techknight
Well-known member
yup you got a bad resistor on there.
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SE/30 horizontal sweep lines....still!
- Thread starter Tekk
- Start date
I've also got this problem (for quite a while, actually). I have replaced all the parts that where recommended to replace by the gamba-site, but still no cigar. I've also measured all the resistors on the neck-board, everything measured fine. Could somebody help me to diagnose the problem? I've got a decent scope and multimeter, so what points on the analog and/or neck-PCB could I check to see if everything is all right? What should I look for?
Well, I just got around to measure all the resistors on the neck-board out of circuit, and they measure all fine. I also replaced C15 on the analog board with a nice PP cap I had lying around in my spare parts drawer, but that also had no effect. What points should I check with the scope or multimeter?
Quick follow-up: I just probed the outputs of the 74LS38 chip with the scope and found the outputs on pin 3 (horizontal sync, got a nice 22.2 kHz square wave, so it works as it should) and 6 (vertical sync, got a short pulse every 15 ms, so seems to be working okay) to be working correctly, but pin 8, the output of the video data, seemed stuck at approx. 1.5V. Strange, that should have displayed packets of video data. The Vcc was fine, so that was not the problem. I probed further, on the input pins on the chip for the video data (pin 9 and 10) and found a very dense wave, but I expected packets of data.
Maybe it's a bad connection? Let's probe the corresponding output on the logic board! Well, same stuff, a fuzzy, dense wave of things. Hmm, maybe the problem is on the logic board after all.
Maybe it's a bad connection? Let's probe the corresponding output on the logic board! Well, same stuff, a fuzzy, dense wave of things. Hmm, maybe the problem is on the logic board after all.
I was wrong on the fuzzy line stuff, I just hadn't set up the scope properly for such small data.
This is what I got directly on the logic board, probe on pin 13 of the UG6 IC, the video data output:
The signal isn't all that strong, only around 100mV P-P.
This is the signal on the inputs of the 74LS38 IC on the analog board, due to a couple of resistors on the logic board, the amplitude is a bit less than the output of the UG6 IC, around 70mV P-P.
On the output of the 74LS38 I get the 1.5VDC, so I think I'll try the spare logic gate, maybe that one will work.
This is what I got directly on the logic board, probe on pin 13 of the UG6 IC, the video data output:
The signal isn't all that strong, only around 100mV P-P.
This is the signal on the inputs of the 74LS38 IC on the analog board, due to a couple of resistors on the logic board, the amplitude is a bit less than the output of the UG6 IC, around 70mV P-P.
On the output of the 74LS38 I get the 1.5VDC, so I think I'll try the spare logic gate, maybe that one will work.
techknight
Well-known member
Thats not anything. That is just noise. Your missing the video signal. A real signal is much greayer in amplitude, but it also contains the video signal for a line and has a front/back porch inversion.I was wrong on the fuzzy line stuff, I just hadn't set up the scope properly for such small data.This is what I got directly on the logic board, probe on pin 13 of the UG6 IC, the video data output:
The signal isn't all that strong, only around 100mV P-P.
This is the signal on the inputs of the 74LS38 IC on the analog board, due to a couple of resistors on the logic board, the amplitude is a bit less than the output of the UG6 IC, around 70mV P-P.
On the output of the 74LS38 I get the 1.5VDC, so I think I'll try the spare logic gate, maybe that one will work.
Now that's some useful info! I'll check the power rails on the UG6 IC and linked components, maybe it just doesn't get its juice.Thats not anything. That is just noise. Your missing the video signal. A real signal is much greayer in amplitude, but it also contains the video signal for a line and has a front/back porch inversion.I was wrong on the fuzzy line stuff, I just hadn't set up the scope properly for such small data.This is what I got directly on the logic board, probe on pin 13 of the UG6 IC, the video data output:
The signal isn't all that strong, only around 100mV P-P.
This is the signal on the inputs of the 74LS38 IC on the analog board, due to a couple of resistors on the logic board, the amplitude is a bit less than the output of the UG6 IC, around 70mV P-P.
On the output of the 74LS38 I get the 1.5VDC, so I think I'll try the spare logic gate, maybe that one will work.
I've just checked all the rails of the PAL's, but they all were just fine.
The UG6 chip got a approx. 15.3MHz clock on pin 1, which would be the system clock /2, so I assume that is correct.
See it for yourself (it is far from a perfect square wave because my scope is too slow in digital mode for this kind of signals):
BTW, you can right-click the images and choose to view them in a new window or something to get a bigger version.
After that I went on to measure the vertical sync on pin 14 of the UG6, which is a 60Hz pulse train which sounds plausible, so I assume that one is correct as well.
While I was at it I also measured the vertical sync on pin 18 of the UI6, which gave a nice 22.3kHz square wave:
So... it seems UG6, the composite-video-creation IC doesn't make a video signal, which causes this nasty pattern on the display. Is UG6 the one I should replace (anyone with a scrap board who can unsolder it for me?) or is there a possibility something else is causing this?
The UG6 chip got a approx. 15.3MHz clock on pin 1, which would be the system clock /2, so I assume that is correct.
See it for yourself (it is far from a perfect square wave because my scope is too slow in digital mode for this kind of signals):
BTW, you can right-click the images and choose to view them in a new window or something to get a bigger version.
After that I went on to measure the vertical sync on pin 14 of the UG6, which is a 60Hz pulse train which sounds plausible, so I assume that one is correct as well.
While I was at it I also measured the vertical sync on pin 18 of the UI6, which gave a nice 22.3kHz square wave:
So... it seems UG6, the composite-video-creation IC doesn't make a video signal, which causes this nasty pattern on the display. Is UG6 the one I should replace (anyone with a scrap board who can unsolder it for me?) or is there a possibility something else is causing this?
techknight
Well-known member
I knew your horiz and vert signals where good otherwise you would have no raster.
You need to stare at the SE/30 schematics for the video section and find out where the video bitstream from the vram mux serializer gets lost.. and check the traces. If good, replace the IC.
You need to stare at the SE/30 schematics for the video section and find out where the video bitstream from the vram mux serializer gets lost.. and check the traces. If good, replace the IC.
