Hello everyone,
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I'm glad to report the HP 3403C works again!
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I've replaced the duos of LDs A/D chips, but this was not enough to revive the unit. It kept blinking +1 although it had lost its tendency to sometime display garbage.
I turns out I lost the +12V supply rail which I previously had. On the test point of the supply had a dead short. Taking boards out I found that it was on A7 (the board between the converter and the motherboard). Looking at it I found that both C1 and C2 looked really weird has they had a ball of solder on them and you could see inside of them. I've taken both them out and although C2 is apparently healthy the C1, which is the smoothing cap of +12V, was shorted. Good thing it was regulated.
With them out the unit works again and is not too far off reality aswell. Very happy about it.
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My beginner mistake was that I made the power supply measurements with basically all of the boards out. However I think the new chips were necessary as the display was not working even when A7 was out.
I'll order new capacitor for that board, and i'll also recap the power supply aswell.
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Here it is reading the probe test of my oscilloscope together with its friend the Racal Dana.
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Thanks everyone!
