S21 Hashboard – Tester detects 107/108 ASICs

[nighthawk91]

Hi everyone,

I’m working on an Antminer S21 hashboard and I’m running into an issue I can’t fully diagnose.

My tester detects 107 ASICs out of 108, but all the signals look good all the way down the chain:
CLK is present on every domain
RST is stable
BI / BO signals look clean
RO / RI chain looks continuous
All LDOs are outputting the correct voltages
No short circuits on any voltage domain

Because the tester stops at 107 chips, I know one ASIC is failing, but the problem is that everything looks correct up to the last chip.

❓ My question is:
If all the signals (CLK, RST, BO/BI, RI/RO), LDOs, and voltages look good until the end of the board, how can I identify which specific ASIC is causing the “107/108” detection issue?

🔧 Additional info:
I already checked for weak solder joints, missing resistors, and continuity issues around the chain.
I also verified that the last RO signal reaches the connector.
No overheating components detected.

👉 What’s the best diagnostic approach in this situation?

Should I:
Inject a low-voltage signal to trace the RO chain?
Replace the last detected ASIC + the next one as a pair?
Use thermal camera to identify the cold ASIC?
Check the small bypass resistors between chips more carefully?
Any advice or methods you use for S21 chain detection errors would be greatly appreciated.

Thanks in advance.

[philipma1957]

well first off lets get a know good board and use the tester on it.

If it shows 108 chips then you know the tester is not the issue.

Sometimes meters and testers are bad.

If the tester shows good I suggest buying a board like this one.

https://epicblockchain.io/wp-content/uploads/2024/02/ePIC-UMC-Data-2_29_2024.pdf
get it at

https://altairtech.io/product/epic-umc-universal-mining-control-board/

this board will show this image for chips

This is a 2 board t21 and it shows all the chips are working but that some are weak

in your case it should show a chip with a very low percent maybe 20% or less.
if you click on the chips it will tell you the chip number

good luck


note chip number 19