Automated Band Saw

[Systemtek]

Hi,

Looking for some suggestions but I have a lead to follow too. I'm helping out a friend with an old piece of machinery.

It's an old automated band saw that is stopping after a short but varying time and the LCD display is mostly showing solid blocks but some characters after the boot screen.

In the electrical cabinet on the machine there is a linear power supply, a PLC, contactors etc and a UI with an LCD display, hard switches and a basic keypad. The board running the display and keypad is getting 5.02v at it's terminals which is then fed to a filter.

The filter has been modified! The input goes to a ceramic and an electrolytic but the trace taking the power from the ceramic to the electrolytic has been cut and a resistor (or inductor?) has been bodged on bridging the gap. 4.8v makes out onto the board after the filter and I tested VCC to ground on a few of the ICs and they are all receiving this with no more drop. I only had time to check the data sheet on the flash chip which state +- 10% in a 5v input.

My first guess looking at the symptoms before I arrived was a poor filter on the controller and or an inductive load (maybe with a failed flyback diode) sending noise back through an output.

The electrolytic tests as having 0.2ohm ESR in circuit and correct capacitance but is old and when I connected the ESR meter one of the 3 or 4  times I tested it, it showed 0.25pF. Maybe a failed connection inside the electrolytic? Combined with the modification to the input filter is changing the electro a good course of action? I have an electrically suitable cap to try it with but may need order a different shape to fully complete the repair if I get good results.

Also when I tested AC across the power supply filter and the input to the UI board with my meter a stable voltage regularly and consistently appeared but this was not there using my friends meter and the DC voltage is stable in both cases so it doesn't seem to be heavy loading and dry caps causing the AC to appear.

This 'UI' board is actually a stack of 2 PCBs one of which is attached to the display and has black blobs on the ICs.

Could it be that the keypad is contaminated and causing it stop maybe?

all any ideas welcome, thanks