Bottlehead Forum
General Category => Tech Tools => Topic started by: mkane on February 06, 2015, 04:19:33 AM
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I'm trying to get a hum pot reading but get nothing. I don't think I have the leads plugged into the correct sockets. We have ground a 3 red choices. 10A max fused-mAuA-Vhz. Which is the correct hole for the red lead? I have the dial set on V with squiggly line above the v. thx HELP
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I unscrew the speaker 5 way binder posts and put the meter lead through the hole on the shaft there.
On your meter you have a common socket that the black lead goes into. There is also a current (10A max fused-mAuA) input, just put glue in those holes; they will get you in trouble. And there is a Vhz hole where you measure volts AC/DC, Ohms and Frequency. This is the one you want to use.
You set your meter on V AC (squiggly line) and the lowest scale you have. If your meter doesn't go low enough you won't get a reading at all.
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Thanks. Meter shows mv. I'm doing this with the volume all the way down. I'll give it a go again
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I saved this from a Grainger post. It will help!
Short the inputs, I use a pair of alligator clip jumpers, one for each channel. Attach your meter to the speaker leads. I put the meter leads through the holes in the 5 way binding posts and tighten the plastic part down on them. This is a hands free method to read the magnitude of the hum voltage.
Set your meter to AC volts, the lowest range if it has ranges. Turn on the amplifier and let it warm up for 15 minutes. If you are just measuring hum, this is what you have. Posting be sure to include the milli-volts (mV) reading.
Start moving the hum balance pot one direction. If the voltage goes up change directions. When you get it to lower find where it begins to climb again while moving in the same direction. You have just passed the null point. Now very carefully make minor adjustments to the pot to get the lowest number, milli-volts, you can on the meter. It is best if you use a 4 digit digital meter. This gives you the greatest accuracy.
The stock pot is wire wound and the limit of resolution is "one knuckle." That means one wrap to the next wrap of wire. They are good pots but don't have the resolution that a 10 turn pot has. The half way point between knuckles shorts two windings together. This is why I went to the 10 turn pots for my Paramours in the hum balance circuit.
BTW, I made RCA shorting plugs from a pair of cheap interconnects. Just cut off the plugs leaving an inch or so of the wire, Strip the wires and twist together to short signal to ground. Add shrink tubing if you want to get fancy.
Cheers,
Geary
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If your meter doesn't go low enough you won't get a reading at all. [/font][/size][/color]
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So adding a jumper wire means connecting red/black together?
I think here's my problem. Range is 400mv-400v. What a waste of $65. 10 years ago I would have took it out in the shop and squeezed it in a vice.
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(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F002_zpsy4arhgnq.jpg&hash=068b03401c795a01d25908fb9c5637d1f07aed42) (http://s285.photobucket.com/user/MKane63t/media/002_zpsy4arhgnq.jpg.html)
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That means the lowest range is a 400mV scale. It can read mV with a resolution of 0.1mV. It should be able to read this just fine. My meter at home has the same range, and I have done this measurement with it.
To make sure you are setup correctly, download a 60Hz tone to your phone or computer. Plug your device into one of your inputs and see if you are getting output. Have you used the amp in your system? It might be that you can't make this measurement because of an error on your wiring rather than a problem with your meter. Getting signal out is more important than maximizing hum performance...
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Thank's for the reply. Been using the amp for a while. Just wanted to do this test for shitz & giggles. Been doing it by ear.
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Still nothing.
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My old Paramours are down at 1mV each with the inputs shorted and the hum pot tuned in as best as I can.
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mm shows nothing. The amps darn near silent with an ear up to the speaker, tuned by ear.
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It just occurred to me that if you have the amp wired for 4 Ohms, you may be able to get the noise down below 1mV.
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I have the adjustable ohm setting feature and it is set @ 4 Ohms. Still not showing anything after switching to 8. Maybe I have an extremely quiet unit.
