SVD wrote on 2021-11-30, 11:30:
Yeah that's what I thought about the 8V being wrong, thanks for confirming it. There is 12 chips on the video input board alone 😒 There is also some chips on the mainboard but that is a lesser amount. I think I should check the voltage again, this time over the chip and not to ground. If it still is 8V-ish then I need to order chips.
Is it problematic to change the chips to a modern equivalent? 74s22 => NTE7413 74s05=>SN74HC14N Or are they equivalent?
Thanks for the tip about the power supply, is it also possible that the 18v should be 12? I dont know if a dead capacitor or voltage regulator could throw the whole supply off like that but i guess its possible. I should probably check all the diodes, transistors and caps also.
It usually is OK to substitute 74-series chips, but you need to know what you are doing. There are two different conventions about how you define high and low. The "old" convention is called TTL convention. You need to pull down an input to below 0.8V to make it low, and you might need to pull quite strong. On the other hand, inputs rise back to high on their own, but if you need it quicker, you can push a little current into it. As soon as the input is higher than 2.0V, it is guaranteed to be read as high. The "new" convention is called "CMOS" convention. This one is symmetrical. Everything below 2V is guaranteed low, everything above 3V is guaranteed high. You don't need to pull strongly to keep an input low. You should avoid mixing TTL and CMOS logic, although connecting CMOS outputs to TTL inputs will work. CMOS outputs can pull strong enough either way. TTL outputs might not reliably go high enough to make CMOS inputs high.
So you need to be careful when substituting chips: 74xxx, 74Sxxx, 74LSxxx use TTL levels for input and output. 74HCxxx use CMOS levels for input and output. If you want to replace a 74LSxxx chip and can't get a suitable LS replacement, you can also use a 74HCTxxx chip. The "T" in the type number tells you that the inputs are TTL compatible, although the chip itself is built using HC (high-speed CMOS) technology. A 74S22 ("dual 4-input NAND gate with open collector output") can not be replaced by a NTE7413 ("dual 4-input NAND fate with schmitt trigger inputs"), because you may not replace open-collector output gates by standard (totem-pole) output gates. You also should avoid replacing 74S series chips ("schottky") by 74LS series chips ("low-power schottky"), because the low-power chips are slower and have less strong outputs. I failed to quickly find a substitute for the 74S22 that will work in any circuit.
The 74S05 again is a chip with open collector outputs, so you need to replace it with a chip that also has open collector (or open drain for CMOS chips) output. The 74HC14 is thus wrong for two reasons: It doesn't have open-collector outputs, and it's a HC chip that may be incompatible with TTL inputs. You may substitute a 74S05 by a 74S06 or a 74S16, though. Again, I fail to find easily available modern equivalents.
The 18V output might be supposed to have 12V, but it might also be supposed to 16V, 18V or 19.5V. Without specifications for that supply, we don't know. In my experience, there is no "set of standard voltages" in classic CRT monitors. Power supplies like the one in your monitor typically use an opto-isolator that is used to communicate "we have enough voltage on the output, push less energy, please" to the primary side. Overvoltages might be caused by the optoisolator being weak or defective. That's something you don't find easily when you use standard component test procedures.
