Harvester keypad. No lights

[will62]

Re: Harvester keypad. No lights

When I apply low voltage with multimeter in Diode test mode, on good board I get full R / G / B light on each pin, but same test on bad board, I barely get any light (very dim) and sometimes it seems colors get mixed even when Im carefully with leads.

Could be a bad diode, but also the shortcircuit could be in the feeding driver IC (TLC)

If you want to rule out the TLC, check for continuity between the cathodes on an led and the out puts on U1 and U2. Once you find out which output pin(s)
are driving that led, use a soldering iron to lift the pin off the pad. Then recheck led.

[edugimeno]

Re: Harvester keypad. No lights

Look into getting a tweeter soldering iron they cost a little bit more than a regular soldering but for you are doing it should work for you

Here is one version of what I am talking about

https://www.ebay.com/itm/938D-220-11...gAAOSwdNRc5EOp

Thanks but this seems suitable for simple SMD components (2 leads), but how can this help for a 6 pin LED or a 28 pin IC?

I've sometimes had access to hot air stations but right now all I have is a normal soldering iron...

[edugimeno]

Re: Harvester keypad. No lights

hot air needed - those chips have a center pad under them.

But the leds are mostly plastic, won't hot air melt the plastic before the solder?

[sam_sam_sam]

Re: Harvester keypad. No lights

You heat the board around the device use a small tip with enough air flow to keep the heating element not to burn out

[stj]

Re: Harvester keypad. No lights

But the leds are mostly plastic, won't hot air melt the plastic before the solder?

i meant the driver chips

[edugimeno]

Re: Harvester keypad. No lights

I observed something that might be a hint to what''s going on.
When I used the multimeter on diode test mode and applied it into the individual LED pins in the working board, I would get the proper RED-GREEN-BLUE colors, one by one, full lit, in only that LED

When I tried this on the failing board, I would get either a mix of colors, really dim, or no light at all, OR, SOMETIMES, I would get ANOTHER LED dimly lighting up in some color.
LEDs are individually addressed each one on different pins (OUTx) from the LED Driver chip. How would current to one led make ANOTHER LED light up? I can only think of internal shotcuts in the LED driver IC, so when I apply voltage to one LED, it gets back into the IC thru OUTx, internal shortcuts connect it to OUTy, which goes out and drives that other LED.
Does it make sense? Either that or internal shortcut inside the PCB layers...

What do you think?

Thanks!

[budm]

Re: Harvester keypad. No lights

1) What Voltage do you get on the Anode of the LED's (good board and bad board) with ref to GND?
2) What RESISTANCE do you get when test the LED's on the good board and bad board?