CPC 6128 - white screen, black border

[Audronic]

@mmldrm

Where in the world are you Located ?

Scotland. Near Edinburgh.

Ok Thanks
Perhaps somebody near you would have a Diag Cart or some suitable tester, Try putting a callout and see what happens.
as its been a VERY long journey so dar  Keep Safe. Ray

[mmldrm]

Ok Thanks
Perhaps somebody near you would have a Diag Cart or some suitable tester, Try putting a callout and see what happens.
as its been a VERY long journey so dar 

It sure has! And I'm incredibly thankful for all the help, but yes it's been.. a journey..

Where would be the best place to put a call-out?

[Audronic]

@mmldrm 

Here On the forum in this Saga.

Keep Safe

Ray

[Rabs]

Ok Thanks
Perhaps somebody near you would have a Diag Cart or some suitable tester, Try putting a callout and see what happens.
as its been a VERY long journey so dar

It sure has! And I'm incredibly thankful for all the help, but yes it's been.. a journey..

Where would be the best place to put a call-out?

Hi, on the South Coast, so a bit far but happy to help. Message me, maybe I can loan you a lower DiagROM.

[mmldrm]

R124 but ROMDIS is enough as I checked mine.
Of course you won't see vertical bars.

Just before I do this test, I checked the D0-D7 values when I first switch on. All data pins were high, except D4 and D7 which looked like this. I measured this both at Z80 and on pin 2 of the RAM chips themselves.


Thought I should share, in case it helps. e.g. maybe it is a sign that IC126 and IC123 are the faulty ones.

I will do the ROMDIS test tonight if I can get time to do it.

[Bryce]

Ok Thanks
Perhaps somebody near you would have a Diag Cart or some suitable tester, Try putting a callout and see what happens.
as its been a VERY long journey so dar

It sure has! And I'm incredibly thankful for all the help, but yes it's been.. a journey..

Where would be the best place to put a call-out?

You could have drastically reduced the journey time by just replacing the RAM's from the very start. The problem is obviously related to RAM, but from all of the IC's involved in reading/writing to/from RAM, the RAM IC's themselves are many, many 1000's of times more likely to fail than every other chip involved. So 99.9% of the time swapping the RAM's will resolve the issue without any need for time consuming testing.

Bryce.

[mmldrm]

You could have drastically reduced the journey time by just replacing the RAM's from the very start. The problem is obviously related to RAM, but from all of the IC's involved in reading/writing to/from RAM, the RAM IC's themselves are many, many 1000's of times more likely to fail than every other chip involved. So 99.9% of the time swapping the RAM's will resolve the issue without any need for time consuming testing.

To be honest I feel worst for the forum members who have tired of seeing this topic. I've learned quite a bit about electronics, the layout of the CPC boards, and the general principles of 8-bit home computer design.

On the other hand, yes, if I had realised that replacing the lower RAM only gave a 50/50 chance of fixing 'the RAM problem' then I would probably have had both bank replaced. That misunderstanding is on me.

If the budget had been mine, I would probably have replaced the RAM earlier. I have now suggested buying a Dandanator and diagnosing using that. At least he can use it later to load games, or sell it on eBay for a minimal loss.

Actually, if it's another £50 to get someone to replace the upper RAM vs. £45 for the Dandanator (and probably then £50 for the RAM installation) then maybe I will recommend replacing the upper RAM. I just didn't want him to spend more money and still have no definite diagnosis of the fault, if it continued to fail.

I also didn't realise that the chances of it being a non-RAM IC were quite so incredibly low.

[mmldrm]

R124 but ROMDIS is enough as I checked mine.
Of course you won't see vertical bars.

Please ignore the previous message. I halted the ROM with ROMDIS and the PAL swap trick, and checked again with the scope. After altering the horizontal scale on the scope, I now realise that I'm seeing a series of 1s and a series of 0s.

And yes, D7-D0 does indeed read 0011-1001.

This is on the upper RAM. The new socketed RAM ICs are removed.

I've attached what I am interpreting as a constant refresh of 1, and 0.

[mmldrm]

Hi, on the South Coast, so a bit far but happy to help. Message me, maybe I can loan you a lower DiagROM.

Thanks! I really appreciate the offer. Unfortunately the ROM chips aren't socketed and you can probably tell I'm trying to minimised required soldering, as I would be (to some extent) learning on the job if I do it myself.

So I'm having to decide between Dandanator (or equivalent) to override the ROM, or blindly replacing/socketing the upper RAM.

Nevertheless, thanks for the kind offer.

[McArti0]

And yes, D7-D0 does indeed read 0011-1001.

D7-D0 on pin14 RAM too?

[McArti0]

Test at ROMDIS Hi pin14 RAM D5,D4,D3,D0.

[mmldrm]

And yes, D7-D0 does indeed read 0011-1001.

D7-D0 on pin14 RAM too?

Sorry, forgot to check RAM outputs. I have attached RAM I/O for D0-D7. I think the lo values look strange on all four (D1, D2, D6, D7) but hi values look ok?

(Happy to test while I wait for my friend's response on Dandanator and replacing upper RAM.)

[McArti0]

D1 ,D2,D6,D7 shouldn't have Hi peaks, and freq is not properly.

Try now after ROMDIS Lo , remove CAS1 at PAL , Set CAS1 to Hi and measure out D7-D0

All Hi or Lo peaks should be disapear

Set H=500ns and click ZOOM

[Bryce]

D1 ,D2,D6,D7 shouldn't have Hi peaks, and freq is not properly.

Try now after ROMDIS Lo , remove CAS1 at PAL , Set CAS1 to Hi and measure out D7-D0

All Hi or Lo peaks should be disapear

Set H=500ns and click ZOOM

I wouldn't be so worried about the frequency, because cheap scopes are really bad at accurately measuring the frequency with so little data, especially when it's not a sinewave. But the voltage levels are definitely not good. It's definitely time to swap out any original RAM's.

Bryce.

[mmldrm]

D1 ,D2,D6,D7 shouldn't have Hi peaks, and freq is not properly.

Try now after ROMDIS Lo , remove CAS1 at PAL , Set CAS1 to Hi and measure out D7-D0

All Hi or Lo peaks should be disapear

Set H=500ns and click ZOOM

"ROMDIS lo" means a normal startup process, yes? I set CAS1 to hi and put the 8x Bank 0 RAM ICs back in. (They were removed in the previous test, when the PAL trick was forcing Bank 1.)

I measured both banks, D0-D7. The signals look good to my eyes. But I am suspecting I misunderstood your setup instructions.

[mmldrm]

I wouldn't be so worried about the frequency, because cheap scopes are really bad at accurately measuring the frequency with so little data, especially when it's not a sinewave. But the voltage levels are definitely not good. It's definitely time to swap out any original RAM's.

Thanks. I'll tell the owner that the other bank needs replaced, and let him decide whether I do it or we pay someone to do it.

[McArti0]

ROMDIS Hi ofcourse , my error.

without RAM bank 0.

[McArti0]

Thanks. I'll tell the owner that the other bank needs replaced, and let him decide whether I do it or we pay someone to do it.

RAM is probably good.

But the voltage levels are definitely not good.

[McArti0]

Good condition CPC 6128, value 0039h in all RAM. D4,D5,D7

[mmldrm]

Good condition CPC 6128, value 0039h in all RAM. D4,D5,D7

I will try to do the same tests here. 

I do have two probes if it helps with tests.