Quote:
Originally Posted by Alex KL-1
An array is a simple form of IC not having power supply sometimes, so rarely have any processing. Basically the cause can be external to it (sometime trowing 20V there).
Anyway, the pin 9 must have higher voltage than the others, or the internal diode conducts.
The problem must be with any of the outputs if it don't reacts with the input.
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And so I measured the logic circuit approaching IC250 and I found a problem with inputs on IC604, which directly feeds the network of transistors leading to the inputs of IC250.
Pin 1 of IC604 is LOW when it should be HIGH. This is causing Pin 5 to be LOW when it should be HIGH.
I traced the input for Pin 1 IC604 back, and it's connected to the collector of Q613, and also to the 12V line through some resistors.
The Base of Q613 is connected to the output of D609.
D609 is switched on and off by the Auto/Manual switch for the horizontal and vertical size and position. The switch goes between GND and Open Circuit.
D609 measures correct on it's other side (toward Q615), but it reads 2.7V on the side towards Q613 when it should read 0V.
I can see that the input of Pin 1 on IC604 being wrong will cause certain problems, but I can't tell if this problem is because D609 give incorrect voltage, or because the 6.5V which should be present on the other side of Q613 is not there for some other reason.
I also cannot see why this possible "cascade" problem would create 24V on Pin 10 of IC250. If I had to make a guess, I would tend to believe there is an issue creating the 24V on Pin 10 of IC250 and that is leading to, or also causing the loss of 6.5V on other side of IC250.
I measured all other logic ICs in this circuit (IC601, 602, 603, 605, 606) and I found that all voltages were correct on all pins of all ICs.
I replaced Q613 with a new part of the exact same transistor (they are still available) and that didn't fix it. The old transistor measured to be a bit weak and measured different from new part, so I thought it was worth a try.
I have replacement ICs for logic coming in the mail, luckily they are all also available and very cheap.
This is what I am seeing, with incorrect voltages in red:
