Replace display trouble

Thread Starter

hide24

Joined Jul 9, 2019
73
Hello,
Im trying to change a 4002 display of my synth.
The old one has 7 pins (2 more on the other side for the backlight with a small thin white cable to connect)
The new one has 8 pins and the pin 15-16 are the backlights.
How can I connect the little white cable to the new display? I weld to the pin 15-16 or the other side which says A2-A1-K2-K1

Thank you very much
 

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Thread Starter

hide24

Joined Jul 9, 2019
73
I tried to put the white cable to pin 15-16 and when I turn on the backlight also turn on for half second and turn off again.
 

jpanhalt

Joined Jan 18, 2008
10,515
Hello,
Im trying to change a 4002 display of my synth.
The old one has 7 pins (2 more on the other side for the backlight with a small thin white cable to connect)
The new one has 8 pins and the pin 15-16 are the backlights.
How can I connect the little white cable to the new display? I weld to the pin 15-16 or the other side which says A2-A1-K2-K1

Thank you very much
Except for the backlight, what does the display show when the loom is lighted? Most displays with backlights can still be read when the backlight is off.

How can an 8-pin display have pins 15-16. Yes, those are the typical pins used for backlight in 16-pin displays. Are they numbered 15-16 in both displays?

A2-A1-K2-K1 Is that a label or 4 pins? Ax = anode (+) Kx = cathode (-) Can you put a voltmeter on the cable to find which is positive and negative or is the cable also labeled?
 

Thread Starter

hide24

Joined Jul 9, 2019
73
The display showed characters even the backlight was not on.
However now my synth won't turn ON anymore! I'm shocked. Could be the display damaged all the synth or burned the board or something?
I didn't touch any cable or anything.
 

ericgibbs

Joined Jan 29, 2010
11,122
hi,
Are the two white wires and/or PCB marked with voltage polarity symbols.?
E

Update:
BTW: it is possible that the old LCD PCB has a series current limiting resistor for the LED.
Check the track on the new PCB for a resistor.
 

ericgibbs

Joined Jan 29, 2010
11,122
hi hide,
Next step.
Disconnect the white wires, does the synth now turn On.??

If No, then most likely you have blown the Synth low voltage supply.
E

Let us know the result.
 
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Thread Starter

hide24

Joined Jul 9, 2019
73
Thank you!
Now I checked and one of the fuses was blown. I replaced and now the synth turn on again! ;-)
The only thing is...I can't get the backlight turn on, only the characters.
(I attach the pictures)

The first four photos are from the Old Display.( At the left , has two points where the white cable was weld. )

The LAST 2 pictures are from the new display which in the pin 15-16 what I did is , weld the white cable with an extension to those pins.
Is it correct?
 

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ericgibbs

Joined Jan 29, 2010
11,122
hi,
Do you have model or type number for the new display.?
Also a photo of the back view of the new display, ie : the last image in your group.

E
 

jpanhalt

Joined Jan 18, 2008
10,515
You can drive the backlight with a separate supply temporarily. All you need to know is the voltage (usually 3.3 V or 5 V). Once you do that, find our how much current takes. If the present supply cannot meet that current, then a small adapter can be made. However, the fuse could blow for other reasons, such as a misconnection to the four pads.
 

ericgibbs

Joined Jan 29, 2010
11,122
hi hide,
The datasheet states pin 15 is the LED anode and pin 16 is the cathode.
Try a 47 ohm resistor in series with the Anode white wire.
Lets know what you see.
E
 

Thread Starter

hide24

Joined Jul 9, 2019
73
Thanks Jpanhalt!
What do you mean by separate supply temporarily? Could you show me a pic please?

Thank you EricGibbs!
Tomorrow I will buy the diode and see if it works!

The connections I did until now are correct?
Or usually that white cable should be connected in another place if the new display that I don't know? Which is the correct way?
 

jpanhalt

Joined Jan 18, 2008
10,515
1) Measure the voltage supplied by the cable. It will likely be 3.3 V or 5 V. Is there a brightness control on the device? If so, the voltage you read could seem to be anything. It will not read the peak voltages of a pulse (see #2).

2) My suggestion was to use another supply. For 3.3 V, 2 AA batteries in series will suffice. For 5 V, 3 AA batteries or 4 X 1.2 V batteries in series will work. Adding a 47 Ω resistor in series, as Eric suggested, would be good protection. It doesn't matter where the voltage comes from for the backlights. They are usually independent of the rest of the circuit. HOWEVER,it is likely the backlight power from the device's PCB is regulated in some way, e.g., a transistor that is pulsed (aka PWM) to control brightness. The added resistor will prevent burning out the backlights or drawing too much current from the display driver. One assumption is that the reason the backlights came on and then turned off is that they were drawing too much power. A separate supply will avoid ruining the board until you find out what they need in terms of power.

3) Another approach is to use the existing cable to supply them with a resistor to limit current and see whether they stay on.

Without some testing, there are some unknowns. For example, maybe the original board was designed for 5 V (i.e., had a resistor built in) and the new board is designed for only 3.3 V without that resistor or a much smaller resistor. Supplying 5 V to the new display could have resulted in an over-current situation.
 

ericgibbs

Joined Jan 29, 2010
11,122
Tomorrow I will buy the diode and see if it works!
hi hide,
It is a resistor not a Diode.
Using a 47 Ohm resistor may give a low level LED light for testing, you may have to reduce the resistor value to suit.
The datasheet states pin 15 is the LED anode and pin 16 is the cathode.

Note: If the LED's do not light at all you may have blown them when you initially tested and blew the fuse.!
E
 
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