Series LED string brightness variation

[gentles]

I thought I understood LEDs but this confuses me.
I have five series strings with 8 LED's in each string, 18 volt supply and 200 ohm resistor on each string. They are medium brightness red LEDs with forward voltage of 1.7v at 20 ma. Four of the series strings work just fine. But in one of my strings, 7 of the 8 LED's lit up, but one in the middle of the string was dark. (At first I thought it might be shorted )  I replaced it with another LED, which also didn't light (). I then pulled out the dark LED, put 20 ma through it with a series resistor, and it lit fine. () The voltage across it was 1.7v at 20 ma. I then put it back in the string and measured the voltages across each LED in the string: 1.67v, 1.64v, 1.65v, 1.57v(the dark one), 1.67v, 1.73v, 1.67v, 1.65v. So one LED has a higher forward voltage (1.73v). I haven't replaced that one, but could it be causing the LED with the lower forward voltage to go dark? (It's a series string, so they're all getting the same current!!!)

P.S., I just wired up another four strings of 8, and one of them has the same problem???

Any suggestions would be appreciated.

[justDIY]

perhaps your power supply is sagging under the load, not able to provide enough current.  use an ammeter in series with a good led string and the bad led string and get compare those numbers.  maybe your resistor is defective and limiting more current then expected ... 200 ohms is not a standard value, 180 or 220 ohms are standard values, or I guess you might have two 100 ohm resistors in series?

[gentles]

Let me give some more information:
I have five series strings each with 8 LED's in series with a 200 ohm resistor. I know 200 ohms isn't standard, but that's what it is (5% tolerance). The five strings are in parallel with each other. The voltage drops across the five 200 ohm resistors are 3 Volts which means about 15 ma going through each string. That's a total load of 5x15 = 75 ma. It is a 15v, 600ma DC power supply that has a measured output voltage of 17.5 volts when it is running the 5 series strings.

So why is one LED dark even when it has 15 ma flowing through it, and the other 7 LED's in the series string are lit???

[justDIY]

Do you have a higher voltage supply?  Despite your readings, my opinion is you don't have enough voltage.  All the small red leds I've worked with seem to need at least 2v, some even need a bit more.  I agree that your reading of 3v across a 200 ohm load means the string is drawing 15ma, but maybe that is an average reading, and not a true reading.

Get ahold of an 18v supply, two fresh 9v batteries in series might work, but I think they'll sag.

[BVnursery]

One being dark would indicate its shorted.  Replace it and all should be well. Remember LED's are made by the millions automatically, some wont be 100%

[gentles]

One being dark would indicate its shorted.  Replace it and all should be well. Remember LED's are made by the millions automatically, some wont be 100%

I already tried that. Read my first post on this topic again...