How to test LDR's?

[53crew]

I have an amp I bought used, and I want to be sure all the LDR's are switching properly. Does anyone have any tips on how to test the LDR's and make sure they work? I have the truth table for them and the schematics for the amp. Is there a way to do this with an ohmeter just measuring for continuity?

Do the LDR's fail very often on amps from around 1992?

Thanks.

 

[Evolu]

Looking at the schematic the LDR's are shown as a NO contact.
These contacts will go NC with +VE.
Set your Ohmeter to continuity, failing that set to lowest ohm range available, they will show as a short with +VE.

I have no idea at what current / voltage the LDR's will switch.

 

[msi]

I'd test the LDR side of the vactrols. If the ldr is off it should have resistance measuring in the range of I think 50 megohms. When they are on they measure 100 ohms or so I think. I'd have to check the data sheet. Just be sure not to electrocute yourself by doing something stupid. If the channels are changing fine then you don't have to test those ldr's. Meaning the only ones really needing testing are right after V1 (the v1pad), the ldr in the orange tone stack, and the two in the Feedback loop. If your talking about a tremoverb, the first ldr is doubled for the red and orange channel, and one of the feedback loop ldrs is doubled.

 

[Boogiebabies]

Happy reading.

http://optoelectronics.perkinelmer.com/content/manuals/gde_vactrolsintro.pdf

Like MSI said, it will have an on resistance and an off resistance on the photo resistor side. Also, the LED side will have continuity (Beep Mode) when on. As far as longevity, I have 45 LDR's at work in amps ranging from 13 to 24 years old.
Only two amps have had one LDR replaced and this was due to the switching speed. Both are VTL5C4's.

 

[tobytheplatypus]

3 of the most knowledgeable and technical replies in less than a day.
god **** i love this site.

 

[53crew]

Thanks, guys.

I'll give it a go.

Cheers.