48K Issue 3, noisy display.
Has anyone here experienced and been able to resolve a problem that looks like this:
Photograph of display.
It is in fact quite a bit worse than it appears in that photo. The picture generally waves and jiggles about (but with no loss of synchronisation) and the pattern in the centre of the display (including the border) slowly and smoothly scrolls upwards.
The video feed was taken as a composite signal with the modulator disconnected. This Spectrum has all new electrolytic capacitors and a new 7805. Apart from the poor display, it seems to work correctly. The pattern rather makes me think the noise is digital in origin.
I've used an oscilloscope to look at the 12V line at the LM1889 and found what looks like a rough square wave with an amplitude of about 100mV p-p with about 100mV p-p of noise superimposed on it. The same measurement on an issue 2 Spectrum that gives a good display finds almost exactly the same.
I've got some further tests in mind, but I thought I'd ask here in case anyone has encountered this before.
Photograph of display.
It is in fact quite a bit worse than it appears in that photo. The picture generally waves and jiggles about (but with no loss of synchronisation) and the pattern in the centre of the display (including the border) slowly and smoothly scrolls upwards.
The video feed was taken as a composite signal with the modulator disconnected. This Spectrum has all new electrolytic capacitors and a new 7805. Apart from the poor display, it seems to work correctly. The pattern rather makes me think the noise is digital in origin.
I've used an oscilloscope to look at the 12V line at the LM1889 and found what looks like a rough square wave with an amplitude of about 100mV p-p with about 100mV p-p of noise superimposed on it. The same measurement on an issue 2 Spectrum that gives a good display finds almost exactly the same.
I've got some further tests in mind, but I thought I'd ask here in case anyone has encountered this before.
Post edited by Zorn on
Comments
I suspect the noise on the signal confuses the analog to digital conversion in the display as it has to quantise the signal. On an analogue display it's just grey and a bit fuzzy.
As I pointed out, I have replaced all of the electrolytic capacitors. I suppose I may check some of the others in the video circuitry.
Maybe the ULA itself produces bad signal quality. In than case it is very difficuilt to improove the signal. But it maybe also improoved by compensation - needs time and patience ...
Regards Ingo.
My old Amiga 600 RF out started to show similar noise then I found some of it's capacitors leaked all over the board.
So I guess, guesser's recommendation is a good one.
By the way, Eightyone (the emulator) is emulating this exact effect :)
If that doesn't help, replace the two transistors right below the RF metal box.
Finally if that doesn;t completely solve it, replace the 7805.
The problem turned out to have a very simple cause, but it took a while to find. After experimenting with extra power decoupling and testing several components I decided to attack it more methodically with an oscilloscope probe.
Below is an example of the composite video output of the problematic Spectrum (top) along with the same from one that gives a good display:
Some significant interference of a digital nature is very obvious during the horizontal sync. pulse. Moving one step back through the circuit, I found the video signal to be perfectly clean at the base of TR2. This meant that either TR2 was behaving in an exceptionally strange way or the noise was somehow being added to the final output signal.
I followed the traces from TR2's emitter around the board and over to the edge connector, looking for anything that could be bridging it to another trace. Nothing was particularly obvious. So, I started looking at the signals on traces that run next to it, looking for one with pulses approx. 500ns wide and found one!
Here is the composite signal at the output along with the Z80's A13 line (pin 3):
I think that makes it rather obvious where the interference must have come from. I measured 870 ohms resistance between A13 and the video trace, but could see no obvious sign of anything bridging them.
I noticed that both traces pass underneath C26, so I removed this capacitor to have a look underneath. The board looked a bit grimy, but there was nothing obviously wrong. I cleaned the board and the body of C26 (after finding it to be conductive!) and then my meter indicated an open circuit between the two affected traces.
With C26 back in place, this Spectrum now produces an excellent display.
I wouldn't be surprised if dirt under C26 is a common cause of poor displays on Spectrums because it is located just inside from the edge connector.
Well done, and thanks for sharing!