Power feed problem

I have a Central Machinery (HF) power feed on the X axis of my RF45 clone mill and it's acting weird . Works fine on the fast move button
but the speed settings are messed up . It doesn't start turning until I get to "2" on the dial and by the time it's up to 3 it's going full speed . It just started yhis , today is the firt=st time I've seen this . When it was working right it would start before I got up to one and would ramp up perfectly as i turned it up . Am I looking at a bad rheostat or maybe just dirty ? Should be clean , that unit is sealed up tight - but ya never know .
--
Snag
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That sounds like a bad SCR.
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On 7/13/2020 5:25 PM, Michael Terrell wrote:

OK , I don't know a lot about SCR's , but I have some experience with electronics . How do I test it ? I was planning on starting by checking the resistance of both sides of the control pot ... but I have no idea how to test an SCR .
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Snag
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The easiest is to just replace it. Use the same part number. The sensitivity varies all over the map, with different types. They are usually cheap, and a lot cheaper than a test fixture.
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"Snag" wrote in message
On 7/13/2020 5:25 PM, Michael Terrell wrote:

OK , I don't know a lot about SCR's , but I have some experience with electronics . How do I test it ? I was planning on starting by checking the resistance of both sides of the control pot ... but I have no idea how to test an SCR .
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Snag
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On Tuesday, July 14, 2020 at 5:57:09 AM UTC-4, Jim Wilkins wrote:

+1
If turns out to be the SCR, let me know. I have some SCR's I salvaged from the scrap yard. I forget the specs, but they are good for about 1000 amps.
Dan
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In principle, it's easy 8-). First, test it like a diode, however if it conducts either way it's faulty.
Then test it forward biased and momentarily connect gate to anode. It should start conducting and stay that way.
There are two traps in this: One is that an SCR generally needs at least two forward diode drops, 1.4 volts, to work at all. The other is that "latching" the SCR on requires some minimum current, maybe 1% or less of running current. Far more than a normal diode tester provides. A car battery and taillight bulb would probably work.
HTH,
bob prohaska
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