Author Topic: Valhalla 2701c schematics + firmware V3  (Read 23623 times)

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Offline RaymondMack

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Re: Valhalla 2701c schematics + firmware V3
« Reply #75 on: August 22, 2021, 08:09:53 am »
I played around with the Tina-TI simulations again and found a few mods that look interesting.

Blue:   C6,7,8 = 1uF, C4 = 0.5uF (original filter, fastest settling time)
Green:  C6 = 1uF, C7= 2uF, C8 = 0.5uF, C4 = 1uF (lowest LF noise)
Maroon: C6,7 = 2uF, C8 = 1uF, C4 = 0.5uF (sharpest attenuation)
Gold:   C6,7 = 2uF, C8 = 0.5uF, C4 = 1uF (slow, but decent LF noise and attenuation)


Both the LF356 and OPA189 have the lowest LF noise with the "Green" filter (swapping C8 and C4 values and increasing C7 to 2uF). This also has equal or better attenuation than the original filter but has about 67% longer settling time.

The "Maroon" filter increases C6,7 to 2uF and has slightly (72%) longer settling time and more LF noise but the sharpest attenuation.

The Gold filter has the longest settling time (twice the original filter) but sharper attenuation and nearly as good as noise performance as the "Green" filter.

The primary downfall of each filter mod is the increased settling time, but since the 2701Cs are so noisy I think they might be worth testing out.

Time to reach 99.999% of input
Blue:   2.45s (original filter)
Green:  4.08s, delta = 1.63s
Maroon: 4.20s, delta = 1.75s
Gold:   4.89s, delta = 2.44s


------------------------Attenuation-----------------
Filter |  -10dB   |  -20dB   |  -40dB   |  -80dB   |
-------|----------|----------|----------|----------|
Blue   | 2.32 Hz  | 5.74 Hz  | 13.03 Hz | 54.97 Hz |
Green  | 1.30 Hz  | 3.54 Hz  | 11.43 Hz | 48.85 Hz |
Maroon | 1.19 Hz  | 2.75 Hz  | 7.68  Hz | 36.14 Hz |
Gold   | 1.03 Hz  | 2.54 Hz  | 8.38  Hz | 40.19 Hz |
« Last Edit: August 22, 2021, 08:20:24 am by RaymondMack »
 

Offline Electrole

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Re: Valhalla 2701c schematics + firmware V3
« Reply #76 on: August 22, 2021, 10:32:56 am »
The noise simulations are very interesting. They definitely show that the filter design is not just about suppressing PWM-noise, but one should also keep an eye on the noise generated by the filter itself. The simulations also suggest that despite the low current noise of the OPA134 the voltage noise kicks in below some 10 Hz. The LF356 seems to have more noise above 10 Hz which could explain the improvements observed when replacing the LF356 with the OPA134. The OPA189 is a cool op-amp, but it's not available in a leaded 8-pin package, so I don't know if it's worth the effort required to put it into use in the 2701C. Personally, I would just settle for the OPA277.

It's also interesting that Le_Bassiste's 2701C wasn't equipped with any EMI measures. In my unit there were already toroidal inductors installed for each of the 8 terminals. There are inductors shown in the Rev J mechanical drawing, so these are presumably added at one point. Unfortunately the assembly drawing reveals no details on these ferrites so I took out one and measured it: It reads close to 5.39 µH + 0.10 Ohm at 100 kHz, and 5.20 µH + 7.55 Ohm at 1 MHz. The 50 µH ferrites used by Le_Bassiste may be an even better choice. If others have a 2701C with no ferrites added, it will be a good idea to add some. I don't think the exact value is essential, but I would go for ferrite inductors that are lossy over a wide frequency range rather than inductors with a high Q. At one point I also considered adding common-mode inductors inside the 2701C but eventually I went for an external filter.

Regarding the buzzing noise from the high-voltage transformer: I have a bit of that too, and the level of noise depends on the voltage setting, which changes the current draw and the firing angle of the switching controller. I do not notice the noise, not even with the lid off, so the transformer in my unit may simply be more quiet.

