I am just looking at an FW4-RI rev B board from Pete Hand. It as clearly had some liquid spilt on the tube side of the PCB and was only lighting the two tubes on the RHS and then in a very odd manner. After a good scrub with some Isopropyl alcohol I started to investigate and, thanks to the relative simplicity of the multiplexed design I was soon able to deduce two obvious faults. One being that there was a break in the PCB trace for one of the cathodes. The other being that there was a break in the PCB trace that connected the HV from one set of high-sided drivers to the other. I 'fixed' these two breaks using wire links for now and that certainly restored the operation of the two previously non functional tubes. The remaining problem is that all segments on all tubes are illuminated dimly whilst it is also possible to discern correct word display on all 4 tubes if you really concentrate. All cathodes have between 80K and 90K resistance to ground and I am assuming that this is the reason for the partial illumination of all segments. Though I also note that all the traces from the cathodes to the HV driver via the relevant series resistors is on the clean/underside of the PCB where I see no evidence of any damage. My questions (at this stage!) are based on my assumption that there should be an infinite resistance between un-used cathodes and ground. I have never used an HV5812 though am familiar with the HV55XX and HV56XX range. Is this a 'common' fault on these boards? Is this a feature of the HV5812 (I am guessing NOT)? As the resistance between all cathodes and ground is between 80K and 90K - I am assuming that this is a fault at the driver level and not some more failed traces between the cathodes on all tubes. What kind of interest is there in these boards? All pointers gleefully received! - Richard
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