Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on August 08, 2013, 08:56:26 am
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Want to make sure I didnt damage pots. Got a new soldering station and tried soldering wick, which is awesome. But I wanna make sure pots are good. Do I turn them all the way up and check for the value in which they are rated?
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Measured resistance between the two outside lugs should equal the marked resistance value of the pot. Measured resistance between either outside lug and the center lug should equal a resistance that varies smoothly from 0Ω up to the marked value of the pot as you slowly rotate the shaft.
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Want to make sure I didnt damage pots. Got a new soldering station and tried soldering wick, which is awesome. But I wanna make sure pots are good. Do I turn them all the way up and check for the value in which they are rated?
The thing that usually goes wrong with pots when they've been soldered and resoldered is that the terminals are crimped to the carbon path and that contact point becomes intermittent. Use alligator clip leads on the legs and wiggle the leg around to see if the connection goes in and out. Pots like that can cause all sorts of weird issues. Sometimes you can fix them by crimping them to get the solid contact back. It can be tricky tho and i'd rather replace them when possible if theres any question. Pots tend NOT to be a black and white issue often and can seem to work fine after you fix them, then later they go wacky again.
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"Re: How to test pots? Just de soldered using wick for first time"
Here is one place where an analog (volt or ohm, but the point is: a test meter) meter is neater than a DVM...but not absolutely req'd.
Pots can fail in about 3 or 4 or 5 ways. All of which you can detect with an volt/ohmmeter, but it is also worth your time IMHO to play with a battery and a resistor and a pot and your meter to acquire a better-than-intellectual feel for what a pot does; how it acts, and how your particular meter acts/reads out the variable resistance as you, yourself, vary it with your fat fingers. Like if you were a kid fooling around with it. Totally serious, this is what we call an "acquaintance" exercise. In general, they are pretty rugged devices. For a pot to work and be your friend it has to:
1: Have an end-to-end resistance sort of resembling its rated value. This can be WILDLY off. +/-20% is very common. One way pots fail is to have the "horseshoe" of res mat'l break...so they open up from term 1 to 3. Under these conditions, it is possible that the wiper will work, for SOME PART of the pot's rotation. Because the crack in the horseshoe mat'l is usually a hairline crack, and the "point" of the wiper will, as you turn the pot slowly, work-work-work-work NOT WORK oncce it gets to the "far" side of the crack.
2: The wiper has to make contact all the way from one end of the pot rotation to the other. It's common for a pot to genrally work but be very noisy and herky-jerky at one or both ends. Sometimes the wiper opens up completely and makes NO contact with the horseshoe. VERY rare in my experience, but I acknowledge that import parts can fail in ways that traditional, domestic parts never even thought of.
3: Pots also have a "taper" which will not prevent them from working if you have the wrong kind...I will not address that here.
4: The wiper has to maintain smooth contact at all positions, or it will produce static or noise in the circuit. This is by far the most common fault---a piece of dirt gets in there and the pot gets "scratchy". Deoxit or other "pot spray" is the cure.