Help me repair a harvester keypad. (no LED lights)

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Hi There. I have this weird device that is not fully working.
It is a keypad used to control several tools in a harvester. It uses CAN bu
s to communicate with the devices in the machine.
When it is powered on, it should flash all the buttons for a second, and th
en only the functions that are active, should remain lit. If you press a bu
tton, it should send a CAN msg and corresponding light behind the button sh
ould turn on/off.

The broken keypad still keeps its control abilities 100%, but you have to w
ork blind, as the leds behind the keys don't light up at all. No powerup te
st flash, no working lights, nothing, but when you press a button, the spec
ific tool is activated, so CAN connectivity seems ok.

Inside, it has 2 boards.
Board A (check pics) has connections to harvester. Only 12V, GND, CAN-H and
 CAN-L are used in a real harvester. It has several transistors, probably f
or functions that are directly driven by the board, probably connected to t
he spare plugs, never seen them in use.
It has a small connector to board B. Connector only has GND, VCC, CANH/L an
d 2 more unknown pins.

Board B has all the logic to drive the CAN signals and the LEDS. LEDS seem  
to be RGB, as they are driven by several pins, and I have seen them flashin
g in different colors. Each key has 2 leds. One helps you locate the functi
on it does at night, other turns on/off whether the associated tools is act
ivated or not. This is the faulty board, when I use this board on another B
oard A, it still fails.

IC's on this board are:
1xTLE4473 GV53, this is a LDO dual output regulator, for 3.3v and 5v (5v se
ems to be inhibited and unused)
2x762T, these are power FET switches. I think one activates TE11 and other  
activates TE12.
1xA1051/3, this is the CAN line driver, to interface with microcontroller
2xTCL5943, these are 16 output LED drivers
1 microcontroller
1x93LC86, this is an EEPROM for the microcontroler

As everything but the lights seem to work fine, I believe the problem is in
 some current driver for the LEDs set, but I can't tell how it could be dri

I have a complete working set and another broken set (only board B is actua
lly failing, as tested exchanging boards)

Can you provide any help on how to diagnose what could be wrong?

I uploaded pics of both boards (you can ignore board A) on both sides, and  
one of front B with annotated IC's markings

Thanks in advance!
Link to photos:

Re: Help me repair a harvester keypad. (no LED lights)
On Sun, 19 Jan 2020 13:41:33 -0800 (PST),

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Good.  The problem is therefore isolated to whatever is driving the
LED lights.  I count 18 LED's.  On board B back, carefully remove  
the black plastic insulator so that you can trace the LED wires.
Try to determine which chip, resistor pack, or whatever is driving  
the LED's.  

I can't read the part number off the photos.  The two TCL5943 seem to
be likely cause of the problem.  However, I can't find anything
with Google for that part number.  Are you certain that they are a
TCL5943 or are they something else?  Can you find a data sheet?
Also, I find it odd that the device would require 32 outputs to  
drive 18 LEDs.  Are there more LEDs somewhere as most of the outputs
on both chips seem to go somewhere.

Plug everything back together and check for power and switching  
waveforms (for LED dimming) on all the TCL5943.  I would be more
specific if I could find a data sheet on this device.  Basically,
start with any LED and trace the signals and DC back through the
TCL5943 chips, and on to whatever powers the chip.

My guess(tm) is one of the chip resistors on the PCB is acting as
a fuse, and it's blown.  I looked for something that looked like
a fuse such as "F1" reference designator, but found nothing.

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Good luck.

Jeff Liebermann
150 Felker St #D
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Re: Help me repair a harvester keypad. (no LED lights)
On Sunday, January 19, 2020 at 6:28:04 PM UTC-5, Jeff Liebermann wrote:
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Maybe it is a TLC5943? 16-bit ES-PWM with 7-bit Device Level Brightness Control  

Re: Help me repair a harvester keypad. (no LED lights)
On Sun, 19 Jan 2020 15:45:11 -0800 (PST), Michael Terrell

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Oops.  Telephone call and visitor interruption induced dyslexia.
Ummm... thanks.

Continuing from where I fumbled the ball:

Start by looking for Vcc of 3 - 5VDC on pin 28.  If that's missing,
look for something open in the 12V to 5V(?) power supply section.  My
guess(tm) is still a well hidden fuse in this area.

The LED current is controlled by a resistor to ground at IREF pin 27.
Make sure this resistor is the correct value.  See Fig 14 (Pg 13) for
resistor value and LED current.  Check resistor with an ohmmeter (with
the power disconnected from the device).

Please verify that all 18 LED's are connected to a common power wire
and that it has VCC on this common power wire.  See VLED on Pg 1
"Typical Application Circuit".

The LED brightness is set digitally by the controller.  Check for data
and the correct levels on all the pins labeled "I" (input) on Pg 8.
Verify that XTEST ping is either pulled high or grounded (but not open
or floating).

Jeff Liebermann
150 Felker St #D
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Re: Help me repair a harvester keypad. (no LED lights)
El lunes, 20 de enero de 2020, 2:16:57 (UTC+1), Jeff Liebermann  escribi?
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Brightness Control  
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Yes Sorry, there was a typo in the LED driver part number. It was TLC.
There are no components in the LED side hidden by the black plastic neither
 discrete nor active. Just traces
I will definitely check those values tonight, thanks a lot!

