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.
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.
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
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.
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.
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.
replying to Joseph Gwinn, Chuck Gambill wrote:
Where can I get this pot I have a ASONG 150 lbs. 110 volt unit really I have 3
on my mill. But the one most used left and right control longitude doesn't work
its full power or no power. Wondering do you install them?
The follow-on is that I found and installed a 5-watt wirewound pot to replace
the failed carbon-film pot, and was happy for a few weeks, until the Asong
unit burned the wirewound pot out too. At which point I gave up on the Asong,
and bought a new unit from Servo, specifically their low-cost Taiwan-built
unit, which works well. Installation did require some lathe work to fit to my
Amazon sells BIG ones at flea-market prices:
(Amazon.com product link shortened)
I haven't tried them because I found old Ohmite ones at the ham flea
The reviews mention low quality brushes, which you might be able to
replace with carbon from a motor shop as I did to fabricate an
unavailable Variac brush set.
replying to Joseph Gwinn, Chuck Gambill wrote:
Hello I have the same problem with a ASONG 150 lbs 110 volt unit are you able to
repair these units with a new pot?
If so I would like to know what you would charge to do this?
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.