NJL 5121DB photo reflector replacement - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: March 16, 2021, 10:21:39 pm
content: I'm trying to figure out why a Tandy 1000 power supply has 0V on the +12V rail. One of the parts it has is this thing called a NJL 5121DB which online says is a photo reflector. Does anyone know where the datasheet is for something like this? Is there an equivalent part I could order at Mouser to replace it?

author: Manul
author_rank: Super Contributor
timestamp: March 16, 2021, 11:23:57 pm
content: It is a simple phototransistor output optocoupler in DIP6 package. Based on quick look, SFH601 should be quite similar and available to buy. But better check pinout first.Is it really failed?

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 12:55:44 am
content: What makes you suspect that part? Are the other voltages ok? Opto-isolator in a power supply is usually part of the feedback loop. If it fails, none of the voltages will be right.

author: alank2
author_rank: Super Contributor
timestamp: March 17, 2021, 01:10:40 am
content: Just trying to look around and see what I can figure out on it.Not that it matters now - I was trying to look at some signals on it using a differential probe and it only took a slight slip of the probe on a dip-8 part to short out the fuse. Putting a new fuse in shorts two, so it has gone from broke to broke worse.I'm just going to order a mean well switching supply and replace it.

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 02:15:05 am
content: Well switching power supplies can be tricky and dangerous, this one sounds like it would have been trivial to repair with a little direction though. I'm sure the chopper transistor has shorted and possibly whatever IC that is. One should be very careful working on vintage stuff like this, it's only original once and if you blow it up that's one less original unit still around.

author: alank2
author_rank: Super Contributor
timestamp: March 17, 2021, 01:09:26 pm
content: The worst thing is that I was trying to be careful. I was just trying to look around and see what I could find. I understand why they make those probes with only the very end tip exposed. I think the probe slipped off a DIP-8 pin and shorted two of them by mistake and a flash and it was all over.The mean well is type of PS that is encased in a metal cage and it is small enough to fit inside the old PS bracket. The toughest part will be putting some ring connectors on the wires and reconnecting the motherboard/two drive power cables.

author: james_s
author_rank: Super Contributor
timestamp: March 17, 2021, 06:38:29 pm
content: It's very, very rare that I poke around in a live SMPS, it's dangerous as you found out, both to the power supply and to the safety of the technician. Almost all repair efforts are done by testing components and deductive reasoning. Most have a single feedback loop tied to a master output and then the other outputs are slaves. There is very little that can go wrong with slave outputs if the master is working. Usually cracked joints, occasionally a bad rectifier. Capacitor failures are also very common, a SMPS is brutal on those.