Teardown: Lambda LS 512 Precision Power Source - Page 1
Forum section: electronics / projects

author: requim
author_rank: Regular Contributor
timestamp: November 03, 2011, 07:55:24 am
content: I received yet another project to work on today in the form of a Lambda LS 512 Precision Power Source. The unit apparently came from Boeing in Southern California, which means it really probably came from Rockwell or Rocketdyne since Boeing basically bought its presence there years ago. The unit has definitely seen better days.I've posted hi-resolution photos to flickr, so you can view the unit in its full glory. I probably posted too many, but I'm new to this whole photograph/video teardown thing. Unfortunately for you I started midstream after I had already disassembled it and cleaned up a few of the components (faceplate, case, and the box looking thing).I'll save the suspense, nothing aside from the other side of the connector is inside the box. It appears that other units/models have two analog meters to display volts and current. Sadly this unit does not.At the moment the unit is slightly out of spec and experiences voltage spikes to 25-35 volts when I change any of the dials. With any luck this is by decay rather than by design and is fixable. Fingers crossed.http://www.flickr.com/photos/54406519@N08/sets/72157628041950366/I'll post photos of it once I've got it back together and with luck working properly. BTW, I'm posting the photos as I type this so it may take a few more minutes until all the photos have posted.

author: amspire
author_rank: Super Contributor
timestamp: November 03, 2011, 12:26:51 pm
content: Very interesting.Made in 1968 or 1969 by the look of it. This would have been an expensive supply at the time.I believe that box next to the transformer is an oven for a zener reference. So if you need accuracy, the unit probably has to be on for an hour to stabilize. Cold it will be fine for normal use.The knobs are very interesting. When one knob switches through zero, then next one moves up by one. That is very cool. Not very common at all. I cannot quite work out how the resistors are wired. If you can measure then and draw the connections for two decades, we could work it out. The resistors are wirewound, and it looks like 3K and 4K are there so it might be a 1k-1k-3k-4k combination to make the decade.If you are able to work out the wiring for a couple of decades it would help. Also check if the switches are make before break, or break before make.The top compartment would be for volts and current meters that was obviously an option. If you wanted to add digital meters, it should be straightforward.With the power transistors, the two FBN36220 are in parallel and are the output regulator transistors. The black one is an NPN driver transistor for the output transistors.The FBN36485 I am not sure - perhaps the oven power controller.I suspect that the IC's on the board will be transistor arrays. This is before the days of high performance opamps.It looks a bit sad, but it is probably not as bad as it looks. The PCB discoloration is just the result of 50 years of cooking from the rectifier diodes. If it has lasted this longer, it has plenty of life yet.It will be really easy to break one of the wires, so make sure you have photos of all the wire connects.Richard.

author: requim
author_rank: Regular Contributor
timestamp: November 03, 2011, 01:02:54 pm
content: How can I check for the make before break or break before make? I'll try to take some better photos of the decade resistors. It's really a difficult piece to get access to due to the space constraints as well as the wiring.I'll probably spend more time today cleaning up the unit and once I've finished I'll re-assemble the thing and leave it on for an hour+ to see whether the readings change. I keep forgetting equipment needs to be left on for a while before it starts to give accurate results. Computers aren't like that so I don't think about it.

