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HP 33120A waveform is distorted. Video inside. What is going on?
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king.oslo:
Hello everyone,

I bought an HP 33120A on eBay, and when I got it, there was a problem with the output waveform. It seems the bottom of the waveform is being clamped somehow.  :-BROKE

I've made a short video introducing my problem. I hope you are able to watch it, and tell me what you think may be going on, because I have traced the problem to the output amplifier, and I cannot quite work out what is happening there which is causing the problem.

This is a link to the service manual: http://cp.literature.agilent.com/litweb/pdf/33120-90017.pdf
The schematic is on page 137. The block diagram is on page 87. (The diagram for the whole instrument is on page 129.)

This is the video:



Please, if you are able to make any suggestion, I will measure/check it out.

Thank you for your time.

Kind regards,
Marius
w2aew:
Interesting that the problem goes away at the amplitude range change.  Is there some sort of output attenuator that switches at the range limit?  It's tough to see the schematic in the video, but it looks like a multi-stage differential amplifier.  Check to see what gets varied (bias voltages, DAC output, bias currents, etc.) when the amplitude is adjusted. 

I hope to look at the service manual over the holidays to see if I can give you specific things to look at.
alm:
Based on the output clipping only as it approaches a certain fraction of full scale, my first guess would be that one of the transistor amplifiers responsible for the negative part of the signal is bumping its head, possibly due to an incorrect bias voltage (eg. failed bias transistor). It would be good to figure out whether the signal is inverted somewhere along the way before you waste time testing the wrong half of the circuit.
c4757p:

--- Quote from: alm on December 24, 2012, 03:57:22 am ---one of the transistor amplifiers responsible for the negative part of the signal is bumping its head

--- End quote ---

I must add this to my growing list of EE anthropomorphisms.
king.oslo:
Merry Christmas everyone,

I found a resistor (R771). It doesn't look healthy. Look at it. The blue label has flaked off. And the trace and pad underneeth has lifted from the board, yet seems to be connected to the circuit. But I desoldered it, measured it, and soldered it back. It was circa 28 ohms (the schematic specifies 26R1). The voltage drop across it is much greater than the resistor R772 which is the low side equivalent. The drop across R771 is aprox 800mV, while the drop across R772 is about 300mV.


--- Quote from: w2aew on December 24, 2012, 03:46:07 am ---Interesting that the problem goes away at the amplitude range change.  Is there some sort of output attenuator that switches at the range limit?  I hope to look at the service manual over the holidays to see if I can give you specific things to look at.

--- End quote ---

There is an output attenuator, but it only has a bunch of resistors, I do not think it could be clipping the waveform like that.

Second, yet most importantly, the error happens in the output amplifier. By the time the signal gets to the output attenuator, the waveform is already distorted.

Thanks.M
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