin 4 (clock): 1MHz

CRT pin 21 (clock) 1MHz

CRT pin 40 (Vsync) 5v

CRT pin 39 (Hsync) 0v

I also have now a faulty Amstrad PCW board to get components from, and I changed the Z80 with same results. I hope no PCW fans will get angry.

How can I be sure what is wrong? the CRT? am I measuring it right?
Thanks!

DrDoom · 16:28, 30 July 22

I keep trying.
I disoldered two 6845 (CRTC) from some old CGA/Parallel Port boards.
Can I piggyback a 6845?

Bryce · 11:49, 31 July 22

Are you sure you have the timebase set properly when reading pins 39 and 40 on the CRTC?

Bryce.

DrDoom · 18:14, 31 July 22

I tested again and no activity reading 39 and 40 on the CRTC. Tryed on diferent timebases.
But, I have now a Dandanator CPC mini, and when connected there's activity in pin 40, but not on 39.
Anyway no image and no keyboard sounds when the Dandanator is connected.

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CPC 6128 not working, hangs after a few seconds