It may be an idea to replace the ICL7650S with an LTC1052. Again, it will not do anything to the wideband noise issues, but if the stability and the low-frequency noise are improved, then why not. I would also consider replacing the socket with a better model, or solder IC5 directly into the board. I will not have time to modify the 2701C any further right now, so I will be looking forward to seeing results from a shoot-out between ICL7650S and LTC1052  :-)

Finally, a note on CE-marking and EMI/EMC: My 2701C does not have any CE-marking, neither does the unit shown on Valhalla's home page. The question is then how much consideration Valhalla gave EMC when designing the 2701C, and how much testing of EMC Valhalla has carried out. I suspect there could be a link between the noise issues we see and potential EMC issues. Without the CE-marking Valhalla is not even allowed to market or sell the 2701C inside the European Union, as far as I understand the legislation. Perhaps that's why there are no links to sales representatives abroad on the Valhalla home page?
 
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Offline 1audio

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Re: Valhalla 2701c schematics + firmware V3
« Reply #77 on: August 23, 2021, 04:23:42 am »
RE EMC/EMI- I have a very early 2701 (pre-a). and it has almost no bypass caps on the digital circuits. Mine had chunks of drafting vellum as insulators for the brackets at each end of the PCB. They had broken down and were arcing at higher voltages. I get the feeling that this may have been someone's PHD or MSEE project that morphed into a business. There are aspects that even a junior engineer would have done (like the caps) that were not. It also has plastic top, bottom and front panels. Nevertheless it seems to work pretty well.

Maybe they were reluctant to make too many changes which would have needed a lot of verification on performance.
 

Offline quarks

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Re: Valhalla 2701c schematics + firmware V3
« Reply #78 on: December 21, 2021, 09:59:05 am »
today I inspected my 2701C and found an annoying clicking noise
most likely I just did not notice it before, but now I used a Stethoscope to verify where it is coming from
Transformer TR201 and (as mentioned above) C201 are the source of the clicking
replacement of C201 with a new Ceramic cap did not help, therefore I tried a MKP and now the noise of the cap is gone
only TR201 noise is still there, but it is very low compared to the before C201 ceramic cap clicking
 

Offline 1audio

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Re: Valhalla 2701c schematics + firmware V3
« Reply #79 on: December 21, 2021, 03:48:16 pm »
many large value ceramic caps make noise and change value with voltage. Same stuff inside are used as tweeters (Motorola Piezo) as well.
 

Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #80 on: February 25, 2022, 02:54:02 am »
I noticed that you started the thread in 2017! What do you think is wrong with your calibrator?


 Spurred on by the very helpful replies here gave me the confidence to tackle this repair. Thanks for your shared experiences.

 So my earlier thought that the HV MOSFET string was the problem as it was getting very hot turned out to be only a symptom of the actual failures.
After replacing the entire HV transistor set TR6-TR12 with no fix, and observing that 1600 V was being applied across the HV transistor string it was no wonder they were getting hot.
Also RLJ was cycling on and off every 1 to 2 seconds.

 So the next tests were static measurements for any obvious component shorts wrong values etc from the HV rectifier output to the output control CCT. Nothing stood out as failed.
Replacing the chopper IC5 made no difference. The reference cct measured okay too.
Moved onto IC1 and opto IC2. replaced both with again no fix  :(.

 Okay obviously being a bit slow on the up take! , I then checked the HV switcher transistors TR201 and TR202 out of CCT as trying to decipher the readings otherwise was not conclusive.
Upshot was TR202 had gone rather leaky C-E (not a full short or open CCT any terminal), and TR201 seemed okay. All other components in that CCT measured okay.
Note that TR201 was an MJE13005 with higher VCE ratings, not the 2N6499 specified in the parts list.
 Finding the 2N6543 used for TR202 proved fruitless from the usual trusted suppliers, so not wanting to take the risk on ebay etc I looked at suitable sub's. There are a number of possibles including the BUX80 which I had one to hand so I tried that plus a new MJE13005 just in case!.

 Result was no fix  >:(, still overheating TR7-12 with maxed out HV across them and RLJ slow cycling on off.
Measuring the output of IC1 showed no switching just ~ -13V, loop not functioning at all.
I removed IC 2 and put a shorting link across pins 4 and 5  to simulate the opto transistor turned full on. The theory was it would turn TR201 and 202 completely off.
The result was such that the HV output dropped from max ~1600V down to just under 100V with no clicking from RLJ or heat issues and now the output could be controlled from 0 up to 80 odd volts  :D.
Proving the chopper control was working.

All components around IC1 tested okay so why no feedback ??. I tested on the bench the opto coupler with the conditions found in the 2701 and found the opto was not turning on properly !, so upping the LED current a bit improved the switching and was still within the LF353 specs so I reduced R23 to 2K and used a CNY117-3 opto which has better CTR than the 4N25.
The result was a now functioning HV switcher with a steady 230V across the HV transistor string and output controllable from 0 up to 1200 V.

