CPC 6128 - white screen, black border

[mmldrm]

This is how I recently simulated the problem on a 464: one of the Dout pins of a single RAM IC is connected to GND - and voilà - the black border / grey screen. Remove the short - and it's back to normal again.

I don't quite know how to apply this information to my malfunctioning 6128, other than it being more evidence that it is most likely a RAM problem, but thanks!

If I don't get positive confirmation of the issue, at least the RAM chips are cheap to replace. So I could try that if all else fails. 

Thanks again!

[mmldrm]

Each pin14 read 2.9v-3.0v, except IC127 which read 1.29v.

I suspect that this shows that data is being read from all eight chips, with a mix of 1s and 0s mostly averaging out at ~3v, and that the 1.29v just happens to be data with more 0s than 1s? i.e. the RAM chips look ok?

Or, if it makes IC127 look suspicious, I am probably happy to try replacing IC127 to see if that makes a difference. I just assumed that if 7 of the 8 RAM chips are working then it would get further than the black and white screen. But I guess it depends which area in memory has a problem, or because the RAM chips work together.

[Rabs]

Each individual 4164 IC is 1bit, and hence there are 8 to make up the 8 bit data bus. It only takes 1 IC to fail and you have corrupt data.

[McArti0]

A side note - one time I switched on to this screen, which was a little different. Sadly, it hasn't happened again.

This is the correct image from cold memories right after turning on. Are you sure it wasn't an image from a wire PAL replacement?

[mmldrm]

Each individual 4164 IC is 1bit, and hence there are 8 to make up the 8 bit data bus. It only takes 1 IC to fail and you have corrupt data.

Thanks, I did think that was the case (having read many old discussions here) but that confirms it.

More and more, I might just buy the 8 RAM chips and cross my fingers that there are no other issues. At least those are inexpensive (around £10 I think?).

[mmldrm]

This is the correct image from cold memories right after turning on. Are you sure it wasn't an image from a wire PAL replacement?

Sorry, what is a wire PAL replacement? If it is a mod, I can check the board for it.

But this Amstrad is from a friend who owned it since he was a kid in the 80s and not really used it since the 80s either. (Either never, or almost never.)

Very interesting that it is the correct image though!   

[McArti0]

Sorry, what is a wire PAL replacement? If it is a mod, I can check the board for it.

Suspecting the RAM, I did the "RAM bank swap trick" by removing the IC118 and replacing it with some jumper wires in the appropriate positions (triple-checked!).

[mmldrm]

Sorry, what is a wire PAL replacement? If it is a mod, I can check the board for it.

Suspecting the RAM, I did the "RAM bank swap trick" by removing the IC118 and replacing it with some jumper wires in the appropriate positions (triple-checked!).

Thanks. When I see PAL, I think of video signals. 

Definitely not that. The IC118 is back in place. 

[Bryce]

Three full forum pages of faffing around with pointless tests. Just swap the RAM and get on with enjoying your CPC.

Bryce.

[mmldrm]

Three full forum pages of faffing around with pointless tests. Just swap the RAM and get on with enjoying your CPC.

I have now ordered RAM, but was hoping to have positive confirmation that RAM is the problem before doing so.

For example, removing the HAL chip (40031) completely makes no difference. So maybe I'm replacing RAM when the 40031 is the problem. That's the kind of thing that has been putting me off buying replacement parts.

[McArti0]

40031
pin7 and pin17 have the same voltage. ~3v

pin8, pin12  have the same voltage.

pin9, pin19  have the same voltage.

ps.

A side note - one time I switched on to this screen, which was a little different. Sadly, it hasn't happened again.

WHEN you seen this image! with 40031 or without?

[mmldrm]

40031
pin7 and pin17 have the same voltage. ~3v
pin8, pin12  have the same voltage.
pin9, pin19  have the same voltage.

Code Select Expand

pin7 (2.18v) - pin17 (3.63v)
pin8 (1.67v) - pin12 (3.73v)
pin9 (0.66v) - pin19 (0.75v)

But should it be pin7 to pin16? (And pin17 to pin20?)

Actually, pin16 was the same as pin17... 3.63v.  And only pin9-pin19 values are close. 

ps.

A side note - one time I switched on to this screen, which was a little different. Sadly, it hasn't happened again.

WHEN you seen this image! with 40031 or without?

With 40031. Still only ever seen it once.  

I ordered 8x RAM chips (and sockets), so I can replace the main bank. But I get the impression it might not just be RAM that is faulty here...

[McArti0]

ramove 40031 and add 3 wire jumper pin 7-17, pin8-12, pin9-19

check by Ώ on multimeter if pin 7 is join to RAM pin15 ic127-ic134 (mutimeter show ~0 )

check pin 8 is join to IC109  pin10

check pin 9 is join to IC109 pin 6

https://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png

[mmldrm]

ramove 40031 and add 3 wire jumper pin 7-17, pin8-12, pin9-19

check by Ώ on multimeter if pin 7 is join to RAM pin15 ic127-ic134 (mutimeter show ~0 )

check pin 8 is join to IC109  pin10

check pin 9 is join to IC109 pin 6

https://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png

With 40031 replaced by 3 jumpers:

pin7 to RAM pin15 - all 8 RAM chips show resistance of 47-48 Ώ
pin8 to IC109 pin10 - 0.5 Ώ
pin9 to IC109 pin - 0.6 Ώ

[McArti0]

Ok. any changes on screen?

next changes not wire 7-17 but 7-16

[mmldrm]

Ok. any changes on screen?

next changes not wire 7-17 but 7-16

Both 7-17 and 7-16 get the same black border, white screen.

Should I have a pin going to pin20? Like 7-16 and 17-20? The YouTube video for this shows a 4th pin.

[mmldrm]

BTW with speaker volume high, there has always been a barely audible pulse, roughly 1Hz. I don't know if that means anything. (Sorry, I only realised it was coming from the 6128 now. I thought it was coming from outside the house, it is so faint.)

[McArti0]

normal.

20-17 or 20-16 for other bank reduce noise. its ok.

[McArti0]

Check RAMDIS to ground and turn on. And check visible condition R124.

[mmldrm]

Check RAMDIS to ground and turn on. And check visible condition R124.

Hi,

Sorry, I was away for a few days. Do you mean these steps:

1. Connect RAMDIS to ground.
2. Turn on 6128.
3. Check voltage across R124

[McArti0]

R124 normally connects ramdis to ground. We want to check if this is really happening.
Then check if the voltage on IC112 pin8 is the same on pin9.

[mmldrm]

R124 normally connects ramdis to ground. We want to check if this is really happening.
Then check if the voltage on IC112 pin8 is the same on pin9.

RAMDIS is grounded (0v)

IC112 pin8 - 4.34v
IC112 pin9 - 4.93v

[McArti0]

Try RAMDIS grounded and turn ON CPC.

[mmldrm]

Try RAMDIS grounded and turn ON CPC.

RAMDIS grounded while turning on, still white screen.  

[McArti0]

Ok. And now when CPC working +5V connect to ROMDIS. IC103 Pin22 to Pin28