Lambda LQD-421 repair - Page 1
Forum section: electronics / repair

author: BigJohn89
author_rank: Newbie
timestamp: April 28, 2020, 11:42:08 pm
content: Hi Everyone! I just came across an old Lambda LQD-421 that I'm trying to get up and running. One side functions OK, but appears to run away on voltage at times - I am already looking at re-capping that side. The other side is stuck at minimum voltage, and will not raise voltage at all. Checking at all three test points Va, b, and c give voltages that aren't too far out of whack, and so far I haven't come across anything burnt or failed - including the main control pots.I noticed some other messages related to repair of this PSU's brethren, but there wasn't much of advice for solutions or things to look out for (other than recapping). Has anyone successfully repaired one of these, and have any kinds of good advice they would like to pass along? I have a schematic for it that I have been tracing, however the unit itself doesn't seem to completely follow the design as printed...Thanks in advance!John

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 12:17:32 am
content: Are your short jumpers on the black terminals screw block on the rear are okay ?Could you post your schematic(s) ?Schematichttp://bama.edebris.com/manuals/lambda/lqd420Its somewat classical topology,not sure if ic101 is an lm723 ??You have a floating supply section on top of the main supply, you have to be sure where you put your meter ground for measurementshave you seen this thread ?https://www.eevblog.com/forum/repair/datasheet-for-lambda-voltage-regulator/The fbt-00-128 ic seems to be an LAS3700 ic ? an unobtanium part, i hope its not busted.The lm723 to-100 case have a strange ressemblance minus the pin numbering ??http://www.ti.com/lit/ds/symlink/lm723.pdfI would not recap the psu immediately, Repair before changing many parts. The design fits many models with some parts change between them, you see the changes in the manual.youre supposed to have :ic 401 12v or 15vdc between pin 10 and pin 1ic 403 5v on test point a16 and a20 for the display supplyAnd as you wroteva = 41.5 voltsvb = 11 voltsvc = 28.5 volts

author: cnqhdszq
author_rank: Regular Contributor
timestamp: April 29, 2020, 02:08:33 am
content: http://bbs.38hot.net/forum.php?mod=viewthread&tid=100352I had repaired a LQD-422-V liner powersupply .First you should check the main regulator, transistor "FBN 36605N" ,if the transistor is OK ,then check the voltage of the volt adjust pot ,when turning the pot the voltage will change ,commonly the output voltage equals the pot voltage . Good luck!!If you can speak chinese ,you may read my repair post (above link).the PIC is my schematic by hand.The resistor in the red circle is open ,both side .One "FBN 36605N" is shorted ,and one 40K pot is open .and the switch of V - I on the front panel is not OK ,and the num ，after I drop a drop of lubrication oil ,it can do it's work.

author: cnqhdszq
author_rank: Regular Contributor
timestamp: April 29, 2020, 02:11:20 am
content: And a 40 UF cap is dried up

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 02:44:37 am
content: @cnqhdszqYou may add confusion here, the LQD-421 is not built like your shown schematic ....

author: duak
author_rank: Super Contributor
timestamp: April 29, 2020, 05:05:19 am
content: John, some of the older Lambda supplies (like the LP-4xx) will damage their voltage control pots if they are connected to a battery and the output voltage is turned down. I don't know if the LQD series do the same thing. Do you see the output voltage suddenly jump up when set to particular voltages? If so, the voltage control pot may need cleaning or may be damaged. I would shut the unit off and then measure the voltage control's resistance - it can be done from the back panel - RP (term 13) to +V (term 14)I think the LAS-3700 chip is a custom Lambda part that doesn't seem to cross to anything else. The chip in the LP-4xx series is a re-marked Motorola MC1466 floating regulator that is in a 14 pin DIP. If the LAS-3700 is bad, it might be possible to substitute an MC1466, but these are obsolete and hard to come by. Lambda is still in business and they sent me a manual when I asked about 10 years ago; maybe they have spares.It might be possible to make up a replacement for the LAS-3700 using a dual op-amp, a current reference and a few other parts.These are not bad supplies, they'll take a lot of abuse and keep on truckin' - I wouldn't give up yet.