 In the end the culprit was TR202 which had failed for whatever reason and the hot output transistors and hot bleeder resistors R209-216 as well as RLJ cycling on/off were a result of the HV transformer being fully on.
The one thing that puzzles is the fact the opto feedback required such a change to the LED drive current to get the switcher working again. Must be a marginal design to start with. Also not what you would call fail safe with MAX HV  output  :o.

 I am in the process of measuring the HV switcher with a MicSig HV diff probe and will post some pictures showing various conditions of TR202 C-E wave forms with no snubber then with the original snubber, and some further experiments with added snubbers across the rectifiers and HV transformer PRI. Inspiration to tinker taken from Electrole's write up (see link he posted earlier).
I will also do some of the suggested mods and use my HP3400 to measure the output RMS noise.
Will update thread as these things are done.
« Last Edit: February 26, 2022, 12:28:30 am by lowimpedance »
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 
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Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #81 on: April 29, 2022, 04:12:34 am »
Noise measurement update.
Readings taken on HP3400A. edit: connected to rear terminals on 2701c

2701c settings
20V range and 10V set.
Noise measured from +V to -V terminals -- 1.5mV RMS
Same level in operate or standby.

Set output to 0V for all ranges.
Noise measured was also 1.5mV RMS except on the 200mV range where the value dropped to 0.7mV
Connecting negative terminal to chassis results in appreciable increases in noise which varies depending on chassis connection point chosen.

Measured noise from any terminal to chassis in either operate or standby results in a value of 210mV RMS.

edit: Forgot to mention the 1.5mV RMS drops to 0.45mV RMS when the 2701c is turned off.

(I have some switcher wave forms to post , need to edit and annotate first)
« Last Edit: May 02, 2022, 05:26:42 am by lowimpedance »
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 
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Offline Le_Bassiste

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Re: Valhalla 2701c schematics + firmware V3
« Reply #82 on: April 30, 2022, 08:03:32 am »
Noise measurement update.
Readings taken on HP3400A.

2701c settings
20V range and 10V set.
Noise measured from +V to -V terminals -- 1.5mV RMS
Same level in operate or standby.

hm, seems extremely high to me. if i get it correctly, the 3400A has a BW of up to 10 MHz. did you accidentally pick up some short wave signals?
An assertion ending with a question mark is a brain fart.
 

Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #83 on: April 30, 2022, 02:20:03 pm »
It may be possible there was some form of interference , I'll see if I can run the test again in a screened environment.
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Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #84 on: May 02, 2022, 05:25:41 am »
Noise measurement update 2 - units placed in screen room (not mine !, but at work !!), so no external interference.
Only equipment powered were 3 DVMs, the 2701c and the HP3400A.
HP3400A input shorted at end of cable to be used for baseline noise level was < 0.1 mV RMS full bandwidth.
The 2701c and HP3400A were not plugged into separate power boards.

One important difference between this set of measurements and the previous was previously the rear terminals were used whereas the below measurements were from the front terminals.
A quick test on the rear gave 1.15 mV RMS on the 20 V range, showing the rear terminals do indeed have a higher noise level.

Noise measured at the front sense terminals on the 20 V range set to 10 V  in either operate or standby was 0.75 mV RMS
This level was consistent on all other ranges above 2 V except on the 120 V and 1200 V ranges where I only measured in standby, not wanting to apply high voltage DC to my HP3400A !
Note that when the 1200 V range was selected the needle on the HP3400A was notably more noisy.

On the 200mV range set at 0.1V the RMS noise voltage measured was 0.34 mV RMS in standby and 0.37 mV RMS in operate (O/P on).

Between +V out and case no difference in level from previous test.
Grounding -V to case also gave the same result as previous test.

edit: Also noting that moving cables or touching the 2701c or HP3400A cases did not affect the measured level.
 
« Last Edit: May 03, 2022, 12:53:04 am by lowimpedance »
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Offline 1audio

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Re: Valhalla 2701c schematics + firmware V3
« Reply #85 on: May 02, 2022, 09:06:42 pm »
I just did a quick check on my 2701 (pre letter) with the instruments immediately on hand.
2701      HP 3404   Fluke 8050
Standby  1.8 mV       .3 mV
1V          1.9 mV       .35 mV
10V        1.9 mV       .33 mV
100V     17.2 mV      7.5 mV

Note, the 3403 has a 100 MHz measurement bandwidth, the 8050 is 50 KHz so there is a significant difference to be expected. Later I'll string some long BNC cables across my shop and plot the noise spectrum for what its worth. Mine may be preproduction since it has essentially no bypass caps on the digital circuitry.
 
