How easy is it to break vactrols / LDRs?

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intellijel

Well-known member
Joined
Mar 7, 2006
Messages
72
I am trying to fix an old MXR phaser that has six BPF stagess each controlled by a VTL5C2 vactrol.

On two other MXR units I found broken vactrols (the led portion was dead) so I replaced them. Is it common for Vactrols to die?


On the unit I fixing right now I already replaced one vactrol but now I am finding there new problems with other stages in the circuit.

I assumed vactrols were really simple devices (just an LDR+LED) but is there some care/repcaution I should be taking with them? Obviously limiting the current into the LED but is it easy to break the LDR portion?

thanks!

Danjel
 
The CdS cell (LDR) has a max power rating. If you exceed it, you can toast it. Worst case would be LED on bright (causing LDR resistance to go down) and "high" voltage applied across the LDR. How that can happen is dependent on the circuit in question. Got a schematic link we can ogle?
 
> the led portion was dead

Then they were dumping too much current in the LED.

Wrong (too big) wall-wart?

Redesign for lower LED current (but it may "need" to be that high for full action).

Stock-up on Vactrols and replace as needed.
 
Thanks for the replies guys.

The MXR phasers are actually the mini vertical mounted rack versions.

They are rated for 15 - 30V supply and draw 33mA of current.

I have ordered a bunch of replacement vactrols and will try to swao those in as needed.


Here is the schematic: MXR Auto Phaser

EDIT: I accidentally wrote that the supply is up to +/- but it is just single ended (gnd and +)
 
> They are rated for up to a +/- 30V supply

No, single 15V to 30V supply.

But if you'd put 60V across them, I would expect the 741 chips to burst first.

Hard to believe a '741 could cook LEDs.

Maybe they are just old. Modern LEDs have very high quality, but back in 1976 they were a bit hit-and-miss.
 

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