author: cnqhdszq
author_rank: Regular Contributor
timestamp: April 29, 2020, 12:30:11 pm
content: Are you sure ?If you spend more 5 minutes on my schematic and your's,you will find them are the same .When I repair my LQD-422-V ,I had no service manual or schematic in my hand ,So I reverse the board .At the beginning I even thought the "FBT-00-128" is LM723 .One day a member send me the same files you post .(sorry for my poor english,but I really want to be helpful )

author: coromonadalix
author_rank: Super Contributor
timestamp: April 29, 2020, 03:05:21 pm
content: @cnqhdszqYou miss some parts in the power stage section vs the lqd-421, sure it has some parts count differences since its not the same (model LQD-422)But the service manual is found and postedThe OP will need to do some check ups as mentioned in previous threads and report back to us

author: BigJohn89
author_rank: Newbie
timestamp: April 29, 2020, 08:58:42 pm
content: Thank you everyone for your help and suggestions! I've been plugging away at it for a while today, and wanted to report back a few things:I have checked the resistance across the control pots on both sides, between RP and +V, and the pots seemed to work properly, though I noticed the resistance creep up even when I wasn't turning the pots. I'll dig into that further, but since I get the same response out of both sides, I am not sure if it is having much of an effect on the side that is not working.I've checked the test point voltages, as well as others listed in the schematic:va = 40.2vvb = 10.4vvc = 27.22vFor IC101 (this is the side that doesn't work, IC401 is the side that is working):Between pins 10 & 1 = 15.27v (IC401 = 15.25v)Between pins 9 & 10, which is also across the two mylar caps on the schematic (listed as 8v) = 9.5v (IC401 = 8.7v)Across C102, which on the schematic should be 15v (and is pins 10 & 1, I just realized) = 15.27vI am confused, because on the schematic there is a 12v designation on the connection along the top going into R101 - what is that 12v in respect to? Is that between that point on R101 and ground? I didn't check that because I didn't want to unintentionally cause more issues with this thing, so I figured it would be best to ask first.I'm going to look further into the datasheet on the IC101, and see if I can find a way to further test it. But if you have any other ideas, I will be very grateful if you shared them with me.Thank you again for all of your help on this so far!JohnEdit: Oh yeah, forgot to mention testing between A16 and A20 came up with just over 5v between the two points. Thanks!

author: duak
author_rank: Super Contributor
timestamp: April 29, 2020, 11:06:52 pm
content: Here's some info and links to more data on the LAS3700:https://www.eevblog.com/forum/buysellwanted/free-for-local-l-a-pickup-lambda-lqd-422-dual-output-power-supply/The voltages aren't insanely wrong. I've got to think about how the LAS3700 works some more. BTW, is it warm or hot to touch? Apparently it has a heater inside to stabilize the reference voltage.I'll bet the resistance rises slightly because capacitors in the circuit are charging.I think the 12.0 V is referenced to the +output terminal. In floating regulators +V output is the pivot that everything is referenced to. The typical non-floating supply uses the -V output.

author: cnqhdszq
author_rank: Regular Contributor
timestamp: April 30, 2020, 01:01:37 am
content: duak, Thank you very much !Finally ,I know what the FBT00128 is ,thank you .

author: duak
author_rank: Super Contributor
timestamp: April 30, 2020, 03:27:37 am
content: Are all the jumpers on the terminal strip on the back the same between the sub-units? Are any missing or different? Is there a connection from one to the other?Is U101 warm or hot to the touch? The voltage between U101 pins 9 & 10 is high. With the power off, what is the resistance between them?Lambda supplies usually cut the output voltage off when the current limit control is set to minimum. If the control is not set to minimum, there could be something wrong with the current limit circuit.With no load and current limit to maximum, what is the voltage across R126? With the negative lead of the meter on +V, what is the voltage on IC101 pins 1, 6, 7 & 8? What is the voltage on IC101 pin 2 as the voltage control is varied?

author: BigJohn89
author_rank: Newbie
timestamp: May 01, 2020, 09:33:57 pm
content: Hi Duak,For the jumpers on the back, they are all in place in the standard configuration, with +V jumpered to +S, and -V to -S, as well as a 3rd jumper connecting two points labeled as ICS on the board.On both sides, IC101 (I think that is what you were referring to - are U101 and IC101 the same?) is definitely warm to the touch, I didn't have a temp probe so i couldn't get exacting figures though.The resistance between pins 9 and 10 was 7.41k, the working side checked out similar as well.With no load and max current, the voltage across R126 was .6 mV. I need to recheck the working side, because I didn't have a figure there...Here are the other checked voltages, working side is in the right-hand column.P1 = -2.483v = -2.681vP6 = -.3mv = -.4mvP7 = .4v = 357.8mvP8 = -.4mv (yes, negative) = 7mvP2 on the non-working side was .516v straight through the scale, with current limit set to none and maximum. The working side was .512v through the scale, unless the limiter was up a small bit, then it dropped to 46.8mv.I'm with you on the current limit circuit as well, and with pin 8 being a negative voltage, I'm going to start working my way back from there to see if I can test out components along the line. It appears I may be looking at the Q1 - Q2 - Q5 transistors, would you happen to have any advice - or am I barking up the wrong tree?I totally appreciate the assistance you are giving me, and can only hope to pay it forward some day!Thank you,John

