Alexa Mini / Preston V+F Issues - Blown Control Unit

James Malamatinas · February 27, 2018

I was working on the Alexa Mini today; the camera was powered by via a battery plate that accepted Anton Bauer onboards and block battery power - both were connected and the camera was turned on showing a reading of 16v.

Also connected to the camera was a Preston V+F Control - this was plugged into the camera via an EXT to R/S adapter (similar to https://tinyurl.com/yd9jya7s).It was turned on and working.

I then proceeded to unplug the block battery from the battery plate (the Anton Bauer was still connected), and simultanesouly the Preston V+F Control died, but the camera did not. We swapped out all of the obviouse, spare cables, tried powering via D-Tap and RS on the battery plate but nothing worked. We ended up sending it to the rental house who could not diagnose the issue but ruled out a blown fuse and said it needed additional investigation.

The Mini EVF was the only othe accessories powered by the camera and was unaffected. The Teradek was powered on at the same time but via D-tap from the battery plate, this was also unaffected.

Can someone suggest how this could have happened? Unplugging the block battery is obviously done routinely and should have no effect, could it indicate some kind of faulty hardware - perhaps the battery plate that didn't regulate the power to the camera (and thus Preston).

I'm at a loss to how it happened. We got another Preston in and that worked fine, we then decided to power down the camera before swapping batteries for the rest of the day so as not to risk blowing any more accessories if there was some kind of issue.

Any insight very welcome, happy to provide more details if needed.

Dom Jaeger · March 2, 2018

Well the voltage input to a Mini is directly passed out the EXT socket, so any input spike would be passed on to the Preston. (Standard Alexas regulate input voltages under 24V up to 24V for their RS sockets, but any voltage input that is above 24V is also simply passed through, so be careful powering accessories that are sensitive to over-voltage if your block battery is 30V for example).

But as far as I'm aware a battery plate like the Wooden Camera ones we use with Minis should take whatever the highest voltage is and pass it to the camera, until both sources are at parity and will both get used. Disconnecting one input source might cause a voltage drop, but not a voltage spike.

Also, Prestons can take a wide voltage input, something like 12-28V from memory, so should normally be fine with whatever is powering the camera.

However Preston did issue a bulletin last year about the dangers of using split-level power supplies, meaning having power grounds at different potentials. This can be caused by powering accessories from a separate power supply to the camera for example, or sometimes by faulty cables. To diagnose something like that you'd need to measure the GND connections of your system setup with a voltmeter to make sure there were no voltage differences. I don't know if that was the cause of your issue, but it seems safe to say you didn't do anything out of the ordinary, and the rental house might need to investigate further.

James Malamatinas · March 3, 2018

Thanks Dom, very interesting about the split level power supply warning and seems like a very plausible explaination. Do you have the full text of that bulletin? I can't find it on their site and the only other place with it seems to be the Steadicam forum where I'm waiting for my user registration to be approved.

Dom Jaeger · March 4, 2018

Sure, here it is:

Preston Cinema Systems Technical Bulletin

The Dangers of Using Split-level Power Supplies and Breakout Boxes

We've recently noticed an uptick in units with PCB traces and/or components damaged when powered by split-level power supplies or third-party voltage breakout boxes. While we have short-circuit protection built into our products, it's impossible to protect from every scenario since we don't have access to the wiring for third-party devices. "Hot Ground" connections pose a threat to all electronics, not just Preston equipment, so it's absolutely critical to be mindful of how you're powering all your accessories on set.

How to Avoid Costly and Time-consuming Repairs

The following precautions can mitigate the threats posed by the ever-increasing array of power supply options:

Avoid using two 12V batteries wired in series whenever possible

If you must, never connect the GND of the center tap to the output. Only use the center tap for +12V output. Use the GND from the bottom battery of the two.

During prep, use a voltmeter to verify there is no voltage difference between each of the GND connections on the system

Power the MDR, LR2, and any other camera-connected accessories from the same source as the camera to ensure all devices have the same GND level

Questions? Contact Tech Support