author: amspire
author_rank: Super Contributor
timestamp: November 03, 2011, 01:39:32 pm
content: Quote from: requim on November 03, 2011, 01:02:54 pmIt looks like each decade has one wire in that goes to single contact on the rear of the front layer (nearest the front panel) and comes out on the single contact on the front of the rear layer. The output goes to the input of the next decade.Here is what I would expect to see, but I am guessing a bit at the values:Highest decade: 0 to 0 to 1MSecond decade: 0 to 900KThird decade: 0 to 90KFouth Decade: 0 to 9K ohmsPot (last decade): 1K ohms wirewoundAs all are in series, you basically have a big variable resistor that you can set between 0 ohms and 2M ohms. One end of this resistor will go to the +ve sense terminal on the rear panel (probably one of the top two shorted terminals on the rear) and the other end will go to the input of the voltage control op-amp. The same input will have another high stability resistor with a value of maybe 680K to the zener which will probably be running at -6.3V. A lot of guesswork here so it may be done quite differently.If you clip a multimeter across a decade and slowly turn a switch, does it go high for a moment - even open circuit, or does it go low for a moment.If something is going open circuit, then that would cause the voltage spike you mentioned.The PCB only has the maximum current potentiometer, so I guess the other two potentiometers will be the oven temperature and something that will adjust the zener reference output slightly to set the 20V. It may be controlling the zener current.The black thing on the heatsink will be a thermistor and as the heatsink gets hotter, it will lower the peak output current.Richard.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 03, 2011, 04:09:51 pm
content: Definitely neat and very worth fixing up. I have something similar on my bench, an Analogic 8200 voltage reference, and I use it almost daily. For applying precise voltages to transistors and doing any sort of high precision resistor comparison task, it can't be beat. Unfortunately, the switches used in the 8200 degrade over the years, and I can't afford new ones. They're digital counters and even though the unit has the same next decade increment/decrement feature, it's all I can do to set a voltage at all. Contact cleaner helped for a while, but I think the oxidation is just too bad. Your mechanical switches should be far more serviceable. There's probably a cap somewhere to limit the spikes, and I'd be checking all the caps in the unit quite carefully. BTW, the decades in my 8200 are a very clever octal system that makes little sense at all until you grasp how they add together. Hopefully yours is more straightforward! If you get it working well, you won't know how you lived without it.

author: requim
author_rank: Regular Contributor
timestamp: November 04, 2011, 04:55:43 am
content: So I probably went a little further in my teardown today then I should have. Ooops. I'm in the process of putting it back together, but wanted to give a partial response.I recorded the values for the resistors in the decades. If you'd still like a drawing of how they're wired inside the decade I can provide that as well. It will just take some time to generate.I've uploaded a few more photos that I took today. Nothing too exciting but there are some new details in some of the photos:http://www.flickr.com/photos/54406519@N08/sets/72157628048790560/As you'll see I also found that there are a set of 5 lights which I assume are all dead which I'd assume were used to illuminate the dials.I have yet to put the multimeter to any parts of it, but I'll post a video to youtube showing the decade as turn it. Perhaps you'll be able to tell what's happening when you see it from the back. Once I've got the unit reassembled to the point where I can reapply power to it, then I'll use the DMM and tell you exactly what happens.The resistors are as follows1st decade:100000.01%DET10-0042nd decade:10000.01%DES10-01110000.01%DES10-01140000.01%DES40-01330000.01%DES30-0123rd decade:1000.025%DER10-0121000.025%DER10-0124000.025% ??DER40-1143000.025%DER30-0134th decade:10?0.1%?(the resistor colors are as follows: brown black black gold brown (the last brown is wide)10?0.1%?(the resistor colors are as follows: brown black black gold brown (the last brown is wide)40?0.1%DEP40-004300.1%DEP30-003The question marks indicate that I could not read the value and made an educated guess.Question: As far as this restoration goes, do you think I should replace the original banana jacks or keep the originals? The originals are quite faded but I assume are in working order.

author: amspire
author_rank: Super Contributor
timestamp: November 04, 2011, 05:15:49 am
content: Lambda was the king of power supplies back in the '60s and '70s, but I have never seen anyone emotionally attached to them. They basically provided a massive range of power supply solutions to industry.So if you want to put new sockets on, or do any other improvements, go ahead. I can't see anyone paying more money for having the original sockets. That said, old banana socket quality is often way better then many of the low grade ones sold now, so if the old ones are not corroded, they may be better then your replacements.Those resistors make perfect sense. The thing about using a 1-1-3-4 set of resistors is you can get all the combinations with just two wafers.Also even though the precision is 0.01%, there is every chance that they sorted the resistors for the first two decades and made matched sets, so it wouldn't surprise me if they matched within 0.001%. Being wire wound, they probably haven't changed value much over the years.One thing I would do is measure the settings on the three pots and write them down - perhaps on a label you stick inside, That way, even if you start fiddling with the settings, you can return it to the current state.Richard.

