Electronics > Repair
HP 8903A repair help please!
bsgd:
Hi all,
So I've been trying to wrap my head around how to fix my beloved 8903A but no luck so far, and now I need help from the experts.
The problem is that the measured distortion is higher than what it should be, and keeps jumping all over from 0.02% up to 0.09% most of the time. I have borrowed a working 8903A so I managed to replace cards and now I know the problem lies in the A3 Notch Filter card. Replacing it with the one from the working 8903A fixes the issue.
Since the problem is a somewhat slight increase in distortion, following the service manual troubleshooting did not help much since all measurements seem to be within spec, so I started 'guessing' what might be wrong and trying to replace a few capacitors etc, but nothing fixed it.
I would like some guidance whether anyone came across such problem before or where to start looking. The A3 board is divided into what seems to be 2 'separate' sections, one for coarse tuning and another for fine tuning, and I still havent come to a conclusion on which part is most likely to be causing the issue. I have 'compared' measurements from the working A3 card with the non working A3 card but found nothing really suspect so far.
It also seems that the problem is not frequency dependant, as distortion still remains jumping around between 0.02% and 0.09% whether I dial in 100Hz, 1kHz or 100kHz.
Here is a pic from the temporary bench setup I have for comparing both.
Thanks in advance.
cncjerry:
Depending on the voltage levels on that card, you can rig up a probe and feed it into a PC audio analyzer. All you need is an fft and when you see the harmonics jump up, that is your stage. I don't think you would see that level on your average scope FFF. Watch out for large DC levels. I don't remember, I have one, but isn't that card mostly for the weighting?
There shouldn't be too many things that could cause this. Bias, offset adjustments would be fixed but resistors change value over time. Interstage coupling would be a little obvious as there would be clipping, I would think, or approaching it. Bad caps in the coupling would be a starting point. If the voltages are off you would have a bias problem. If you can't rig up a problem check the DC voltages accurately in the signal path with the same signal fed into the good one and the bad one. That would help eliminate bias issues. At that level it could be a darlington pair that went off.
I love mine too. I bought the matching signal generator, 8904? I also have the spectrum analyzer, they call it a signal analyzer, and that is handy for finding problems like this as it has it's own source.
macboy:
Have you looked at the monitor output with a scope? This is the distortion after the notch filter (and other filters is enabled). You should trigger the scope off the input signal to get stable triggering. Comparing the monitor output of the two units may be enlightening.
bsgd:
--- Quote from: cncjerry on February 29, 2024, 01:54:46 am ---Depending on the voltage levels on that card, you can rig up a probe and feed it into a PC audio analyzer. All you need is an fft and when you see the harmonics jump up, that is your stage. I don't think you would see that level on your average scope FFF. Watch out for large DC levels. I don't remember, I have one, but isn't that card mostly for the weighting?
There shouldn't be too many things that could cause this. Bias, offset adjustments would be fixed but resistors change value over time. Interstage coupling would be a little obvious as there would be clipping, I would think, or approaching it. Bad caps in the coupling would be a starting point. If the voltages are off you would have a bias problem. If you can't rig up a problem check the DC voltages accurately in the signal path with the same signal fed into the good one and the bad one. That would help eliminate bias issues. At that level it could be a darlington pair that went off.
I love mine too. I bought the matching signal generator, 8904? I also have the spectrum analyzer, they call it a signal analyzer, and that is handy for finding problems like this as it has it's own source.
--- End quote ---
I have tested all resistor in circuit comparing resistance with the known good A3 board and all is almost exactly equal, little variation of less than 5%. I also have recapped the board completely today and even replaced a few crital opamps. No change |O
I will have to get some extender boards to compare voltages.
--- Quote from: macboy on March 01, 2024, 12:24:15 am ---Have you looked at the monitor output with a scope? This is the distortion after the notch filter (and other filters is enabled). You should trigger the scope off the input signal to get stable triggering. Comparing the monitor output of the two units may be enlightening.
--- End quote ---
I didint do that, good idea. Since residual distortion is only around 0.05% would that be visible with a scope?
George Edmonds:
HI
With a typical modern digital sampling scope you will be lucky to see 10% distortion, certainly not 0.05%
G Edmonds
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