What does a dead ASIC look like? 464 Plus

[Overkill]

Hi All,


I have a non-working 464 Plus, I found that both the RAM chips were dead, so I've replaced them but the machine still doesn't work.


When I power it up, I get a grey screen with diagonal lines for a second-or-two, then the TV says no signal, which is exactly what happens on a good working 464 Plus without a ROM cartridge fitted.


The fact that the two RAM chips had gone, makes me think that the computer might have been connected to a too higher voltage PSU, and therefore, maybe the ASIC has been destroyed.  BUT would I get the grey diagonal lines if the ASIC was completely dead?


I have an oscilloscope, and would be grateful for any diagnostic advice on how and what to check next.
 

[chinnyhill10]

You checked the cartridge port is clean? I had a scare with my 464 Plus recently and although the cart edge connectors were clean it turned out something was fouling the socket end. Some contact cleaner (the strong stuff) fixed it.


Just worth checking. I'd thought the RAM or ASIC had blown but its now just fine.

[Overkill]

Good thinking I sprayed it with contact cleaner but maybe I should check it more carefully.

[Audronic]

Good thinking I sprayed it with contact cleaner but maybe I should check it more carefully.

Hi Overkill


Did that fix the Problem ? ?


Thanks    Ray

[Overkill]

I've looked at the contacts and they look fine, I will test them with a meter but I don't think that is the problem.

[gerald]

The fact that the two RAM chips had gone, makes me think that the computer might have been connected to a too higher voltage PSU, and therefore, maybe the ASIC has been destroyed.  BUT would I get the grey diagonal lines if the ASIC was completely dead?

Dead asic / no cartridge will usually give the same result.

I have an oscilloscope, and would be grateful for any diagnostic advice on how and what to check next.

1st check that the Z80 have a clock (pin 6), then that the WAITn (24) is working as well.
If you do not get a clock, check that the main oscillator is working as well.

[Overkill]

I will do some testing later today and let you know the results, thanks.

[Overkill]

Dead asic / no cartridge will usually give the same result.
1st check that the Z80 have a clock (pin 6), then that the WAITn (24) is working as well.
If you do not get a clock, check that the main oscillator is working as well.

I get the clock at pin 6 and 5v at pin 24.


I've tested all the connections to the ROM cartridge and they seem fine.

[gerald]

I get the clock at pin 6 and 5v at pin 24.


I've tested all the connections to the ROM cartridge and they seem fine.

Are you sure of you replacement RAM ?
Any chance you can run this : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc

[Overkill]

Are you sure of you replacement RAM ?
Any chance you can run this : http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/quick-and-dirty-ram-test-for-cpc

That sounds like a handy tool to have, I can program an EPROM.


I've since found that the lumi signal is missing (I think), comparing with a working 6128+ at the RGB out socket, I find no voltage at pin 3 (that's if the attached diagram is correct).

[Overkill]

hmm.... hang-on, I might have got the pin-outs the wrong way around.   Going for my dinner, I'll check again later

[Overkill]

I've tested the output of the RGB socket and pins 1 and 3 seem to be the same as the working 6128+.


I've checked the working temperature of the ASIC and it seems about the same as the working 6128+ 43-45C.


I'll try the RAM diagnostic ROM tomorrow and keep you posted

[Overkill]

Sorry I need more help, see error while trying to program the RAMTEST.BIN

[Overkill]

Ignore above, it was a bad EPROM I think

[Overkill]

Hi Gerald, the computer is the same with your RamTest inserted (grey lines),  I tried the EPROM on my working 6128+ and it worked on that one.


What would happen if the SN74LS27N IC16 was dead?  I'm not sure what that IC does.

[gerald]

What would happen if the SN74LS27N IC16 was dead?  I'm not sure what that IC does.

All could happen, as it's masking IORQn on some address range and could prevent CRTC init.
More on that in http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/amstrad-plus-and-im2-bug/msg147349/#msg147349

Did you check the Z80 as well ?

[Overkill]

I swapped the Z80 with my 6128+

[gerald]

So there is not much remaining to check.
All access to the ROM/RAM need the ASIC to be working.
Only a logic analyser would help confirming the death by checking what really happen on boot.

[Overkill]

It's a shame that IC1 is so hard to swap

[gerald]

Could also check the video out path from ASIC to DAC.

[Overkill]

IC1 to IC15?   

[gerald]

IC15, check that digital RGB input match analogue output.

[Overkill]

I've found something (maybe) comparing the signal on IC15, BLU3 I get a 300mv wave (looks like curved waves that you see at the seaside ), BLU2 I get a good 5v square wave, but BLU1 and BLU0 is at a constant 5v (flat lined,no wave).  On my working 6128+ I get a good 5v square wave on all of those.


The OUTB at IC15 is a 600mv wave that is similar to the working 6128+.


I know I'm a novice, but the fact I get a good wave pattern at BLU2 makes me think that IC1 must be working, at least a little  ??

[gerald]

Is the NSYNC similar on the din output when you compare with the working one ?

[Overkill]

They are identical, a 5v square wave.