General > General Technical Chat
HP 33120A waveform is distorted. Video inside. What is going on?
king.oslo:
The waveform is not distorted before the output amp. It has is a few mV offset, but not clipped or distorted. I think the problem is in the output amp too. But now I have measured all the resistors, replaced more than half of the transistors and measured the rest of tem. Is my best bet to canalization the output amplifier with hot air, soak it in flux and rebuild the whole amplifier from scratch? |O :-+
Adjusting R710 offsets the output signal a bit, so it lifts itself a bit out of the clipped range, but only by about 3 percent of the amplitude.
Thank you for your time time :)
Kind regards,
Marius
Pat Pending:
Since the output amp is symmetrical try taking voltage readings at node pairs relative to AGND, they should be similar in magnitude assuming the offset can be zeroed.
king.oslo:
Hello again,
I made the decision to change both Q713 and Q718 (the big TO-39 output transistors with heatsinks), and I was very, very pleased to observe that the waveform was no longer being clipped.
But the offset functionality still doesn't work. When I offset the waveform, I expect that the current-injection-opamp should shift the output up, and both its inputs should be equal beause it is in negative feedback. This is not the case. The opamp doesnt have the same waveforms on both its inputs. I reckon I will try and replace it. One time in the past whilst probing the output of this opamp, I managed to short its V+ power rail to its output with the scope probe. I remember seeing on the scope that the offset of the output waveform made a small jump of about 10mV. A few days after I found that the offset functionality wasnt working.
Hopefully after I change this opamp, I will once again have a working instrument.
Thank you to everyone whom helped me; thank you! %-B
Kind regards,
Marius
fran:
Hi all,
I know this is a zombie thread, and I am new here but I am seeing a similar problem to the OP so I thought it best to post here rather than start a new thread.
I picked up this hp3320a locally, and it has been working fine until this week - the symptoms are clipping of the top of the sine wave (fairly constant once you go above a lowish amplitude). It is clearly happening on sine, square, sawtooth waves but its not visible on square wave. Amplitude is accurate enough, as is frequency.
So I pulled the manual and started looking around. The signal into +amp_in and -amp_in are clean. I have a DC offset of ~200mV - this jumps in steps as the relays switch to each output range - doubling to 400mV, then 800mV etc.
So I'm a bit confused as to where to go next - and I'm not very experienced at this, so apologies in advance if I have missed anything.
Measurements:
U702 - AD711 - +/-15V present on pins 4 and 7. Pin 5 (output) has -6.5V , pin 2 (-input) -2mV, pin 3 (+input) -2mV.
R702 - which I think is in the feedback circuit - one side has 2mV (which must be the U702 side) and the other side has 6.1VDC.
Q701 and Q702 - see photo, I have clean signal on the base of these, but some distortion on the emitter of each. I'm seeing +/-13V on the collector of each, and -6mV and -3.6mV on the base of each.
I've done the transistor checks (in circuit mind you) around the amplifier and all seem OK - ie no dead shorts.
The resistors around the output (R834/R815/R835 all 69r) at K804 have gotten hot at some point, and I lifted these off the board and they all measured perfectly.
I pulled Q713 and Q718 which seemed to be the OP's problem, but they measure fine in a cheap transistor tester.
I'm a bit stuck as to where to go next to try save this nice piece of test equipment, any help is much appreciated.
strawberry:
feedback loop measurements are tricky because feedback will compensate actual problem
output transistors are most likely
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version