[UK-CONTEST] G-650C Rotator problem

[Ian White, G3SEK]

G3SJJ wrote:
>I haven't had to fault-find or repair a rotator before so this one has 
>stumped me. Beam
>and indicator fixed at 290 degrees. Dial lights up but no rotation of 
>indicator or beam.
>Motor voltages appear reasonable :
>
>AC Volts             Left        Right
>YLW to RED       30          15
>YLW to GRN       15          30
>
>
>Pot DC Volts across          BLU WHI =  1v but 10v when dissed rotator
>Return Resistance across  BLU WHI = 330 when dissed from controller
>(125m of HD  7way "trailer" cable to rotator)
>
>My thinking is that it is a DC system failure in the controller but I 
>don't have a cct
>diag and also the pcb has a number of surface mount components. This is 
>a real nuisance
>coming just a few days before the FOC Marathon. Another week and it 
>would have done me
>until spring time. At least the 10m beam is pointing to States.
>
>1 - Anyone willing to loan a G-650c controller for the weekend?
>2 - Are replacement PCBs available
>3 - Any helpful suggestions?

Let's attempt No. 3...

I don't know the G-650 in particular, but there are two general types of 
controller. In one type (like the HAM series and my KR-600) you can 
drive the rotator independently, regardless of whether the indicator 
works or not... and if you can't, there is a rotator fault.

In the other type, the indicator is part of the control system so the 
fault could be in either that or the motor. However, in an emergency you 
can still get inside there and drive the motor independently, and 
eyeball the beam direction.

Is 290deg an end-stop - in other words, is the rotator at one of its 
limits, or just part-way? Since the indicated and true directions agree, 
it seems quite possible that the indicator part is OK, so you should 
concentrate on getting the motor going.

Probably the next step is to check the motor connections from the shack 
with an ohm-meter. Usually there are two identical motor windings of a 
few tens of ohms, with the centre tap coming back through the limit 
switches (if any) in the rotator. If there are limit switches, there 
will be two switch wires, one of which may be shared with one end of the 
pot. When both limit switches are closed, there will be a near-short 
between those two wires.

So, by trying various combinations you should be able to find:

* The two ends of the motor windings, connected in series, total 
resistance R.

* The centre tap, showing R/2 to each of the two ends

* Another wire showing a near-short to the centre tap (assuming there 
are limit switches out at the rotator, and both are closed)

Also worth checking is the motor capacitor, which is probably in the 
control box - I don't think anyone since CDE has been daft enough to put 
it inside the rotator!

If all of those tests are OK, then you should be able to drive the 
rotator directly from the transformer using your own manual switch.

I covered some of these topics in 'In Practice' for July and August 
1994, and could fax those if necessary.

-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek