final alignment
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Author:  KB3SZZ [ Mon Apr 02, 2018 2:29 pm ]
Post subject:  final alignment

Hello: My kit building experience went perfectly until I got to the very last stage of VR1 - VR 4 alignment. I can get
VR1 to give me 150 mA and VR2 brings it to 300 mA. After that, little/no change occurs when twisting VR3 or VR4, which is puzzling.

After setting VR1 and VR2, I redid the ohmmeter test and got about 333 ohms for the "VR1 to gate of Q1" and "VR2 to gate of Q2" test. I also tested the resistance for "VR3 to gate of Q3" when I twist VR3 clockwise and can see that it rises just as well, same for VR4 - so I suspect the VR's are all good. (All VR to gate measurements started at about 52 ohms when I started.)

My solder joints look good, but something must be off. If these clues give enough info for where I should start my troubleshooting, I would appreciate any input.



Author:  jimveatch [ Tue Apr 03, 2018 7:37 am ]
Post subject:  Re: final alignment

Hi Ed,

Good troubleshooting so far. There's a couple things I'd like to know to help figure out what's going on.

When you are turning VR3 and VR4 is the gate voltage of Q3 and Q4 (respectively) rising. The VR's should vary the gate voltage from 0V to 5V. It would be highly unlikely that it the voltage doesn't come up because the same 5V source is also driving VR1 and VR2.

Also check the drain voltage on Q3 and Q4 it should be the same as your power supply, 13.8V. If it's not then the most likely cause is something wrong with L5.


Author:  KB3SZZ [ Tue Apr 03, 2018 8:22 pm ]
Post subject:  Re: final alignment

Thanks, Jim. Just to make sure about the voltage test - I have the HR 50 turned on, but no test leads are in place. Then I place the black DMM lead on GND and the red DMM lead on the gate. If this is correct, I am only getting readings of about 0-5 mV (not V) as I twist the VR.

The drain voltages DO, however, check out at 14.2 V (same as my power supply) - with black lead on GND and red lead on drain.


Author:  jimveatch [ Wed Apr 04, 2018 5:04 am ]
Post subject:  Re: final alignment

Hi Ed,

In order to get the gate bias voltage, you'll need to put the HR50 into PTT mode and key it by grounding the center pin of the RCA jack.

The drain voltage is present all the time (including when the amp is switched off).


Author:  KB3SZZ [ Wed Apr 04, 2018 5:05 pm ]
Post subject:  Re: final alignment

Jim, everything you suggested checked out perfectly with the voltages, so I decided to borrow another DMM from a friend and start over with the alignment on the off chance that my DMM was functioning incorrectly and...that was it. Apparently, the fuse was bad from something I did in the long ago past and I was exceeding the limits for the mA range, which is not fused. I never would have discovered it without your help, so thanks once again!

It is perfectly aligned now!

Best 73!

de KB3SZZ dit dit

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