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feb 9

Actuator Replacement

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This article discusses the replacement of the actuator of a 'BABY' Ciro-Mazzoni magnetic loop antenna. Whilst the antennas are very reliable, occasionally an actuator will fail and require replacement. In order to ensure that the replacement results in a fully working antenna, a number of points need to be considered. The first is that the position of the actuator must be such that when it is driven to either its inner or outer electrical limit that this occurs before reaching the hard mechanical limit. Inside the actuator are two normally closed limit switches. If the actuator is driven to either of its limits one of these switches will open placing a reverse-biased diode in series with the armature and blocking the flow of current. To exit the limit it is only necessary to reverse the polarity of the drive signal, whereupon armature current will flow in the reverse direction, and the actuator will return to its normal operation. If the actuator were to encounter a hard mechanical limit this would stall the motor, and then the only thing limiting the current would be the armature resistance itself plus any cable resistance. The very large current that would flow in this case would almost certainly damage the motor driver integrated circuit, and so must be avoided at all costs. Equally, as well as avoiding hard mechanical limits, it is necessary to ensure that at the inner limit the resonant frequency of the antenna is well below the nominal low frequency limit of 6,600kHz, and at the outer electrical limit its resonant frequency is above the nominal upper limit of 29,800kHz. In addition, as the resonant frequencies are somewhat temperature sensitive this too has an effect and must be taken into account. The initial replacement instructions required you to accurately measure the distance from the edge of the clamp to the end of the actuator housing, but this did not always result in a satisfactory position. The latest method involves closing the capacitor to a position that is 15mm from its fully closed position and then tightening the clamp. The actuator must be at its inner electrical limit when this is done, and it is normally supplied in this position. It is most important not to rotate the actuator rod! In my case I made a simple jig out of a piece of wood with two large diameter countersunk headed screws driven in until the flat portion of the screw heads were protruding exactly 15mm, and then by holding the capacitor closed on this jig, the clamp screws could be tightened securing the actuator in its optimum position. With the antenna then mounted temporarily in a Black & Decker 'Workmate' table I measured the resonant frequency at the inner electrical limit and found it to be 6,484kHz. With a bench 24V current limited power supply I drove the antenna to its outer electrical limit and the resonant frequency measured again, and this time it was 30,314kHz. As these frequencies were well outside the nominal 6,600 - 29,800 kHz tuning range the actuator was correctly located. The armature current when driving the actuator with 24V was between 350mA and 400mA. With the actuator positioned away from either electrical limit the armature resistance was about 3 to 5 ohms, depending on where the armature had stopped. The resistance can be expected to vary somewhat depending on where the brushes land up on the armature segments when the motor stops. When correctly positioned, the Mk-II controller can easily determine both the 'sense' of the motor connection, as well as the type of antenna when using the Auto-Detect mode. However, see a later post of mine where I ran into some quite unexpected trouble! Adrian, 5B4AIY

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  • gm7eey
    6 giorni fa

    Anyone suffer from poor tune on 20m when the Midi loop gets wet? When I build this one I noticed that the Gamma match tubing had a badly seated cap, when I removed it to inspect a few drops of water came out of the tube. Am going to take the loop down today and inspect this again as its been quite wet recently but am interested to know if this is a known issue. This is a used loop and as far as I know it was stored indoors prior to my assembly and when I did the build, the loop was only outdoors for about 2 days before it went on the mast. Was rather surprised to see the water and trying to tun last night when raining, 20m was nothing below 2.1, today now its dry kinda back to normal at 1.6 ish. Real concerned that water is getting into the co-ax section and thats bound to be bad news. 73s Pete
  • Robert Carswell
    ott 23

    Hi I have a brand new Baby Loop and have a problem straight away. After assembly I went through the installation First Turn On process. Set the loop type and moved to using the star key and setting the frequency as advised to 7050 pressing the enter key looks as though it will do something but the ATU just shuts down and restarts. I have attempted the Auto Detect routine and it fails with same issue the ATU just shuts down and restarts. The display shows 18000 khz with a SWR of 1.6 which is presumably after the factory tested the loop. The firmware is 3.29 and the board is 1.06.0663. I have downloaded the firmware again as per the manual and it doesn't appear to do any upgrading as presumably it already knows the firmware installed on the ATU is version 3.29 I have checked the twin core cable I am using with a 12volt battery and it checks out with the volt meter. I have checked the antenna cable which is mini 8 and that checks out. I have attempted the procedure for manual testing of the motor control as per the manual on page 70 entering the frequency of 7050 pressing enter which shows TEST: Duty 21% 7050khz SWR: 9.9 Pressing 9 to open the loop just shuts down and restarts the ATU During the manual testing I have checked across the motor cable plug on the rear of the ATU and there is no voltage at the terminals during any of the setup procedures or the manual testing of the motor control. This is a brand new never used before Baby Loop and fails at the first turn on stage. I have contacted the supplier but being the weekend there is no one available to help Any one any idea of the issue?
  • jeffsmithchicago
    set 19

    I received the Baby loop last week and assembled it. After setting it up a few feet away from the desk I decided to connect it and run some tests to give it a shake down. It worked perfectly considering it was only a few feet away from my radio desk and then it just seemed to die with no error messages or anything. The loop is "open" all the way and will not move at all. When manually entering a frequency it goes thru the normal message of tuning but does nothing else, no motor sound or anything. Any advice to get me going in the right direction?

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