Hi folks.
Work was recently having a clearout of broken/old electrical equipment, and one of the items was a 2U rackmounted 2.5KW power supply made by EMC.
The power supply itself appears to work fine, the reason it was going out was the voltage display on the front panel is broken. The power supply otherwise works as it should.
I'd like to fix the display, so i've set about doing some testing to try and see whats going on.
There are two almost identical displays on the front of the unit, one for current, one for voltage. I first off swapped the liquid crystal display between the two units, confirming the display itself is good. On close inspection i see the voltage display does come on, its just very dim (you need to REALLY squint at it to see it) and is displaying "-1" rather than a reading.
I've then spent some time looking at the inputs to the display board, which it turns out is pretty simple, a 30vDC supply (labelled as +15v and -15v on the mainboard, but they dont seem referenced to chassis ground) then a voltage input. The voltage input is 1:1 proportional with the main output voltage.
The board appears to have a TC7126 ADC which has all the driver components build in to run the LCD segment display. The TC7126 has a maximum supply voltage of 15v, so i figured the board must be doing something to reduce the 30v input voltage down to a lower level. I measured the voltage at the chips V+ and V- terminals on the working display and got just under 9v. However on the broken display, i measure only 2.8v.
Theres no fancy switching components on the board, its all just passives. From what i can tell, both input rails are fed directly into a 10k resistor then go from there directly to the microchip ADC. Measuring the output of these 10k resistors gives the same 2.8v reading that the chip is seeing.
I'm struggling to see what exactly could be wrong with this thing, so i've taken some photos of the board and i'm hoping someone can give me some pointers (i've removed the LCD for extra clarity).
https://www.dropbox.com/s/9cggcn9ub0rl2uo/2015-06-13%2014.47.10.jpg?dl=0https://www.dropbox.com/s/61uafvtho5igkh8/2015-06-13%2014.47.23.jpg?dl=0The positive rail comes in on Pin 3, and goes directly to R5. The negative rail comes in on Pin 1 and goes directly to R9.
The Capacitor C3 is missing in the photos becuase it was reading funny while installed, so i removed it for testing. Capacitance checked out OK once removed so i've now refitted it. The other electrolytic tested OK for capacitance in situ. Oddly, C3 on the other display tested fine in situ. A pointer perhaps?
I realise i could simply get a cheapo voltage display from ebay and clag it in, but i'd quite like to actually fix it properly.
Cheers
Kevin