13 AUG 2012
ECONOMICAL KR400RC INDICATOR REPAIR
KR400 FAILING POTENTIOMETER - M0GHZ EFFECTS REPAIR.
The potentiometer in my aged rotator had become intermittent, resulting in the indicator on the controller wandering around the dial or indicating the wrong direction. I anticipated this to be a simple repair by fitting a new potentiometer to the rotator, however, the Yaesu service department advised that the part was obsolete (I wonder what their new rotators use?).
Peter G3IBI (www.g3ibi.co.uk) sells rotator spares but had no stock either. He suggested that Farnell Pt No 1055288 might be suitable, but at £59 including delivery from the USA and VAT the repair of an old rotator was becoming uneconomic, particularly as I had a second rotator which was also a bit dodgy. The pot is a 500Ohm 360o rotation item, which is rather unusual. It has a 6mm dia shaft and 9mm dia mounting bush.
Waters & Stanton were found to list a 500Ohm pot on their web page for £40 but they didn’t have any stock so I don’t know if it is compatible. I though that I might be able use a 10 turn 5k pot as an alternative (500 ohm for one rotation) and was encouraged when I found someone else had suggested this on the internet.
Farnell Pt No 1144803 is a 5k 10 turn pot with the correct size bush and shaft and available from the UK at a much more reasonable price of £10 + delivery + VAT. I managed to buy two on Ebay for £16 although these were found on arrival to have a 6.34mm shaft and 9.5mm bush!
The defective pot was removed, the gear bush drilled out to fit the larger shaft and the mounting hole enlarged and the pot fitted. It was adjusted to 250 ohms and the rotator set to mid position when reassembled. The rotator was tested on the bench, and although the indicator worked smoothly, the calibration was not accurate over 360o and completely inadequate for VHF working, let alone microwaves.
After examination of the circuit diagram, it was noted that the controller indicator is driven by a motor coupled to an identical 500ohm pot. The control circuit drives the motor until the resistance of each pot is identical; hence they need to be identical for correct calibration.
Using the second pot purchased the control box pot was also changed (I can now use the removed pot in my other rotator!). This entailed the same process of drilling out the pot gear bush and mounting hole. The rotor calibration was then found to be accurate over 360o. The repaired rotator has been working reliably for some months now.