Tech 48, I've read your instructions regarding how to fix this and I plan on doing this fix myself as I've already had the HU at Kenwood for the same problem about a year ago. I went without my HU for about 3 months! Not this time.
1. I need to know what kind of solder you would recommend for this fix. I used to work at an electronics plant several years ago and we used every kind of solder back then depending on the board that we were working on. What solder size (i.e., thickness) would you recommend? Should it have flux?
2. Do the resistors you mentioned (i.e., the ones with reference code R43, R44, R46, R47, R48, R49, R50, R51, R55, R57) say "2R7" or "2.7"? I'm looking at the manual right now and it suggests that they say "2.7" as opposed to "2R7". I just want to make sure I know what I'm soldering before I see it.
3. Lastly, should I resolder ALL of these resistors or just a select few of them?
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Re: Kenwood X590 - LCD Display Gone
Resolder all the resistors. They say 2.7 in the manual, but the print on the component is 2R7. For solder use 60-40 rosin core with flux. The thickness should be a small diameter, but most any solder will work, you will only be putting a very small dab of solder on each solder connection. Just enough to make it melt and reflow, no more. This is no guarantee that it will fix your unit, but it is the most common cause. You may find that the display being out for yours is something different. If you have a voltmeter you can put the radio on a bench and check the voltages (DC) on those resistors when it is powered on. You will need a small power supply capable of about 3 amps to power up a car stereo. I don't remember what the voltage are supposed to be, but you should start at one end and have a high voltage and as you go from one resistor to another, the voltage will go down in equal increments. It is a voltage ladder and with all the resistors being the same value, you should have the same voltage across each resistor.
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The first thing to check would be the voltages on the speaker wires. They should be about 6.5 to 7 volts on each wire. Then check the surface mounted resistors on the front right side of the main board. Take out the CD deck and reflow the solder on them. Make sure you do this part with no power source connected. The surface mounted resistors have "2R7" on the top of them and are located in the right front as mentioned before, while looking at the unit from the front facing you. If you do not find the surface mounted resistors it could be that I am thinking of another model of Kenwood, but I am pretty sure they are also in this model as well. Use just a touch of solder on each side of the resistors, there are about 6 to 8 of them all connected in series. After that, reconnect the CD deck and connect the power source to the red and yellow wires and the black wire is your ground, connect that as well. The display should light up. Let me know what you come up with on the voltages of the speaker wires and if the display starts working again. I do not have access to my service manuals at this time, it may be about 2 weeks before I can get them. So I may not be able to get you completely fixed up until I can get my service manuals. A schematic is sometimes the most valuable tool when fixing electronics.
Are you talking about the circuit that lights the display? If so, you need to resolder the surface mount resistors in the front right corner on the top of the board. Front right while facing unit with top off and CD deck out. you will see a series of surface mount resistors and they all are connected in series. All of them need to be resoldered. That will make the display work again.
Method of clearing the programmable security
1. While "– – – –" is being displayed, press [->> ] key for at
least 3 seconds while pressing [AUTO] or [TI] key.
This makes "– – – –" display disappear.
2. Using the remote controller, input "KCAR".
Press remote controller  key 2 times (Input for "K") and
then press [->> ] key.
Press remote controller  key 3 times (Input for "C") and
then press [ ->>] key.
Press remote controller  key once (Input for "A") and then
press [->> ] key.
Press remote controller  key 2 times (Input for "R") and
then press [->> ] key.
3. The security is released and the unit enters the STANDBY
4. If a wrong code is input, the unit goes into the Code Re-quest