Lambda LLS6120 (repair part 2) Help needed - Page 1
Forum section: electronics / repair

author: rodd
author_rank: Regular Contributor
timestamp: July 20, 2017, 03:27:24 pm
content: Hi folks,The is the continuation of my previous post.I decided to open a new one since I am focusing a different problem.The primary stage of this supply won't oscillate.I have already checked Q101, Q102, Q103 and the components around them.Then I removed IC101 and latter Q103, but no response at all.The voltage @ Q101 is abou 340V ( I don't remember the exact value now).I tried to understand the schematics but I could not figure out how this works, since it appears to me that there are 3 transformers windings in series: SR101, T201 and T102 and a "short" between SR101 and T102.Any help would be very appreciated.Roger

author: Armadillo
author_rank: Super Contributor
timestamp: July 20, 2017, 03:50:14 pm
content: Have you checked the diac D101?

author: duak
author_rank: Super Contributor
timestamp: July 20, 2017, 06:04:33 pm
content: I hadn't realized until after I submitted my reply to the other posting that this circuit is self-oscillating. Since the switching devices Q101 & Q102 are naturally off and aren't biased on by anything there must be a starter circuit to get oscillation going. It looks like R107 & R108 charge C115 to some voltage when Q103 is turned off. When the voltage on C115 reaches Diac D101's breakdown voltage (30 V??) it conducts and generates a pulse to Q102's gate thru R106 to kickstart it.I would check the voltage across C115 - if it's much less than say 30 V, check out R107, CR12 and Q103 to see if they are holding the voltage down. With U101 removed, A103 will be on and there won't be a starting pulse. I'd then check the the gate of Q102 to see if there's any voltage whatsoever either static or a pulse. I could see that any one of the parts in the circuit could be bad and prevent starting. If the diac is bad and can't be bought easily I believe old fashioned light dimmers use them to trigger the triac.If you want a better description of how these things are supposed to work (in general) try looking for "self oscillating half brdige converter". Simplified versions are used quite a bit in lighting power supplied and ballasts.Cheers,Duane

author: rodd
author_rank: Regular Contributor
timestamp: July 20, 2017, 09:56:45 pm
content: Hi Duane,Thank you for your suggestions.I will try them as soon as I have some time to go back to the bench.Now that I have the right topology, I can Google it and learn more about the circuit. I have spent more than 4 hours trying to search for circuit references without success!Regards,Roger

author: rodd
author_rank: Regular Contributor
timestamp: July 20, 2017, 10:52:32 pm
content: The diac is a 1N5761A, which seems to be opened.I tested it with a bench power supply in series with a mA meter.I went up to 40V and no current was passing through it.It will take some days until the ordered parts arrive.I will double check my junkbox again for possible replacements.Roger

author: Kjo
author_rank: Regular Contributor
timestamp: November 22, 2017, 09:17:37 pm
content: @RoddDid you ever find schematics to the front panel boards?

author: Armadillo
author_rank: Super Contributor
timestamp: November 25, 2017, 11:20:31 am
content: ehm!. no response, probably preparing for world cup!

