Re: Help to repair my HP654A Test Oscillator
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What's your secret Jeremy?? I have a couple of 200CDs but they are nothing to write home about regarding distortion.
Bob K6DX
On Saturday, December 14, 2019, 10:19:52 PM PST, Jeremy Nichols <jn6wfo@...> wrote:
My650A did fairly well distortion-wise but my favorite is the 200CD. With a few tricks and a little tweaking I was able to get it down to 0.04%, well below spec.
Jeremy
On Sat, Dec 14, 2019 at 9:43 PM Bob Albert via Groups.Io <bob91343= [email protected]> wrote:
I know this doesn't help but I did want to say that, in all my years of working with HP gear, this circuit is about the worst design.? I have repaired my 651A a couple of times and have never been able to get it to meet its distortion spec.? Mine works okay but I got tired of messing with it and it's no longer in line for use.? My main repair seemed to center about that FET in the middle of the diagram.? I tried a few different types without much success.? I ddi finally get it to oscillate on all ranges.? The distortion runs a bit over 0.5% worst case and the frequency calibration isn't as good as it could be, but it remains a useful instrument.
Perhaps you'd like to play with it.? The main thing I like is the nice attenuator.? Back in the day, I had a 650A that was a marvelous piece of gear.? Too bad this one didn't match that.
Bob
On Saturday, December 14, 2019, 08:35:49 PM PST, Don Bitters via Groups.Io <donbitters= [email protected]> wrote:
Almost certainly 50Hz AM modulation - 20msec per cycle.? Does touching the 645 chassis change the amplitude of the modulation, if so check the chassis ground at the power cord input, you may have an open ground pin.? If not check the power supplies for 50/100 Hz modulation, percentage of voltage of the supply, the worst looking supply is probably the culprit. Don Bitters
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