Repair of an Asong AS-250 X-axis power feed

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The Asong AS-250 is famous for burning out its speed pot, and mine was
no exception.  I replaced the speed pot assembly twice with the factory
repair part (provided by Shars Tool), and the new pots also failed after
a short time, so we have a design problem.  Shars was going nuts with
the resulting warrantee repair hassle.

Autopsy of the failed post showed that the carbon-film resistor element
was burned. apparently by pulling too much current from the wiper.  The
failed pots claim a 2 watt rating, but this is not what one can pull
from the wiper it would seem.

So, I replaced the carbon-film pot with a 10 Kohm 5-watt wirewound unit
made by CTS, about $3 each (Digi-Key CT2159-ND).

This required fabrication of a new shaft extender-adapter of brass, to
connect the 0.250" pot shaft to the 6mm knob socket about 3" away.

The power drive now works, after about two years of neglect.  We will
see if this fix lasts.


Joe Gwinn

Re: Repair of an Asong AS-250 X-axis power feed


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Design problem, indeed!  I can see this happening if the wiper can get
grounded while one end is hooked to full PS voltage.  Usually, the
engineering folks design in a current-limiting resistor in series with
the pot so you don't get unlimited current flowing through the wiper
and adjust other component values for full range of control.  If you
can get the schematics, you can see if this was done.

Stan

Re: Repair of an Asong AS-250 X-axis power feed


In article
 stans4@prolynx.com wrote:

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There may be a series resistor, but it isn't easy to tell, as described
below.  Nor is it important, as all units seem to fail this way.

The original Servo unit that this is a knock-off of probably used those
bulletproof MIL-SPEC molded carbon composition pots that were universal
in WW2 military equipment, and were widely used well into the 1980s.  I
bet their tolerance for wiper current greatly exceeded that of a
carbon-film pot, just on mass and bulk alone.

I did measure the wiper current:  It's about 1 mA at the slow-speed end,
rising to about 7 mA at the high-speed end.  This with no mechanical
load on the unit.  I assume that the waveform is weird, a series of
oddly-shaped pulses, because AC, DC, and AC+DC readings (from a
Tektronix model TX3 RMS-reading multimeter) were a little odd.

Thanks to Shars Tool for providing the dead boards and pot assemblies
from which I made the little "extension cord" used to measure the
current.
 

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A schematic came with the unit, and there were only two problems:  
First, the circuit board didn't match the schematic, not even matching
in counts of the active components.  This triggered closer examination
of the schematic, which yielded that the schematic itself made no sense
- it was an artist's impression of a schematic, and violated even the
grammar of schematics.  I didn't have the energy or interest to trace
the circuit board.


Joe Gwinn

Re: Repair of an Asong AS-250 X-axis power feed


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LOL.  I did something similar.  The height control POT for the hydraulic
jackplate on my boat failed.  Nobody stocked the part.  I contacted the MFG
who said it was a custom part "even though its listed in their catalog" and
they would be glad to sell me one for $40, but there is a $300 minimum order
and 8-10 weeks lead time.  I speced a different pot and bought it retail for
$11.20.  Used the extender off the old pot.  Works great.

http://bbcboards.zeroforum.com/zerothread?id=459557

So far no problems with the substitute pot.  Some good basic knowledge about
components and the confidence to apply it sure comes in handy sometimes
doesn't it?  Glad yours is working.


Re: Repair of an Asong AS-250 X-axis power feed



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Good luck with your repair as well.

It seems that your problem is with an abusive manufacturer.

Joe Gwinn

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