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Hi Guys,
I am trying to fix some problems on my 11302, but I am stuck on a problem that seems very simple, but which I cannot overcome: a simple voltage divider. I am probably doing something wrong, but I cannot guess what (I added an album among the photos with the simplified schematic diagram).

The 11302 "reads" the CRT cathode voltage using a voltage divider, selected by a 4051 analog multiplexer. It should be simple: the voltage divider is made up of a 20 M ohm and a 20 kohm resistor (the former is actually 2 x 10 Mohm resistors). So the output should be one volt for every 1000 volts in the input (well, 0.999).
For me this is not the case: the divider output is significantly higher than it should be (with -1.6 V in the output, the input is actually -1.510 V).

Easy, you say, one of the resistors has changed its value over time, but it is not so. I desoldered one extreme of them and they are all very precise. Another hypothesis was that the CRT mesh was causing some problem, but it is not: I disconnected it and nothing changed. I neglected the C1100 and C1420 capacitors because they could only lower the voltage, not make it greater.

The last thing I didn't check yet is the dirt on the lower part of the board, but before disassembling it I wanted to hear your suggestions, if you like.

I will be very grateful for any help, I am getting crazy. Thanks Gianni

The noted symptom on-screen is consistent with the divider giving too high an output result - this causes the HV to drop, so the display blooms oversize. Also, that "spikey and noisey" TP condition during the fault is probably what you'd expect when the regulation loop goes open from loss of feedback or something hitting a limit. The problem I think is that something is wrong with one of the parts, causing intermittent jumping of the apparent divide ratio. It can be one of the resistors' elements or connections going bad, or a bad solder joint, or the mux.

Remember the 4051 is a bunch of CMOS transmission gates that are actuated by changing gate voltages internally. Being leaky can cause different troubles depending on the power supplies and signals used. If it's a fully bipolar switching arrangement, a fault leakage current may be of either polarity, and would actually cause an offset, not a scale change - but you don't know yet until getting some measurements. Since the signal is negative, the Vss would have to be negative and bigger, and there's probably a plus Vdd analog supply and plus 5V logic (or one in the same), so faults could go either way. I'd recommend again that if you have not eliminated the 4051, to do so and be sure.

I'd still suspect possible resistor or contact oddities, but since you've done a lot already to eliminate those, that leaves I think only the mux, or maybe something downstream from that. The CMOS parts have pretty good transient voltage protection, but they can still randomly fail, and that signal is part of a HV environment, and you don't know what sorts of things may have happened in there before. Good luck.

Ed