Modifying Bitmain T21 Miners: From Three Boards to Two

The T21 is a strange miner with that 3-phase power supply... such a hassle.

Yeah, but you can do some cool stuff with it if you mod it right. Here’s what I did: 1. Grab a 240V power supply from Bitmain or a local source. 2. Get the bus bars, but make sure they fit the S21. 3. Use the epic controller for better performance. 4. I pulled out the middle board and used a spacer, just some wood wrapped in aluminum tape.

Wait, so you're running two boards instead of three? How's that going?

Yeah, I turned 2 T21s with 3 boards into 3 T21s with 2 boards and it’s one phase now. But I noticed one unit is underperforming, probably due to it downclocking too much.

Nice. What downclock settings are you using?

First I went down to 66 at 160 MHz. Now I’m at 99 with 240 MHz and the two boards are giving me a hash rate of 66. The weaker unit is actually better at the higher clock with lower volts.

Interesting... so the tuning really affects performance.

Yup. The weaker unit is catching up with the others at 120th or 290 MHz. It's getting close to the stock 19 watts per TH. Plus, cooling’s not an issue since I’m only using 2 fans.

Are you planning to push it further?

I’m going for 150th next, which should get me 100th on 2 boards. One unit is even hitting better than spec at 17.9 watts per TH.

That’s impressive. What’s your max target?

I’m aiming for 180, which is just under the stock 190. But we'll see how the cooling handles it.

Why bother with all these mods? Seems risky.

Bitmain gear is overdriven and tends to break. I want to test these out more, but my power rates are about to jump. Might send them to my guy in Iowa for the summer.

Quick clarification: you mentioned turning it into single-phase, right? It’s actually called single-phase with 2 hot legs, correct?

Yes, that’s right. One leg gets pulled, so it's still single-phase.

Thanks for clarifying that. Good luck with the rest of your mods!

Out of curiosity, how many wires does the 3-phase PSU have? Is it 4 or 5?

Let me check... I found one of the wires, and it has 5 3 hot, neutral, and ground.