HP 339A oscillator problem

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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.
 
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
 
Do you have an oscilloscope available to look at the output for signs of obvious clipping?

If no scope available, would you report a few DC voltages? First, I assume the that the shorting diagnostic test that I suggested earlier in NOT in place. Would you advise the reading you observe on TP3, 4, 5, and 8?

Steve
 
If it turns out the level control jfet (VCR2N) is bad some folks have tried alternate parts in the 339A. Here's what Dick Moore had to say. "Subbing in the VCR2N, PN4091 and PN4092 revealed that the VCR2N and the PN4091 produce essentially identical results, and that the PN4092 works the best of the three."

These parts are all out of production, however having additional options may increase your chances.
 
BSST, yes I have a scope and now I have the oscillator running I see the waveform is very clean with no clipping. All the function tests are OK except the distortion check. I cannot get -15dBV on the meter, advancing the osc Vernier slowly from zero the meter abruptly jumps to full scale.
Chamberman. Yes I set the osc amplitude as 5. 14 in the manual, I think the osc section is now OK, the output level is about right into a 600 ohm load.
Steve
 
If it turns out the level control jfet (VCR2N) is bad some folks have tried alternate parts in the 339A. Here's what Dick Moore had to say. "Subbing in the VCR2N, PN4091 and PN4092 revealed that the VCR2N and the PN4091 produce essentially identical results, and that the PN4092 works the best of the three."


That would make sense looking at rDS(on) or rds(on):
VCR2N = 60 Ohms (Vishay/Siliconix)

PN4092 = 50 Ohms (Central)
PN4091 = 30 Ohms (Central)
 
Hi Microx. I'm not sure what to make of the abrupt change in meter reading. The oscillator "vernier" is the amplitude knob? Is the behavior similar on all ranges? Any associated activity with the arrow LEDs?

I'm finding it a bit difficult to follow the test procedure you're using. May I suggest that you advise what section of the service manual you are pursuing, control settings, etc, and add a a brief narrative of what you're observing.

We are burdened with a 6 hour time zone difference. I'm at work and feel guilty if I spend more than a few minutes on the forum. If you are still having trouble, perhaps we can a less delayed exchange Saturday or Sunday and have something approaching interactive troubleshooting.

Good luck.
 
That is to say test 3-52 in the manual. I set up as per the manual but I cant get lower than -4dB, if I drop a range on the output and then slowly advance the level control that is when the meter jumps to full scale. Anytime you can help me is OK with me including the weekend, I am very grateful for your help.
 
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Thanks for the bit about the frequency arrows. I'll think about that and see if I can conclude anything. A thought: of course the 339 has a built oscillator. But might you have a second audio generator available? It need not have have low distortion, but might be useful if available.

Talk to you later.
 
yes , I have several audio oscillators and I have tried input to the distortion
amplifier and I get no led arrows lighting when I vary the nulling frequency to match the external osc. nor do they light when I connect the internal osc. to the distortion input and change the multiplier. I would expect them to blink on and off as it searches for the null. Food for thought.

U6 quadamp maybe in the error detector

U2 and U4 have been changed in the past
 
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The abrupt jump you reported in post #16 might be normal behavior. Refer to the A2 schematic and see notation near Q3.

I'm going to make a wild guess at the problem. If wrong, we will have to proceed more methodically.

Return to test 3-52 and measure distortion. If I understood, you saw only 4dB reduction in amplitude? Is the waveform at the monitor output still a clean sine wave at the fundamental? That is to say, like the input, but reduced in amplitude only modestly?

If this the case, I suspect an amplitude nulling problem. Referring to A4, would you report voltage seen at TP1 and U3C-pin 8 (i.e. - input)? These two voltages should be equal. If they are equal and are about 9V, then an amplitude null problem is a near certainty and chief suspect is the opto-coupler E2 in the A3 module. Now we need to confirm it has failed. Use a scope to measure signal amplitudes at TP4, and U3-pin 6, and U3-pin-2. Presumably all three are clean sine waves. Of course pin 6 should be nulled to 0 if all were working properly. Knowing these three amplitudes should allow us to ascertain the U2 photocell resistance.

If I've ventured into the weeds, let me know what you do observe and we'll take it from there.

Good luck. TTYL.
 
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