I think if you don't have a good desoldering tool, you should probably invest in a set of hollow stainless needles. The idea is that you melt the joint, then push the needle through the hole around the pin. Solder won't wet to stainless, so what this does is separate the pin from the hole. These sets were available for $1 delivered a few years ago, and might still be sold cheaply from some vendors.
Wow, thanks for the tip. I had never heard of such a thing before! I see i can get a set for less than 5 pounds on ebay. So will buy a couple of types and see how i get on. Hopefully they could be useful for future work (i have a large 4x DIP DRAM desoldering job ahead that i have been putting off because i know it might not go well with the tools i have to hand...)
Many LCD modules have an unpopulated footprint for an ICL7660 charge pump circuit for exactly this purpose. See if yours does.
Yes, the replacement i bought has an unpopulated area with pads for a SOIC, which is filled in on the original with the charge pump. I'm going to order the components from digikey and see if i can work out the correct jumper positions to get it working. Will also contact winstar to see if they can advise, but not holding out much hope for that...
FYI, i really suggest against “forceful rocking” — you could easily rip out a plated through hole. If you don’t have the tools to desolder a pin header (or other multi-pin device) properly, you’re much better off cutting the header pins so you can desolder them individually.
Yeah, i'm against using force to remove components, but in this case i had no better option, and the PCB i was rocking was the LCD one trying to get the header pins up off the extremely tight through-holes. It worked out ok, but i do know better, despite all my seeming naïveté. I'm going to look into the needles that helius recommended, as that might save any rough stuff in future.
I discovered this myself recently. The contrast control on multiplexed, passive LCDs is the end of a chain of resistors that set the drive voltages for multiplexing the LCD segments. The other end of this resistor chains is tied to Vdd. In 5V modules the end of this resistor pin, I'll call Vo, is left for the user to connect to a pot between 5V and ground. However, it is just as effective to just use a pot between this pin and ground without a 5V connection.
It was making the connection between there being no voltage 'put' on the pin and the connection to ground working well, that made me realise it's not about voltage going in, but the difference between the two ends. A tricky point, and not obvious at all from first glance at the datasheets i've been working from (don't get me started on the winstar one, the word datasheet is a compliment to that POS).
One thing I didn't see when reading the posts is what the original problem was that prevented the original display from working? I read that you discovered the negative voltage circuit on the display which is missing from the replacement displays. But what was wrong that prevented the original unit from working?
The short answer is: i don't know. It could have been something as simple as a bad solder joint. The fact the contrast trimmer had no effect made me panic and suspect LCD failure, but that turns out not to have been the case. I contacted the manufacturer and they told me to look at an SMD ferrite bead in series with the power line as a point of failure, but when i told them what voltages i'm reading they said it looks good and is unlikely to be that component. So, I'm waiting on some dupont jumpers to arrive after which i can do a solderless install of the original LCD and see if it is in fact working in circuit or not... From my readings and being able to trace the circuit, it looks hopeful. Will report back.