author: requim
author_rank: Regular Contributor
timestamp: November 04, 2011, 05:27:50 am
content: If the decade is at fault for the voltage spikes should I consider replacing the decades and reuse the resistors?

author: amspire
author_rank: Super Contributor
timestamp: November 04, 2011, 05:38:25 am
content: With the nature of that type of decade switching, there will be glitches either up or down as you switch. If you want this as dual use precision voltage source and general purpose supply, what you can do is add to the top section a digital volts display (maybe volts and current meters), and a 10 turn 20K potentiometer.Also add a switch so you can switch between the 20K precision decade resistors, and the single 10 turn 20K pot.You would end up with the single knob you can smoothly turn up and down, and you will also still have the voltage source that once calibrated, can be used for testing and calibrating.It is really hard now to get good quality switches at an affordable price, so if the current switches work, do not touch them.10 turn pots though can be very affordable.Richard.

author: requim
author_rank: Regular Contributor
timestamp: November 04, 2011, 05:45:14 am
content: So I guess if I just want to use it as a precision source then I don't need to worry about it. Now I've learned the difference between a precision source and a standard power supply. I have 4 Agilent / HP DC power supplies so I think using it just for a precision source would be fine for my purposes. As it is I have too many power supplies.Rewatching Dave's teardown of the Krohn-Hite DC Voltage Standard I noticed that my unit also uses a standard Kelvin divider instead of the Kelvin-Valley? divider he mentions.So is your thinking that the spikes are just something I have to live with if I keep the original design? And that all the unit needs (assuming I didn't break/kill it) is to be calibrated?

author: amspire
author_rank: Super Contributor
timestamp: November 04, 2011, 06:00:01 am
content: You have a twist on the Kelvin divider. Instead of needing 9 resistors per decade with a simple switch, you use 4 resistors with a complex switch which save money on the expensive 0.01% resistors.. You definitely do not have a Kelvin-Varley divider.The Lambda is perfectly good as a power supply once a voltage is set, and it should have far more stability and accuracy then the other power supplies. It is a good thing to have.What Dave has is a device to output precision voltages at at very low currents. You have a device that can output precision voltages at currents up to 1.8 A.You just have to remember not to change the voltage with something connected.Richard

author: amspire
author_rank: Super Contributor
timestamp: November 04, 2011, 06:36:08 am
content: It just struck me that those two chassis mounted pots line up with the holes on the PCB. So all the adjustments are simple. One for setting the zero volts, one for calibrating the output, and one for setting the 1.8A short circuit current.If you feel the urge to calibrate it fully, get this reference from Geller:http://www.gellerlabs.com/SVR%20Series.htmYou set the supply volts to 10V. Connect up the Geller reference to 15V from another supply and connect the Geller 0V to the Lambda 0V. Put your most sensitive multimeter between the Lambda + and the Geller + and measure the voltage difference. Adjust the Lambda Max Vout for zero volts on the multimeter.The Lambda should then be more accurate then your multimeters.Richard

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 05, 2011, 03:10:00 am
content: It should be possible to get smoother switching by adding capacitors. You'll need to use very low leakage ones to avoid throwing off the accuracy.

author: requim
author_rank: Regular Contributor
timestamp: November 05, 2011, 08:00:13 am
content: I may have to investigate using caps to smooth out the switching. I'd be willing to do that if I can figure out how to do it.So I'm putting the unit back together and I decided to test the decade resistors. Please keep in mind that I did not solder or desolder any of the decades just the two wires to connect into it. I decided to ohm test the decades since I figured it was essentially the same thing as the decade resistor that Dave built as a kid and that he reviewed a few blogs back. It appears that the decade is off by about 14 ohms. Should I be worried? Or is this what the adjustment pod is for on the side? And will the existing pod allow me to adjust it 14 ohms?

