Author Topic: 6128 screen has lost vertical sync?  (Read 477 times)

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Offline nockieboy

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Re: 6128 screen has lost vertical sync?
« Reply #10 on: 16:01, 11 May 18 »
Some thoughts :

- The video sync going to the monitor is a composite one. That is, horizontal and vertical are sync combined.
- The horizontal sync coming from the CRTC is used to generate interruption to the Z80. The FW uses the interruption to do the keyboard scan and the sound queue management. If the keyboard and sound is fine (del key test), then the CRTC is fine and its connection to the GA also.
- The composite video sync is going straight from the gate array to the DIN connector via a  220 Ohm resistor (R137).
If you have a scope, you should have something like this with 5V amplitude on pin 5 of the gate array (40010): https://en.wikipedia.org/wiki/File:Videosignal_vsync.jpg


Ah, interesting... Well, it seems the keyboard was working - I couldn't read what was appearing on the screen thanks to the lack of hsync, but characters were definitely appearing.


I have a GT65 monitor that is exhibiting similar issue (no sync) when powering the CPC, but not when the CPC is powered with an external supply. I've never tried to diagnose the exact problem (I've no use for a monochrome monitor), but it could be that your monitor is the problem, not the CPC.


Not so sure about this - as I mentioned in the OP, I also have a CPC464 which displays just fine with the same monitor, excluding pretty much all monitor failures I can think of.  I did just go back and plug the 6128 into the monitor without the 12V connector, though, to match the power draw of the 464 more accurately (in a rough 'n' ready way), but the display was still corrupted.


I have an oscilloscope, but won't get the chance to test the 6128 until late next week unless I get any spare time before then.  Will post an update when I can.

Offline nockieboy

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Re: 6128 screen has lost vertical sync?
« Reply #11 on: 11:44, 17 May 18 »

Disclaimer:  I'm not an electronics expert.  I just about know which way around batteries are supposed to be fitted.

Okay, managed to get the oscilloscope running.  Haven't got pics unfortunately as I only have two hands to hold the probe, ground lead, etc.  But from testing the MC6845P, it seems to be putting out reasonable hsync and vsync signals.  At least the sillyscope is able to lock on to a repeating signal.


I've also taken a look at the gate array - in my 6128, it's an Amstrad 40010.  HSYNC (pin 11) is putting out a nice clean signal, looking something like this:


   _      _     _
  /      /      /
 |      |      |
 |      |      |
-    ---    ---   ---  etc


(Apologies for the crap ASCII art representation...)


HOWEVER... VSYNC (pin 13) isn't outputting any discernible signal that I can lock on to with my oscilloscope - I'm not expert, as I mentioned at the start, but it sure looks like there's nothing but garbage coming out of pin 13.


I will try again later if I can get the other half to take some pics for me, but it seems the 40010's VSYNC may have died..?

Offline Bryce

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Re: 6128 screen has lost vertical sync?
« Reply #12 on: 11:55, 17 May 18 »
What's the make/model of the scope you're using?

Bryce.

Offline nockieboy

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Re: 6128 screen has lost vertical sync?
« Reply #13 on: 15:30, 17 May 18 »
Hitachi V-212.  Simple 20MHz 'scope, nothing special.

Offline Bryce

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Re: 6128 screen has lost vertical sync?
« Reply #14 on: 15:51, 17 May 18 »
Ok, so it doesn't have an "Auto" button which would try to make sense of the V-Sync. Is the V-sync pulsing at all or can you better describe the garbage? Maybe even take a "screenshot" (photo of the scope screen).

Bryce.

Offline gerald

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Re: 6128 screen has lost vertical sync?
« Reply #15 on: 18:11, 17 May 18 »
Ok, so it doesn't have an "Auto" button which would try to make sense of the V-Sync. Is the V-sync pulsing at all or can you better describe the garbage? Maybe even take a "screenshot" (photo of the scope screen).

Bryce.
But it has a TV-H and TV-V trigger mode and that should be enough  :)

Offline Bryce

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Re: 6128 screen has lost vertical sync?
« Reply #16 on: 22:38, 17 May 18 »
So it does (just checked the manual online)! That should work.

Bryce.