I have been getting so much help on this forum, so figure it is my time to contribute.

Purchased a 95 2 months ago and stereo (Pioneer) has no backlight, so really annoying using it in the dark. Decided to dig in with help of father-in-law (with super steady hand and soldering master).

Photo 1 is photo of bulb. It is an incandecent bulb encapsulated by a green rubber sleeve. One has to desolder two pins to get it out. Unfortunately for us, the bulbs were fine (we tested it using a regular 9V DC battery). So now it is a power source problem (circuit is not allowing power to the bulbs).

Going back to the car we made sure when we turn on lights there are power going into the stereo (forgot the pin for the backlight). So it's not a connector issue either.

Digging deeper, my father-in-law started diagnosing power sources. We plug a 12V portable battery pack (the ones you jump start cars with) to the stereo unit to see where the short is. We had to remove the LCD board and the tape deck board to get access. Turned out a resistor on the bottom, main circuit board (near the corner) is lose. In photo 2 and 3, the lose resistor is the inner circle, and there is an identical resistor next to it (in the outer circle).

Very gingerly my father in law soldered it back in, and voila, backlight is working.

But unfortunately, during the process we must have bent the flexible circuit board for the LCD, and we ended up losing 2 segments but none are significant.

If it's just burnt bulb we would have been done within 1 hour and less than $1. But we had to dig in so much more, ended up taking 4 hours but it was free (resistor not bad, just lose).

Hope mine is the exception, not the norm.

 

This is awesome. I replaced my display last year, but still have no backlight. I hope my problem is in the same place, I'll try sometime soon. Thanks!

 

Be real careful with the flexible LCD circuit. In fact put some tape (Scotch or black insulator) on it while working to minimize potential shorts or damages.

 

Wow thanks for awesome write up. My back light comes and goes. I haven't done anything about it as of yet but will look into removing the deck to see if this is my issue as well. I am hopeful it is just a connector issue though.

 

Great DIY!
Just wanted to thank you for doing this DIY, because I had the same exact issue on my 2LS, and I was about to purchase a new/used headunit. After doing some research and reading your post, I decided to tackle the problem myself, since I do posses moderate soldering skills. After following step-by-step instruction, (as well as looking at great tutorial images) I solved the problem by doing exactly what you did. Once again, thank you for doing this DIY.

 

can i expect a similar repair for my temp/clock light? i even want it brighter thaqn normal, if possible?

 

ck90211,

Your discovery is spot on. I am adding to your post for the sake of others as I could not find a single post elsewhere identifying this root cause. Funny thing is I found this after I fixed it via a board swap. However, It wasn't until I saw this thread that I took a closer examination of R964 and sure enough there was a minute crack in the solder joint.

To reiterate, I had the exact same problem occur (no back lights, nothing) with the Lexus Premium Sound System (Pioneer) radio in my 1996 LS400. The radio still functioned fine otherwise. The radio is marked P1703 on the tape deck input. Toyota part number KEX-P8156ZT, 86120-50361-1.

Prior to testing the unit, I picked up the same radio from a salvage yard to swap parts if needed. When I tested the lamps with a regulated 5 vdc power supply on the original radio, they were all were functional! Hmm? Rather than try component level repair, I simply swapped the main circuit board labeled CNP3426-D with identifying mark 3 ( assume rev 3) with the parts radio marked CNP3426-D with identifying mark 4 (assume rev 4). The main difference I noticed was the absence of surface mount resistors R963 and R964 on the silk screened pads on rev 4; instead these were replaced by standard resistors soldered on the back side as jumpers. The resistors are very close to two power transistors B942 which get hot and thermal cycle the solder. So yes, very hard to spot but there was a crack near R964 on mine also. I could have re-flowed the joint, but same problem could occur again. I was glad to have the later rev 4 board that clearly addressed the manufacturing oversight of undersized heat sink/copper traces and poor layout. Overall though, the build quality of the radio is very high quality and compact. Way ahead of its time.

Here are a few pictures. Sorry, I should have taken a pic of the rev 4 board with the jumper resistors in place, but it is already back in the car.

 

this is great, my radio and climate lights are out, i will check this, are the bulbs replacible in to leds? if any ?

 

I'd say that this was one of bad examples of a poor design. A designer should compare and calculate the difference of expansion rate between the board and the resistor.

 

The light bulbs are available from Pioneer. They can be replaced, but are soldered in place. They last a very long time, can last the life of the vehicle. It would be rare for all the bulbs to be out at the same time. You can easily test the bulbs with 5-8V power supply once the board is removed from the radio. It is possible to have a few bulbs that need replacement, but simply replacing the back light bulbs will not solve the problem if your situation is the same as described in this thread; i.e. none of the bulbs receive voltage from the 8V regulator circuit.

Keep in mind LED's require appropriate current limiting resistors and a different type of dimmer circuit than used with incandescent lights. Also matching the brightness level of the rest of the instrument panel would be a challenge.

 

True. The engineer must also factor in heat dissipation of surrounding components and ambient temperatures under real operating conditions. It is not easy to calculate. Rev 3 circuit design probably would have been fine with a properly sized copper or aluminum heat sink, rather than using the less efficient steel case. If the external bypass resistors (simple work around addressed in Rev 4) has same problem again, I will replace the original steel heat sink/mounting cage on the rear of the radio with one made from copper sheet of same gauge.

 

if you re-flowed the joint using a better solder mix why would the problem reoccur? i know today's solder lacks lead (ROHS compliance), but wouldn't it suffice for a fix?

 

Excellent point. 63/37 Sn/Pb flows at 188 °C and Sn-3.5Ag-0.74Cu-0.21Zn flows at approx. 220 ˚C. Modern lead free alloys can also form a stronger joint. That may be enough to fix the issue and is simple to do.

 

Both of my resistors are loosing. I did solder it again and and it's works great. It's awesome. Thanks you all.

 

That's good.
But a simple resoldering will cause the same problem again because of the poor design.
I would solder additional wires to both ends and the board with some small loop that absorbs the stress.