author: amspire
author_rank: Super Contributor
timestamp: November 05, 2011, 08:17:58 am
content: Do you mean that at a setting of 00000, it has a resistance of 14 ohms?

author: requim
author_rank: Regular Contributor
timestamp: November 05, 2011, 08:38:40 am
content: Yes. That's exactly what I mean. I am still in the reassembly process so I still have not resoldered the leads to the decade switches and I'm getting 14 ohms with all decades set to 0.

author: SgtRock
author_rank: Unknown
timestamp: November 05, 2011, 10:05:34 am
content: Dear Conrad Hoffman:--When you get to the point that nothing else will work to get your switches going, you might consider using DeoxIT D100L, applied ever so sparingly to the metal only. It is very slightly acidic (dilute oleic acid). I have had some good results using it. I make this recommendation, only as your last resort before parking or tossing."It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge. " Enrico Fermi 1901 1954Best RegardsClear Ether

author: requim
author_rank: Regular Contributor
timestamp: November 05, 2011, 12:10:29 pm
content: So I found the error of my ways. Well part (all?) of it. It seems the millivolt range turns the opposite direction from the rest of the knobs. So when I put the face back on it threw it off. In addition it seems that once you get past a certain point it starts going back up again so it takes a bit of fine tuning to find zero. So it looks like I'll have to disassemble it partially again to fix the dial.Now I'm letting it warmup and measuring it with a 6.5 dmm and it appears to be off by about 0.12 mV at the moment. I assume I should be able to zero it out pretty easily with the adjustment dials. I will of course wait till after both units have been on for a good period of time before doing so.I also ordered some contact cleaner which should get here by Friday so I can clean off the contacts on the decades since they all seemed dirty. All in all it looks like I got a great unit. I am wondering why the initial calibration was off prior to my disassembly as well as the voltage spikes. I'm not seeing any voltage spikes now which I can't figure out why they went away since I was only cleaning stuff.

author: amspire
author_rank: Super Contributor
timestamp: November 05, 2011, 01:13:53 pm
content: If the offset error is about -0.12V, that is an interesting number, as the error showing on the Fluke meter on ebay was -0.11.It could be your voltage calibration is spot on, and your offset only has to be adjusted to zero.Richard.

author: requim
author_rank: Regular Contributor
timestamp: November 05, 2011, 03:03:15 pm
content: Ah.. the difference being that I was measuring in mV when I quoted 0.12 and not volts. No matter, I've had it on for a few hours and it is reading ~-0.54 mV when set to zero which tells me that I can probably zero it just by the way I put the dial back on the smallest digit. Perhaps I should consider replacing the pot used for the variable resistor on that digit since it does not end at 0 and 10 but instead goes more like 2 .. 0 ... 12 instead.

author: requim
author_rank: Regular Contributor
timestamp: November 05, 2011, 08:20:09 pm
content: After leaving both the LS 512 and my 34401a on for 6+ hours it settled around -.6 mV. I was able to adjust it to 0.012 mV with all the dials set to 0. At the top end it seems to be off by about +6mV and I'll need to fiddle with the pot for that setting as well. So all in all it appears to work great. As I said the voltage spikes are gone, but I can't figure out why that is.I ordered some contact cleaner and I'll break it down again to gain access to the contacts as well as the mV dial since it's off by about two mV from zero. Other than that it is pretty much perfect. And best of all I didn't have to replace a single part. Though I do need to buy some rubber feet since it is missing the ones I assume it had.

author: amspire
author_rank: Super Contributor
timestamp: November 05, 2011, 08:36:55 pm
content: Well done!You definite have a great bargain now.The Lambda guys never designed anything pretty, but they did know how to design stuff that was solid and functional.I think the interlocked rotary switches are brilliant.Richard