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PostPosted: Tue Jan 10, 2017 12:55 am 
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Joined: Fri Dec 16, 2016 4:57 pm
Posts: 13
I am not getting any current draw when I align VR1. I do not think that any of the VRs are bad because I measured 380 Ohm resistances in two of them during test three and was able to correct those to 52 Ohms by turning them fully counter clockwise 25 turns. I have been through several of the posts on this topic on the forum and I would like to explore the "not measuring current" possibility to start with. Can you give me the specific hook up instructions for measuring the current in the amp? I don't want to fry anything. As an aside, I did measure the voltages at the drains of the mosfets with the unit powered up without test leads and got 13.8 Volts at each, which is where the power supply is set. Please suggest any other voltage checks that I should do, I would like to trouble shoot this as best I can. I suspect that I am not measuring current properly. I am getting a small 5 mA current with the test leads attached, but I think that that is an issue with my DMM. Thanks,


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PostPosted: Tue Jan 10, 2017 6:36 am 
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Joined: Mon Jul 23, 2012 6:25 am
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Hi,

Before you set the bias current you need to make sure that you can measure the current accurately. If there's any question about the test equipment or set up I usually recommend this test:

1. Turn of the amp and you power supply.
2. Connect the negative lead of you power supply to the black side of the PowerPole on the HR50.
3. Connect the positive lead of your power supply to the positive lead of you ammeter.
4. Connect the negative lead of your ammeter to the red side of the PowerPole on the HR50.

(interesting note, you can use a clip lead to banana plug to connect with the PowerPole)

If you turn on your ammeter at this point it should read 0 mA.

Turn on the power supply, after possible a second or two to charger up caps, the meter should still read 0.

Turn on the amp, The current will go to about 75mA for a few seconds then settle down to right around 45mA.

If you don't get this far, there's probably something wrong with your meter.

The next step is to turn off the HR50, the current should return to 0 mA.

Now connect a clip lead from the positive terminal of your power supply to which ever side of the power switch makes the amp come on.

At this point the current should still read 0 mA because you are supplying the current for the control circuitry through the new clip lead, not through the ammeter.

If all this is working, you should be OK to set the amp on 30M and set the bias.

73,
Jim WA2EUJ


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PostPosted: Wed Jan 11, 2017 5:23 pm 
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Joined: Fri Dec 16, 2016 4:57 pm
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Jim, it looks like I definitely had a problem with my DMM. Got a new DMM and passed the current measurement process that you outlined. I was able to align VR2, VR3, and VR4 with no problems, it took about 25 turns but they came in right on spec. VR1 would not respond even in excess of 60 turns, either at the start or once I had the others aligned and attempted to bring the current reading back down. I think that I have a bad VR1 in addition to the bad DMM that I replaced. What do you think?


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PostPosted: Wed Jan 11, 2017 5:55 pm 
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Joined: Fri Dec 16, 2016 4:57 pm
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Jim, additional information: the resistances measured in Test 3 were 54 Ohms for VR1 and 50,51, and 51 for the other VRs respectively.
John, KC2THY


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PostPosted: Fri Jan 13, 2017 2:22 pm 
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Joined: Mon Jul 23, 2012 6:25 am
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Yes it sounds like a bad pot. You can tell for sure if the gate resistance to ground is about 50 ohms then turn the pot CCW a few turns. If the gate resistance is still 50 ohms and hasn't increased, the pot is bad. Send me an email with your address and I'll put a new one in the mail. wa2euj (at) arrl (dot) net.

Jim


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PostPosted: Sat Jan 21, 2017 4:23 pm 
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Jim, I emailed you at the ARRL address with my mailing address.
Thanks,


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PostPosted: Sat Jan 21, 2017 7:51 pm 
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It must have gotten stuck in the filter. I searched for it and found it. I'll have the pot in the mail on Monday.

Jim.


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PostPosted: Wed Jan 25, 2017 4:03 pm 
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Joined: Fri Dec 16, 2016 4:57 pm
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Received the pots, installed one at VR1. Alignment was quick and easy on all four VRs, right on specification.
Thanks Jim,
John KC2THY


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