464+ working but black screen

[evenmore]

Hi everyone,

I have an Amstrad 464+. it seems to be working correctly, but I don't get any picture.
I do have a CPC 464, which works correctly on my Scart TV.

I can hear the Burning Rubber music playing, I can also run BASIC and press the DEL key and hear sound.
I have two Scart cables, one powered and one normal. Both show a black screen.
I have tried a couple of different power supplies, one 5v 2.4 amp and one 5v 3 amp.
I have also tried an OSSC upscaler with both Scart cables, which reports that the 464+ is outputting a signal but the image is black.
The OSSC overlay menu is visible, so the problem does not seem to be with my TV.

I opened it up and there doesn't seem to be any obvious issues with the circuit board, although I am not an electrician.

A video of the machine running on my TV is available here.

Thanks very much for any suggestions!

Regards,
  Chris

[Bryce]

Hi evenmore,
     do you have a multimeter to check some signals? If so, you should check the pins 2 to 9 and 11 to 14 on IC115 (AMS48464 - Video DAC) to see if the ASIC is outputting a video signal.

Bryce.

[tibbs1972]

Hi,

I'm a bit of a newbie to the plus range, started working on mine last week.

I found out that newer TV's might not handle the 464+/6128+ RGB output through the scart connection. 

Also, I tried a Scart to HDMI upscale, but that definitely didn't work, it was grey and garbled.

I was lucky, I had a 10year old Murphy TV that detected the RGB signals thorugh the scart lead. A possible cause might just be the TV can't handle the signals. 🤔

Regards
Tibbs

[evenmore]

Thanks everyone, I will get my dad to help me test the board with the multimeter.
It's strange that it should work for my CPC464, but not the Plus.
That makes me think it is something to do with the computer rather than the TV.
It is an old TV, over 20 years or so.
I will let you know what happens!
Thanks very much for the advice!
Chris

[eto]

Hi,

I'm a bit of a newbie to the plus range, started working on mine last week.

I found out that newer TV's might not handle the 464+/6128+ RGB output through the scart connection.

Also, I tried a Scart to HDMI upscale, but that definitely didn't work, it was grey and garbled.

I was lucky, I had a 10year old Murphy TV that detected the RGB signals thorugh the scart lead. A possible cause might just be the TV can't handle the signals. 🤔

Regards
Tibbs

then they probably don't detect that they should switch to RGB. They work in default mode and try to decode a (not present) composite video signal.

You need a Scart cable with power injection that puts the proper RGB detect signal on one of the pins.

You can find more detailed information on the Wiki.

[eto]

Thanks everyone, I will get my dad to help me test the board with the multimeter.
It's strange that it should work for my CPC464, but not the Plus.
That makes me think it is something to do with the computer rather than the TV.
It is an old TV, over 20 years or so.
I will let you know what happens!
Thanks very much for the advice!
Chris

What cable do you use? Can you take a picture of it? In the video it's not fully visible and looks slightly suspicious .

[McArti0]

I saw with my own eyes how scart didn't work because there was no audio connected.

TV has strange whims.

If you have disassembled the Plus cable, open it and show photos of what is soldered there.

[McArti0]

I have a tricky question yet: do these cables work well with this CPC464 without plus? 

[evenmore]

Thanks everyone, I will get my dad to help me test the board with the multimeter.
It's strange that it should work for my CPC464, but not the Plus.
That makes me think it is something to do with the computer rather than the TV.
It is an old TV, over 20 years or so.
I will let you know what happens!
Thanks very much for the advice!
Chris

What cable do you use? Can you take a picture of it? In the video it's not fully visible and looks slightly suspicious .

The cable from retro computer shack is powered. I'm not sure where I got the unpowered one.

[Bryce]

I saw with my own eyes how scart didn't work because there was no audio connected.

TV has strange whims.

If you have disassembled the Plus cable, open it and show photos of what is soldered there.

