Author Topic: Lambda LLS6120 (repair part 2) Help needed  (Read 567 times)

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Offline rodd

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Lambda LLS6120 (repair part 2) Help needed
« on: July 21, 2017, 01:27:24 AM »
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
 

Offline Armadillo

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #1 on: July 21, 2017, 01:50:14 AM »
Have you checked the diac D101?
 

Online duak

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #2 on: July 21, 2017, 04:04:33 AM »
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
 

Offline rodd

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #3 on: July 21, 2017, 07:56:45 AM »
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
 

Offline rodd

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #4 on: July 21, 2017, 08:52:32 AM »
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
 

Offline Kjo

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #5 on: November 23, 2017, 08:17:37 AM »
@Rodd
Did you ever find schematics to the front panel boards?
 

Offline Armadillo

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Re: Lambda LLS6120 (repair part 2) Help needed
« Reply #6 on: November 25, 2017, 10:20:31 PM »
ehm!. no response, probably preparing for world cup! :-//
 

Offline Kjo

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Re: Lambda LLS6120 (repair part 2) Help needed (LLS6018)
« Reply #7 on: December 06, 2017, 04:38:17 AM »
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....


 


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