The Radio Information Board

I have a tk-760-h (fcc id says h-1) that the panel lites went out on. The desk mike seems to have quit working also. I read thru the previous posts and wasn't sure what applied. This panel has 3 leds at each end, and sounds more like an early -g than a -h, from what i have read, it is not equipped for button lites. I do have 5 vdc on one mike connector lead, but are not sure if that is the correct voltage. Any ideas? Thanks

( just measured across the 3 leds on both ends and got 0.518vdc, is that correct? )

There is a transistor Q405 that appears to control the LED lights. It also links back the SB line on the Mic.
I can try to copy part of the schematic if you need more help.

No markings on the board except some ic's so a partial schematic would be much appreciated. Also is the"sb" line the one voltage line on the mike connector or is there more? Is it 5v or 12. Thank you for your help.

If I remember correctly, each side of the display is illuminated by 3 series-connected surface-mount LED's, and are controlled by a constant-current power supply through Q405. I've seen a few cases where at least one LED in each series string went open, leaving the whole display dark. Usually replacing the open LED's will fix it.

Also, that model was known for circuit traces going open on the control board,IE
mic audio and A+ for a dtmf mic would go open,usually fixed with small wire.
mod

Still lost on this one. I can't find q405. The highest vdc+ i can find on the mike connector is 4.95 vdc+. From what i can gather the desk mike requires 8vdc+, it isn't working. I also can't find a pinout for the 6 pin mike connector. I can find a good ground for what i suspect are the mike and ptt grounds but no others, the highest vdc i can find is 4.95 vdc+. The board number on this jewel is 172-0440-42 b/2 (x57-495). The radio is a vhf tk-760h. Thanks

Looking at the mic jack from the front with the tab down,pin 1 is on the left and it should have 13.8 vdc.
Get a voltmeter and work backwards from pin 1 to see where it opened up.
Pin3 is PTT and should have approx 5 vdc.
mod

O.K., here's the latest.I have 1.02 vdc on that line, all the way to the ribbon connector. I traced that across the main board to r-247, which has 13.8 vdc on the supply side. R-247 measures 440 ohms (within specs). I get 13.8 on the output when the control board is unplugged, it drops to 1.02 when the control board is plugged in. I can't see where the line splits off for the panel lites. I don't see anything burnt, fried, or even warm, on the control board. I suspect something is drawing down the vdc coming in. Shorted q-405? Haven't found it yet. I need a picture of Q-405's location, Thanks again

I think at this point you will need a service manual....
mod

aardvark wrote:O.K., here's the latest.I have 1.02 vdc on that line, all the way to the ribbon connector. I traced that across the main board to r-247, which has 13.8 vdc on the supply side. R-247 measures 440 ohms (within specs). I get 13.8 on the output when the control board is unplugged, it drops to 1.02 when the control board is plugged in. I can't see where the line splits off for the panel lites. I don't see anything burnt, fried, or even warm, on the control board. I suspect something is drawing down the vdc coming in. Shorted q-405? Haven't found it yet. I need a picture of Q-405's location, Thanks again

R-247 (4.7) indicates 13.61 volts going in, goes to pins 13 and 14 of the ribbon connector.

Pins 13 and 14 on the control head indicate 13.08 volts

That trace goes directly to the mic jack (j401) with a stop at R455, C442 and D405

out of R455 (12.87V) goes into Q405
out of C442 goes to another leg of Q405
out of D405 goes to the same leg of Q405


meh, this is confusing......



lets try this.....







and.....








hope that helps....

edit:
photobucket downsizes the photo's. You can see the 5meg ones better. If you want those email me at ............................

My brand new to me kenwood tk-760 H has intermittant lights...

Power up, no lights. Give the radio a rap with my knuckles and one side lights up. whap it again both sides light up with every combination in between...


This seems like a cold solder joint to me and have been touching bits and pieces with a hot solder iron without success... Could this be inside one of the LED's themselves?

*Nevermind*...


Imagine my surprise when I touched one side of one of the three LED's with the soldering iron and it came sailing clean off the board...

Found the cold solder joint on that side alright. Was a pain getting it lined back up again but got it done... Touched the rest and now I have steady light!