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BPI 3000c Distortion Analyzer Auto-Null Problem
BPI 3000c Distortion Analyzer Auto-Null Problem
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Old 9th July 2018, 02:36 PM   #1
ejmathews is offline ejmathews  United States
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Location: Westport, CT
Default BPI 3000c Distortion Analyzer Auto-Null Problem

I have been repairing a BPI 3000c auto-nulling distortion analyzer and have had to replace quite a few parts to get it working. Overall, things are looking pretty good but I’m having problems with the auto-nulling circuit for frequencies above 7 kHz. The unit should be able to read down to .005% distortion from 20-20,000 Hz. It now works to spec below 7 kHz but won't go below 2% at 20 kHz.

The auto-nulling has an auto-tuning notch filter circuit and an auto-gain circuit both based on optocouplers. Although they both work at lower frequencies, the gain setting reaches it limit on higher frequencies and so about 2% “distortion” remains in the signal after auto-nulling. The remaining waveform significantly shows the fundamental wave. Something is limiting the ability of the circuit to null correctly above 7 kHz. The tuning component appears to be working correctly but the gain circuit runs out of gas before the waveform is fully nulled.

I have spent quite a bit of time on this problem and haven’t figured it out. I have schematics and can explain the circuit topology fairly well but obviously I have some limits to my understanding of how it works.

Does anybody have familiarity with this unit?

Thank you,

John
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Old 12th July 2018, 01:06 AM   #2
1audio is offline 1audio  United States
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BPI 3000c Distortion Analyzer Auto-Null Problem
I don't have experience with that unit but I do have some experience with those types of problems. Is there a trim on the nulling circuit? Can you get a null at 20 KHz even if you sacrifice at 1 KHz?
The problem will usually be either low gain in the notch at HF or the phase detector is not detecting the phase correctly.
Can you share the schematics or the relevant part of the schematics?
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Old 12th July 2018, 02:06 PM   #3
ejmathews is offline ejmathews  United States
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Default BPI 3000c Auto-Null

Thanks for taking a look. I have set the nulls at 1 Khz and have not tried at 20 Khz. Attached are the schematics of the two main boards that affect the THD circuitry and the overall unit topology.

John
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File Type: jpg BPI3000c_p14.jpg (420.4 KB, 46 views)
File Type: jpg BPI3000c_p16.jpg (503.7 KB, 44 views)
File Type: jpg BPI3000c_p22.jpg (481.9 KB, 43 views)
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Old 14th July 2018, 03:35 PM   #4
1audio is offline 1audio  United States
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BPI 3000c Distortion Analyzer Auto-Null Problem
A few suggestions-
1) get replacement optocouplers. its not easy to crossreference but these guys seem to have the best available today: http://lunainc.com/wp-content/upload.../06/NSL-32.pdf
2) Check the caps for the HF steps, they are all small values so there may be some detuning going on.
3) Opamp upgrades could improve the performance however they can also backfire since the circuit was tuned to the limitations of the opamps in it.
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Old 16th July 2018, 12:36 PM   #5
ejmathews is offline ejmathews  United States
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Thank you, Damian.

I have replaced two of the three optocouplers without seeing any changes. The third is on the main board and I will also replace it. After that I'm going to look at the cap values and also the signal contacts on the main frequency selector switch.

I have avoided op amp upgrades because of concern over upsetting the original circuit tuning. I am thinking that op amp frequency response is an unlikely contributor to the problem at audio frequencies. However, there's something lurking in there and I am driven by curiosity, as much as anything else, to figure out what it is.

John
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