author: duak
author_rank: Super Contributor
timestamp: May 01, 2020, 11:40:08 pm
content: Hello John,No hay problema.Hmm, the manual I have calls it IC101. I think I got it from here:http://bama.edebris.com/manuals/lambda/lqd420If you're postive that the voltage across R126 is negative, it could be a clue that current is going through it backwards. According to Table 1, R126 should have a value of 0.3 ohms - do you measure that?With the negative lead of the meter on the +V output terminal, what is the voltage on IC101 pin 5? Q101 base, Q5 base and Q1 or Q2 base? IC101 pin 5 should have about 3 V and as you work through the chain the voltage should drop by about 0.6 V every time it passes through a part.Cheers,

author: BigJohn89
author_rank: Newbie
timestamp: May 03, 2020, 05:42:48 am
content: Hi Duak,Thank you very much for your advice and taking the time out to assist me in this!I think I made some progress, and added a little confusion for myself as well! I have verified twice that I have an LQD-421, and the schematic reflects that - so far. Now, going into the base of Q101 is a diode, CR124 (FBL-00-105) which is also connected to one side of R134 as well (at least on the schematic). On the schematic however, Note 13 states that CR124 should be jumpered (and not present) on the LQD-421, and that R134 should be a 1.8K resistor. In real life, CR124 is present and R134 is missing completely. This is also the same on the other (working) side as well. I have tested this diode in circuit (not the most reliable way, I'm sure) and got completely different measurements from the same diode on both sides. I also tested CR138 on both the working and non-working sides and got very similar readings as the diode on the working side. In those cases, I received a forward resistance, however reversing the polarity gave me an open circuit. CR124 on the non-working side gave me resistance with both polarities, with numbers that were jumping all over the board (usually 3-ish Mone way, and 700K the other). I haven't pulled it yet, as I wanted to research further to make sure I wasn't missing anything else first - that and I don't have a suitable (or know of one yet) replacement anyway.That being said, I checked R126, and the value on both working and non-working sides is 1.1.I went ahead and checked the voltages, here they are for both sides (non-working, then working):P5 = -2.52v then slowly moved to -2.477v and stayed steady | 2.448vQ101 base = Testing here caused my meter to go "flat", it kept blinking but no voltage. | 2.065vQ5 base = -135mv | .926vQ2 (or Q1) base = -134.5mv | 279mvI also tested the voltage at the junction of C119 and CR124, which tested at -2.474v. The same junction on the working side tested at 2.43v.Thank you again for any assistance you (or anyone else reading this) can provide!John

author: duak
author_rank: Super Contributor
timestamp: May 04, 2020, 04:07:05 am
content: John, I've got some good news and some bad news...I think IC101 has expired. Pin 5 should have about 3 V relative to +V output terminal to cause the pass transistors Q101, Q5, Q1 & Q2 to conduct and pass some current. Looking at the Functional block diagram or the test circuit shown in the above files I see that both the V and I amplifiers can cause Pin 5 to have a voltage on it. The V amplifier's + input is higher (0.512 V) than its - input (-0.3mV) so its output should be high. Likewise, the I amplifier's + input is higher (357 mV) than its - input (-0.4mV) so its output should be high too. Logically, pin 5 should be high too, but it's not.According to the Catalog Pages info, U101 is supposed to heat itself to 115 C. Is it hot? Careful if using a digital thermometer (finger).Clutching at straws now, the voltage between U101 pins 9 & 10 is high. It may be that the resistance between the pins is too high. Try comparing the value between the two channels to see if they're different. With any luck, either R101 or R102 is bad and this is why U101 acting up.If all else fails, it should be possible to replace U101 with a current reference and a couple of op-amps. It would even be possible to use a few transistors with some loss in performance, but it would be useable.Cheers,

author: coromonadalix
author_rank: Super Contributor
timestamp: May 04, 2020, 02:08:03 pm
content: If ic101 is dead after all the tests, the remaining circuit could be used with an lm723, it seems compatible with some drawings and pcb reworking

author: duak
author_rank: Super Contributor
timestamp: May 04, 2020, 10:41:49 pm
content: Using a 723 is a good idea. I would use two; one for the V reference & amplifier and another for the I reference & amplifier. Their "Freq comp" pins would be tied together with a diode to allow one to override the other and reduce drive to the pass transistors. The decision to be made is which of the amplifiers, V or I overrides the other.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 05, 2020, 01:36:35 am
content: Each time i see this psu, i'm seeing the lm723 in to-100 case, it is still sold today and strangely compatible in some ways .... you have tons of schematics floating around and im sure this psu could be converted to a more recent design, while keeping a vast majority of parts.

