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HP 33120A waveform is distorted. Video inside. What is going on?
king.oslo:
Hello everyone,
Since last time, I have replaced a bunch of transistors. The rest were desoldered and measured before soldering back in.
The one marked in red was desoldered, no matter where I measured it base-emitter, emitter-base, emitter-collector, I measured in all the combinations possible, and always got my multimeter showing OL. I replaced it with another PNP transistor I had laying around.
The output is still clipping.
What do you advice that I check next?
Thank you for your time.M
free_electron:
check on R704 ( at the opamp ) that you have an UNDISTORTED signal.
that will eliminate some other problems.
then check the collecotr resisotr sof the endstage transisotrs( like the red one ) as well as the zeners in the collecctor line... verify you got the right voltages there.
Pat Pending:
I'm wondering if the DC offset has skewed the output signal too close to a negative supply rail. So I'd try this:
Set the waveform amplitude to zero, then sweep and measure the DC offset at the output. Should be linear.
Connect the opamp +, R701, R702, R703 node to AGND. Output waveform should be centered about 0V.
king.oslo:
--- Quote from: Pat Pending on January 14, 2013, 10:27:43 pm ---I'm wondering if the DC offset has skewed the output signal too close to a negative supply rail. So I'd try this:
Set the waveform amplitude to zero, then sweep and measure the DC offset at the output. Should be linear.
Connect the opamp +, R701, R702, R703 node to AGND. Output waveform should be centered about 0V.
--- End quote ---
Thanks for your reply. I'd really like to check what you are suggesting. My problem is first of all, I am unsure I understand what you are asking. Do you want me to set amplitude to 0? (this is not possible, it will only go as low as 50mVPP
Second: The offset doesn't change at the output of the instrument. It only changes when the instrument changes range. Consequently, nothing changes when I short NI+ node of the opamp. Furthermore, when I probe the waveforms and ni+ and in- of the opamp, and I notice that the DC offset is only visible at ni+. The DC shows no offset. What are your thoughts?
--- Quote from: free_electron on January 14, 2013, 09:20:09 pm ---check on R704 ( at the opamp ) that you have an UNDISTORTED signal.
that will eliminate some other problems.
then check the collecotr resisotr sof the endstage transisotrs( like the red one ) as well as the zeners in the collecctor line... verify you got the right voltages there.
--- End quote ---
All the resistors on in the output amplifier is the correct value.
I measured the input to the opamp. My Rigol scope says that Vmin is -424mV and Vmax is 432mV. Is this DC offset no problem, you think?
Shuggsy:
--- Quote from: king.oslo on January 15, 2013, 12:15:33 am ---
--- Quote from: free_electron on January 14, 2013, 09:20:09 pm ---check on R704 ( at the opamp ) that you have an UNDISTORTED signal.
that will eliminate some other problems.
then check the collecotr resisotr sof the endstage transisotrs( like the red one ) as well as the zeners in the collecctor line... verify you got the right voltages there.
--- End quote ---
All the resistors on in the output amplifier is the correct value.
I measured the input to the opamp. My Rigol scope says that Vmin is -424mV and Vmax is 432mV. Is this DC offset no problem, you think?
--- End quote ---
Is the signal at that node (or the other side of R704, net name of +AMP_IN on pg.137, the output amp schematic) UNDISTORTED, as free_electron said? That is, as you vary the output amplitude, you should essentially see whatever sine wave you're trying to output, I think... just at a lower gain than the final output stage. Best to measure that with a high impedance termination on the scope (in case you happen to be using a 50 ohm termination for these measurements. Not sure if you are! No worries :)).
Since that is the (non-inverted) input to the output amp stage, if you see a clean signal there then the problem definitely lies within the output amp. You already noted the distortion/clipping exists at the input to the output attenuator, so if the signal is good when it enters the output stage and bad when it exists... the problem is within!
If the signal is clean at that node, then keep going with what free_electron said. If the signal IS distorted, then check back to the pre-attenuator stage... I'd just keep working your way back until you see an undistorted signal, if possible. Always good to check things from both the input and output sides... keeps narrowing down where the issue is.
My money is on some obscure behavior in the output amp or something odd in the pre-attenuator stage. Out of curiosity, does varying R710 have any affect on your issue?*
*EDIT: Just read a little more on the manual. There's a cool write-up on the theory of operation for the output amp on pages 87-88 of that manual. I'm doubtful varying R710 will do much now, but if it's not sealed in some set position then it likely won't hurt to check. Also, it may pay to check the bias current setting resistors of R734 and R727 on the low side of the output stage.
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