CPC-464 freezes on start-up with black background colour

[Rabs]

Screen has "Ready" and currsor.  Firmware and proc in bank 2 ram work. Check joy port or Simulate press key beetwen AY and 147

Still don't understand the black screen, white text symptom.

[McArti0]

Still don't understand the black screen, white text symptom

look here...

Whilst I was testing however I accidentally changed my TV source to a different input and when changed back to SCART the CPC's output was in colour with the correct blue background and yellow text. Cold starting it displays with a black background with white text again until I change my TV from one source back to SCART.

Pin32 40010 join to SOUND. I hear 300Hz interrupts...

ps. The One Wire CPC oscillosphone is a little cheaper than Rigol. 

[McArti0]

CPC's output was in colour with the correct blue background and yellow text. Cold starting it displays with a black background with white text again until I change my TV from one source back to SCART.

Maybe You must unplag CPC pin7 from your cable? Or your cable has too much damping?

[Tezz]

My next check would be the 40009 ROM. Check if it has any stuck Data or Address bits.

Bryce.

OK thanks, I'll look into that. I'm missing an oscilloscope to look at the current state but I did recently buy a TL866II Plus which I've not yet used so I could remove the ROM and check that it's ok along with the 74LS logic although from what I've read testing logic with the TL866II is not conclusive as to whether it is fully operational. It would be nice to have something like SLICE to use but £450 is a bit pricey. It's a shame there's nothing like the old Fluke 9010A functionality around today. I've always taken the lamers approach removing IC's until I find the culprit.

I use a Hantek DSO5102P oscilloscope. Makes investigating a lot simpler.

See https://www.amazon.co.uk/gp/product/B00I5EWF1U/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Thanks, that's pretty inexpensive for what looks to be a decent oscilloscope. I'll towards buying that soon.

[Tezz]

When you join LS145 pin 15 to expansion port pin 1 (sound) you hear in speaker brum 50hz scan keyboard. This is check that's interrupt and soft and z80 works fine.

Oscylosphone is in CPC :-D

I've had time today to finally check that out, (I forgot about mothers day etc happening at the weekend). The test did produce the expected buzz from the interrupt so the cpc is alive. So, the next question is where should I progress from here continuing with the diagnosis

[McArti0]

When you join LS145 pin 1,2,3,4,5,6,8,9,10,11 to expansion port pin 1 (sound) you hear in speaker brum 50hz scan keyboard but lover volume then PIN15. 

Maybe this is bad plug keyboard...

Next try join AY3 PIN7,8,9,10,11,12,13,14 each to each LS145 PIN1,2,3,4,5,6,8,9,10,11. This is like press keys.

AY3 Pin7 and LS145 Pin11 probably is DEL key and beep sound.

[Tezz]

When you join LS145 pin 1,2,3,4,5,6,8,9,10,11 to expansion port pin 1 (sound) you hear in speaker brum 50hz scan keyboard but lover volume then PIN15.

Maybe this is bad plug keyboard...

Next try join AY3 PIN7,8,9,10,11,12,13,14 each to each LS145 PIN1,2,3,4,5,6,8,9,10,11. This is like press keys.

AY3 Pin7 and LS145 Pin11 probably is DEL key and beep sound.

Thanks, I've now tried from AY pins 7-14 to the LS145 and no response.

[Bryce]

Be careful, the LS145 is the "source", but it's an open collector chip, so you will not measure anything without it being connected via the keyboard to the AY.

Bryce.

[Tezz]

Be careful, the LS145 is the "source", but it's an open collector chip, so you will not measure anything without it being connected via the keyboard to the AY.

Bryce.

Ok so a direct wire between LS145 and AY won't actually work. So, I guess I'm left with either the PIO, AY or LS145 to look into

[Tezz]

Will I get key presses registered with a wire between the pins of LS145 and AY with the keyboard membrane connected?

[McArti0]

Yes a wire between the pins of LS145 and AY is the same like key press.

Output LS145 "you will not measure anything" only when you measure between 0V and Pin. When you join a wire to sound line R114 you can hear 50Hz buzz in speaker.

you can check whether the AY3 inputs are high ~5v

AY3 Pin7-14 join to mass 0V should end with chars printout.

Check Output from AY3 Pin21-28 buzz 50Hz when const 0V at input Pin 7-14.

Check Buzz 50Hz at AY3 Pin18,20 control lines.

Check ~0V at ROMEN Pin20 40009

[Tezz]

Yes a wire between the pins of LS145 and AY is the same like key press.

Output LS145 "you will not measure anything" only when you measure between 0V and Pin. When you join a wire to sound line R114 you can hear 50Hz buzz in speaker.

you can check whether the AY3 inputs are high ~5v

AY3 Pin7-14 join to mass 0V should end with chars printout.

Check Output from AY3 Pin21-28 buzz 50Hz when const 0V at input Pin 7-14.

