I had the "x" problem, too. Maybe I still have.
![Roll Eyes](https://bitcointalk.org/Smileys/default/rolleyes.gif)
Here is what I did and tried:
1. Undervolt to ~0.85V with pencil mod, everything fine on various MHz-Settings, no x´s.
2. Undervolt to ~0.78V with pencil, on 2 of my ants the x´s show up. Usually the first 8 in first row, and sometimes even more, both rows. Sometimes I was able to get rid of them by reboot, sometimes they disappered alone. After a lot of trying clocks/timeouts from 162MHz up to 240MHz I was about to give it up. Nothing worked properly. Some days the x´s vanished, then show up again randomly. The pool hashrate also showed that the affected miners ran okay for several hours, then hashrate suddenly dropped from ~120 to 100 or even 80 GH/s and stabilized there. After all, I thought it was the pencil, because I was not able to get exact the same Voltages to all ASIC-sections of the antminer.
3. I removed the pencil, and soldered 4.7k resistors in parallel to our R3 and friends. Now I got a stable 0,776V (Load) - 0,779V (Idle) at each section of the hashboards. The Antminer is pulling now ~10,5Amps @ 11,97Volts DC and is hashing at ~120GH/s with 237MHz. So far so good. (Before it pulled ~13Amps with the 0,78V pencil mod) But when I moved my rig into the garage in november, the damn x´s got back.
Random drops from 120 to 100, on the other ant from 120 to 90 GH/s.
Temperatures in garage were around freezing, and I got it to barely 10 Celsius with the rig (4 Antminers total, and some Litecoin ASICs) running 24/7. Good to work some hours a day on my cars. COOLING INFO: even at 10 Celsius the fans ran so slow that you were able to count the blades on them.
![Smiley](https://bitcointalk.org/Smileys/default/smiley.gif)
ASIC showed around 20 Celsius. Nice silence.
4. Since december the rig is back in my house, in my living room as a little helper to cut heating costs. Suddenly the x´s disappeared after 2 days of (intermittent) running! (I need to shut the rig down when nobody is at home, to avoid risk of fire.)
Ok, so what could have caused the x´s? I believe it is only temperature and voltage that can cause it yet. I think the capacitors on the boards are aged, and some of them fail in cold conditions. Especially with voltmod, maybe the voltage gets very low under sudden load for a short time, causing the ASICs to crash... dont know.
![Huh](https://bitcointalk.org/Smileys/default/huh.gif)
And I do not know how well or bad the pre owners treated the miners, got all my 4 ant´s at the bay.
After a little research, I also think my voltage is a little low, maybe too low to power all the ants properly. Got 2 HP DPS-800GB with backplane that power my rig. The litecoin-section surprisingly NEVER crashed with this setup, at the same 12V rail.
Now I run 1 of the suspicious ants in the garage on a 12,34V PSU (instead of 11,97V from my HP proliant powermonster), to prove if voltage does matter. Actually running 4 hrs without x´s.
Maybe I got it....
Next step is to get closer to 0,75V... a 3,9k Ohm in parallel gets me 2,1k and a resulting 0,76V under load. I will report if it works fine and how much the miner is pulling then in DC and AC.