Replacing an LCD: more to it than than meets the eye

Until this weekend my homebrew QRP SSB/ CW rig (Summit Prowler IV) possessed an un-backlit black on green LCD. Unfortunately the backlight never worked. The transceiver worked well, and I overlooked the dark display, perfectly usable in daylight but useless at night unless lit with a direct table lamp. I justified it in my mind by thinking of it as not a limitation but a feature — it’s a battery saving measure in a portable rig, a design choice, not a construction failure or component fault. A few weeks ago I set it up on a side bench with no direct light and found myself shining a torch on it to tune. Finally the niggling annoyance got to the point where I could ignore it no more.

Summit Prowler IV with nice new blue display. Now ready for those twilight activations.

Time to place it in the middle of the bench and fix it. Replace an LCD. Sound simple enough? Well let me confess that it took almost 8 hours!

With the original LCD, on Mt Nelse Feb 2018.

I started by sourcing two identical LCDs, marked WH1602B-NGA-JP*. I found a datasheet for a WH1602B-NGA-JP which appeared to match electrically and physically. Not an exact match but probably close enough.

This allowed me to work with one on the bench. The datasheet was easy to find and it all matched what I had in front of me. Putting 5v on the anode and cathode of the backlight (without the rest of the unit connected) did nothing. I jumped from pins 15 and 16 to the A and K pads where the backlight connects to the PCB and she lit up. But the minute I connected the LCD pin 1 (0V) the power supply current limiter kicked in.

There was no way I could get the backlight to behave. Finally I isolated the backlight and powered it from a separate supply. This worked but as soon as I grounded its negative to chassis the short condition appeared. After 3 hours I had to go and attend to other duties, but had resolved to abandon this display for a smaller white on blue one. That decision opened up a whole can of worms of the mechanical kind.

The new white on blue LCD (tronixlabs TL1602LCD WB) had same display area but the board was smaller. And the holes didn’t align. And the header was on the top, not the bottom. These display are electrically standardized, but physically they are all different. That doesn’t matter if your display is not panel or case mounted.

How to mount this without rebuilding the entire Arduino/si5351 VFO/BFO/Controller?

I considered making up a custom PCB with the four mounting holes and header pins in the exact same positions. Then, mount the new LCD on top of it and jumper the connections. Easy enough to do but any slight inaccuracy in my handiwork could leave me with a display panel that would not mate with the Arduino board. Or wouldn’t fit in the transceiver cleanly.

Then it occurred to me… why not turn the green LCD into a base plate — that way it was guaranteed to fit. If I couldn’t make it work as a backlit display, I’d make it work as a substrate.

I stripped the LCD components, filed all the surface mount components and copper off, leaving only the 16 pin 0.1 inch header. This turned out to be harder than expected, requiring my chunkiest rasp and best files for a solid 40 minutes.

Having butchered the old display, I cleaned it up, put a bandaid on my stubbed thumb, and mounted the new display on the stripped board. I ran ribbon cable for the 6 plus 4 control, power and data lines used, plus 2 more for the backlight, from the edge connections on the new display to the 16 pins on the old board.

Front view, new LCD attached in front of what’s left of the old LCD, preserved as a backplate only, everything else ground off.

Rear view, the slanted cutouts accommodate the twisted metal cover lugs.

Current draw with the new LCD, acceptable on a decent LiFePO pack.

The offending Winstar LCD. Anyone know how to make its backlight work?

Diagnosis? The two WH1602A-YYH-JTV*displays I tried both exhibited identical behavior. It could be that this particular part needs some jumpering to get the backlight on. The datasheet didn’t say. As these come dirt cheap from a local disposals outlet, it is also possible they are a Winstar special production run. It’s slightly suspicious that the data sheet for the exact serial number could not be found. They could have also been from a faulty batch. Who knows? At a certain point it’s best to redirect your efforts to something that you know can work, and get on with it.

Lesson? When you choose a display for a homebrew project, always buy two, so if you let the smoke out of one, you get a second chance. There are lots of these devices in disposals outlets and on Ebay, so buying a pair is not prohibitively expensive. I have smoked si5351s, Nanos and of course RF power FETs a number of times and always make sure I have spares on hand or can replace these parts easily, avoiding shipping delays.

There are other front panel mounted parts that should be stockpiled. Have you ever burned out a front panel meter while debugging a power supply or transmitter? What do you do if that dead meter was a one-off hamfest special? Find a bigger one? Re-cut the front panel? Patch it?

It’s close to midnight. Back to working the VK ZL Trans Tasman Contest. The shack is dimly lit. This blue display is so easy on the eye 😀.

Postscript: I notice that Pete Juliano N6QW took 6 hours to swap out an LCD for a TFT display in one of his rigs.  Maybe my 8 hours was more typical than I first thought.

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