Refurbishing an old PAG charger

Phil Rhodes · December 24, 2022

People seem to make a living on YouTube with this sort of stuff, so let's document a process.

I have a soft spot for old PAG battery gear. I should be clear that the company now makes far better stuff than this - it's all at least ten to fifteen years old, it's just that some of the old packs were really easy to recell. Creating a battery system with enough capacity to create a 48V bus for big LED lights would be several times more expensive any other way. So, today we're revisiting a truism about buying quality vintage gear, which generally keeps being useful for a lot longer than brand X.

The problem is that one of my AR124 chargers came with a broken display. It's old enough to have some history.

The upside is that the LCD character displays are very much an inexpensive commodity item, and I purchased a replacement for £6.50 on eBay. If anyone can see the immediate problem here, it's that the old display (bottom) has an electroluminescent backlight panel, like an Indiglo watch, while the modern one, at the top, uses LEDs. This means this was probably one of the original, oldest AR124 chargers, since more recent ones have LED-backlit displays from the factory.

So, I modified the charger to properly drive the LED backlight. The relevant area of the PCB involves the transformer marked 5A and the black plastic TO92-cased transistor to its left, which together form a high-voltage inverter. The backlight connects to the smaller white two-pin connector.

The relevant changes included cutting a couple of PCB traces to isolate the inverter circuit, and connecting pin 2 of the connector to the charger's +5V logic supply via a resistor to limit the backlight current to about 25mA. This matches the brightness of other AR124 chargers, and minimises the extra load on the charger's logic supply.

Job done! All that remains is to reassemble the whole thing, and... oh, bollocks.

The problem is that the variable resistor used to set the LCD's contrast bias doesn't have enough range of adjustment. It was fine for the old one, but this display needs a setting we can't achieve with PAG's original design. We might be seeing the fact that modern LCDs of this type are designed to be less temperature sensitive than old ones, which is fine, but it changed the bias behaviour.

The resistor in question is marked 202, which means 20 followed by two zeroes, or 2000 ohms. That instinctively feels low. Usually we use 10K potentiometers for that. Checking the datasheet for the specific display I'm using, sure enough, it recommends 10K. So, I replace the 2K RV3 with a 10K pot, which I have to bodge in a little as I don't have one of the same physical layout. The new one is just above where it says RV3, the old one is below it.