NVR Died

RE: NVR Died

jw0752 — Sun, 03 Nov 2019 16:36:57 GMT

Hi Ralph,

When checking the circuit with your meter be aware that the color of the probes may not accurately indicate their polarity. It's best to use a second volt meter to verify which ohmmeter probe has the positive potential.

John

RE: NVR Died

shabaz — Sun, 03 Nov 2019 07:33:28 GMT

Hi Ralph,

I too think the excellent techniques mention by John will help.

Were you looking for a composite (RCA connector) video output? A source could be camera modules from ebay, they are around $10, e.g. this one (I've not tried this one): https://www.ebay.co.uk/itm/Analogue-520TVL-CCTV-Camera-Module-3-6mm-Lens/383239562473?hash=item593adba4e9:g:eE0AAOSwT5Rd…

Also, if you think there's any chance that the PCB was stressed/flexed (e.g. by wiggling in/out connectors or tightening screws, if you can see any MLCC capacitors on that 12V rail circuitry that could flex, for example if they look like an input cap to a voltage regulator, it could be worth removing it and testing it for a short.

I've marked what looks like caps near DC-DC converters, maybe the lower-right one (closest to a screw hole) could be worth temporarily lifting off (if it is connected across 12V and 0V) to see if the short on the board disappears.

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RE: NVR Died

Gough Lui — Sun, 03 Nov 2019 07:26:04 GMT

I would suggest you try isolating as much as you can.

I'm not sure if you have tried, but see what happens if you try to apply 12V power without the fan and hard drive attached. If the board is still shorted, then it's a good sign that the fault is not in the hard drive or fan. If the short disappears, the fault might not be in the fan and hard drive but could be related to the behaviour of DC-to-DC converter under load.

The next thing is to find a friend with a thermal imager - but if you don't know anyone with one, I find a finger works fine too. My suspicion is that you might have a DC-to-DC controller that has a shorted MOSFET - so try touching each of the chips near the inductors on the board (i.e. bottom edge, near the centre near the SoC) to see if any of them are hot. Similarly with the inductors which might give a clue.

If not, then the next step might be to feel and carefully examine any electrolytic capacitors. If there are any tantalum capacitors, they are a prime suspect as well as some have a power-on-short failure mode which is known.

If you do have a failure in the DC-to-DC controller ICs, it may be a sign that your power brick isn't a great quality unit. It could have been affected by a mains surge or more likely, it may put out spikes or high ripple which may have stressed the converters.

- Gough

RE: NVR Died

jw0752 — Sun, 03 Nov 2019 03:15:00 GMT

Hi Ralph,

The chances of finding the problem when it is a shorted power in line are usually pretty good. I would begin with an ohm meter probe on one of the power input wires and then I would look at all the semi conductors that are close to the power input. We are looking for any that are directly connected to the power rails. There is perhaps a diode that was being used for polarity protection. The diode could be one of the 3 legged SM components as many of these are diodes and not transistors. You mentioned putting power on the input and looking for smoke. Perhaps you could modify this idea and put just a little power in and use an infrared viewer to look for where the heat is going. You may be able to see the warm traces leading up to the shorted component. If you do not have an infrared viewer you might be able to use a small thermistor and move it around while watching its resistance. Traditionally I would cut traces and eliminate circuit branches to trace down the culprit component but these newer SM boards are not as easy to do this with. Still you might look to see how far you can follow the power traces and if there is a place where you can put a bunch of components on one side and another bunch on the other side cut the trace and see which side the short is on. You can always scrape the masking from the trace and put a solder bridge back after the bad component has been removed. If I think of anything else I will add later.

John