BK Precision 4010A Out-of-Tolerance Calibration: A Technical Case Study

An instrument can pass almost every calibration check and still fail one important function.

That is exactly what happened when two B&K Precision 4010A function generators were sent to a JM Test Systems laboratory for calibration.

Both instruments performed within tolerance for most of their checks. However, the voltage-controlled generator (VCG) frequency function failed verification.

The issue was not immediately obvious, and the technician could not find a documented adjustment procedure.

The original JM Test technical brief, written by senior RF technician Mark Price, describes how the laboratory investigated the problem, contacted the manufacturer, made an adjustment and verified the result. 

This case illustrates why calibration involves more than checking a specification and applying a pass-or-fail label.

It requires understanding how an instrument works, identifying the failed function, documenting the original condition and determining the appropriate corrective action.

What Is the BK Precision 4010A?

The B&K Precision 4010A Function Generator is a laboratory signal generator used to produce electrical waveforms for testing and troubleshooting.

The instrument supports sine, square, triangle, pulse and ramp waveforms, along with adjustable frequency, amplitude and DC offset.

What Is a Voltage-Controlled Generator?

A voltage-controlled generator changes its output frequency in response to an external control voltage.

On the 4010A, an external positive DC voltage can be applied to the VCG input to reduce the selected frequency.

For example, the 4010A operating instructions describe a change from approximately 200 kHz to 2 kHz when the appropriate positive control voltage is applied. This represents a frequency reduction of 100:1. 

VCG operation is useful in applications involving frequency control, signal sweeps and electronic circuit testing.

However, the function must respond correctly to the specified control voltage.

If the frequency changes too early or too late, the instrument may not meet its performance requirement, even though its other functions appear normal.

The Calibration Problem

Two BK Precision 4010A function generators arrived at a JM Test laboratory for calibration.

According to the original technical brief, nearly all parameters passed verification.

The exception was the VCG frequency function.

The laboratory's documented check required the output frequency to decrease from 200 kHz to 2 kHz when the control voltage reached a value between 9.0 V DC and 11.0 V DC.

Instead, the two instruments reached the frequency division point at lower voltages.

That made the result an out-of-tolerance condition, even though the generators passed their other calibration checks.

What Does Out of Tolerance Mean?

An instrument is out of tolerance when its measured performance fails to meet an applicable acceptance limit.

That limit may come from:

• Manufacturer specifications

• Customer requirements

• A calibration procedure

• An engineering specification

• A quality-management programme

• A particular operating or measurement range

An out-of-tolerance result does not necessarily mean the complete instrument has stopped functioning.

For example, a function generator may produce acceptable waveforms and output levels while failing one voltage-control check.

A multimeter may perform correctly in most ranges but fail one current range.

A pressure gauge may pass at low pressure but fail near the upper end of its range.

The important question is which function failed, by how much, and whether that function was used in work requiring the specified performance.

How JM Test Investigated the Problem

The original technician first searched the available technical material for information about adjusting the VCG function.

No suitable adjustment instructions were found.

The technician then contacted B&K Precision.

According to the original JM Test article, the manufacturer suggested adding resistance in series with the VCG input to change the control-voltage response.

The technician subsequently implemented an adjustable resistance arrangement, evaluated the effect and verified the generator's performance.

One instrument required a different adjustment component to obtain the desired response.

After the work, the VCG frequency division point was brought to approximately 10.0 V DC, near the middle of the documented acceptance range. 

This was a historical laboratory solution for specific instruments.

It should not be interpreted as a universal modification instruction for every BK Precision 4010A. Any internal adjustment or modification should follow current manufacturer guidance, approved laboratory procedures and qualified service practices.

Why Manufacturer Support Matters

When an instrument fails calibration, the cause is not always straightforward.

The problem may involve:

• Component ageing

• Internal drift

• Damaged circuitry

• Incorrect setup

• Calibration procedure differences

• Environmental conditions

• Measurement uncertainty

• Instrument configuration

• An adjustment that is no longer correct

• A genuine instrument fault

Manufacturer information can help distinguish between normal adjustment requirements and deeper equipment problems.

