Help with LED circuit!!

[rshadley]

I have a Kenwood KR 9600 that has had the dial lamps, meter lamps and stereo indicator lamps replaced with LED's. The meter lamps and stereo indicator lamps work fine. About 1/2 of the dial lamps in the middle flicker and glow a little dimmer than the first two and the last one. Using the LED calculator, it appears that the stereo indicator lamps is drawing about 10ma of current and the intensity is perfect. The supply voltage is listed at 8 volts, actual is around 7.5 volts. The positive side has a 470 ohm resistor and assuming that the white LED light is rated at 3.6 volts would indicate that the current draw is 10ma. The dial lamps are set up as follows, each of the 8 volt 300ma lamps were replace with a pair of the white LED lamps in series. The 6 series of led lamps are wired in parallel. Set numbers 1, 2 and 6 are fine. Sets 3, 4 and 5 flicker and are a little dimmer. There are no resistors connected to the LED's. Their appears to be a diode on the negative lead of the voltage supply but it is small and I can not see the silver band, so I am not 100% positive that it is a diode. Anybody have any thoughts as to why the middle pairs of LED's would flicker and glow a little dimmer. I am thinking about removing the diode and connecting a resistor to each one of the positive ends of the 2 LED's wired in series. Using the LED calculator and a target of 10ma per pair of LED's would result in a 1/8 watt 30ohm resistor. Any thoughts on this approach?

 

[Bob]

given what you have written it's hard to determine exactly what is wrong
1. are the string of 6 paralleled identically soldered and numerically distributed? (looking for bad solder joints - just reflow)
2. pull the middle ones and 1 end one and test separately with 9v battery if you don't have exact equivalent supply voltage
(looking for bad LEDs, bad joints between the series LED, flaky wires used if wires were used,...)
3. put a VOM/DMM on each series string, and in the middle, DMMs should change voltage reading with the flicker.

rather than rebuild that engine. try a 47uf @ voltage 2x max supplied voltage, and clip (using probe clips) into
the circuit - after the power supply rectifier diode. it seems that whatever dropping resistor is missing to convert
a) whatever transformer output to 8v or b) possible additional resistor to convert 8v to whatever the LEDs need.
find this one and place cap before it and after the rectifier diode.

Easier if you post a schematic - less guesswork. or pix

 

[rshadley]

thank you for the response!
1) The negative lead of one of the led's is soldered to the positive lead of the 2nd lead(no wire) to form the series of 2. They are evenly distributed. I will remove the circuit board and post some pic's as soon as possible. I will aslo trace back the power supply and see if a resistor was added there and update on my next post. I really don't want to rebuild the engine. If I test 2 white led's wired in series(assuming they are rated at 3.6 volts ea) with a 9 volt battery do I risk the chance of destroying the led's?

 

[Bob]

as I think about this, try measuring the voltage in the middle of all 6 series'd LEDs (to ground).
looking for more clues.

look up the datasheet for your LEDs and check max voltages. or put a pot in the circuit with the
9v test - you want the same voltage flowing through the LEDs as if it were in the circuit.

how bad is the flicker? does it alternate between the two (that are in series?).

if you have extra LEDs, wire 6 in parallel and add dropping resistors and clip them to the unit.

didn't think to ask why you used two LEDs to replace one lamp. if its intensity, dial up the
voltage or buy brighter LEDs. If its dispersion then do the same - find LEDs with a wider
dispersion.

 

[rshadley]

I am not sure on the datasheet of the LED's. I had a local service tech go through this unit about five years ago and the lighting system was a mess and he must have decided that this was the best fix. I think he used 2 LED lights in place of the original 8 Volt 300ma incandescent lamp for dispersion reasons as well as the two LED's rated at 3.6 volts wired in series gets you close to the 7.5 volts of the actual supplied voltage to the circuit. The Kenwood KR-9600 used a rubber boot for the dial lamps that were inserted onto two pins making contact with the bulb wires. I don't know if he removed the pins or the previous owner did as he attempted to replace the dial lamps as the originals have become obsolete or hard to find. It appears that the two led lights wired in series was a perfect fit for the place where the two pins existed. I was checking the internet and it seems that most white led's draw about 3.6 volts. Armed with the LED calculator and knowing that I had an 8 volt circuit and a single led with a 470 ohm resistor for the stereo indicator lamp I started to make some assumptions. So you can see that I am going in blind to this little adventure. I did not notice the lights flickering until I had the unit home for awhile and never brought it back for him to check out, but I believe that they were flickering since day one of the repair. The flickering is not bad, I think the two lights in the series flicker together. The problem that bothers me the most is the middle of the tuner section is slightly dimmer than the outer edges and it would appear that there is a fix because it is only the middle 3 that flicker.

 

[ferninando]

u can get cool led testers on the bay. plug i n led and dial in brightness and current u want and bobs ur uncle.
no math no measure.cheep too. have about 10 diff settings.

 

[Bob]

yikes. you're re-engineering/fixing 3 levels deep.

without pictures, and assuming all 6 strings are identically soldered in parallel, then any flicker is within
a single string of series'd LEDs, but it's not clear whether those original 2 pins are soldered to the voltage-supplying
rails/wires or simply plugged in - OR where there was a bulb is now a pair of LEDs and are "plugged" in or soldered.
A picture would eliminate incomplete descriptions and faulty responses.

plugged in tells me there's crap in the "sockets".

if all the connections are NOT soldered then there's an intermittent contact somewhere and this makes the problem
a mechanical connection problem not an electrical. this would make sense assuming the LEDs are all identical, and the
dial bulbs suffer some mechanical disturbance through proximity or direct physical connection.

the biggest problem is matching the bulb connector female or male socket sizing to the replacement LED wire sizing.
slip fits means crud gets inside.

I think you need to determine the specs on the LEDs (one working by itself is not necessarily representative of
the actual electrical parameters). try pot in series with LED in series with 9v battery. change pot resistance to what might
be called "average" or designed in brightness. measure voltage across pot, and with battery off, measure resistance
across pot. this will decide the actual draw. then apply this to the led with the balance of the 9v not used.