Hi, a few more news from the front:)
I measured the input voltage right from the usb port on the Antminer (Black and Red dots according to Walking Glitch's picture).
First i noticed that there is massive voltage drop of about 0,3 V. SO my miners getting about 4.9-4.8 V instead of 5,1 or so.
That was my problem, i am using passive cooling, and need USB extension cables. I used cheap and long ones (3 Dollar, 2.5m). Then i measured the resistance of the cable: 100-150 Ohm.
Chinese crap:)
Now i bought quality ones, and getting 5,1 V before Antminer starts mining.
Second thing I noticed, the miner gives massive ripple to the usb ports (usb Hub or in my case PCI-USB 3.0 Card).
If the miner 1 suddenly increase current consumption you will notice voltage drop on other miners if they are sharing same power supply.
For example if on miner1 the input voltage peaks to 5,2 (and so the hashrate), the miner2 will experience voltage drop to 4,8 V etc.
That is why you can see this bouncing effect in you mining software.: some miner getting faster while others are getting slower at the same rate.
To prevent this i added additional Tantalum capacitor of 470 µF on each miner (see Picture). As the result, the miners stabilize MUCH faster, HW drops even more.
Next i once again tried Hardware hack:
10K+3,7K gave me stable 2,6 GHS. But saddly enough i could only run one Miner on my PC. If i switch another one, they both stop mining... (not sure why, since my PSU can provide 25 Amp on 5V rail, perhaps too high ripples/interference)
After using 2,2K instead of 3,7K I managed to get stable 2,2 -2,4 GHS with two devices at the same time. Since i only can cool two miners right now, i dont know if running 4 Miner on this mode will be stable. Right now they are running at nominal 2GHS, while hacked ones perform 5-15% better.
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One more thing: I noticed if input current falls under 4,7 V the miner stops mining and switches off.
Guys, who cant run their miner past few hundred MHS should measure if there is any voltage drop on the input....
PS
Still no idea how to bring antminer to advertised 4GHS....
Maybe some separate power supply, not just regular USB3.0 Port with maximum of 900 mA. For example connect GND and +5V from other source, and D+ and D- to USB ports.