In the 4010A case, contacting the manufacturer provided information the technician could not find in the available reference material.

That helped the laboratory investigate the issue without assuming that the entire instrument had failed.

Calibration, Adjustment and Repair

These three terms describe different activities.

The original JM Test technical brief described the work on the 4010A as an adjustment rather than a repair. 

However, the classification of physical instrument modifications can depend on the manufacturer's instructions and the service provider's quality procedures.

An instrument should be verified again after adjustment or repair to determine whether it meets the applicable performance requirements.

Why As-Found Results Matter

As-found data shows the condition of an instrument before adjustment or repair.

For the two function generators, the initial VCG results were 8.3 V DC and 8.8 V DC.

Those values matter because they document the failure rather than simply reporting that the instruments were adjusted.

As-found data helps answer:

• Which function failed?

• How far was it outside the acceptance limit?

• Was the error consistently high or low?

• Could the error have affected previous work?

• Was the instrument drifting over time?

• Should the calibration interval be reviewed?

For an instrument used in production testing or customer acceptance, this information can be important.

A failed as-found result may trigger an assessment of earlier measurements made with the equipment.

It does not automatically mean all previous results were invalid.

The impact depends on the instrument's use, the magnitude and direction of the error, and the acceptance limits of the work performed.

Why As-Left Results Matter

As-left data shows how the instrument performed after any adjustment or repair.

In the reported 4010A case, the VCG response was adjusted to approximately 10.0 V DC.

That result demonstrated improvement in the failed parameter.

A complete return-to-service decision should also consider whether the remaining required functions were verified and whether the instrument met the applicable acceptance criteria.

Good as-left documentation helps identify:

• The final calibration condition

• The adjustment performed

• The final measured result

• The specification used

• Any remaining limitations

• Whether the instrument was accepted for return to service

JM Test Canada's calibration services information includes as-found and as-left data among its calibration documentation features. 

Why One Failed Parameter Should Not Be Ignored

A common mistake is to assume that an instrument is acceptable because most of its functions passed.

An instrument's fitness for use depends on its application.

For example, an organization may use a function generator for:

• Electronic circuit development

• Frequency-response testing

• Equipment troubleshooting

• Educational laboratory experiments

• Production testing

• Research and development

• Signal generation

• Verification of other instruments

If the VCG function is part of the required application, a failed VCG check may have a meaningful impact.

If that function is never used, the organization may assess whether the instrument can be restricted to specific tasks, subject to its quality requirements.

Any limitation should be documented and communicated clearly to the user.

Measurement Uncertainty Still Matters

Passing a calibration specification does not mean a measurement is perfectly exact.

Every calibration measurement has uncertainty.

A calibration laboratory must consider the capability of its reference standards, the test setup, repeatability, resolution and other relevant sources of uncertainty.

In Canada, NRC's metrology guidance explains that traceability requires an unbroken chain of comparisons to a recognized measurement reference, supported by stated uncertainty and appropriate documentation. 

For the BK Precision 4010A example, the acceptance range is known from the historical article.

However, that article does not provide the complete uncertainty budget or decision rule for the measurements. The published case should therefore be treated as a practical technical illustration rather than a complete accredited calibration report.

Canadian Calibration and Traceability Context

Canadian laboratories and industrial customers may need calibration results supported by documented measurement traceability.

The applicable chain can lead through NRC Canada, NIST or another recognized national metrology institute.

Where accredited calibration is required, the customer should check whether the exact measurement type, range, method and uncertainty are included in the provider's accredited scope.

The Standards Council of Canada operates a Testing and Calibration Laboratories Accreditation Program based on ISO/IEC 17025. 

NRC also confirms that, following the discontinuation of its Calibration Laboratory Assessment Service on April 30, 2026, SCC is responsible for the relevant ISO/IEC 17025 laboratory accreditation functions.

For older electronic test equipment, customers should confirm that the laboratory can support the instrument and the functions they need calibrated.

Why Older Electronic Test Equipment Needs Careful Assessment

Legacy test equipment can remain useful for many years.