author: BigJohn89
author_rank: Newbie
timestamp: May 05, 2020, 12:36:24 pm
content: Thank you both very much! I did run some more tests on the device, R101 was a little low at slightly over 5\$\Omega\$, while the working channel was closer to 7, but using R102 to compensate made no change in the voltage between pins 9-10 or coming off pin 5 whatsoever. I did pull and test CR124, and while I had suspected it originally, it tested out fine. So, at the end of the day, it does look like the FBT128 is a gonerSadly, my engineering skills are much, much worse than my repair skills, so I have a lot of reading ahead of me to figure out how to make this work with the lm723! If you have any advice, I am always open for suggestions. It would be good to not only bring this device back from the dead, but I'm sure there are plenty of others like this one out there, so this may help keep those out of landfills as well.Again, thank you very much, I really, really appreciate all of your help on this!John

author: coromonadalix
author_rank: Super Contributor
timestamp: May 05, 2020, 02:45:53 pm
content: I do understand you want to keep it genuine, it can be doneI have personally designed and built some lm723 based psu's ... hacked some pyramid 12v power supplies ...I dont know if someone has made a FBT-00-128 clone ?? or a small pcb ??The problem is the fbt-00-128 aka LAS3700 contain the cc and cv crossover circuitry ...

author: Kosmic
author_rank: Super Contributor
timestamp: May 06, 2020, 12:01:20 pm
content: You can find some sellers on ebay carrying what look like NOS FBT128. 10$ per chip. It's unclear if they are really new or recycled parts or just fake.Coincidently, one channel of my LQD-421 just died this week. I traced the problem back to the FBT128. Ordered some from ebay. I should be in measure to tell you if they are really new parts in a month or 2

author: coromonadalix
author_rank: Super Contributor
timestamp: May 06, 2020, 02:22:53 pm
content: Apparently theses chips / ics heat a lot while working, that must be the failure reason over time

author: duak
author_rank: Super Contributor
timestamp: May 06, 2020, 06:02:58 pm
content: I've attached a conceptual replacement for the LAS3700 chip using two 723 regulators. The left side shows the Voltage Control section where the 723's reference is basically multiplied up by the ratio of the front panel pot's resistance to RVscale. If RVscale is replaced by a current source, the regulation would improve because the closed loop gain would not change as much as the voltage setting was varied.The right side show the Current limit section. The error amplifier in the 723 compares the voltage across the current sense resistor R126 to a variable threshold voltage generated from the 723's reference voltage and the existing circuitry. When the sensed current exceeds the threshold, the amplifier output goes low and through Dclamp pulls the Voltage control output low, reducing output current until an equilibrium is reached.The advantages of using 723s is that they have a current source for a load for the error amplifier making wired-OR circuits easy to implement. They also have voltage references in the right range. Their error amplifiers probably have similar voltage gains and gain-bandwidth products to the LAS3700.OTOH, for best performance, it'd be better to use current references and perhaps op-amps instead of the 723s although there will be more details to plow through. In particular, the combining/crossover circuit and feedback stability.I see from the LAS3700 catalog data sheet that it operates at 115 C (!!!) due to its internal heater. I'm sure it'll be stable but maybe not so reliable. I've got at least one Lambda supply that uses this chip so sooner or later I might have to replace it.Cheers,

author: coromonadalix
author_rank: Super Contributor
timestamp: May 06, 2020, 08:56:34 pm
content: @Duak that seems doableThere is a project like this ideahttp://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htmThe same version pumped up to 10 amps in attached picture, could be adjusted for less amps.But its not designed as @duak suggestedTo keep a constant current control and or the constant voltage leds / operation there are op amp based psus with the "or function"see this schematichttp://www.seekic.com/circuit_diagram/Power_Supply_Circuit/Constant_voltage_and_constant_current_regulated_power_supply_circuit_diagram.htmlyou see the two diodes at the op amps output, it the "or function" switching between cc and cvOr this one with relay switched xformer secondaries, some circuitry could be removed and you could still have the cc cv leds ...http://www.seekic.com/circuit_diagram/Power_Supply_Circuit/High_precision_Constant_Voltage_And_Constant_Current_DC_Power_Supply_Circuit.htmlThe major problem with lm723 designs, they wont go down / less than 1.25 volt for the voltage output, this has to be negated with an -1.2 volt supply in the voltage adjustment potentiometer ...

