HP 339A oscillator problem

[BSST]

If I recall, much earlier TP1 was about 8V, but now it's very near 0. Sort of opposite extreme. I can't recall TP2. I'll go back and review.

 

[BSST]

Good night. Much progress, I think.

 

[microx]

Progress indeed, many thanks and goodnight

 

[BSST]

I don't know if you've looked at the "phase shift amplifier." This will also be key to proper nulling.

Use your scope channel 1 to monitor and sync on A3 TP4 and probe TP1 on the second channel. Use the 339A generator as the source. You should see a sine wave of equal amplitude and 90 degrees phase shift re TP4 at all frequencies. Any departure from 90 degree offset means a problem problem in frequency resistors (or caps) in either generator or phase shift amplifier. If you have a frequency counter, you can use it to confirm oscillator frequency is correct. If no counter available, the external generator might help distinguish.

Talk to you tomorrow.

 

[BSST]

Good afternoon.

Re the phase shift amplifier: upon further thought, if you see constant amplitude at TP1 and sine waves at U1 pin 3 that are 3dB lower amplitude than TP4 at all frequencies, probably all is well with this section. A less tedious check.

 

[BSST]

I just now considered more carefully the data you provided in post #73. The details of TP1 don't seem plausible in a functional U3B given the values of R21 and C8, irrespective of waveforms presented by U2. I was going to ask for corroborating evidence by asking about waveform at R21, but it's generated by part the same opamp. Same story re TP2 and U5.

You can poke around for confirming symptoms, but both are likely failed.

 

[microx]

Sorry for delay,watching F1 race. On A3 TP4 andTP1 two sine wave signals equal amplitude but not at 90deg phase shift, I would say only about 60deg. Signal on pin 3 of U1 is about 3dB down on TP4. I have a counter also in the scope, 339A osc is slightly low on CAL position ie 1kHz is 980Hz but this is easily corrected with the frequency Vernier.

On A4, TP 3,4 and 5 sine wave similar amplitude. Tp1 garbage TP2 DC only. Circuits from Tp1 and TP2 drive E1 and E2 leds so DC would make more sense than garbage. Also the three trim pots do nothing as far as I can see

 

[BSST]

I have a friend who is an avid F1 fanatic, so I fully understand.

I should have specified more carefully. That you don't observe 90 degrees is concerning, so I'll elaborate/belabor this point.

This circuit is a so-called all-pass network. If you examined phase shift versus frequency, it would be 0 degrees at low frequencies, 180 degrees at high, and in fact it's exactly twice the phase shift the phase shift produced by the R-C lowpass filter driving U1 pin 3. So it should be 90 degrees at the lowpass "corner" frequency, i.e. = 1/(2*Pi*R*C). So in the 339A at 1kHz, R is R1=28.7K, C is C3=5600pF. The formula yields 990.26 Hz. So I'm at a loss to explain what you saw. I'm hoping you measured phase shift at U1 pin 3 and took it to be 60 degrees.

 

[BSST]

Maybe U5 is ok. Does R55 voltage = TP2? What is U5 pin 1 Voltage?

What frequency is garbage on TP 1? What does U3 pin 1 look like?

 

[microx]

If I understand well, if I lay the two traces over each other in phase they would merge into one. The peak of one does not coincide with the trough of the other but somewhere in the middle so there would be incomplete phase cacellation

 

[BSST]

If you are referring to the processing of nulling for best notch depth, yes. For perfect cancellation, the signals have to be equal in amplitude and matched in phase.

But if you're referring to the required precision in the phase shift amplifier, things aren't quite that onerous. The two loops (amplitude and frequency/phase) can still sort things out. It's just preferable that the two balanced demodulators (U2 and U4) operate with reference signals that are 90 degrees offset in phase (in quadrature, in engineering parlance).

 

[microx]

So what should I see exactly with the two traces from TP4 and TP1 overlaid on the scope, in phase would be coincident 90deg looks how

 

[BSST]

If you exactly centered one trace so that there was equal amplitude above and below the display's center line, then shifted the other channel's offset to that its peak fell on the center line, the zero crossing of the first would occur at the peak of the second.

 

[microx]

Exactly my understanding, the peak of the second trace is only about 60 percent of the way between the peaks being in vertical alignment and lying on the zero point of the first trace

 

[BSST]

Not sure what to make of that. Same situation on all decade ranges? Set the "tenths" frequency knob to 0 to make sure it's not involved. BTW, in units knob, if you test 1 through 6 positions, you excersise all resistors. 7 through 11 are paralleled resistors.

 

[microx]

I will check on all ranges tomorrow and let you know,
thanks again and goodnight

 

[BSST]

TTYL

 

[BSST]

I wonder if the frequency switch sections might be out of mechanical alignment.

We know that the oscillator section must be appropriate to the knob indication because you confirmed with a frequency counter. Back in post #55 you found that the notch was way lower than indicated and I recall being surprised it being ~14% off. And now we have phase-shift that doesn't quite compute. The switch decks in the notch-filter are connected to the frequency knobs by shaft couplers. I speculate that the unit-digit frequency switch section might be mis-aligned. ~14% might become ~4%.

Do you recall if you set frequency as 1.0 x 1K or as 10.0 x 100? I suggest trying both settings and measuring ohmmeter resistance between TP4 and U1 pin1. I'm hoping these measurements will reveal resistance at odds with the schematic.

 

[anatech]

Hi BSST,
Sort of breaking the switch, I don't think what you are suggesting is possible.

-Chris

 

[BSST]

I'm speculating that the mis-alignment is introduced within the shaft coupling. But maybe the shafts have flats and such mis-alignment isn't possible? I'm grasping at straws here.