Sammich LCD backlight is not working

[vintagestar]

Hi,

My Sammich LCD backlight is not working. It is a high-power backlight LCD. I have already put a shunt in JBL in the “12V†position. I have also Put a shunt in JR4 to increase current. The backlight was working before. However, when I take it out and turn on the synth. The back-light is not working anymore. Is the LCD broken or there is some kind of bad soldering on the PCB? What should I check on the PCB to be able to find what cause the problem? Thanks

Regards,

vintagestar

[Wilba]

You can test the header on the PCB by changing to low-power mode (JBL=5V, no shunt in JR4) and putting a LED in top two pins labelled B- and B+, put LED cathode (short lead) into B-. This will test if the brightness pot and transistor are working.

You can test the LCD backlight by supplying power to those pins (which match B-/B+) on the LCD... always test using a resistor in series. However, if you can get a LED to light in the header of the PCB, and still not light the backlight the same way, then perhaps the LCD backlight has failed.

[Danchenka]

I've been having a similar problem for months now. I wonder if it's the same cause. I hope we can both get our sammichSIDs working perfectly:)

[Snoozr]

I've been having a similar problem for months now. I wonder if it's the same cause. I hope we can both get our sammichSIDs working perfectly:)

For what it's worth, your LCD may have come with a little tech sheet -- it might be worth reading through this. My experience has been that LCD's are a bit delicate as I've cooked two so far.

[Hawkeye]

totally agree with Snoozr, i bought mine from a cheap retailer in china and the datasheet was wrong (backlight polarity had to be switched). So if it never worked before, you can always try that :-)

[Danchenka]

For what it's worth, your LCD may have come with a little tech sheet -- it might be worth reading through this. My experience has been that LCD's are a bit delicate as I've cooked two so far.

It's a displaytech that I got from Wilba himself. Dozens of people have used them with no trouble. I doubt it's the LCD. (And just to be sure I tried a second (high power) LCD and had the same problem.)

I'm not sure if my problem is the same as vintagestar's or not, but it does sound somewhat similar. We are both using high power LCDs and the backlight worked at first and then after awhile stopped working.

In my case, replacing the BC337 will get it up and running, but after about half an hour the backlight goes out again, and the only thing I can do is replace the BC337. I think I've already gone through 7 or 8 BC337s. Its behavior is pretty consistent. It's not the LCD that's getting cooked. It's the transistor.

Wilba has gone to pretty great lengths to help me troubleshoot the problem. Regardless of whether my problem gets solved or not, I owe him some beer just for his time alone.

Sorry if I'm hijacking vintagestar's thread, but does anyone have any idea what could be causing my problem?

And vintagestar, have you tried replacing the transistor?