This blog does get around, especially for those who are searching for “Eagnas repair”. Back in July, a comment came through from a reader asking if I knew what value potentiometers were in the tension head and whether they were available. I replied saying that the value is written on the trimpot and they were generally available.
Later I emailed the reader as I realized that he is from Australia as I have repaired units like these for other people. Eventually he got back into contact and would drop the tension head to me when he was passing through Sydney sometime in the future. This did happen on the 4th of this month, he had already purchased some trimpots that should be suitable.
The original trimpots had been roughly treated in the past, not by this reader but by a previous owner. It was a relatively easy manner to desolder the trimpots, then install the replacement trimpots after bending the leads appropriately to fit the circuit board. Coincidentally, both trimpots were marked P102 which is a 1K-ohm and these happened to be the ones that he purchased.
As you can see, the original trimpots look a little worse for wear. The newly installed ones look perfect. The next step was to reassemble the unit, mainly put the cover back on, then feed the six wires out the front panel to a double pole two position switch with center off. This was a problem since my previous wiring for the switch had slightly different colours – so off with the cover, then trace the wiring to determine which way the switch should be wired.
Eventually, I worked it out. One pair of wires, the red and the black are the motor wires. Then another pair – blue and yellow are the +24V, so are power. The remaining two wires were brown and red. Fortunately, the reader had put cable ties on each group of three wires, because these are to different poles on the switch. The switch works as a reversing switch, applying power to the motor in one direction – to tension, and then to reverse the motor direction to release the tension.
One group of wires is brown, red and yellow – so the red is the motor goes in the middle then brown on the II position, and yellow in the I position on the switch. The remaining group of blue, black and red – again black is the motor so goes in the middle, then this time blue must be the opposite side to the yellow, so goes in the II position, then red finally in the I position.
The final test is to power on, and check that when I click the switch to the I position, that the gripper turns clockwise. Nothing happened – ok, what is going on. I got the cover off, then looked at the small circuit board behind the motor – I had to remove a couple of connectors from that board, and I noticed one connector wasn’t lined up, i.e. wasn’t plugged in properly. My fault – when I pushed it back in, I though the pins had engaged but they hadn’t. It was a tight fit, so this time, carefully I manoevered the connector until the pins had gone in, then pushed it home. A visual inspection showed both connectors were lined up – great.
Power up again – after connecting the switch, and fantastic – the I position moves the gripper clockwise, and switching to the II position, the gripper moves anti-clockwise then stops when it reaches its home position. Normally this will have released the tension on the string, but if it hasn’t, the red button can be pressed which will turn the gripper anti-clockwise another time.
Once reassembled, it was time to check the calibration. These trimpots are used to adjust the gain in the various amplifiers on the circuit board. I mounted it onto my test bench then started with low tensions and working up to the high tensions – and once I was sure that things were working – I set it to 55 lbs and did a tension test.
It is working, so with a few small adjustments, I did a chart from checking tensions from 20 lbs to 60 lbs. This was then made into a calibration chart that could be used to work out what setting is needed to get a particular required tension. Generally though, this kind of tension head is not very accurate and the tension can vary by a pound or two, even from one string pull to another.
Of course, all of this did not happen on the same day, but it is an idea of the sort of effort that is needed. By the way, this kind of tension head is best calibrated on the stringing machine that it is to be used on. The reason for this is that if the string is not horizontal which is where I have calibrated it for, then the resulting tension can be low or high.
I.e. if the string is sloping downwards towards the tension head – the tension being pulled is slightly low, but if the string slopes upwards towards the tension head – then the actual tension will be higher. The reason for this is that as the gripper turns and pulls the string tighter – the gripper assembly which is pivoted on a L shaped bracket, will lift causing a microswitch to be activated. The microswitch is located near the front left corner – a clever use of gravity actually. Now, if this tension head was used on the moon, what tension would be get if we tried to tension for 55 lbs?
P.S. The small black knob on the right below the LED display, turning this knob moves the wiper of another potentiometer but also compresses or releases a spring. The spring acts against the bracket causing resistance to the pivoting. In this way, turning the knob anti-clockwise will compress the spring, and give a higher display – so the result is a higher tension. The calibration trimpots are used to set the display reading to be close to the actual resulting tension.
So, on the moon with one-sixth of the earth’s gravity, the resulting tension would obviously be lower. The weight of the gripper mechanism and the motor and gearbox would probably be 15 lbs on Earth, so the tension we would get on the moon would probably be 12.5 lbs less – does that sound right?