Looking at the photos, either your camera makes crappy pictures :D or there's something broken in the Speccy's video circuitry.
I assume no composite video mod (like here) has been done on this one, so you're using the TV modulator's output? Has the TV been tuned to the correct channel? (where I live, in the Netherlands, TV modulators usually output on UHF ch. 36, but it may be different for equipment sold in other countries).
Next thing would be to check the power supply voltages. You should have at least 8~9V DC input, and 5V output (to be checked at the 7805 regulator). And the Spectrum internally generates +12 and -5V from this (check at supply pins of the 4116 DRAMs - see schematic). The -5V isn't needed to obtain an image, but the +12 is used by the video circuitry.
If voltages are okay, you could have a broken transistor in video output stage, broken video encoder IC (LM1889, very common - should be easy to find replacement), or an ULA with damaged video output(s?). Could be other parts in the video circuitry as well, but most of these are passive (resistors, capacitors) and don't often die.
Do you have tools to check the supply voltages? Any chance you can test the ULA in another Spectrum? You might want to do the composite video mod on this machine, and thus take the TV modulator out of the equation. And perhaps: (blind) type in some commands to make a 'beep' sound (see manual), do your hear anything?
That picture (a screen with rapidly moving artifacts) is a sympton that the lower memory is not working at all. When the ULA wants to read that memory to build the picture, it finds nothing (floating bus) and hence, the value read is whatever the CPU was fetching from ROM, that becomes visible through the bus-divider resistors.
My suggestion: check the voltages at low memory (5V, 12V, -5V) and see if there are ok.
Looking at the photos, either your camera makes crappy pictures :D or there's something broken in the Speccy's video circuitry.
It's surprisingly difficult to take pics of a TV!:-o
I assume no composite video mod (like here) has been done on this one, so you're using the TV modulator's output? Has the TV been tuned to the correct channel?
No composite mod, it's totally standard. Yep, tuned correctly - booted an Atari ST up before to check I wasn't being stupid!
Next thing would be to check the power supply voltages. You should have at least 8~9V DC input, and 5V output (to be checked at the 7805 regulator). And the Spectrum internally generates +12 and -5V from this (check at supply pins of the 4116 DRAMs - see schematic). The -5V isn't needed to obtain an image, but the +12 is used by the video circuitry.
If voltages are okay, you could have a broken transistor in video output stage, broken video encoder IC (LM1889, very common - should be easy to find replacement), or an ULA with damaged video output(s?). Could be other parts in the video circuitry as well, but most of these are passive (resistors, capacitors) and don't often die.
Do you have tools to check the supply voltages? Any chance you can test the ULA in another Spectrum? You might want to do the composite video mod on this machine, and thus take the TV modulator out of the equation. And perhaps: (blind) type in some commands to make a 'beep' sound (see manual), do your hear anything?
That picture (a screen with rapidly moving artifacts) is a sympton that the lower memory is not working at all. When the ULA wants to read that memory to build the picture, it finds nothing (floating bus) and hence, the value read is whatever the CPU was fetching from ROM, that becomes visible through the bus-divider resistors.
My suggestion: check the voltages at low memory (5V, 12V, -5V) and see if there are ok.
I'll try the beep test next! :) Any docs on checking voltages? I have a multimeter which should work I think, just don't know where to place the probes - I'm more Software than a Hardware!! :-P
Search this forum for the TMS4116 datasheet, it has the pin outs of the 4116 memory which is the easiest place to test for the correct voltages (this is lower memory). There's a link to the data sheet in one of the threads.
Comments
Looking at the photos, either your camera makes crappy pictures :D or there's something broken in the Speccy's video circuitry.
I assume no composite video mod (like here) has been done on this one, so you're using the TV modulator's output? Has the TV been tuned to the correct channel? (where I live, in the Netherlands, TV modulators usually output on UHF ch. 36, but it may be different for equipment sold in other countries).
Next thing would be to check the power supply voltages. You should have at least 8~9V DC input, and 5V output (to be checked at the 7805 regulator). And the Spectrum internally generates +12 and -5V from this (check at supply pins of the 4116 DRAMs - see schematic). The -5V isn't needed to obtain an image, but the +12 is used by the video circuitry.
If voltages are okay, you could have a broken transistor in video output stage, broken video encoder IC (LM1889, very common - should be easy to find replacement), or an ULA with damaged video output(s?). Could be other parts in the video circuitry as well, but most of these are passive (resistors, capacitors) and don't often die.
Do you have tools to check the supply voltages? Any chance you can test the ULA in another Spectrum? You might want to do the composite video mod on this machine, and thus take the TV modulator out of the equation. And perhaps: (blind) type in some commands to make a 'beep' sound (see manual), do your hear anything?
My suggestion: check the voltages at low memory (5V, 12V, -5V) and see if there are ok.
It's surprisingly difficult to take pics of a TV!:-o
No composite mod, it's totally standard. Yep, tuned correctly - booted an Atari ST up before to check I wasn't being stupid!
I'll try the beep test next! :) Any docs on checking voltages? I have a multimeter which should work I think, just don't know where to place the probes - I'm more Software than a Hardware!! :-P
Thanks again!!
baz