Optocoupler in BF amps

[KingCameron]

Does it matter which direction I install the optocoupler in BF amps?

It seems that one side would be the LED and one side would be the light senstive resistor...does it matter which goes on which points?

How do I tell which side is which?

[Enzo]

Of course it matters. Two wires are from a light bulb and two wires are a variable resistance photocell. The tremolo circuit drives hte bulb to pulse on and off, while the variable resistance is in the circuit to make the volume go up and down.

If we reversed those connections, we'd have the trem circuit putting a voltage across the photocell resistor, and there would be a dark light bulb in the signal path sitting there.

[KingCameron]

I thought so...now what's the answer

As I said, it seemed so.

soooooo, what's the answer? How do I tell which side is which?...so that I can properly install it

[KingCameron]

Thanks but no thanks...LED side is marked

I was able to find the information out from the manufacturer.

For those who would like to know in the future....

The LED side of an optocoupler usually has longer leads and will be the designated side. In the case of Fender amps, they will mark the LED side of the optocoupler with a designator.

The resistor side will usually have shorter leads and will be the unmarked side

[Garydean]

The "roach"
The non-polarized optical (lamp) side usually has longer leads or the leads may be bronze or the package may have a silver dot on the optical side. That side goes to the 12AX7 and the B+. The other side, the resistive side, will have shorter leads....usually... and one lead connects to ground the other to the intensity pot.

[KingCameron]

Garydean is a cool dude!!

Thanks bud!
I appreciate your coolness...sending some good mojo your way

[Robert M. Martinelli]

Just a footnote:
Even if the pinout is unknown, it's pretty easy to state which side is the LDR and which one is the LED/buld lamp, with a DMM set to measure junctions ( diodes ) the LED will give a reading only when forward biased, and the value shown is usually the threshold voltage. A bulb lamp will read the same value even reversing the probes, and by changing the DMM to Ohms it will be possible to read the filament's resistance. The LDR should read a very high resistance ( in the MegaOhms range ) when the emitting side of the trem roach is not lit.

To test the LDR side it's usually enough to forward bias the LED ( in case of a LED trem roach ) with some 1.8/2VDC, with the DMM set to Ohms the resistance reading across the LDR should fall considerably when the LED is lit. The same applies to a bulb lamp trem roach, only, the voltage/current to turn on the lamp ( and observe a change on the LDR side ) will be different.

Hope this helps

Best regards

Bob

[Bruce / Mission Amps]

Hold it... he said BF amps

Black face Fender amps do not have LED optocouplers... they have a neon bulb laying on top of a CAD cell... called an LDR, light dependent resistor.
Just measure the two wires on either end of the roach with a DVMM in hi resistance and which ever is infinite ("open") is the neon bulb and the other is the variable resistance.
It makes no difference which left or right wire of one end goes where.... just like it doesn't matter which direction you install a resistor or really, a neon bulb with respect to it's leads.

[Robert M. Martinelli]

Hi Bruce,
My apologies....you're right, I completely missed to read that and so I neglected/forgotten to deal with neon/LDR assemblies....
Thanks for making the picture complete!
Regards
Bob

[cjlectronics]

I am working on a BF Super Reverb and the vibrato doesn't work. I have replaced all the caps in the vibrato section and swapped the tube out. Still no tremolo/vibrato(whatever it is) I suspect the optocoupler. Is there a good way to test the bulb? When I flip the standby switch ON, I can see a flash from the bulb... sort of like an arc. What voltage should I be measuring across it?

[Jazz P Bass]

Fender Vibrato

The flash should be a blink.
You should see it blinking, dependant on speed setting.

[52 Bill]

Do you have a foot switch or at least a shorting plug plugged in the amp?

The oscillator will not run without one.

[Jazz P Bass]

OptoCoupler voltage

The plate of the oscillator tube gets its B+ voltage through the optocoupler.
The schematic indicates 350Vdc on the plate.