Advice wanted: debugging a DC/DC converter - Page 1
Forum section: electronics / beginners

author: david.given
author_rank: Contributor
timestamp: February 14, 2025, 01:26:52 pm
content: I have a 1995-ish Husky FS/2 handheld industrial computer, which I recently found to no longer start up. It runs off three AAs and uses what I think is a buck converter with two mini transformers to generate the 5V supply it uses internally, and I think this has failed --- the 5V power rail shows up at about 0.8V below the input battery voltage and there's no sign of oscillation on the transformers or the attached inductor.My belief is that a failing internal Nicad battery (for running the CMOS clock; not the same as the external AA batteries) caused some kind of damage, but I don't know what. What voltage is reaching the internal power rail is just the battery supply leaking through the non-functional converter. I've done the usual maintenance like replacing the electrolytic capacitors (no change), removing the dead nicad (no change), replacing the nicad with a dummy load (no change), and injecting 5V directly to the power rail (CPU starts but doesn't initialise properly; something in the reset logic is clearly depending on details of the power supply).Sadly, it's not using an easily identifiable IC to control all of this, and instead appear to have built their own regulator using opamps, mosfets and a single big power transistor. And it's all surface mount, making removing anything for testing difficult. Tracing the circuit is hard partly because it's all a horrible maze of surface-mount analogue components but also because there are internal tracks --- I've attached some pictures. (I'll note that the thing I labelled as a Zener is, I think, not a Zener after all. It's just a white ceramic-ish tube with ZA on it.)Does anyone have any advice on things to try? This is rapidly getting out of my league!

author: jwet
author_rank: Frequent Contributor
timestamp: February 14, 2025, 04:45:12 pm
content: I can try to help. I was an FAE for Maxim in the mid 90's and worked with customers like Husky/Itron etc on things like this.My best advice would be to start at the batteries and see if you draw a rudimentary schematic- just the big stuff, you don't need to account for every little R and C. Pay specific attention to big FET's and how they connect to the magnetics, etc. What the secondaries of the magnetic go to. The output of a standard boost converter (if that's what it is- I don't think so) should sit at a diode drop below the batteries - something like 3.8 for three fresh alkalines. At the time this was designed, it wasn't unusual for gadgets like this to use a flyback power supply. This is like a boost but uses a transformer and allows you to have multiple secondaries. You can look a the Tandy Model 100 Schematics/Service manual on line- this is a good example of what you might have. This was done because they often needed 5v and a LCD bias voltage, usually positive 15 or so but sometimes negative.The two black transformers/inductors show that say "Wound Products" are kind of critical to understand. How many pins are on them? Can you google their data sheet etc. The thing that you call "Core" is an inductor that looks like it was a bodge to pass EMI or something. If you have SMT equipment (hot air), remove those Wound products things and ohm them out to get a handle on primaries/secondaries.Look with magnification at all the part number of stuff in that corner. There is an SO-8 close to the corner of the board near the "core" that could be a boost controller. Those other parts that I can see are 74HC125 which are tristate buffers- they may not be power stuff or they may be part of this the supervisor that will tell the uP to boot up. ID the reset pin on the uP and work your way back to its source in the power supply.I'm a little surprised that feeding 5v in didn't work. It could be that that PS makes other voltages that aren't present. Look for clues. Capacitor voltage ratings can be a clue to a higher voltage LCD supply.See if you find any service data. Equipment for the US market has to pass a part 15 FCC test for emissions. This is public data and can be found on the FCC website often with a schematic. If this is a UK unit, there may be an equivalent, poke around with google.Industrial designs like this don't change very fast so often if you can find a similar unit older or newer or a related schematic- it can be a big help. Designers are basically lazy and they don't fix what works. Usually the models are for more RAM or a new CPU, etc- power tends to stay static.Good luck- If you post progress, I'll follow. I check in the morning US east coast and after 9 pm usually. Good luck- things don't look bad. Stay at it- you can do it.

