Need a cross for Tektronix part 156-0885-00 aka S0C123A

Hi. I’m looking for a replacement for an optoisolator ic on a 2445 scope. The original part number is a Motorola S0C123A. Tek number is 156-0885-00. Does anyone have any old data books or a 1982 or newer ECG master crossref that has S0C123A in it?


I found a document online which claims it’s a Motorola MOC1005. It seems fairly ordinary with the exception that it’s rated for 7500 Vac of isolation, which is higher than a typical optoisolator.

We have a couple of parts that are very similar:



Okay. I have since seen so many different things about that part. NTE3041 seems to cover it, but NTE3041 covers SO MANY different part numbers which makes me wonder what is the actual difference in any of them? I saw someone mention a 4N35 on a different site. Datasheet for 4N35 also is for 4N36 and 37. No real difference that I saw. Vishay says they don’t have info for S0C123, but the other number I found is TLP631, and they say that is comparable to CNY17-3. Funny thing about all of the different numbers I can find, they all cross to NTE3041.

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Ugh…I also found a Motorola book that suggests a 4N25 as a equivalent of the MOC1005…OMG…do you think you can have a look and see if you see any reason for me to be overly concerned with details here?

it is the closest option that I find also there is no direct option, the voltage is a bit higher but it may work.

Hello KB5VXX,

ECG was absorbed by NTE back in 2001. Perhaps you remember the old yellow cross reference. It was a tissue-thin paper book about an inch in thickness. The purpose of ECG was to provide a minimum number of semiconductors to serve as the substitutions for the maximum number of parts. Consequently, the TV repair shop with a complement of ECG components could reasonably expect to repair most sets that came into the shop.

That cross reference is alive and well at

As a rule, most designs are conservative and don’t push the semiconductor to the limits. The semiconductors are operated well within their design maximum ratings. As a result, a general-purpose semiconductor can be substituted for many different values. NTE identifies this as “4,700 NTE devices cross referenced to 384,373.”

Personally, I’ve had success with the NTE products and have no reason to believe your outcomes would be different.

Best Wishes,


P.S. I have a 2246 on my bench. It’s a nice scope. I also have an older 547 that doubles as a space heater with something like 80 tubes aglow.

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Oh yeah, I remember it well. Wish I still had the first copy I ever got in 93. Well, I’m gonna take everything I’ve found and what I’ve been told here, look it all over and then take a stab at ordering something. Whether it’ll be a 4N25, MCT2 or CNY17-3 IDK yet. I have so much info and not a clear understanding of what I’m really doing I’m just grasping at straws. Once I replace that and recap the LVPS I’m hoping I can bring this 2445 back to life. I may have an issue with the other ic in the feedback circuit. I’m seeing 13 volts at one of the inputs and I’m pretty sure it’s only supposed to be 5 volts there. Not sure if that cdip lm324 is messed up or not.

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I’ve got you by a few years. It was about 87 for my first catalog. :slight_smile:

With regards to 13 VDC on pin 9 of the LM324: it that a rail voltage?

Good luck with your repair.



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That’s what worries me. That should be 5 volts coming from the inverter, so I’m not sure whether the inverter is throwing 15 ish volts at that point, or whether the 324 is leaking it’s supply voltage through that pin. I’m not sure how to test it outside of the scope and really kinda scared to put it back in scope before I can figure out how to test it. If that power supply is throwing wild voltages like 15 volts down a 5 volt rail it may already be too late for the rest of the scope.

Good luck to you KB5VXX.

That wouldn’t be the first machine with a regulator failure. That reminds me of this video with a shorted regulator pass element. Let’s hope that isn’t the problems you are chasing. If only the designers had added a crowbar circuit.



Getting back to that optoisolator, my suggestions above were given based on the info I found on this page: Common Design Parts Catalogs - TekWiki, and specifically this document:

If you look on page 328 of that document, they show the MOC1005 as an option for the 156-0885-00. Based on that, I found a datasheet for the MOC1005 here and searched our product line for the best matches to it.


One of those is what I’m gonna go with I think. The suggestion of TLP631 came from a Tek tech with a newer RPR that isn’t on the web, it was a 156-0885-05. I may order a couple of the CNY17-3 along with your suggestions based on the MOC1005. Just in case. I can always figure out something to do with the extras, especially if I keep being a bit too careless with probe placement… I think I figured out some of my voltage issues. I think. I built the circuit in simulation and played with it a bit to see what’s going on. It doesn’t help that the regulation is completely out of whack from no U1040 input, along with nothing to pull that output down coming out of the 324. I’m going to start building a cart and add the parts you suggested, throw it back together after receiving the parts and see what it’ll do. By the way, thanks for that link. I’m certain that will prove useful.

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