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HP 339A oscillator problem
HP 339A oscillator problem
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Old 8th July 2018, 12:35 PM   #1
microx is offline microx  Spain
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Default HP 339A oscillator problem

Hi all, I have no output on TP1 so I guess that U1 has died. If not what else fails and what should I use for U1. Also should I remove the board, lot of work, or change the chip with the board in place.Steve
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Old 8th July 2018, 01:28 PM   #2
Chamberman is offline Chamberman  United States
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Is the +/- 15V present?
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Old 8th July 2018, 05:27 PM   #3
microx is offline microx  Spain
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yes +15 -15 are correct
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Old 9th July 2018, 11:39 AM   #4
Chamberman is offline Chamberman  United States
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If you're going to begin replacing parts then you may want to look at performing the mods outlined here by Davada post #1184. That thread will also answer your question as to what opamp to use for U1.

These upgrades greatly simplify the oscillator circuit and drop the noise substantially. I performed them to my old 339A with great results.
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Old 9th July 2018, 06:54 PM   #5
microx is offline microx  Spain
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Thanks for the info. Chip is out of board and waiting for order from Digi-Key. I hope the FET is ok, its listed as a selected part.
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Old 12th July 2018, 12:27 PM   #6
BSST is offline BSST  United States
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If TP1 showed ~0V DC, the opamp probably wasn't the problem. If this is still the case with new opamp in place, a quick divide-and-conquer diagnostic test is to short drain and source terminals of the FET. The oscillator should start and be driven into clipping.

On the other hand, if TP1 is stuck on either rail, suspect a bias path problem, most likely associated with frequency tuning switches. TP1 and the minus input of the opamp should be at the same DC potential.

Good luck.
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Old 17th July 2018, 03:42 PM   #7
microx is offline microx  Spain
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Default fresh problem

OK, well I have got the oscillator running -U1 and U2 changed- and now I seem to have too much output. On the 3v osc range I have a max of 7v at the output and the distortion check routine cannot be set up, the meter jumps to full scale and I cannot set -15dB, could this be the control FET Q2, I dont have one and it is listed as selected. How critical is it,
Steve.

thanks for your help
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Old 17th July 2018, 03:53 PM   #8
Chamberman is offline Chamberman  United States
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Are you running it into 600 ohms?
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Old 17th July 2018, 04:16 PM   #9
microx is offline microx  Spain
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I have tried 600 ohms in series and shunted but on the 3v range I cannot get -15dB and if I drop a range the meter needle jumps to full scale as I slowly advance the Vernier from minimum. The waveform is very clean on a scope.
Steve
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Old 17th July 2018, 06:14 PM   #10
Chamberman is offline Chamberman  United States
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I used a 600 ohm resistor across the osc outputs.

Did you recheck the osc amplitude as outlined in sect 5-14 of the manual?
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