author: david.given
author_rank: Contributor
timestamp: February 14, 2025, 10:51:51 pm
content: Here are some better pictures with all the major components labelled. Sadly, that 8-pin chip next to the inductor is actually a comparator, not a control chip of some kind. My hunch is that the opamp on the other side of the board next to the little trimmer is doing most of the work.> The output of a standard boost converter (if that's what it is- I don't think so) should sit at a diode drop below the batteries - something like 3.8 for three fresh alkalines.That's what I'm seeing; I'm feeding in 3.6V (I have the original external nicad battery pack, now long since dead, which you can use instead of AAs), but I've tested it up to 4.5V and it's consistently about 0.7V below the input. Just to clarify --- you're talking about the output when the converter isn't running, right? If it is running, I'd expect the output to be 5V!> The two black transformers/inductors show that say "Wound Products" are kind of critical to understand. How many pins are on them?Eight each; one pin at each corner, and two extra at two opposite corners. Of course, I don't know how many are structural and not electrical. There are two of them, a WOUND PROD LTD 4000-906 and a 4000-953. Sadly, WOUND PROD is the least googleable name ever, and their product codes aren't very helpful either; I was unable to find anything. It does occur to me that it's possible these _aren't_ transformers and are potted DC-DC converters, but I would expect to see more labelling on them for that.Sadly, my desoldering skills aren't sufficient to remove them intact for inspection.> I'm a little surprised that feeding 5v in didn't work. It could be that that PS makes other voltages that aren't present.The CPU is running and sometimes things appear on the screen --- I even got a boot screen once (it crashed shortly afterwards). This does at least prove that the screen just runs off 5V. The CPU reset line looks fine, but it's fed from a custom chip, so I have no idea what triggers it. I did try grounding it while the machine was running but it didn't reboot, instead instantly crashing (I could tell because the current went up to 50mA). There are a couple of HIN241 RS232 transceivers, but these generate their own weird voltages internally. So yeah, it's odd. My suspicion is that the reset logic is looking for some kind of power-good signal from the power supply which it's not receiving if I do direct 5V injection, causing it to not start up properly. Sadly this rules out just bypassing the power supply circuitry.> See if you find any service data. Equipment for the US market has to pass a part 15 FCC test for emissions.That's a really good idea. This unit is British, but they are also sold in the US... and I'm blowed if I can find anything resembling an FCC ID.If this isn't a buck converter, what else could it be?

author: jwet
author_rank: Frequent Contributor
timestamp: February 15, 2025, 05:16:20 pm
content: Much better pics. Let me study and I'll get back. In the mean time, try to rough out a schematic of some kind.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 16, 2025, 08:57:53 pm
content: Do you mean Ni-Cd Battery?Make sure you recycle it properly.Cadmium is toxic and very hazardous to the environment.

author: jwet
author_rank: Frequent Contributor
timestamp: February 22, 2025, 03:34:36 pm
content: Any progress on your Husky Terminal?You mention that the reset comes out of a custom IC. It wouldn't surprise me if the power function wasn't in here too- it has some power adjacent functions like the reset. Husky was big enough to get customs of something like a power supply that was used over multiple projects (it takes a few million to make sense). Does the gate of the FET and feedback go here?It is a boost converter- the .8v below the input gives it away. Track back to the FET and big inductor. There isn't much to a boost just a FET, Inductor, Diode and Cap. Trace those out- see what the gate is doing- it should be getting hammered with pulses continuously (below output setpoint), look at its drain, it should be pulling down close to ground when gate is high. Frequencies of boosts in those days were 30Khz to 200 Khz. Inspect the board with a 10x magnifier and see if the NiCd leakage ate some traces. Clean board, do repairs. Most likely problem. Consider replacing NiCd with NiMH- less toxic and less likely to leak.A few mysteries to me.What the heck are the transformers by Wound products? Does the unit have an EL backlight? That would be what you'd need for that. Is the keyboard backlit too? That could account for the pair.The no boot problem could be a symptom of a single data or address line getting eaten by NiCd juice. It could be intermittent. Acid eats tracks from the outside and gets under solder mask- it thins it out in width and often in Z. Inspect the small traces at 10x and pay attention to vias- that caustic stuff can get down in vias and won't evaporate and will ruin them, check continuity end to end. Clean the board with IPA. Clean any areas exposed to NiCd (Potassium Hydroxide) with a mild acid like Vinegar and then rinse with clean water- distilled if possible.Good luck.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:42:06 pm
content: I think the thing you labeled "Zener" might be a fuse.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:46:13 pm
content: EPROM chips don't reliably retain ther contents for more than 20 years.Your device is about 30 years old.