Lambda LPD-422A-FM power supply issues. - Page 1
Forum section: electronics / repair

author: XOIIO
author_rank: Super Contributor
timestamp: July 22, 2015, 12:42:09 pm
content: Hey all, so I was having the following issue with my lambda PSU, something I picked up for a good deal, and was working great until this arose.https://youtu.be/2UUoRw-VnakNow, I've cracked it apart, and found a couple different things, first of which being that black thing on the standoffs.Yeah, that's a resistor, and still measuring good. (going to replace it obviously) Apparently the other side had a similar issue, taking a peek you can see someone's repair work on it.Now, aside from that, the caps look in pretty rough shape, so I desoldered one of the leads on each one and went about testing. Unfortunately, I haven't worked on anything this old, and most of the pads got ruined, and one particularly stubborn one had the trace rip up with it.Bodge wires will fix it but man I am pissed at myself that this happened, I thought my soldering iron temp of 275-ish wasn't too bad, but I guess for this it was. Maybe it also has to do with the fact that it's an old tape made one, or that it flexed a bit as I tried to remove the caps.Anyways, here is a picture of all three.Pretty retro things in there, and while original would be nice, a working power supply would be nice, and while side 2 was fine, I figure I should maybe replace them as well, but I'd like second opinions.Here is what I measured up. I'm not sure if MFD is for microfarads or milifarads due to the readings I got. I forgot to measure the absolute smallest one that you can see peeking out from the wires.Largest size cap (rated 500MFD) - Side 1 (bad) = 740 milifarads - Side 2 (good) = 680 milifaradsMedium sized cap (rated 70MFD) - Side 1 (bad) = 72 microfarads - Side 2 (good) 77 microfaradsSmallest sized cap (rated 75MFD) Side 1 (bad) = 103 microfarads - Side 2 (good) 94 microfarads.So I'm not really sure what to draw from these, If we assume that MFD is for microfarads it seems that the largest caps are totally gone, but the smaller ones are in decent shape so far, though a tad high, and I could get away with just replacing those.I'm thinking I should just replace them all to extend this thing's life as long as I can and get it over with, I just want to be sure I'm using the right capacitance. It's not in perfect shape but it's pretty good and decently compact for a dual output supply, and the analog gauges are really nice and retro. I want to ad in some nixie colored LED's to them as illumination, and maybe some on the inside as well so it would show through the vent holes, I think that would really tie this thing together.Also, check out the cool heatsink on this can.

author: XOIIO
author_rank: Super Contributor
timestamp: July 22, 2015, 01:07:04 pm
content: Oh yes, I'm also wondering in this sort of application, would there be any harm in using higher capacitance capacitors? I have a feeling I will have trouble finding 70 and 75 microfarad ones locally, as it is these will likely end up being much smaller physically. I can't see why it would but I haven't done any design work, so perhaps the specific capacitance does matter in a power supply.Edit: even digikey doesn't have much, same with mouser, I think I have no choice but to go up in capacitance. I might even have to bodge in some radial caps. At least it's not something you'd see much.Edit 2: you know actually, if those caps are bad, at least the big ones, I could hollow them out to hide more modern ones inside, I'd just have to be careful since they would bend easily.. hmmm.

author: XOIIO
author_rank: Super Contributor
timestamp: July 22, 2015, 05:19:35 pm
content: Sigh, why must murphy constantly accompany me?Went out and spend $40 on caps, put them in, repaired the damage to traces, and the good side works fine. The bad side however, even worse. Now I have no control over the voltage at all, and it's going wilder than ever. The whole board has darkened spots from heat or overcurrent draws, maybe putting in a new capacitor with probably way less resistance than the old one finished that side off.Really pissing me off since this was the one good power supply I had to read current on. I've traced every connection I can from damaged areas and it all checks out, something must have fried. The only schematic I can find seems to be for one half of the supply, the boards aren't identical on both sides unfortunately, and there is just the one page, so any specific parts or things with notes have no more info.I'm guessing it has to be something on the input side that's gotten fried causing it to flip out on it's own and not accept any input from the potentiometer at all, but with the physical state of the board and caps I wonder if it's even worth fixing. I may just have to cut my losses and try to find another one for a decent price, which I doubt I'll have any luck with.Edit: seems it will be $150 for a replacement one, hell of a lot more than I payed for this :/

author: XOIIO
author_rank: Super Contributor
timestamp: July 22, 2015, 05:47:21 pm
content: Well, here's the only schematic/bit of technical information aside from a manual that I could find, I'm too tired right now to get anything useful from it, and considering my electronics knowledge is basically hobby level, I doubt I'd get much anyways, but maybe someone can point me in the right direction, but I still feel like something has fried. Just not sure what it would be.

author: madires
author_rank: Super Contributor
timestamp: July 22, 2015, 05:57:55 pm
content: Quote from: XOIIO on July 22, 2015, 05:19:35 pmMy guess is that some semiconductors are broken too. How many output channels does your PSU have? If it's just a single one, the circuit diagram would be for the complete PSU.