Check Buzz 50Hz at AY3 Pin18,20 control lines.

Check ~0V at ROMEN Pin20 40009

Thanks for your further help, 

connecting ground to AY pins 7-14 has no response with chars output.

AY inputs (pins 7-14) all show 4.46v. 

AY pins 18 and 20 do produce buzz. 

ROM pin 20 shows 3v.

Thanks again 

[McArti0]

Pin 20 3V ROM swithed between RAM and ROM. A little more RAM then ROM.

Check voltage at A11,A12 Pin1,2 of Z80 . it should be close to ~4.5V. Main Loop read program from 1BXX,1CXX, 1DXX (but here is I-reg too)

Check voltage at A0, Pin30 of Z80 . it should be close to ~2.5V

[Tezz]

Pin 20 3V ROM swithed between RAM and ROM. A little more RAM then ROM.

Check voltage at A11,A12 Pin1,2 of Z80 . it should be close to ~4.5V. Main Loop read program from 1BXX,1CXX, 1DXX (but here is I-reg too)

Check voltage at A0, Pin30 of Z80 . it should be close to ~2.5V

Z80 pin 1 : 2.13v
Z80 pin 2 : 2.31v
Z80 pin 30 : 1.61v

(Btw I have previously tried swapping a known working Z80 with no change to the issue)

[McArti0]

This voltages is an attempt to estimate whether the correct character reading program is being executed. Without any tools. I check this at home, but i think volt. at Pin2 is too small.

[Tezz]

This voltages is an attempt to estimate whether the correct character reading program is being executed. Without any tools. I check this at home, but i think volt. at Pin2 is too small.

Thanks for looking into it, very much appreciated. Will get there in the end!

[McArti0]

Check this scenario. Cold cpc. Press and hold DEL and turn on cpc. Are You hearing sound? 0,5 SEC? 0,2 SEC? 0,01sec? 0,00....

[McArti0]

I also have a working long board 464. 

You can compare voltages Addresses line.

[SerErris]

What is the current status? 

Is there still the power on message on the screnn but yellow on black and then nothing happens?

That is beyond ROM not working. ROM is working otherwise you would not get there. Same is true for Ram. If you would have a defective RAM chip, you would see it in the picture.

Is the sound chip getting a clock signal at all? It is very unlikely that it would not, because the screen works and if the clock from GA would not get out, the CRTC also would not work. But no clock signal on the AY would result in no keyboard and no sound.

Measuring Voltage on Address or Datalines is not really helpful other it is maybe completely stuck.
But as we see the power on message, there is neither a problem with addressing nor with data bus. Also Ram works (read and write) so even GateArray control and the Latch and Buffer working fine.

The GateArray might be defective as it potentially cannot display blue anymore. If it can only display Read and Green you would get black background with Yellow letters.

That is what is coming to my mind.

An Oscilloscope would be very helpful to understand what is going on. Then measuring address lines or datalines in any way would be more helpful.

[McArti0]

A12 Pin 2 i have  ~2,25V its always 1 from PC and 0 from I-register. its correct. 

Black screen is not true.

however I accidentally changed my TV source to a different input and when changed back to SCART the CPC's output was in colour with the correct blue background and yellow text. Cold starting it displays with a black background with white text again until I change my TV from one source back to SCART.

Tezz did not test with a buzz speaker test Pin 21-28 of AY3 when he connects pins 7-14 to ground.

Voltage on nCPU/CLK AY3 PIN15 is ~2.5V when clock is working.

[McArti0]

I wonder what happens to AY and PPI when AY 3 Pin 21-28 is out and PA is out for a long time.

[McArti0]

AY has CLK because this the same as nCPU. nCPU work because Screen is visible and CPU work because scaning keyboard. BC1, BD1 (Pin18,20) has buzz on speeker.

[Tezz]

Check this scenario. Cold cpc. Press and hold DEL and turn on cpc. Are You hearing sound? 0,5 SEC? 0,2 SEC? 0,01sec? 0,00....

Ok, I've tried this now. I don't hear any sound even a millisecond blip at all holding down DEL with a cold start. 

I received some 4164 RAM this morning so I've swapped out the single 4264 for good measure with MT RAM well known reputation although it was tested.

To clarify, the 464 starts with the correct startup being displayed but with a black border and black background with white text and no keyboard response. If I change the TV source away from SCART and back to SCART whilst it's in this frozen state the display is then in the correct colours with blue background and yellow text, still frozen.

Keyboard has been tested on my other long board 464 and I've swapped in known working Z80 and Gate array with no change. Original Z80 and gate array are back in place.

[McArti0]

AY3 probably died

Check Output from AY3 Pin21-28 buzz 50Hz when const 0V at input Pin 7-14.

[Tezz]

There's one AY-3-8912A currently listed on eBay https://www.ebay.co.uk/itm/256299151144