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Here's my hook up's.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F002_zpssq9lhyde.jpg&hash=08b0b0549e2243ef9ba3edbe8d83ea9d6b5090af) (http://s285.photobucket.com/user/MKane63t/media/002_zpssq9lhyde.jpg.html)
Home made shorting jack;
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F005_zpstjqizoui.jpg&hash=3395ceb412133d050aaa4842ba9d20cd5167d086) (http://s285.photobucket.com/user/MKane63t/media/005_zpstjqizoui.jpg.html)
hand's off setting;
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F003_zpsdlkovgqx.jpg&hash=8c2c7b52b631007938f80ae68ab8aa4ac17038c5) (http://s285.photobucket.com/user/MKane63t/media/003_zpsdlkovgqx.jpg.html)
Hands touching hum balance knob;
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F004_zps1o5m9xvp.jpg&hash=c0277dcb51eca5d29485e5beaec474b80ccfd96e) (http://s285.photobucket.com/user/MKane63t/media/004_zps1o5m9xvp.jpg.html)
Seems as though the decimal point's in the wrong place. I'm an electrical idiot. The amp's deal silent at the speakers.??
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Is that with the hum pot adjusted to minimize the voltage reading? (You should be able to get it lower than that on the 4 or 8 Ohm taps)
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Can't seem to
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Touching the volume control should not bring the hum down. There is something wrong here.
The Stereomour should get below 5mA with careful hum pot adjusting.
So... I think you need to touch up all the solder joints on your inputs, selector switch and volume pot.
BTW, Klein are good tools.
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It's the hum pot control that my fingers resting on that bring's the readings down. The tubes are JJ2a3-40 & psvane 12at7 if that make a difference.
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It might be a faulty hum pot.
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Well this is good - at least you are getting a reading now. Congratulations on that!
This looks like one of those things that will be a long slog to find out what's going on - just a heads up!
Since the voltage is higher than expected, and the touching behavior is unusual, I suspect there is some other noise source louder than the filament hum. So I want to go looking for what it might be.
Before going further, I assume this is an older Paramour, on the 6x10 chassis, with C4S, and using the old plate choke as a power supply choke (the old upgrade). As a caution, make sure the safety ground wire from the power cord is in fact connected to the chassis plate and to the signal ground, and that the power socket has a good safety ground.
Assuming you don't have an oscilloscope, perhaps you can connect a speaker and listen to whatever noise it makes, while touching / not touching the potentiometer shaft. Can you hear a difference? What does the noise sound like? Do you get the same effect touching other parts of the chassis plate? Can you hear a difference as you turn the hum pot? If so, where in the rotation is the quietest? How many degrees either way can you rotate the pot before you hear an increase? Have you done this on both monoblocks?
Obviously what I am asking for is more data - we don't have enough on the forum here to make good guesses, so I won't make any just yet.
By the way, when I make low-voltage measurements, I twist the meter wires together for as much of their length as possible, to preclude measuring the ambient magnetic field instead of the circuit - worth a try.
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The unit is a Stereomour, approximately 1 1/2 yrs old. I believe I've found the problem. The power cords ground terminal was broken where it plugs into the wall if this makes any sense. I replaced it with a different cord and the touching phonomina has gone away. I can get the measurement as low as 2.5 and then it goes to 0.0. With the volume knob turned all the way up the speakers (Cornwalls) are silent.
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi285.photobucket.com%2Falbums%2Fll58%2FMKane63t%2F001_zpsomm4k5xx.jpg&hash=b7cb7027091581314a22f409d728b72785d4e55d) (http://s285.photobucket.com/user/MKane63t/media/001_zpsomm4k5xx.jpg.html)
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That is good it is solved. It is not good to have the safety ground open.
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^^^^^^^^^^safety ground open????? You mean my discover?
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Yes! Good catch. Good call Paul.
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:)Now if the quickie would get here.
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There is also a current (10A max fused-mAuA) input, just put glue in those holes; they will get you in trouble.
That gave me a good chuckle.
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Good thing my fingers won't fit.
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. . . That gave me a good chuckle.
The current input has to be a very low impedance so it will not affect the circuit that it is inserted into. If you get the meter probe in the current input and go to measure the voltage at an outlet it smokes the current circuit in the meter and most likely much more.
Working in industry I used the current input for tuning 4-20mA loops. But I can't think of a reason a hobbyist would use it.
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I'll plug em' with something in the morning.