how do i troubleshoot an LDR?

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dietnews

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Joined 2013
#1
hi, i have an old broadcast console whose headphone amp is currently not functioning. it uses an LDR (vactrol 21L564) to set the gain on an op-amp amplifier. i bypassed the LDR and fed a signal through a 1meg resistor and the amplifier circuit worked fine. this makes me suspect the LDR has failed.

however, i am not sure how to test it or how to find a suitable replacement. i'm attaching a copy of the amplifier schematic and the power supply that drives the LED side of the LDR.

any help would be greatly appreciated.
thanks!

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#2
An LDR is a light dependent resistor. Measure its value. Add some light, and measure again. If it has been packaged with an LED to form an optocoupler then you will need to check the LED too, as that is the only light source you can use without breaking open the package.

Anyway, it may be that the LDR has gone open circuit.
 
#3
the LDR has the LED built into the package. it appears to be an open circuit across the photoresistor leads. i havent had success finding any information on this part anywhere online so i have no idea what specs to look for in a replacement. oh well.
 
#4
Outside security lights (the cheapo domestic ones) often use an LDR for the daylight sensor. You could perhaps make a replacement complete with LED in a light proof tube.

Are you sure the LED's are OK ? Check the volt drop across them when they are conducting.
 
#5
Hi,

Bit hijacking this thread... apologises!

I have a DIY Lighter Note LDR, got it 2nd hand. I find it as a very nice pre with my Hypex ucd180 power amp.
Unfortunately I just found out that the the signal flow is pretty strong even though the pot is turned all the way down.

At the moment I can´t try it with different sources but the problem is very obvious with the following setup:

MacBook Pro (vol. max) -> Airport Express -> Beresford DAC (fixed out) -> Lighter Note -> Hypex.

Should I perhaps try adjusting the onboard trimpots?
I appreciate if someone have some hints for the troubleshooting.

Best,
M
 
#6
I wouldn't adjust anything at random, at least not without knowing the function of the presets and having devised a means to return them to where they are now.

One possibility is that you are over driving the unit (MacBook on full volume).

If you get no joy here then I would start a new thread specifically about the device in question.
 
#7
Hi,

Thanks for the quick reply! I was just also thinking about the full volume on my c-puter...
I´ll wait and see with different sources in near future as well.

In case of more problems, new topic is to be started.

Have a nice time!

M
 
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