It is common for an IR Emitter to fade and not put out enough light to trigger the receiver.

Best thing you can do (if you don't have a spare) is swap the emitter with one in another position to see if the problem moves.

You can also use a digital camera or cell phone camera to see how much light it is putting out compared to others. It should show up as a light pink/purpleish light in your camera but usually only looking straight at them since they are usually very narrow beams.

I understand that you can see these light up with a camera that doesn't block IR light, and yes there is a certain cool factor to it if you don't already know this ... but for troubleshooting it's not really that useful. The best way to check the transmitter is to put your volt meter across it and measure it's voltage drop, this is actually faster and easier that trying to look at it and attempt to judge its light level since you don't have to remove the transmitter check it.

I could be wrong, but I've actually never heard of this in a semiconductor device, they are either on or off. From a purely technical aspect I would find it more common for a connection or failing current limiting resistor to be dropping more voltage than it should which would result in less voltage available to the IR LED, less voltage = less current hence less light from the IR LED.

Well, I work with a lot of equipment that uses both IR reflective and cross beam setups and I can assure you two things:

Light intensity output does diminish (especially when driven at or near their max ratings).

And measuring voltage drop only indicates the LED is still conducting. Very hard to interpret the voltage drop vs. Output except for dead short and open as you mentioned.

True, it is tough to compare the IR ouput when viewed with a compatible camera, but it's about the only non-technical test I could suggest. I have diagnosed plenty of marginal devices that way. It's a quick and easy task. Just not definitive.

I'm sorry I have to disagree, since one side or the diode is at or very close to ground potential when turned on, the voltage drop across it will be the remainder of its supplied voltage, since I=E/R the higher this voltage, the higher the current draw and the brighter the LED.

Common failure of LEDs is dimming over time. They often (if not usually) don't burn out, they get dimmer and dimmer. I've seen opto transmitters that were too dim to register at the receiver. Replace the transmitter and all is good.

Thanks for the clarification on the dimming effect of these transmitters, this would mean that the resistance of the diode is changing over time, I would be very interested to know what the resistance of this particular LED measures, as well as its forward bias break over voltage.

Did you replace both the transmitter and receiver, and have you measured the voltage across the transmitter yet?

Voltage on the transmitter between A and K.

Their wavelength also matters. When in doubt, replace the pair with matching wavelength pairs.

I would have once agreed that optos don't "fade" with time until I read this in a Sharp Optoelectronics datatsheet:

optos_(resized).jpg

The machines we are all working on are in the main WELL past five years old so it seems LED degradation is a fact according to Sharp anyway.

If you have replaced the optos then it's time to start investigating the opto driver board. This is the 7 or 10 opto board. Measuring voltage across the transmitter may not tell you much since it's only about a volt there. The 12v is input into the opto board.

True, it can be a problem on the opto board, or wiring connectors. I typically don't start with measuring the input to the opto board since if it's bad you will have more than one bad opto and likely a message on the DMD stating to check your 12V, but it's an easy to check to make and while you have your meter out anyway, it doesn't hurt to check it.

Since you have to start someplace, I usually start with what doesn't work and work my way back towards things that do work. Yes there is only a small voltage on the LED as it's rated voltage is 1.7V, but if there is 1.2V on the LED, I'm sure it will be too dim to operate; if it's low, work back to find why the voltage is so low, how much voltage is leaving the driver for this specific LED? If that voltage is low there is only 1 component that can make a single opto voltage low and that's the current limiting resistor. So you check your voltage on the resistor, if its good you know it's the header or connector pin is causing the problem.

I also think the older LEDs used some IR-reactive epoxy; which leads to the LED's own lens to become opaque to IR light.
I go SO tired of debugging Opto issues on my STNG; that I went in and replaced every opto with a new emitter. Gone are those brownish lenses.... In their place; blue clear lenses.

It wasn't that expensive - I ordered like 30 of them from Digikey at a good price. The hardest part was getting at the optos in the machine.

Also; keep in mind that most light already has IR in it's spectrum - Sunlight, Fluorescent tubes, incandescent ... that can play a role in your test procedure unless you are testing in the dark.

And some LED flashlights don't trigger optos at all.

Leds are generally a single wavelength

Start working your way back to the board as mark said.
That is my best idea as well.
It was also a great idea to dismount them both and hold them close to insure they trigger. Sometimes the bulb is mounted not perfectly square with the board or the plastic holders get bent allowing them to work and not be seen by each other.

This 1.35V -- is it measured directly across the emitter pins?
If so -- as an emitter, depending on the specific device used - there would be upto about a 1.7V drop across the opto. The 1.35 does not sound unreasonable at all. If you were to put 13V directly across the emitter, it would put out one brilliant pulse of light... then nothing. But since it would be in the IR range, I doubt you would see the pulse.

There should be a current limiting resistor between that 13V source and the LED. This will limit the maximum current through the LED. In the process of limiting the current, there will be a voltage drop across the resistor. If you were to measure this voltage across this resistor - it would be 13 - 1.35 = ~11.65V.

The difference you see is the additional .08 voltage drop across the LED itself, this is a common amount of loss for a working LED. The next measurement I would make is the voltage at the current limiting resistor to your ground braid, the difference between that point and the 1.35V you measured at the LED is the combined total loss of all cables, connectors, etc... this should be a negligible amount. Another good check is to see what the voltage on the other current limiters are since they are driven by the same source, power the same type LEDs and have similar return paths, they should all be close to the same voltage readings.

Have you replaced the LM339s on the opto board ?

I think you are putting too much emphasis on the voltages. The LM339 works by converting the analogue signal into a digital one for the switch matrix. Over time the positive switching becomes a little 'muddied' and the switch matrix doesn't get a clean signal.
To be honest if it were my machine the LM339 would have been the next thing I changed after the opto. I have had so many problems with them in the past I ended up buying a lot of 50.

There are opto receiver LEDs out there that are misassembled from the factory. If they are installed according to the markings or lead length, they behave as you are describing. If you've ruled out everything else, switch the leads if possible or unsolder the receiver LED, turn it around 180 degrees, and resolder.

Search for my thread that explains the manufacturing problems with these LEDs

Great that you sorted it out - have a read of this short thread (it explains the background to your issue):

https://pinside.com/pinball/forum/topic/how-to-determine-the-c-e-of-black-opto-receiver-leds#post-3538582

Thank you so much for documenting this!

It has saved me a huge amount of work as I had EXACTLY the same problem.

After a week of troubleshooting the game was fixed in 5 mins after just reversing the wires on the opto receiver.

.