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HP 339A oscillator problem
HP 339A oscillator problem
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Old 11th August 2018, 10:36 AM   #131
microx is offline microx  Spain
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correction , U5a pin 1 has a clean sinewave
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Old 11th August 2018, 03:14 PM   #132
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I'm concerned that U5 pin 9 doesn't appear to track TP2. Perhaps it changed during the duration of the measurement. If you connect voltmeter leads between these points, difference should be less than 10mV regardless of null quality. Failure of this check suggests a problem with U5D, Q2, photo-coupler LED, etc.

If you monitor TP2, does it vary smoothly with changes in oscillator frequency? Over what frequency range is nulling achieved? What is null depth? I.e. is nulling loop working at all? Waveform you describe at U3A pin 1 sounds plausible. When loop is nulled properly, its average voltage will be 0.

I'm on a long road trip today and tomorrow, but I'll try to check in when possible.

Good luck.
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Old 11th August 2018, 03:24 PM   #133
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Sorry meant to say U5A pin 1, but same would be true of U3A pin 1. In analogous TP1 fashion, is it relatively stable with changing oscillator frequency and producing a good lull when the frequency is tuned for best results?
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Old 11th August 2018, 05:58 PM   #134
microx is offline microx  Spain
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U5 pin 9 does not track TPS, several volts difference.
TP2. If the 339a is set up for a null with external osc. and I switch it off and then on again the voltage on TP2 starts at about 2vdc and then rises very slowly to 8 to 10 v .
U5 has been replaced, I will check Q2. The photo resistor I will remove and test.
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Old 11th August 2018, 08:32 PM   #135
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I quick test for opamp life: use scope probe at an angle to join out and minus pins simultaneously. Make unity gain follower. You might need second probe or VM to monitor what + input is doing.
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Old 13th August 2018, 08:13 PM   #136
microx is offline microx  Spain
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The 339a will have to go on the back-burner for a few days due to flooded swimming pool pump room. At least it will be easier to fix!
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Old Today, 11:01 AM   #137
microx is offline microx  Spain
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Default more progress

Flooded pump room sorted so back to the beast,

Board A4,
U3, U5 , U6 changed for TL074CN. Before the change I had a null with ext osc under the frequency set by the 339A, after the change the null is at higher ext osc frequency. Maybe the TL074 has gain difference, suggestions...

Board A2
Meter crashing to full scale, random readings etc. Traced to U8, changed for TL074CN, fixed.
Noise on meter when engaging 80kHz filter, changed U1, fixed.
I have now changed all the quadamps.
Rigged up a test of the two frequency leds, one poor the other open circuit. Jury rigged two leds to find that with a null from ext osc the leds work perfectly, high and low, with knife edge sensitivity.
Still on it when time permits.
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Old Today, 01:23 PM   #138
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Sounds like great progress.

On board A4: TP2 and R55 voltage should always track each other within a few mV, despite misbehavior elsewhere in the instrument. Same applies to TP1 and R22 voltage. If this is the observed behavior, then the optocoupler current sources are working properly and trouble lies elsewhere. I'm going to assume this is the case until advised differently.

If the instrument were working properly, the analyser should be able to null over a range of a few percent, centered around the frequency specified by the knobs. I take "knife edge sensitivity" to mean null tracking is very restricted? And higher than knob frequency? These symptoms lead me to guess that the TP2 is "stuck" near ~ 8V, where the cell resistance is near minimum. Hence, null frequency is higher than nominal and tracking range is very restricted. In this scenario, the problem probably lies somewhere in the circuitry bounded by TP2, TP3, and T4.

With knobs set to 1.0 x 1kHz, perhaps you can tabulate voltages versus frequency: TP2, TP1, null depth, and anything other behavior that seems noteworthy.

Best.
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Old Today, 03:00 PM   #139
microx is offline microx  Spain
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nowhere near, TP1 .687vdc R22 10.65vdc. R22 was open circuit replaced now TP1 118mv R22 0v.
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