How to know if it’s a bad 14V boost circuit or bad LDO’s

I’ve always heard the first thing that fails on an L3 hash board is the 14V boost circuit, I can say that I 100% agree with this. Of all the hash boards I’ve repaired, at least 75% have been fixed by installing an external boost circuit. Normally this problem is evident once you measure the voltage across D1 (anything less than 14V spells out a problem), however sometimes that voltage can be deceiving.

I recently worked on 3 different hash boards that came on two L3+ systems I bought off Ebay. The sellers were clear and honest about there being a bad hash board or two, so I knew what I was getting into. Two of the boards measured over 14V at D1, which normally would tell me the boost circuit was fine, however I now know it wasn’t. The other was more telling, 10V at D1, popped a boost on it and it’s been hashing away fine.

After measuring 14V at the boost I proceeded to measure all other relevant voltages on the hash board, that includes not only checking 14V, but the 10V from the buck controller (measured at L2), 12V input to the board, and the voltage across pins 2 (GND) and 5 (1.8V output) of each LDO. The first 10 domains measured correct at ~1.8V, however domains 11 and 12 came in at 1.1V and 0.5V respectively. My go to was to try and replace the LDO’s since that’s a common failure, however this time it didn’t fix them.

Note – The LDO’s at the last two domains require an LDO that takes a higher input voltage (~5V on 11 and ~4.5V on 12.) Some versions of the LDO work across a wide enough input voltage range that they can be used on all domains, however the most common (LN1134 marked usually as 4VK4) only works with the first 10 domains. This device only works up to a 3V input and will fry if you put it on the last two domains.

After failing to fix the problem my first thought was, “what if, under load, the 14V is dropping.” This would point to the common problem the on-board boost circuit has, and even the inductor (L1) can’t help keep the voltage constant when the output current is less than the draw of the LDO’s. So I popped off D1 (to isolate the on-board boost circuit) and installed an external boost circuit.

I plugged in the hash board, viola, it came right up and started hashing away. I went back and verified the voltages at the last two LDO’s and they both measure a clean 1.8V output. I put together a quick video to show the steps I went through:

https://youtu.be/5abmjlW9_4I

Lesson learned, even if the boost circuit says ~14V, it doesn’t mean under load this isn’t dropping. You really can’t fully test this unless the board is actively hashing and the LDO’s are drawing current from the on-board boost. If you measure 14V but don’t get 1.8V from the last two LDO’s, consider it might be the boost circuit instead of bad LDO’s, or even both…

13 thoughts on “How to know if it’s a bad 14V boost circuit or bad LDO’s”

  1. Hi, thank you for the awesome guide. I have also a not detected l3 hashboard on which im not able to find the problem. The 14v and 10v circuit just measures around 3v and 0 asics are detected. Could you maybe give me some hints on how to proceed? Thank you very much

      1. Thank you for the fast reply.

        So if im correct – the buck converter should be U73 on the v1.41 boards. Im just not curious because on my board it doesnt look square like on the most pictures, its more rectangle (it says 744LGSGN967 on it). Could you tell me which is the output so i can measure the actual voltage? I already ordered some LM27402 like you mentioned in the L3 spare parts thread.

        Are there special ways to check for a correct functioning PIC? I just made a dump of a second v1.41 Board with the same PIC (16F1704) and wrote it to the broken board. It succeded without any errors and with no change of the situation sadly (but i never checked if its actual functioning because of missing knowledge).
        I also could borrow the PIC of a working board (just for testing) or could it be damaged because of maybe other faults?

        Best regards and thank you 🙂

        1. Correct, they changed the device on newer versions of the board. The output circuit is still the same however, you should measure the output voltage in between Q1/Q3 and Q4/Q5 (center channel of these devices is the output.) I’m not sure of the specific device you mentioned, I couldn’t find any data on that.

          As far as the PIC, you can check the code by simply using a hex editor to look in it. Generally if it’s all 0’s it’s bad, but copying code from a working similar revision board to the suspect board works well. Really the best way to check if the PIC is programming is checking the 10V output when you initialize the board, either through the test firmware (hitting the test/ip_sig button) or as the board initializes normally when booting firmware. The buck converter should start up just before you begin hashing.

          1. I just transplanted at first just the pic, still 0 ASIC and after this the (if im correct) buck converter (U73) aswell, of a known good hashboard to the bad one – it works, chain has 72 ASIC. So seems the buck converter is faulty because if it would have been the pic it should have worked without transplanting U73 right?
            I just uploaded a picture of U73 cause you said you are not sure what device i mentioned. I hope that the LM27402 is the right one i ordered..
            ( https://www.dropbox.com/s/h0lblffpjcriqu1/65401_y.jpg ) you can delete this post or the link.
            Best regards, and thank you for the help!

          2. Yup, that’s the correct component, from your last post I wasn’t familiar with the device ID you put down. If you’re not getting 10V out of the buck converter after starting, or running the test fixture, then that’s probably your issue for sure.

  2. Awesome. Im just a bit confused if it will fit, cause the lm27402 is more square and the lgsg more rectangle. Have you already used the lm27402 as a lgsg replacement already?

    1. I would only replace with the same IC package style. I’m not sure if there’s any additional components required if you go from the SO package to the square package.

      1. Ok, ill try to get the same chip and post the link if i get one so you can update the parts list.
        Thank you very much for the super fast reply and help!

        1. Hi again,
          i just received the right lgsg and replaced the faulty one (i tried the lgsg on a known good hashboard first to make sure its working – which it does). sadly i still get 0 asic and no boot. I checked the Pic which is functioning (exchanged it from the faulty board with the known good one). now im a bit lost cause the 3v on the 14v and 10v are gone but now i have 0v cause of the 0 asic – it says not receive CHIP_ADDR register value. could it be that the 1st asic is faulty? if so, could i verify this by bypassing it? maybe you have some hints where to start again.

          Best regards

          1. If you’re not getting 10V or 14V then it’s basically telling you it’s not ready to hash. That could be due to temp sensor failure, PIC failure, or possibly an ASIC failure. Even if an ASIC is bad it usually can’t tell until the PIC initializes so it can power on the buck controllers (10V and 14V) to power on the LDOs to then test and run the ASICs. So if you’re not even getting to that point then you need to look earlier in the chain. I would insure the PIC, 10V buck controller, and temp sensor are reflowed with good continuity.

  3. hi, im coming closer to the problem =) i just tried a 10v step-down board which made the hashboard come back alive showing all 72 asics. so its definitely not a faulty asic or pic etc. The problem therefore is somewhere at the 10v circuit.

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