author: edavid
author_rank: Super Contributor
timestamp: July 22, 2015, 07:12:11 pm
content: 0. Did you ever think of mentioning the model number? Like maybe in the title?1. MFD = microfarads2. The caps were fine3. Service manual is here:http://www.davmar.org/pdf/LambdaLPD.pdf

author: XOIIO
author_rank: Super Contributor
timestamp: July 23, 2015, 02:11:43 am
content: Quote from: edavid on July 22, 2015, 07:12:11 pmThe model number is clearly visible in the video, within the very first second actually. I figured people might at least look to see the issue which I was talking about which is why I provided it but apparently that isn't sufficient, especially considering this thread has 200 views, but the video 11. I'm surprised so few even bothered to see what the issue I had was.Thanks for finding a complete service manual though, I could only find a user's instruction manual and that one bit of a schematic.Don't know why I assumed milifarads were smaller, that's what I get for starting something after being awake 12 hours already, didn't even occur to me to look up a conversion sheetSeems that they would be over spec then? that or they are dried up and it's messing with my meter. I'd wager that's more likely.High voltage spikes at output do point to a capacitor issue though so I was on the right track, however it's actually some of the much smaller ones, coincidentally around all the darkened areas.Quote from: madires on July 22, 2015, 05:57:55 pmIt's dual output, which is why that schematic is really just half complete, the good side of the PSU has a fuse for the mains input, as well as other things that this side does not, so it's a good thing my issue was with this side.Page 43 of the pdf edavid found starts the schematics, complete this time.High voltage spikes at output do point to a capacitor issue though so I was on the right track, however it's actually some of the much smaller ones, coincidentally around all the darkened areas. I just need to fix the input issue first.edit: Got images of one from an ebay seller and the caps look much more modern, but original in the way the leads are bent and the heat shrink used on them, I'm guessing mine is a bit older.

author: XOIIO
author_rank: Super Contributor
timestamp: July 23, 2015, 03:45:00 am
content: Quote from: wilfred on July 23, 2015, 03:39:52 amModified the title again, I just wish that there were more characters allowed considering how long the model number is. Originally I just posted it to get a second opinion on the state of the capacitors as I figured that would fix it, but murphy came along and made it more complicated of course.

author: XOIIO
author_rank: Super Contributor
timestamp: July 23, 2015, 05:42:04 am
content: It's got to be a blown transistor or something, I've sat there for an hour buzzing out things on each side to compare where they hook up to, and making sure that all the connections I have made have good continuity to where they originally lead, and it's identical. I was hoping some stupid mistake would jump out at me but sadly that's not the case, I think the manual mentions voltage test points, I'll have to have a look at those and see if something is off but as far as I can tell so far, it's got to be something else, or a combination of things with all the heat damage there is. I need to print out the schematic and test everything one by one, maybe that will help narrow it down.

author: madires
author_rank: Super Contributor
timestamp: July 23, 2015, 11:47:54 am
content: Yep, it's a good idea to check each stage step by step.

author: Vgkid
author_rank: Super Contributor
timestamp: July 23, 2015, 04:25:28 pm
content: Good luck finding those blown power transistors. They are even rare on ebay, I own a working lambda ls fm.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2015, 05:40:57 pm
content: If you understand the scematics is possible to replace blown transistors with modern (cheaper, may be better) equivalents. This may require replacing a pair (e.g. both of the TO3 ones), but two 2N3055 (not modern, but a comon TO3) are still cheaper than one excotic one.So I see no need for spcial high priced parts to make it working. A slightly higher power Zener diode for the upper auxillary supply may be a little mode difficult, but still not that excotic.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 28, 2015, 03:18:43 am
content: theres a few tests you can make, test the 11.7 v (preregulator) if it's fine,The upper section : v comparator, v reference, the or gate, preregulator are on a floating ground technique, you have to make sure where your ground meter is when you measure things. The ground for that is the pin 15 on output terminals.For the power part : the ground is at c109, it passes thru cr116 and cr119 to create around -1.2v of negative voltage at the circle named 74, that way the supply can goes down to 0 volts.If the fuses blow it kills the upper section.You can check at square "F" if something moves in voltage, it's and OR gate who will permit the voltage regulation OR current regulation modes, Q5 and q6 are the voltage section, Q2 and Q3 are the current section.Check r29 if it is ok, it is your shunt current measurement resistor.Check Q8 predriver, q9 and Q11 outside the circuit, measurements can be influenced ...Finally be careful with those types of pcb's, the pcb traces are easy to damage, and can lift-up pretty easily

author: USMC_Spike
author_rank: Regular Contributor
timestamp: February 23, 2019, 01:27:23 am
content: It makes no sense to start another thread about this, so I hopeno one minds if I continue this post.I guess this is the time where I say the ME TOO!I had a working unit like this for awhile now.Until today. I've got a scooter with two 12V batteries wired in series.for 24 volts DC. It just so happens the connector for the batteriesis for the end points. And it has terminals for the + - series connection.As my batter charger for the scooter fried, I was going totry and charge these batteries from my Lambda LPD-422a-FM.I planned to use a higher voltage and current limit the batteries.The batteries are 10Ahr.You can figure out the rest. I put the power leads from theLambda into the opposite polarity of the batteries.When I did this I hear the tiniest of pop noises from the powersupply.Then the faint smell of the sacred smoke...but no smoke seen.I looked at the connections and and made them correct.then I changed the leads to the other channel for the lambda.Switched on the power and saw that the smoked channel wasrunning full out, both the Voltage and the Ammeter werefar off to the right...in Excess of full operating voltage.I checked the pots but they had no affect either full CCW or CW,the meters were past the highest of each meter. So I poweredoff. I found this thread and got the manual found the schematicand the trouble shooting pages.R30a, and R30b measure just fine, on both A and B sides.So the next obvious choice might be The diode CR23 or CR24. Note 11 doesn't help much.Checked that diode, it conducts in both directions...in circuit.Then again so do the other two of the same type. But in different values.Well I guess it's time to start desoldering and testing. So much for an easyand quick fix.

author: USMC_Spike
author_rank: Regular Contributor
timestamp: February 23, 2019, 05:02:27 am
content: No worries.I just pulled the fuse on that A side and put it back together.I'll trouble shoot it some other time.I've got bigger fish to fry.