I just picked up a used PVS-14 to use as a backup unit.  After receiving it and testing it out, it powers on and works just fine, but when comparing it to my other PVS-14, it seemed dimmer at max gain.  I also noticed it seems to power on and be nice and bright at first, but quickly starts to dim on its own after a few seconds sort of like its autogating, but without any change in ambient light.  Also my other unit shows no similar dimming.  It doesn't dim out to black, but it ends up at a noticeably dimmer output vs. the other unit.

I opened it up and everything looks right to my untrained eye, but when I tested the voltage at the main board contacts with several fresh batteries, it always reads about 2.05 volts, when I believe it should be about 3 volts here.  If so, seems like it might be the problem?  The intensifier tube is ITT manufactured in 2007 by it's markings.

Any ideas on a possible cause, and possible fix other than replacing the battery housing/circuit board?

Plugs are fouled. Clean then good then try again. On both the tube and the contacts.

Started writing up a general troubleshooting guideline and then got curious and popped off the lower battery housing on a couple of PVS-14s. Imagine my surprise that two read 2.07V and 2.25V at the tube contacts. Fresh batteries and all. Both were lower housings made by Carson (I think) and I never noticed any performance issues with those units. Measured two other lower housings, made by ITT I believe, and they read right around 3.0V.

Only trimpot on the board is in the gain adjustment circuit. Now I'm curious to probe around and see if they were designed to output around 2.1V (why?) or if it's a common failure mode.

Could you take a pic of your intensifier tube? Just curious. Looks like a metal housing I haven't seen before.

I wonder if the output has a AC component that might not work well with certain tubes?

Everything looked clean, but gave it a shot.  No change.

Interesting.  Most boards I see online are green PCB, are the red ones Carson manufactured or something else?  The only trim-pots I could find are the two on the tube pigtail, and one on the underside of the battery compartment board.

Here are some additional photos of the tube.  Didn't want to take it all to bits but it gives and idea.  The silver metal housing on the tube appears to be the grounded connection for the power board.

Good thought. Checked my board with a scope and there's only a few mV of ripple. Doesn't mean it couldn't be an issue with D_Man's board but considering I had two otherwise healthy boards that read low,  doubt it.

One board on my end looked exactly like yours, exact same layout, just green. Another had a different (simpler) layout, also green. Was told both are Carson. The other boards have a different layout and the switch is wired with individual wires, not the flex board. Told those were ITT. Hope someone will chime in and add to this.

The trimpot on the underside of the board is part of the gain circuit. Still haven't gotten to tracing out the circuit and probing it but am assuming the voltage is set with a pair of fixed divider resistors. Probably possible to change out one of those resistors and up it to 3.0V, hope you like soldering surface mount parts tho. If it's a backup unit, picking up an older, less popular, and cheaper dual AA cell lower battery housing is an option.

Thanks for the additional pics. Haven't seen a tube with a conductive ring near the front. Neat.

BTW, do you hear a very slight autogating whine when the tube is powered up? Found conflicting information regarding the part number on the back and would like to clear it up for my own notes.

I found similar conflicting info on it having autogating or not.

So with the battery compartment off, I direct fed 3V to the pads on the tube using a cr123 cell.  Powers on nice and bright and does not do the same dimming after start-up.  Looks very comparable to my other PVS-14 (which has an Omni VII eqiv. tube) at full gain.  And while it's not as loud as the whine on my other unit, it does have an audible autogating whine.  I tried to test looking around at various light sources in the distance to see it, but it was hard to tell while while juggling the open unit with a loose battery and two wires held together with a plastic spring clamp.

Swap battery compartment with the other PVS 14 and see what happens.

I have another ITT 1.5 to 3 volt convertor and it use a MAX1674EUA DC/ DC convertor.
I would remove the board and use a magnifier to check for bad solder connections.
The circuit is pretty straight forward and the components are unlikely to fail.
It should be 1.5 volt in 3.3 volts out.

The (alleged) Carson battery compartment uses a TI TPS61010 boost converter chip, output variable between 1.5-3.3V. Pair of surface mount resistors form a voltage divider for setting the output voltage. Resistor between ground and the FB pin is 200k and the resistor between FB and Vout is 634k. With a FB pin voltage of 0.5V (per the spec sheet), the set output voltage is 2.09V. I'm measuring 2.07V, well within the tolerances of the resistors and bandgap reference. For whatever reason, it was originally built to provide ~2.1V to the tube and is currently functioning as designed. I'm confident that D_Man's battery compartment is, likewise, functioning as designed.

I'm stumped why it was built to supply ~2.1V. Simple fix would be to replace the 634k resistor with a 2.5M resistor. Alternatively the 200k resistor could be replaced with something close to 127k. Would recommend the latter instead of having a needlessly high impedance divider.

Remotely plausible that the feedback resistors for output voltage and the low battery indicator threshold were swapped by accident per the original design. As built, the low battery threshold is 0.9V and, as mentioned, a set output voltage of ~2.1V. Supposing the resistors from those two dividers were mixed up, the alternate values would be a low battery threshold of 1.0V and an output voltage of 2.5V. My stash of surface mount resistors is low so instead of waiting for an order to come in, I'll perform that swap and live with a tube voltage of 2.5V. Don't like it sitting at 2.1V as is.

did you check your batteries? people have stored batteries for a period of time, and even though they are 'new' unused, they may have gone bad.if the batteries had been stored for a while, the voltage may drop under load, so check that - connect the battery to an incandescent light bulb or other load, and measure the voltage for a while.

after that, connect the battery compartment power leads to the intensifier, with the volt meter to measure the voltage coming off the battery compartment. with the objective lens of the intensifier covered to prevent light damage, turn it on and see if there is a voltage drop, and if the tube gets dimmer.

That indeed is really odd... I wonder why they'd do that. 2.1V is on the extreme low end for most tubes operating voltage. Maybe they wanted to really squeeze that battery life, extend it as much as possible.
All tube power circuits I have seen to date feed the tube anywhere between 2.5V and 3.3V.