Re: Help me repair a harvester keypad. (no LED lights)
Ok here some measurements on TLC driver chip:
Vcc is ok: 3.3v
Xerr is pulsing LO-HI BUT this is also happening at the working board, at t
he same rate if is flashing red to indicate "CAN bus not detected". So this
 is not indicating a failure
Iref is 1.2v while working. Resistor measured to 3.3K, same as marking on c
Voltage measured at outputs (OUT0-OUT15) varies from 0v to 3.2, 3.5v, pulse
s, etc... Mainly same on working and failing board, but like half of the LE
D outputs in failing board have like 0.3-0.5v less than their corresponding
 in the working board.  

I would check the LED but I have no way to tell what model it is and whethe
r it is driven color by color (R1-R2 / G1-G2 / B1-B2), or with a common ano
de and 3 cathodes, or with serial data... I can't find any marking. So I ca
n't tell what's the supplied anode voltage

Any other suggestion for further tests will be appreciated
Thanks again

Re: Help me repair a harvester keypad. (no LED lights)
On Mon, 20 Jan 2020 13:04:37 -0800 (PST),

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I think you just found the problem.  The LED's on the working board
are drawing current from the TLC5943 chips and therefore will show a
lower voltage.  The LED's on the failed board are not drawing current
and will therefore show a higher voltage.  What this tells me is that
NONE of the LED's are connected to a common bus that goes to either
3.3V, 5.0V, or ground.  This is why I suggested that you remove the
black plastic insulator from the LED board.  Find this common bus that
goes to all the LED's and check if it is connected to one of these bus
lines.  Compare the bus DC voltage and waveforms with the working
board to be sure.  My guess(tm) is that this common bus goes nowhere.
Maybe a blown trace, blown fuse, blown series protection resistor
acting as a fuse, or failed connection.

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You didn't mention that there were different color LED's.  The hard
way to do this is to trace out part of the PCB and produce a
schematic.  A better way is to assume that whatever has failed, has
simultaneously affected all the LED's, irrespective of color.  My
guess(tm) is still that it's the common bus that connects all the

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Yes.  I should have suggested this earlier.  Remove the LED board and
shine a bright flashlight from behind the PCB.  Any broken or fused
trace will be instantly visible, unless the broken trace happens to be
under a component.  This will also sometimes show bad solder
connections.  Try shining the light from both sides of the PC board.

Jeff Liebermann
150 Felker St #D
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Re: Help me repair a harvester keypad. (no LED lights)
I also measured SIN and SOUT on both IC's, both boards, giving same pattern
 (digital tiny flicker around 3.1v).

I checked the LEDS to determine their internal wiring. Found that 3 corner  
leads  are common anode and 3 remaining are cathodes for R / G / B. When I  
apply low voltage with multimeter in Diode test mode, on good board I get f
ull R / G / B light on each pin, but same test on bad board, I barely get a
ny light (very dim)  and sometimes it seems colors get mixed even when Im c
arefully with leads.

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

At this point Im stuck between bad diodes or bad driver. I guess I should r
emove one LED and test it isolated, but Im afraid Im going to fry the board
 as I don't have any good and thin solder tip neither any special tool to d
esolder 6 pins at a time

Any other hint on how to diagnose?


Re: Help me repair a harvester keypad. (no LED lights)
Jeff, I'm 90% sure the issue is in the LEDs. Im still awaiting for a reply  
from the guys who demanded this repair, in order to confirm whether the LED
s have failed one by one, or half of them first and then the other half (1  
driver chip at a time)

Yes the LEDs are multicolor. There 2 leds under each key. One is used to he
lp find the proper function, specially at night, for that key, and the othe
r is used to show that the function is engaged or disengaged.

I manually tested some of the LEDS and found they are RGB LEDs, with indivi
dual colors sddressed (not serial data), where 3 of the corner pads are Com
mon anode, and the 3 remaining are R-G-B cathodes.

Common anodes from upper LED on each key are driven by a Power FET IC (762T
 chip on top left coerner) , and bottom LED's common anodes on each key are
 driven by the other 762T on top right, this way (my guess) they drive high
 one set of LEDS, send color, then drive the other set and send color, usin
g the same LED driver chips.

I can't really remove much  of the black tape on the LED side as this plast
ic also keeps the touch buttons in place, its just those smal circular piec
es of curved steel that are held in place by this plastic.  

When I used my multimeter in diode test mode directly to the LED pins on th
e working board I got the right individual full-lit colors: Red, Green Blue

When I tried this on the failing board, I would get mixed colors and really
 dim light, sometimes, and this only happened on the failing board, and cou
ld be a hint on what's wrong, when I applied this voltage to R-G-B pins, so
metimes other surrounding LED would also show some DIM light...Maybe curren
t flowing back in to the driver chip, which might have inner shortcuts, and
 back out to other LED? I guess I am going to have to desolder either some  
LEDs or one LED driver chip, which Im trying to avoid as all I have is a re
gular soldering iron and many of the times I have desoldered an SMD chip I  
either lifted a track or burned something else...:)

Thanks again!!!

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