CPC 6128 - white screen, black border

mmldrm · 19:02, 13 April 24

Quote from: McArti0 on 18:59, 13 April 24Ok. And now when CPC working +5V connect to ROMDIS. IC103 Pin22 to Pin28

Connect IC103 Pin22 to Pin28, +5v to ROMDIS, and switch on?

McArti0 · 19:06, 13 April 24

NO First ON next Connect

mmldrm · 20:36, 13 April 24

Quote from: McArti0 on 19:06, 13 April 24NO First ON next Connect

Thanks.

I powered on, connected +5v to ROMDIS, then connected IC103 Pin22 to Pin28. No change on screen at all.

(I must admit, I am very close to buying a cheap oscilloscope if we are close to running out of ideas/options here.)

eto · 21:45, 13 April 24

Quote from: mmldrm on 20:36, 13 April 24I must admit, I am very close to buying a cheap oscilloscope if we are close to running out of ideas/options here.)

have you replaced the RAM ICs already? Or did you try the Amstrad diagnostics ROM?

McArti0 · 21:48, 13 April 24

we cant see anything because we have all colors white.

BUT ...

When you start CPU. Next Disable ROM by const +5V on ROMDIS

Voltages on working good bank RAMs are:

Pin14 IC127 ~2V
Pin14 IC128 ~0V
Pin14 IC129 ~0V
Pin14 IC130 ~2V

Pin14 IC131 ~2V
Pin14 IC132 ~2V
Pin14 IC133 ~0V
Pin14 IC134 ~0V

This is address 39h pushing on stack after doing FF (RST 38) reading from bad gate.

bingo!

mmldrm · 14:48, 14 April 24

Quote from: McArti0 on 21:48, 13 April 24we cant see anything because we have all colors white. BUT ...

When you start CPU. Next Disable ROM by const +5V on ROMDIS

Voltages on working good bank RAMs are:

Pin14 IC127 ~2V
Pin14 IC128 ~0V
Pin14 IC129 ~0V
Pin14 IC130 ~2V

Pin14 IC131 ~2V
Pin14 IC132 ~2V
Pin14 IC133 ~0V
Pin14 IC134 ~0V

This is address 39h pushing on stack after doing FF (RST 38) reading from bad gate.

bingo!

Thanks. So... I used expansion port VCC (pin27) -> ROMDIS (pin43) for . It was near, and I had no hands helping me.

Pin14 on all 8 ICs (127-134) are wild. None of them are stable values.

McArti0 · 15:09, 14 April 24

Check voltage on All pins 2.

mmldrm · 17:27, 14 April 24

Also all unstable values.

Interestingly, while my second pair of hands (daughter) was holding VCC to ROMDIS, we got a bunch of yellow screen artefacts. Maybe she lost connection or connected VCC to RSET or RAMDIS, but I thought I would share.

McArti0 · 19:24, 14 April 24

It can't be a weak connection.

mayby beter point for RomDis is here....

Bryce · 20:05, 14 April 24

Quote from: mmldrm on 17:27, 14 April 24Also all unstable values.

Interestingly, while my second pair of hands (daughter) was holding VCC to ROMDIS, we got a bunch of yellow screen artefacts. Maybe she lost connection or connected VCC to RSET or RAMDIS, but I thought I would share.

More likely she was coming in contact with some part of the clock source (before TTL conversion) which would cause multiple resets and similar effects.

Bryce.

mmldrm · 22:59, 14 April 24

Quote from: McArti0 on 19:24, 14 April 24It can't be a weak connection.

mayby beter point for RomDis is here....

I have attached an image of what I did with ROMDIS while checking the DRAM pins. Just to confirm it was correct.

mmldrm · 23:00, 14 April 24

Quote from: McArti0 on 19:24, 14 April 24It can't be a weak connection.

mayby beter point for RomDis is here....

In your latest image, what should I do between R112 and diode D201?

Should I take +ve from pin40 (IC201) and connect it to ROMDIS on the expansion port?

McArti0 · 06:23, 15 April 24

Place between r112 and diode is the same ROMDIS but i think better to hold probe. Pin40 of 765 is better to hold 5v

mmldrm · 20:01, 15 April 24

Quote from: McArti0 on 06:23, 15 April 24Place between r112 and diode is the same ROMDIS but i think better to hold probe. Pin40 of 765 is better to hold 5v

Thanks. We did this and there was nothing on-screen. I think this is correct because without ROM the board does nothing?

I measured pin2 and pin14 of IC127-IC134. All are still fluctuating between 0-5v, same as last time.

McArti0 · 20:16, 15 April 24

Quote from: mmldrm on 20:01, 15 April 24All are still fluctuating

but how? does ic127 have a different voltage from time to time?

What have you now in PAL 40031 socket?

