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Testing 4 wire servos

Old 02-29-2016, 11:55 AM
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jayel7
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Default Testing 4 wire servos

I have a Kraft Gold Series KP-6B system from the late '60's that I would like to get working, and I'm starting with the servos. These are the 4 wire servos, the receiver and servos use a center tapped 4 nicad pack. I want to use my modern, 3 wire, servo tester but don't know how to connect it. Can anyone help?
Thanks - John
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Old 02-29-2016, 01:36 PM
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grotto2
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Might work. What kind of servo tester?
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Old 02-29-2016, 03:22 PM
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jayel7
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Originally Posted by grotto2 View Post

Might work. What kind of servo tester?
Thanks grotto2, that was my first thought too. But I wonder if the Kraft rx signal is referenced to the center tap, i.e. is 2.4 volts instead of the 4.8 volts? Hate to burn one out trying. My servo tester is just a no-name, you dial up the position and the pulse width displays in the lcd; one of the most useful tools in the shop! It can be used with a separate battery if needed.

John
Old 02-29-2016, 04:09 PM
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grotto2
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Yep, referenced to black, ground. You can get some good information from www.vintagercfiles.com
The reason I asked about the tester was to see if you would have any issues if it supplied it's own power.
Old 02-29-2016, 05:13 PM
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jayel7
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Originally Posted by grotto2 View Post
Yep, referenced to black, ground. You can get some good information from www.vintagercfiles.com
The reason I asked about the tester was to see if you would have any issues if it supplied it's own power.
O.k. - I'll give it a try! The vintagercfiles will be a big help too.
Thanks,
John
Old 03-01-2016, 11:34 AM
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Originally Posted by jayel7 View Post
O.k. - I'll give it a try! The vintagercfiles will be a big help too.
Thanks,
John
Yes, the above diagram is correct.

You may have a problem testing your servos on a tester as many of the older servos required a pulse voltage at or near supply voltage. Many of the later IC or processor based testers output a pulse voltage quite a bit below supply so the servo may or may not respond. If necessary a couple of transistors or a TTL gate could be configured as a buffer to improve the output pulse. When working on this old stuff (some years ago) I usually just used a receiver output as a test source.
Old 03-01-2016, 12:15 PM
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jayel7
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It worked! - guess I got lucky. All the servos are working now that I've exercised the pots a bit, didn't have to do anything to the tester.
Interesting info -thanks.

John

Originally Posted by Truckracer View Post
Yes, the above diagram is correct.

You may have a problem testing your servos on a tester as many of the older servos required a pulse voltage at or near supply voltage. Many of the later IC or processor based testers output a pulse voltage quite a bit below supply so the servo may or may not respond. If necessary a couple of transistors or a TTL gate could be configured as a buffer to improve the output pulse. When working on this old stuff (some years ago) I usually just used a receiver output as a test source.

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