Vintage Synth Repair

on 3/26/05 10:10 AM, synthflake at gemjamstudio@... wrote:

> 
> 
> Ok, a closer look at the display and the backlight is working, just
> not nearly as bright as my Korg 03R/W and Yamaha TG500. Roland told
> me the $115 LCD is now discontinued and telesisbacklighting.com says
> they can't fix those but wanted me to check to make sure I was
> driving enough current for the backlight. Looking inside the unit
> there is a 1/4watt resister (axial) right next to the header where
> the LCD ribbon cable plugs in, I'm wondering if this may be a current
> limiter for the LCD backlight? Seems logical that in a mass of SMT
> devices a single precision 1/4 watter right next to the connector...
> Does anyone have a schematic of the JV880 that might show a LCD
> backlight intensity select resistor?
> 
> Synthflake

Okay. I have a schematic( or as some of my customers call a sketch-0-matic)
There is a resistor designated R116, one end tied to Vcc(+5 volt dc) and
the other tied to pin15 of the ribbon cable to the LCD. The schematic does
not give the
value(thanks, Roland). Get this, neither does the parts list, "describing"
R116 on main board, ERD2FCJ8R2P. I am guessing the value to be 8.2 ohms.
So should be easy enough to measure the value of the resistor you found.
If the display has only this one 16pin ribbon cable and also not a separate
2 conductor cable , chances are good that the backlight is not a EL type
but a LED type. Given this, pin 15 is where the LED's get their current.
To positively determine this, try lifting one end of R116; if the backlight
disappears, then that's it. Now then, brother, you are on your own if
want to change the value of it. If you do change it, advise conservative
value to 7.5 or 6.8 ohms. Also keep in mind that I deconstructed(guessed)
the value based on the part's list description, better yet to determine
the value by the color code. Good Luck. Russ