author: Kjo
author_rank: Regular Contributor
timestamp: December 05, 2017, 05:38:17 pm
content: I'm not sure the OP ever solved his problem.... But rather than starting a new thread for my LLS6018, I think it better to expand this one since all LLS6000 supplies are very similar. A brief review of these Lambda LLS supplies is in order before diving into the problem I had with an LLS6018. The LLS series is from the 80's & 90's. Lambda made 7 different classes of LLS from the LLS3000 to the LLS9000. While the internals varied, the external form was quite similar. The primary reason for the different classes was efficiency. the LLS3000 were ~45% while the LLS9000 were over 90%. Interestingly they all appear to use the same front panel control. I have attached a Front Panel schematic from the LLS5000 class, as it looks identical to the one in my LLS6018.WORD OF CAUTION!!! Working on these supplies can kill you. DO NOT debug without using an isolation transformer! It allows you to connect test equipment safely and to make ONE mistake with your finger. (Two mistakes, shame on you!)On to debugging my LLS6018 and hopefully helping the OP with his. This supply died on power-up. No lights, no fault indicator, no output. Just a relay click inside. These are not easy to work on because the two internal PCBs are back-to-back with a PCB 2 PCB pin connector that is hard to bypass. Also, at least in my case, the schematics available are for a version of the power PCB that is just a bit older and slightly different.I have attached my version of the schematics for the rectifier block and the fault logic block. First, I want to go through the function of IC101, as there seemed to be some confusion from the OP on what it does. It is indeed a quad comparitor of the LM339 variety. There are 5 functions provided by the comparitor circuit. First is IC102 (U2) shunt voltage regulator. R114/115 double the internal 2.5V ref to create a 5V reference bus that goes to the inputs of all 4 comparitors.Comp-A is a HVDC bus failure detector. Pin 6 measures the HVDC bus, and if it falls below ~190VDC pin 1 switches high and turns on Q103. Q103 clamps off the startup trigger DIAC and Q102 gate. This stops all self oscillation and transformer action, shutting down all secondary power generation. I think this was implemented to insure that the LLS6000 would turn off if the mains drop below ~70VAC.Comp-B is an overvoltage/overtemperature fault latch. The 5V ref bus is tied to 2 crowbar SCRs optically coupled to the analog PCB to sense PCB temp and the FP OV pot setting. If triggered, the SCRs clamp the ref bus to common, pin 2 goes high triggering Q103 and CR112 creates hysteresis that latches the fault condition until power is removed.Comp-C just drives the FP LED indicating that a fault is present.Comp-D controls the start-up behavior. As Duak noted, DIAC D101 sends RC timed pulses into Q102 to initiate self-oscillations in Q101/Q102. If the supply can self-oscillate, there had better be something to stop the DIAC from tripping Q102 or there will be chaos! That is the job of comp-D. If Q101 can oscillate, it sends charge pulses through CR108 to charge C108. When C108 reached ~160VDC pin-13 goes low and CR109 clamps off the RC charging end of the DIAC. It will no longer fire and the self oscillation of Q101/102 is insured.On to my problem....With normal 120VAC input the LSS6018 was dead but I measured 170VDC of the HV bus. (This before I fully understood its operation). The fault light was off, vref=5V, and IC101-A was on, stopping oscillations. On a hunch, I used a variac to allow changing the HVDC bus voltage. (I really didnt know the proper HVDC voltage at this time, though I should have figured it out.) Above 200VDC the oscillator started and the supply limped into action. Though the analog unregulated voltages were not ideal. It was clear that the HVDC bus was suppose to be much higher. (Actually over 300VDC!)Then it dawned on me... this is a dual voltage range supply. It was behaving on 120VAC as if it was set for 240VAC! (The main power converter uses a conventional voltage doubler on 120VAC controlled by a slide switch. (The available schematics dont show my revision of the doubler accurately. See attachments). I took out the back panel selector switch and found it rather flaky. I just soldered in jumpers, as Ill never plug it into 240VAC. Still the doubler was not working. After considerable time making continuity checks on the PCB traces, I could see that the common node of C105/107 (main power capacitors) was isolated from all other traces!!!After pulling C105/107 it quickly became apparent that one of the plated-thru holes on the common node had failed and isolated the common node from R103/104/S101. A jumper wire quickly fixed this and the supply fired up like new! I suspect that the C105/107 lugs, when pressed into the PTHs caused some damage that took 30 years to show up.Moral....Good schematics and right-to-repair are vital. Schematics should have at lease nominal voltages if not component values.Two failures can be more than 2X problem to find.One often assumes it is component failure, and that can lead down unproductive paths.I hope the OP comes back with a progress report....

author: rodd
author_rank: Regular Contributor
timestamp: June 30, 2018, 12:16:06 am
content: Quote from: Kjo on November 22, 2017, 09:17:37 pmHi,Sorry for the delay to answer.I have abandoned this repair attempt for a while.Answering Armadillo, unfortunately I haven’t come across the front panel schematics yet.I will post if I find it.Enviado do meu iPhone usando Tapatalk

author: zmetzing
author_rank: Contributor
timestamp: January 02, 2019, 09:29:04 pm
content: I'm currently working to repair a LLS-5018 supply. I'll post some pictures when I figure out where the failure lies. Right now, I've measured a good HVDC bus (>300V) on J103/J203 pin 4, but no +5VDC for the front panel logic.I'll continue to diagnose after I replace the 4A fuse I blew by putting my o-scope ground on the wrong pin.

author: zmetzing
author_rank: Contributor
timestamp: January 11, 2019, 06:13:02 am
content: Well, I know _why_ it doesn't power up, but I'm puzzled as to the exact cause. When 10VDC is applied at the "low voltage" side of R265 (a 1-2W 150k),simulating the voltage that would be seen after the leakage bootstrap resistor, that part of the circuit draws 9mA. Since 345 / .009 ~= 38k, this is an impossible startup current.What puzzles me even more is that I can't locate anything that might be drawing that kind of current, even after replacing C232 for good measure and disconnecting R261 (gate drive resistor). The IMLLS5000.pdf, available on the TDK-Lambda website, also shows an IC211 op-amp being fed by the leaky resistor bootstrap. However, this seems to have been deleted on my version of the power supply.Edit: IC211 was replaced by Q204, a transistor marked "Motorola L199 K8805", which I found via ahttps://www.wbparts.com/rfq/5961-00-930-5326.htmlcross-reference to be a 2N3702. Not having one of these, and looking at the heat discoloration of the PCB under nearby R264, I decided to replace it for good measure. I substituted a PN2907A, which is a PNP with equal or slightly better ratings. Collector goes to local GND, base to the resistor divider described in the next post (later found to be faulty), and emitter pulling down on the COMP input to disable the switching regulator during a low HVDC bus.

author: zmetzing
author_rank: Contributor
timestamp: January 12, 2019, 07:14:56 am
content: .. and the winner is a cooked R276 resistor in the HVDC low-supply sense path. This resistor is 150k and forms a voltage divider with R277 (4k), and pulls 2.2mA. That gives us 3/4 of a watt across a resistor which looks like about a 1/2 watter to my trained eye.Replaced with two 100k (1W) in series that I happened to have on hand. Supply works again!Edit: Yes, 200k > 150k, but the point of this resistor divider is to raise the voltage at the base above (COMP - 0.7) to stop the transistor from pulling COMP low and disabling the switcher (UC2845N). As long as COMP is above 1.4 volts, the internal PWM control will be enabled and the switcher will be live (see TI datasheet figure 12), so the base of the PNP must be above 1.4 + 0.7 or 2.1 volts.