Optoisolators, info request

[Metatron_X]

Does anyone know how to test optoisolators/LDR's to see if they are working properly? I think I fried one today.

Can I use any LDR to replace a bad one? The ones I have are VTL5C6 and I'd like to find something really solid to replace all 6 of them at once. Can I just solder in any type or are there other factors involved?

[teemuk]

Drive the LED, measure resistance in dark and illuminated modes?

No, in most cases its safe to start from assumption that you can't replace the LDR with just any other LDR.

Factors involved include at least:

- pin configuration
- maximum cell voltage
- power dissipation
- on and off (dark) resistances
- saturation, speed and hysteresis
- resistance vs. illumination slopes

Once you know what characteristics are important and in what range they should be (which all depends on application they are used in, eg. tremolo or limiter/compressor LDR may require totally different characteristices than a switching LDR) you may find a bunch of substitutes. But if it's possible then it's likely easier to just replace with the spec'd part.

[Ronsonic]

Usually you can test in circuit by switching whatever it is that changes the control voltage, measure it to be sure it's there and then a simple ohmmeter measurement at the resistive end. The LED side can fail either short or open. The resistive end can go open.

Like teemuk says, there are differences between and among them.

These are usually very reliable, there's no reason to do a wholesale replacement.

[Metatron_X]

Good info, thank you.

Since these LDR's ARE in the signal path don't they affect tone? I would think that just like every other component they can add or detract from the end result based on a myriad of factors.

I made a booboo when I soldered in my Resonance control cap and accidentally created a micro connection to a nearby jumper that leads to one of the opto's on the board - There's very little wiggle room on this board for sloppy mistakes and I crossed the line. Not only did this insert some low frequency oscillations into the circuit but I think it contributed to early opto failure. I might be wrong about the opto failure as I don't know enough to say that with confidence but I do know that my distortion channel has been cutting out since the soldering accident. Yesterday it decided it didn't want to work any more and stopped cutting back in. The clean channel is loud and proud but the distortion channel is nearly nonexistent accept for some distant buzzing that gets louder until it's on 9-10 and then gets quieter again. I swapped out all tubes as a rule-out. It did this yesterday after I soldered in a new set of cathode resistors in the power section which is on a separate board from the preamp.

I've already replaced the electrolytic caps, OT and coupling caps so it's unlikely that there is a problem there since it was working fine after all of that surgery.

[Metatron_X]

These LDR's are used to switch from clean to distorted channel & effects loop on and off for a Crate BV120H (Yes, the dreaded blue doodoo!).
I can provide a schem if it's allowed and needed.

I found the spec sheet here:
http://www.alliedelec.com/Images/Pro...es_9800014.pdf


It seems to me that what would be important in the case of a switching LDR is the resistive value. More resistance stops the current through a given path and less resistance allows current through a given path. I'm making that assumption but I'm relatively new to this stuff so I'm making an edumacated guess.

In this case the VTL5C6 reacts to higher input amperage with lower resistance:
1mA - 75k ohm
10mA - 10k ohm
40mA - 2k ohm

I set my meter to the 20k ohm resistance range and measured the resistance of the photo-resistor legs with the power on and on standby.
OC1, OC2, OC3 and OC4 are all part of channel switching. OC5 is the only one dealing with the effects loop being off or on. The only one I'm concerned about is OC3 as it's showing a negative ohms reading regardless of switch position. Are these all normal readings given their purpose?

These are the readings I obtained for each opto:

OC1
ch1 on : 0
ch2 on : 1.79k
fx on : 0
fx off : 0


OC2
ch1 on : 2.89
ch2 on : 0
fx on : 0
fx off : 0


OC3
ch1 on : -.77
ch2 on : -1.53
fx on : 0
fx off : 0


OC4
ch1 on : 0
ch2 on : 1.12
fx on : 0
fx off : 0


OC5:
ch1 on : -
ch2 on : -
fx on : 0
fx off : 1.2

[Metatron_X]

OC3 photoresistor has one leg bleeding to ground like it's supposed to which might account for the weird readings I was getting. I didn't realize that until I stared at the schem for a while.

[Metatron_X]

I tried lifting one leg of OC3 and cutting it out of the circuit to no avail.

I'm all out of tricks. Does anyone know where I should start in diagnosing this problem? The second channel has no volume, it's just a faint buzz in the distance. The pots are good, I already checked.