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Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #86 on: May 03, 2022, 02:10:07 am »
I ran a CAL cycle on the 2701c which went smoothly, cross checking on the three DVM's.
While performing the CAL on the 1200V range I noted a rather long settling time for the 1000V reading, (longer than Valhalla indicate), which is possibly thermal related ?
For other owners reference.

Additionally below are some screen shots of the main switcher wave forms across C-E of TR202 after repair, see post above.
First pic shows waveform with no snubber in cct. (R205 removed). note the huge spike at turn off.
Second shows 120V range with snubber cct. installed, original values plus additional HV transformer primary snubber. (O/P at 100V operate)
Third shows waveform when 1200V range selected. (O/P at 1000V operate)
Last pic shows what happens to the waveform when an additional 10nF is added to the snubber cap C215. (O/P on 120V range set at 100V operate), was just for demonstration purpose  and was  removed after the  pic. was  taken.
« Last Edit: May 09, 2022, 10:33:32 am by lowimpedance »
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline KubaSO

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Re: Valhalla 2701c schematics + firmware V3
« Reply #87 on: February 25, 2023, 06:02:35 pm »
I got the same beast, with similar signs of overheating here and there, with the HV switcher stuck full-on.

I really want to make it work reasonably well without designing out every single problem on the PCB. I’ve bit the bullet and decided to keep the TO-3 parts, in spite of the cost of suitable replacements. Historical accuracy and all  ;)

Here’s what I’m doing - will report my findings and details in a couple of weeks, after the parts come and I put it together:

  • Replace several parts in the HV switcher that were marginally rated. TR201, several resistors, add snubbers, rationalize the the TR201 C-E overvoltage and dV/dt protection.
  • Replace HV rectifier series 1N4007s with single properly rated diodes.
  • Replace HV electrolytics with film caps rated for 1.5kV or 2kV. Get rid of bleeder resistors there.
  • Install parts for the current source (IT2) option.
  • Clean up a few other things mentioned in this thread - filter response, mess around IC11, etc.

It looks like substitutes for everything for the current source can be found on DigiKey - even a matching heatsink. The relays are still being made!

Long term plan is to make it work, get it calibrated, and sell as soon as I find a suitable smaller replacement. That thing is monstrous.
 

Offline lowimpedanceTopic starter

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Re: Valhalla 2701c schematics + firmware V3
« Reply #88 on: February 27, 2023, 01:28:25 am »
If you make sure the switcher transistors are good then you can test by removing the opto  IC2 and place a shorting link at points C-D, (disables the HV drive to the Mosfet string), That will allow tests of the chopper up to approx 80V.
If that is good then I would look at the Opto drive current to make sure switching is happening.

As for the IT2 current option , do you have access to the appropriate firmware or are you going to try the one uploaded previously in the thread ?. Also keep in mind that when you do load the extra components you will need to exchange the cal NOVRAM IC103 p/n X2210 for a X2212 version to cater for the current option cal constants. The X2210 is not large enough, see parts list.

« Last Edit: February 27, 2023, 03:55:12 am by lowimpedance »
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Offline KubaSO

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Re: Valhalla 2701c schematics + firmware V3
« Reply #89 on: March 25, 2023, 03:40:32 am »
Thank you for the pointers. Yep, got the larger Xicor NVM, and will try with firmware from here - or just disassemble what’s there and add it. I’m willing to bet that the cal data for the IT2 option could be squeezed into the original NVM, although haven’t confirmed it yet. Those Xicor parts are getting rarer and rarer on the secondary market, so avoiding the need to replace them would be ideal.

Anyway, made some progress:

  • Replaced the awful 4x47uF electrolytic string and its balancingheater resistors with 9uF/2kV foil capacitor bank (16x0.56uF/2kV). 8 of those on top of the PCB, 8 on the bottom. They fit perfectly in the board space formerly occupied by the electrolytics and the heaters.
  • Replaced the 0.25uF output capacitor with 1.68uF/2kV (3x0.56/2kV).
  • Installed IT2 parts. R301 made using four 10Ohm/0.1% precision wirewounds (two string of two in series paralleled for lower heating).
  • Fixed the HV pre-regulator. TO-3 device was dead. BUX98A replaced it. Added snubbers and overvoltage protection (unidirectional TVS). Works as it should. Still have to replace the buzzing base snubber ceramic capacitor with a foil type - I’ve changed the value that was there but only had a similar dielectric ceramics on hand. Ugh.
  • Replaced the MOSFET gate potential string with resistors rated for the dissipated power, installed at an angle from the board, so they fit in the same footprint, just 3D
  • Replaced all electrolytics. They weren’t inspiring confidence. For everything but the 5V rail I got nice 105C types. The 5V rail was replaced by the same thing, just fresh. Probably that one was unnecessary, but hey, was ordering parts so why not.
  • Tantalums don’t inspire confidence either. Will replace with small electrolytics with 0.1uF ceramics in parallel
  • Replaced all 4N400x diodes in LV circuits with UF4007 to lower rectifier noise.
  • Replaced HV preregulator steering diodes with avalanche rated types. Replaced the series pairs of HV rectifier diodes with single 2kV rated ones
  • Made some quick checks on 1kV range against a 34401. Seems to be within 15ppm throughout the range. No calibration nor adjustments. Not bad.
  • There was nice ozonated corona smell coming from the HV transformer since I got this unit. Eventually, that transformer developed a short from one of the secondaries to the electrostatic shield at the 100ohm point from one end (they are 900ohm end-to-end).  |O So that one’s good and toast. Will not attempt a repair - no way it’ll be as good, and this is HV and I have zero tooling to work on those. Nope.
  • Sent RFQs for replacement transformers to be custom made, qty 2. I’ll let everyone know how much they are, in case anyone wanted to get some as well either as spares or for experiments.

So - not great, not terrible. The only stock options for a transformer I know of would be Hammond plate voltage transformers, and those have no electrostatic shield, so no-go for this application.

Right now the plan is to get this thing fully working and calibrated, and then build a smaller replacement with a little bit more volt-nuttery applied, and once that works, sell the beast.

2701C one has no shields, no ovens nor thermal masses… it’s probably pretty good for the low cost PCB construction it got, but with a bit of mental elbow grease a replacement can be much smaller. I really need something that will actually fit on the shelf, have a low power standby to keep the ppms from escaping the voltage reference ;D, and fix the low-hanging fruit problems like output 10Hz-1MHz noise. It’d also be nice not to have to use custom wirewound resistors, even though I could order those custom as well if I was inclined to duplicate their feedback string. Not sure yet. And I need something that can be used with external reference as a “digital KV divider”, and hacking 2701C for that is not optimal, even though I tried and got it to do what I needed. It emulates strain gages well enough :)
« Last Edit: March 25, 2023, 03:49:44 am by KubaSO »
 
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Offline alm

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Re: Valhalla 2701c schematics + firmware V3
« Reply #90 on: March 25, 2023, 08:30:46 pm »
Right now the plan is to get this thing fully working and calibrated, and then build a smaller replacement with a little bit more volt-nuttery applied, and once that works, sell the beast.
Good luck! The closest existing thing might be Ian Johnston's products: https://www.ianjohnston.com/index.php/onlineshop, but they only have a single range. I'd expect the higher ranges with the switches and amplifier to add the most bulk and costs. An older unit like that was the Fluke 515A.
« Last Edit: March 25, 2023, 08:33:46 pm by alm »
 

Offline KubaSO

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Re: Valhalla 2701c schematics + firmware V3
« Reply #91 on: March 25, 2023, 09:40:53 pm »
Ian’s source looks super nice but I really want to make something classical with FET switches and PWM and filters, mains powered. And it is gonna have a 1000V output for sure, plus an external reference input to stand-in for strain gages, and of course a current source so it will be good for cal-ing both strain meters and current loop process meters.  I need that functionality sometimes.

I’m also thinking of using several LM399s either in series or paralleled depending on output range, to avoid amplifying the noise at least up to a 100V output or so. I’ve got a couple dozen of them and they’ve been sitting on the shelf for almost 20 years. For shame. Not much longer though. Got enough good latching reed relays that I can be switching all of them all day long :) Dang. Day is too short. Is there volt-nut group therapy available? I may be on the way there… the ppms are calling.
 

Offline KubaSO

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Re: Valhalla 2701c schematics + firmware V3
« Reply #92 on: March 31, 2023, 12:20:24 pm »
Looks like I got all the transformer quotes I was waiting for. I’ll have 4 made. The extra 2 are yours for the cost of shipping from US or Poland (your choice)+a beer - so if anyone needs one, they should be available in 6-8 weeks.
 


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