McArti0 · 20:23, 15 April 24

Remember that we do not expect any changes on the screen because we do not have any colors set. All colors are white.
Before measuring, connect +5V to RAMDIS for 1 second, but the PAL socket should not be 40031, but 4 wires.
1 Turn ON
2 connect +5V to ROMDIS const
3 connect +5V to RAMDIS 1sek
4 measuring Pin14 and pin 2

mmldrm · 20:32, 15 April 24

Quote from: McArti0 on 20:23, 15 April 24Remember that we do not expect any changes on the screen because we do not have any colors set. All colors are white.
Before measuring, connect +5V to RAMDIS for 1 second, but the PAL socket should not be 40031, but 4 wires.

Ahh ok.

I had +5V to ROMDIS for the whole time I was measuring the RAM pin2/pin14 voltages.

I also had the 40031 IC in the socket.

I will do the tests again, with the 4 wires replacing the 40031 IC.

Quote from: McArti0 on 20:16, 15 April 24but how? does ic127 have a different voltage from time to time?

Yes, IC127-134 pin2/pin14 voltages were always moving between ~0.2V and ~4.8V. But I will retest with 40031 replaced with the 4 wires.

Should I keep +5V between r112 and diode while I am measuring the RAM voltages? Or remove it after 1 second, then test RAM?

McArti0 · 20:55, 15 April 24

rOm Dis long lasting connected

rAm Dis 1sek.

measuring voltages ... pin14 all memories and write where is ~0V

mmldrm · 20:41, 17 April 24

Quote from: McArti0 on 21:48, 13 April 24Voltages on working good bank RAMs are:

Pin14 IC127 ~2V
Pin14 IC128 ~0V
Pin14 IC129 ~0V
Pin14 IC130 ~2V

Pin14 IC131 ~2V
Pin14 IC132 ~2V
Pin14 IC133 ~0V
Pin14 IC134 ~0V

This is address 39h pushing on stack after doing FF (RST 38) reading from bad gate.

bingo!

Ok, I finally got a helper.

Pin14 IC127 ~2.5
Pin14 IC128 ~0.04
Pin14 IC129 ~0.04
Pin14 IC130 ~2.5
Pin14 IC131 ~2.5
Pin14 IC132 ~2.5
Pin14 IC133 ~0.04
Pin14 IC134 ~0.1 (I think - maybe 0.04)

Pin2 IC127 ~4.2
Pin2 IC128 ~4.2
Pin2 IC129 ~4.2
Pin2 IC130 ~4.2
Pin2 IC131 ~0.6
Pin2 IC132 ~4.2
Pin2 IC133 ~4.2
Pin2 IC134 ~0.7

So they are very clearly hi/lo values, if a little off (2.0v vs. 2.5v). Is pin2 ok at 4.2v hi?

Is this looking like RAM is ok but maybe RAM management isn't?

(Also, I finally bought the oscilloscope this morning. It won't arrive until around 24/4. Maybe we will have a solution before it arrives? 😂 )

mmldrm · 20:43, 17 April 24

Also, I know this has probably been a terrible process, but I am really enjoying learning about the 6128 internals. Thank you.

McArti0 · 21:30, 17 April 24

Yes RAM has been written!!! by 0011 1001 = 39h next this is instruction ADD HL,SP no write to RAM.

But why on Pin2 6Fh?

mmldrm · 22:56, 17 April 24

Quote from: McArti0 on 21:30, 17 April 24Yes RAM has been written!!! by 0011 1001 = 39h next this is instruction ADD HL,SP no write to RAM.

But why on Pin2 6Fh?

Fantastic! This gives me some hope. (Even though I already bought the replacement RAM. Oops.)

Tomorrow I can do the same test again, and make sure the pin2 result is definitely 11110110 / 0x6F. Maybe I made a mistake when writing the results.

mmldrm · 17:38, 18 April 24

Quote from: McArti0 on 21:30, 17 April 24Yes RAM has been written!!! by 0011 1001 = 39h next this is instruction ADD HL,SP no write to RAM.

But why on Pin2 6Fh?

...because I screwed up!

I just retried and got the same results on both pins.

Code Select

Pin2 TAKE TWO
Pin2 IC127    hi
Pin2 IC128    lo
Pin2 IC129    lo
Pin2 IC130    hi
Pin2 IC131    hi
Pin2 IC132    hi
Pin2 IC133    lo
Pin2 IC134    lo

Pin14 TAKE TWO
Pin14 IC127    hi
Pin14 IC128    lo
Pin14 IC129    lo
Pin14 IC130    hi
Pin14 IC131    hi
Pin14 IC132    hi
Pin14 IC133    lo
Pin14 IC134    lo

So 0011 1001 = 39h both times!

McArti0 · 17:47, 18 April 24

Ok. Now you need to check why the ram is not being read.

Give us sharp Photo your mainboard.

mmldrm · 18:48, 18 April 24

I removed the IC118 wires so you could see everything.

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CPC 6128 - white screen, black border