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K6JRF's Page formerly W6FZC US Tower MA-850 Tower Motor Repair

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(Dec 7, 2019)

US Tower MA-850MDP The premier MA-Series crank-up Tubular Towers with their slender design, makes a clean, flagpole-like appearance. The MA-850 MDP models include a heavy duty 1/2 HP motor drive with positive pull down. The MDP models provide motorized raising and lowering of the tower. The picture shows the tower with a series of monoband antennas mounted on it. On top @ 94ft is a C6 2mtr colinear for 144Mhz. Next, at 92ft, is a 4el 20Mtr yagi on a 42' boom. At 74ft, is a 80Mtr dipole made by Create Design (CD-78). At 54ft, is a 2el 40Mtr beam. Recently, when I tried to lower the tower, nothing happened!! It wouldn't come down as the motor wouldn't 'start'. This page shows what happened and how it was repaired.

MA-850 Tower Motor Failure When I tried to activate the DOWN switch, the motor just "hummed" indicating that it was getting AC line power (120Vac) but would not "start". From what I know about this tower having it for 20+ years, it seemed to indicate that the motor capacitor has most likely failed causing the motor to not start. The picture shows the motor - - appears to be a Marathon 1/2 HP running on 120Vac line power. However, the label is not visible and even repeated application of a paint remover, "Goof Off" gave a little more visibility for parts of the label but not enough to make out the complete motor part number. The bottom "hump" houses the motor start capacitor. The specs (found from a similar motor, tnx US Tower) are "460 - 552uF non-polarized capacitor". So I removed the metal can and you see what was found in the picture.

As you can see, the motor start capacitor is no longer . . a capacitor! It has 'oozed' and what you see is dried aluminum electrolytic fluid. So that certainly shows what's wrong. Now what was the value of this capacitor?? Using similar motors the cap appears to be a nominal 500uF, 125Vac non-polarized capacitor.

The cap was removed and the metal "can" cleaned. The picture on the left shows the motor leads; the picture on the right shows that the electrolytic fluid 'ate' the metal cap leads and there's nothing left. I did measure the value as apx 77pf . . yes, 77pf . . not 500uF!!

Parts Source for Motor Start Capacitor Researching for suitable parts, I found two (2) sources that appear to be suitable replacements. From Grainger: 460-552MF non-polarized motor start cap for apx $22.00 Size: 1-13/16" x 4-3/8" here From Mouser: 460-552MF non-polarized motor start cap for apx $22.00 Size: 1-13/16" x 4-3/8" here I've bought parts from Mouser for years and I prefer them as my parts supplier. The part arrived in perfect condition. The picture shows the measurement; 545uF @ 125Vac.

Motor Start Capacitor Replacement It's raining with very light showers so I installed the replacement motor cap between the showers. The picture shows it 'mounted' into the metal can and all the previous debris cleaned out. BTW, I use CLR, Calcium, Lime & Rust Remover as it's an excellent cleaner for calcium and rust. After cleaning I used a "brake cleaner" fluid to remove the remaining gunk! No problems with the installation, the motor works perfectly. The real good news is the tower now comes down and goes up! Special thanks to Brad Troxell and Jody Cash of US Tower for help in identifying the motor and start cap.

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