Kenwood TR-7950 LED Back Lights

by Tony King, W4ZT

 

My old Kenwood TR-7950 though still a good radio had seen better days with the back lighting for the LCD display.

The little bitty (grain of wheat) bulbs are difficult for some to find (hobby shops have them) but they do get hot, melt plastic, and burn out.

I had replaced them several times so this time I figured that this was going to be the last time.

The pictures below give you an idea of what I did.

 

Here's how it looks after the blue LED modification is complete.

 

http://w4zt.com/tr7950/P2120001m.jpg

The lamp behind the channel knob was the easiest.

 

http://w4zt.com/tr7950/P2120003m.jpg

You can see the two holes where the bulbs were recessed. See how the left one is discolored from the heat of the bulb. There was melted plastic in that hole.

 

http://w4zt.com/tr7950/P2120004m.jpg

The head doesn't have to be removed but it does have to be loosened and folded downward. You will have to disconnect the connector to the volume/squelch/power control and to the display board.

 

http://w4zt.com/tr7950/P2120005m.jpg

There is another bulb on the right, between the two connectors, mounted on the opposite side of the circuit board right next to the edge. It provides illumination for the DTMF pad. In this case I replaced it with a white LED. I changed the series resistor to a pair of 680 ohm 1/4 watt resistors in parallel.

 

http://w4zt.com/tr7950/P2120006m.jpg

The whole radio opened up.

 

http://w4zt.com/tr7950/P2130008m.jpg

This photo was taken with no flash and a small amount of ambient light.

 

http://w4zt.com/tr7950/P2130013m.jpg

As you can see here, the display is evenly lit and looks nice.

 

http://w4zt.com/tr7950/P2130014m.jpg

The blue LEDs makes the entire radio look new.

 

http://w4zt.com/tr7950/P2130015m.jpg

You can see the white LED lighting the DTMF key pad in this picture.

 

The source of the LEDs that I used was The Electronic Goldmine     http://www.goldmine-elec-products.com/
(thanks to Gene, N0MQ for reminding me to put this link on here)


Click "order on line" then “view categories" then "LEDs". Select "T 1 3/4" and you will find a huge assortment of LEDs to choose from. The ultra bright LEDs are the ones I used and provide the best light for current consumption. Most LEDs are 20 ma devices however you will want to check with your LED vendor for the optimum current for light output and LED life. You will want to pick appropriate series resistors for your LEDs to limit the current through them to the rated current.

 

Written: March 28, 2005

 

Tony King W4ZT (formerly WA4UPE) lost his battle with cancer on Sunday, February 8th, 2009 at age 62. He will be missed. This article is provided in his memory.