author: duak
author_rank: Super Contributor
timestamp: May 06, 2020, 10:10:41 pm
content: @coromonadalix, the second supply looks very good. It uses the Harrison/hp/Lambda floating regulator principle with a slightly different way to handle V-I crossover. It also has a floating auxiliary supply that circumvents the 723's 2 V minimum output voltage problem.On a different note, I was trying to think of any differential input amplifier chips that could have their outputs tied together wired-OR style and remembered that the LM393 comparator has an open collector output. While many balk at the thought of a comparator used in the linear mode, as long as it doesn't have built-in hysteresis it should be useably linear. It's got a reasonable amount of voltage gain (50,000 min) from a less linear VAS with two poles - hopefully the existing compensation networks will work with some adjustment. The lower gain and the fact that it was designed as a comparator may allow it to crossover between voltage and current limit modes more cleanly than an op-amp might. If I didn't have so much yard work to do, i'd try it out now.

author: BigJohn89
author_rank: Newbie
timestamp: May 08, 2020, 08:30:09 pm
content: Thank you all again, very much! You have given me a lot of stuff to read and to learn from. I'll admit that I went the route Kosmic did, as I found some of 128's online and took a gamble on ordering those first. If this part doesn't work, I am going to see about hoping I can work out a good drop-in replacement circuit using the LM723. If I can keep this PSU out of the dumpster, and help others keep theirs out as well, we'll all be better off!Thank you again for all of your help - you guys rock!

author: Kosmic
author_rank: Super Contributor
timestamp: July 01, 2020, 03:58:38 am
content: Quote from: Kosmic on May 06, 2020, 12:01:20 pmFinally received the two FBT128 I ordered on ebay. They look brand new. Was produced in 2004 ?And as I expected, after replacing the FBT128 on channel 1 of my LQD-421, it's now fully operational.Old one on the left, new one on the right.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 01, 2020, 04:26:55 am
content: Hum since this puppy is heating, would it be wise to put an heat sink on itCongrats for finding theses parts and do a succesful repair

author: Kosmic
author_rank: Super Contributor
timestamp: July 01, 2020, 03:32:15 pm
content: Quote from: coromonadalix on July 01, 2020, 04:26:55 amIt's an oven. The goal is to keep the voltage ref at a constant temperature. If you put a heatsink on the chip you are probably going to destabilize the whole thing.

author: duak
author_rank: Super Contributor
timestamp: July 01, 2020, 10:45:25 pm
content: Good-oh! These are actually nice supplies that take a lot of abuse.I'll echo Kosmics comment. These chips are supposed to run hot! The LAS3700 catalog data sheet says 115 C (!!!) due to its internal heater. More than I would have thought.

author: BigJohn89
author_rank: Newbie
timestamp: November 21, 2020, 06:58:49 pm
content: Hi Everyone! My apologies for leaving this one hanging open, I ended up really sick for a long while, and am only just starting to get back into things. I did get a replacement FBT128, and put it in. Everything seemed to run better - I was able to adjust the voltage through the scale, however when I turned up the current limiter, I smoked the pot for the voltage selector. I spliced in a temporary pot to allow me to switch voltage and re-run tests, and as soon as I nudged the current limiter again, the new temp voltage pot smoked. Before I start tearing this down to check everything, I was hoping you may have some pointers to send me off in the right direction. Or, is this a sign of a bad FBT128? I ordered two, so I can replace it - but if it is a failure somewhere else, I would rather know that first before replacing a possibly still good new part.The other channel works like a charm, so at the very least I can still use that one. Thank you all very, very much for your help on this!BigJohn89

author: djerickson
author_rank: Regular Contributor
timestamp: February 19, 2021, 01:24:05 pm
content: I repaired a Lambda LQD-422 a few years ago. I just put together a web page discussing the details of the repair.I was also able to fix the offset errors on the digital readout. It looks like this was a design problem on early units.http://www.djerickson.com/lambda/Cheers!Dave Erickson

author: coppercone2
author_rank: Super Contributor
timestamp: September 21, 2022, 04:24:18 am
content: fixed mine finally, the 5V reg was low (this caused the display to fluctate and be dim). If you have a bad display thats dancing, be sure to check the power rail. I had both capacitors and a bad power rail, so replacing caps helped some, but restoring the power fixed it totally. I thought the reference was bad at first, and that the LED grew dim over the years, which was not the case.