Wow, haven't seen that before! What TV needs audio for the RGB to work??

Regarding the question in your second post: The DIN connector is different, so he couldn't use the same cable on a non plus 464 without converting the cable.

Bryce.

[McArti0]

I saw with my own eyes how scart didn't work because there was no audio connected.

TV has strange whims.

If you have disassembled the Plus cable, open it and show photos of what is soldered there.

Wow, haven't seen that before! What TV needs audio for the RGB to work??

29PT8306 

[evenmore]

Hi evenmore,
    do you have a multimeter to check some signals? If so, you should check the pins 2 to 9 and 11 to 14 on IC115 (AMS48464 - Video DAC) to see if the ASIC is outputting a video signal.

Bryce.

Hi Bryce
I got my dad to check with the multimeter. Pins 2-9 and 11-14 all outputting 5v.
Pins 15-17 (OUTR, OUTG, OUTB) all showing 0v.
Any suggestions? Does this mean the chip is bad?
Thanks again!
Chris

[Bryce]

That sounds like the DAC is broken. Pins 2 - 9 and 11-14 are actually inputs, so either the ASIC is setting them all to 5V (in that case the ASIC is dead), but even then, that would be the colour white, so if the DAC was working the screen should be white. So the DAC is definitely not doing what it should and the ASIC is possibly also damaged. Sounds like the computer was either connected to the wrong voltage or some connected the power the wrong way around. Either way, this isn't something that can be fixed because neither of those IC's can be obtained unless you removed them from a working Plus.

Bryce.

[McArti0]

On BASIC run INK 0,0,26:BORDER 0,26 (and press enter of)
This blink white/black WHOLE SCREEN.

Slow Speed of blinking You can set SPEED INK 255,255

If DAC is healthy you will see changes on multimeter.

But most likely the DAC is dead.

[evenmore]

That sounds like the DAC is broken. Pins 2 - 9 and 11-14 are actually inputs, so either the ASIC is setting them all to 5V (in that case the ASIC is dead), but even then, that would be the colour white, so if the DAC was working the screen should be white. So the DAC is definitely not doing what it should and the ASIC is possibly also damaged. Sounds like the computer was either connected to the wrong voltage or some connected the power the wrong way around. Either way, this isn't something that can be fixed because neither of those IC's can be obtained unless you removed them from a working Plus.

Bryce.

Thanks very much for your advice! Not really what I wanted to hear!
Hopefully my dad was pressing the right parts with his multimeter.
He talked about needing to touch one end to a negative source, I suggested the keyboard grounding pins?

I could have connected the Plus to an incorrect power supply while trying to get it set up a few years back
I have a stack of them in my cupboard for all sorts of things.

Thanks very much for your help. I'm not sure where to go from here, whether it would be worth buying another machine.
It's a pity there are no replacement chips.

Regards,
  Chris

[eto]

I suggested the keyboard grounding pins?

What keyboard grounding pins? On the keyboard connector? There is no real GND.

Just use GND on the motherboard. Lots of ICs have a GND pin, e.g. the RAM ICs here.

[eto]

neither of those IC's can be obtained

Can the DAC be replicated with a R2R ladder? I think there was once a discussion around that.

The STe and Amiga also use R2R ladders for 4096 colours. Is that an option to look at?

[McArti0]

Can the DAC be replicated with a R2R ladder? I think there was once a discussion around that.

No problem with one amplifying transistor per channel.
You can repeat the RGB output arrangement from the CPC x3 with one more resistor for LSB.

There is only one question: what voltage levels should be at the output in PLUS.

[McArti0]

simple-R-DAC

[Bryce]

I suggested the keyboard grounding pins?

What keyboard grounding pins? On the keyboard connector? There is no real GND.

Just use GND on the motherboard. Lots of ICs have a GND pin, e.g. the RAM ICs here.

He probably means the brass grounding spring that presses against the lower keyboard plate.

Bryce.