However, older instruments may present challenges:

• Original service documentation may be difficult to locate.

• Manufacturer support may be limited.

• Replacement parts may be unavailable.

• Some internal adjustments may require specialized knowledge.

• Certain functions may no longer meet original specifications.

• Repair may cost more than replacement.

• Updated customer requirements may exceed the instrument's capabilities.

B&K Precision lists the 4010A as a discontinued model. 

That does not automatically make an existing unit unusable.

It does mean that any calibration, adjustment or repair plan should consider the equipment's age, condition, supportability and intended application.

What to Do When an Instrument Fails Calibration

When an instrument is found out of tolerance, the next decision should be based on the failed parameter and the work the instrument supports.

A quality or maintenance team should:

1. Review the as-found results and failed test points.

2. Confirm the tolerance, specification and decision rule used.

3. Identify work performed with the instrument since the last acceptable calibration.

4. Assess whether the error could have affected previous results.

5. Determine whether adjustment, repair, restricted use or replacement is appropriate.

6. Verify the final instrument condition before returning it to service.

7. Document corrective actions and update equipment records.

The amount of follow-up required depends on the risk.

A minor failure on non-critical equipment may need a straightforward corrective action. A similar failure on equipment used for product release, safety-related decisions or regulated testing may require additional review.

What Should a Calibration Certificate Include?

A useful calibration certificate for an electronic function generator may include:

• Instrument manufacturer and model

• Serial number and customer asset ID

• Calibration date

• Test procedure

• Functions and ranges tested

• Applied reference values

• Measured results

• Acceptance limits

• As-found results

• As-left results, where applicable

• Adjustment or repair notes

• Measurement uncertainty

• Reference standards

• Traceability information

• Environmental conditions where relevant

• Conformity statement and decision rule, where applicable

• Accreditation information, where applicable

A certificate that simply states "Pass" may not provide enough detail for investigating an out-of-tolerance condition later.

Canadian Industries That Depend on Electronic Calibration

Electronic signal generators and related instruments support work across Canadian industries.

Telecommunications: RF and communication systems require accurate signal generation, frequency verification and electronic troubleshooting.

Manufacturing: Production test equipment must perform consistently to support quality and acceptance decisions.

Utilities and power: Electronic measurement equipment helps with maintenance, controls, instrumentation and equipment verification.

Aerospace: Test procedures may require documented calibration and traceability for instruments used in maintenance or manufacturing.

Research and education: Laboratories use signal generators and measurement instruments for experiments, development and training.

Industrial maintenance: Older electronic test equipment may remain in service for troubleshooting and instrumentation checks.

The consequence of a calibration failure depends on how the instrument is used, not simply how old or expensive it is.

JM Test Systems Canada Calibration Support

JM Test Canada's calibration services page lists electronic instrument calibration capabilities and describes calibration documentation that can include traceable certificates, calibration stickers, QR-coded records, as-found and as-left data, and historical records accessible through a customer portal. 

For older instruments such as the BK Precision 4010A, service availability should be confirmed before shipping equipment.

Customers should provide the model, serial number, required measurement functions, known faults, calibration requirements and any previous service records.

Where specialized adjustment or repair is required, the laboratory should confirm whether it can perform the work and what limitations may apply.

Practical Takeaway

The BK Precision 4010A case shows how an instrument can be mostly within specification but still fail an important calibration parameter.

Two function generators failed the VCG frequency verification at 8.3 V DC and 8.8 V DC, below the documented acceptance range of 9.0 to 11.0 V DC.

After investigating the problem and obtaining manufacturer guidance, the technician adjusted the instruments and brought the measured response close to 10.0 V DC. 

The broader lesson is that a failed calibration should lead to investigation and documented corrective action.

Understanding the failed function, recording as-found results, applying an appropriate service procedure and verifying the as-left condition can help organizations make better decisions about older test equipment.

JM Test Systems Canada offers electronic instrument calibration services where available. Contact the Canadian team to confirm service capability for legacy function generators, applicable ranges, certificate requirements, adjustment or repair options and turnaround time.