optocoupler question

[iknowjohnny]

In this tonestack of a PV classic 30 amp theres a opto switch used to turn on a boost circuit. Would i be correct in assuming the "150" it shows inside the optocoupler in the schematic refers to the connection having 150R resistance when the switch is on?

[nuke]

Don't really know. Could be Peavey's internal part number. 150 ohm seems on the low side, but who knows. The hard you drive the LED, the low the LDR goes.

If you can get the actual number off the part, or from the parts list, it may be possible to find the original datasheet.

[Phil_S]

I believe the schematic shows 3 parts to the optocoupler. U5 is a 24 or 28V incandescent light bulb, unknown current rating but certainly not very high, maybe 40mA. 150 indicates there is a 150Ω resist0r between the U5 bulb and the LDR. There is no specification for the LDR (light dependent resistor.) I guess the 150Ω resistor is there to keep total R at a minimum of 150Ω as the LDR is 0Ω when U5 is dark. You should hope and pray that this part doesn't blow because finding another is almost impossible. You might be able to build your own, but without the component specs, it is a guessing game. I mention this because you should be extra careful working on anything near this thing.

[didit]

I suggest it's a vintage vactrol, and one might find a https://xvive.com/audio/product/vtl5c1/ works as replacement part in that application, however this is barely more than a guess.

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[Astronomicum]

The schematic symbol is a light emitting diode. What is missing is the switching circuit and most likely the dropping resistor so the diode works on -36V. An LDR resistance increases when dark, decreases when illuminated. In my experience, an LDR resistance does not drop to zero when illuminated and the high/low resistance depends on range for that model. 150 ohms as the low resistance when illuminated seems reasonable to me. The dark resistance I would expect to be 100s of kilohms.

[Astronomicum]

The schematic symbol is a light emitting diode. What is missing is the switching circuit and most likely the dropping resistor so the diode works on -36V. An LDR resistance increases when dark, decreases when illuminated. In my experience, an LDR resistance does not drop to zero when illuminated and the high/low resistance depends on range for that model. 150 ohms as the low resistance when illuminated seems reasonable to me. The dark resistance I would expect to be 100s of kilohms.

[iknowjohnny]

I believe the schematic shows 3 parts to the optocoupler. U5 is a 24 or 28V incandescent light bulb, unknown current rating but certainly not very high, maybe 40mA. 150 indicates there is a 150Ω resist0r between the U5 bulb and the LDR. There is no specification for the LDR (light dependent resistor.) I guess the 150Ω resistor is there to keep total R at a minimum of 150Ω as the LDR is 0Ω when U5 is dark. You should hope and pray that this part doesn't blow because finding another is almost impossible. You might be able to build your own, but without the component specs, it is a guessing game. I mention this because you should be extra careful working on anything near this thing.

Actually i already found the part # in case i need one and they are all over the place cheap. One ebay auction was like 10 of em for $10.

[sluckey]

Actually i already found the part # in case i need one and they are all over the place cheap. One ebay auction was like 10 of em for $10.

What is the part number?

[nuke]

Actually i already found the part # in case i need one and they are all over the place cheap. One ebay auction was like 10 of em for $10.

Unfortunately, the original manufacturer quit making the original vactrols decades ago. Now pretty much, they come from China and made from semi-random parts.

There were different vactrols, some were fast switches, some were slower, some were meant to be slow linear rise/fall time for optical audio compressors.

[iknowjohnny]

Actually i already found the part # in case i need one and they are all over the place cheap. One ebay auction was like 10 of em for $10.

What is the part number?

Vactec VTL5C

[nuke]

VTL5C... what are the rest of the letters. There's a whole family of VTL5x/x with different characteristics.

[didit]

Actually i already found the part # in case i need one and they are all over the place cheap. One ebay auction was like 10 of em for $10.

What is the part number?

Vactec VTL5C

VTL5C1 ..

[nuke]

There's an absolutely excellent data sheet by the original manufacturer, Perkin-Elmer, which covers a bunch of their photo-resistor cells and the entire line of Vactrols, with lots and lots of details.

Sadly, Perkin-Elmer don't make them anymore, and that's mostly because the light-sensitive resistor contains cadmium sulphide, which the EU has banned entirely for some years now. They're still allowed in the US, but most electronics manufacturers signed onto the ROHS thing a long time ago. They're still legal in the US.

Most of what's available today is made in China and while China is perfectly capable of making nice things, many of the currently made vactrols are basically random LDR's and LED's molded into a plastic blob and don't correspond well with the original specs. A lot of the new production ones are not well sealed, and ambient light leaks in through the wire holes in the plastic. Some are so bad, you can apply some current to light the red LED inside, and easily see it shining past the wire leads in a dark room.

The 5C1 has a full input (40ma to the LED) resistance of 200 ohms or less, and a full dark value of 50 megohms, it's a fast switch, but it has a big light history.

You can order some new 5C1, then test them to see if they are close enough to do the job in this particular circuit. Or test the existing one to see if it is still good.

[nuke]

Oh yeah, here's the data sheet:

[didit]

Hello --

Those vintage vactrol family of parts were used extensively across broad range of audio related product; perhaps most notably in compressors and synthesizers, perhaps most notable Buchla's. In this application distinct behaviours are of much less concern. Current ROHS compatible production of good quality will be fine. Xvive has a reasonable reputation. Presuming you're not relying on transition response curves, etc.; which in this application is irrelevant.

Apparently the I50 label on the schematic isn't a resistance spec, it's a misleading ghost from Peavey's internal part numbering.

Testing that existing part is smart, assuming it's not obviously damaged.

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