Weird Power Supply Issue - Page 1
Forum section: electronics / projects

author: timb
author_rank: Super Contributor
timestamp: August 18, 2015, 09:04:23 pm
content: Hey guys, I've run into a weird issue with an old piece of gear in the lab. It's an Analyzer488 from IO Tech, circa 1989. (It's basically a standalone and serial controlled GPIB analyzer/controller/instrument simulator with a gorgeous amber plasma display.)Anyway, it's powered by this huge linear brick supply that delivers 5V@2A through a 6-pin DIN connector. Now, the other night I realized it appears to be identical to a MIDI connector, which I happen to have a metric ass to of just sitting around. (Radio Shack was selling them for like 10 cents and nobody else wanted them, what was I going to do, not buy them?)Anyway, I got the idea of replacing the hulking brick supply with a smaller, more modern switching unit. So, I snipped the end off one of the MIDI cables and buzzed the pins out; the old power supply actually had the pin out for the DIN connector on the top label (how nice of them): 1-5V 2-NC 3-Earth_GND 4-NC 5-GNDI figured common (earth ground) wasn't important and hooked 5V and GND from my 3A Keithley bench supply to the homemade cable, plugged it in, hit the power button and... It sort of powered up? All the panel lights came on, but the VFD never started. According to the manual, this is indicative of some sort of RAM/ROM failure during startup. The bench supply is showing 1.7A, so I know all the logic has to be running. I turned it off and on a few more times, same deal. I plugged it back into the OEM brick and it starts up just fine.I figured, okay, maybe it pulls more than 3A briefly on startup, so I hooked it to my 19" rack mount Power Designs 60V/6A supply; same result. Then I thought, okay, maybe the thin wires in the MIDI cable are just dropping the voltage too much durning high current draw at startup, so I opened the OEM brick and clipped the bench supply leads directly to the output leads. Same result.I've measured the output from the brick supply at 5.1VDC. I've desoldered the output cable from the brick and put my 6A supply on it at that voltage and I still can't get it to start. So what the hell is this supply doing that's so special?I took a photo of both sides of the board, then ran some filters to make a map of the copper side, which I horizontally flipped and superimposed over the component side.The design is sort of weird, the transformer has 7 wires coming out of it going into the power supply board. There's four large rectifier diodes, but they're all on the negative rail. So far as I can see, the positive output is coming directly from the transformer, through an 8000uF filter cap, into the collector of a TIP transistor and out the emitter. I guess the transistor is regulating and rectifying?At first glance it appears to be a standard pass transistor based linear regulator design, but I guess it's just the number of wires coming from the transformer and lack of normal rectification that has me scratching my head on this one.So, can anyone see why this supply would work while none of my bench supplies will?Sent from my Tablet

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 18, 2015, 09:16:53 pm
content: Have you verified that the labeling on the connector is accurate? I have found that a number of these bricks actually are using remote sensing to assure that the voltage at the powered unit is correct under high current conditions. That could explain your symptoms if the device is voltage sensitive during startup. This was the problem for me in when I did something similar to what you are trying. It is even possible they are using the difference between earth ground and the ground returns as a method of remote sensing.

author: timb
author_rank: Super Contributor
timestamp: August 18, 2015, 09:26:15 pm
content: Quote from: CatalinaWOW on August 18, 2015, 09:16:53 pmYeah, I'm getting 5.1VDC out of the brick. The earth ground lead comes out of the wall cord and is soldered to a tab that's bolted to the transformer. The "common" pin from the output connector is soldered to a tab on the other side of the transformer. (The bolt goes through the frame of the transformer and electrically connects the two tabs.)I buzzed all 7 leads coming from the tranny, none were connected to earth ground.Edit: Oh, and the labeling is wrong. Pin 4 is common, not pin 3 as the label states.Edit 2: I suspect the earth ground feed to the unit is related to the IEEE-488 interface. I'd imagine they ground it to ensure the transceivers don't go over their common mode range or something.Sent from my Tablet

author: MarkF
author_rank: Super Contributor
timestamp: August 19, 2015, 06:04:04 am
content: Tim,Here's a reversed engineered circuit diagram. You NEED to check for mistakes.Maybe it will help....

author: timb
author_rank: Super Contributor
timestamp: August 20, 2015, 09:46:22 pm
content: Quote from: MarkF on August 19, 2015, 06:04:04 amWow! Thanks man! I was going to draw a schematic myself, but I've been out of town the last couple of days dealing with an emergency. At first glance it looks correct. This will be something I can try to make sense of on the drive home.Edit: I just realized you re-did the copper overlay too. I should have posted my original copper mask layer so you didn't have to do all that work. Though, yours looks a lot better. How did you do it? I basically normalized the light best I could, converted it to grayscale and ran an auto-trace filter, then filled the areas with a paint bucket. In retrospect I would have gotten a smother result had I used the magic wand to select areas of copper, smooth the selection then fill. Ah well.Sent from my Tablet