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  Notes on the Kenwood TR-7930 and TR-7950
By Mike Morris WA6ILQ
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Photo courtesy www.rigpix.com

The TR-7930 and TR-7950 were an interesting pair of radios that were made in the 1982-1985 time frame. The TR-7930 offered 25w or 5w, and the TR-7950 offered 45w or 5w, both on 144-148 MHz or 142-149 MHz with the CAP mod. Receive current draw was about 1/2 amp, and the TR7950 transmitter could pull as much as 9 amps.

Both radios use 12 volt 60mA incandescent lights to illuminate the display and the DTMF pad. Replacements were EXPENSIVE from Kenwood. Fortunately Radio Shack had an almost identical bulb (and it's close enough that it works perfectly) as part number #272-1092 (two bulbs). Some hobby stores and most model railroad shops have a similar bulb.
Or you can use white LEDs with an appropriate dropping resistor. Tony King W4ZT wrote an article on that and it's below.

One thing to check in your 7950 (and maybe the 7930): There's a manufacturing screwup that I found on a brand new TR-7950 within minutes after I opened the box. On the power output board where the PA module is connected there was (note the word "was") a 470 µf capacitor designated C1. The silkscreen on the board shows the positive terminal going to ground (the ground plane) and naturally the facatory assemblers installed the capacitor to match. When you think about it, this is obviously wrong as ground is the minus lead not the positive lead.

I pluged the brand new radio into the bench supply and turned it on. A minute or so later there was this loud fart from inside the radio and a familiar smell. I powered off the supply and opened up the radio. The cap had swolen up and burst due to reverse polarity (due to incorrect installation). I scavenged a replacement cap and almost blew it up until I caught the error in the silk screen.

If you have a 7930 or 7950 you would do well to pull off the back and inspect the cap installed at C1. Its the only large filter cap on this board so its easy to locate. If it's installed with the negative lead to the ground foil it's OK, if not, or if the case looks bulged, you might want to replace it.


Articles:

LED replacements for the TK-7950   By Tony King W4ZT (SK)
The mod is exactly the same for a 7930.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



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This page originally posted on 03-Jan-2010


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