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

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

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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

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In article

snipped-for-privacy@prolynx.com wrote:

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.

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

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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?idE9557

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.

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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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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?

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On Feb 7, 2016, Chuck Gambill wrote

3

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 Millrite.

Joe Gwinn

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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 market. 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.

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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?

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On Feb 7, 2016, Chuck Gambill wrote

As I said in another answer to this question, I gave up on the Asong and bought a unit from Servo. This was after January 2010, and the unit I had came with the Millrite.

Joe Gwinn

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