Test equipment and power supplies · TMDE family

Agilent 668xA: troubleshooting and error codes

The ServiceForge knowledge base holds 27 documented symptoms for the Agilent 668xA, with 39 ranked causes, 117 diagnostic checks with test points and expected values, and 24 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

No display on front panel

Front panel display is blank or not showing any output after power-on

Ranked causes

1. Front panel microprocessor failure (occasional)
2. Front panel ROM failure (occasional)
3. Front panel EEPROM checksum error (occasional)

3 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Primary interface circuit failure

Primary interface RAM, ROM, or processor not functioning correctly

Ranked causes

1. Primary external RAM failure (occasional)
2. Primary microprocessor failure (occasional)
3. Primary ROM failure (occasional)

3 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: EEPROM initialization

Front panel board or EEPROM replaced; power supply requires reinitialization with proper constants

Ranked causes

1. Front panel board or EEPROM replaced (common)
2. Calibration constants not transferred to factory preset locations (common)

First check

Run EEPROM initialization program Figure 3-18

Test point: Initialization prompt
Expected: Program pauses and asks 'Initialization (I) or Factory Preset Replacement (F)?'

2 more checks with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Post-repair

Recalibration required after component replacement in CV/CC DACs, operational amplifiers, control circuits, readback circuits, front panel board, or EEPROM.

Ranked causes

1. Components in A10 Control Board replaced (CV/CC DACs, operational amplifiers, comparators) (common)
2. Components in A1 Front Panel Assembly replaced (front panel board or EEPROM) (common)

First check

Verify INHIBIT CAL jumper is not installed or is in NORM position

Test point: Jumper at INHIBIT CAL position on A1 Front Panel Board (Figure 3-15)
Expected: Jumper must be in NORM position to allow calibration

3 more checks with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

DAC circuit malfunction

Digital-to-analog converter not providing correct output for voltage or current setting

Ranked causes

1. CV DAC failure (occasional)
2. CC DAC failure (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

EEPROM write error

Cannot store calibration or configuration data in non-volatile memory

Ranked causes

1. EEPROM chip defective (occasional)
2. Calibration process error (occasional)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Overvoltage (OV) circuit not firing high voltage

Overvoltage protection circuit does not engage when output voltage exceeds limit

Ranked causes

1. OV comparator failure (common)
2. OV reference voltage incorrect (occasional)

Secondary interface 5V readback failure

Secondary interface cannot verify 5V power supply output

Ranked causes

1. Secondary bias supply failure (occasional)
2. Comparator readback circuit failure (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Secondary interface circuit failure

Secondary interface RAM, ROM, or processor not functioning

Ranked causes

1. Secondary RAM failure (occasional)
2. Secondary ROM failure (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Temperature readback failure

Ambient temperature sensor or comparator cannot be verified

Ranked causes

1. Thermistor RT500 failure (occasional)
2. Temperature comparator failure (occasional)

2 corrective actions and part numbers are in ServiceForge. Connect ServiceForge to your AI

Averaging CC measurements

Set up voltmeter to take statistical average of 100 readings for accurate dc regulation values

Ranked causes

1. Need to average measurements for accurate dc regulation testing (common)

First check

Program 10 power line cycles per sample

Test point: Voltmeter function key
Expected: 10 power line cycles set per sample

5 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

CAL DENIED displayed during calibration

CAL DENIED message appears on display when attempting front panel calibration, or error code 1 occurs during GPIB calibration.

Ranked causes

1. INHIBIT CAL jumper is installed (common)

First check

Verify INHIBIT CAL jumper position

Test point: Jumper on A1 Front Panel Board (Figure 3-15)
Expected: Jumper in INHIBIT CAL position if CAL DENIED appears

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Bench series supply with temporary GPIB board

Bench series supply initialization and factory preset calibration loading using temporary A2 GPIB board replacement

Ranked causes

1. Bench series supply requires GPIB board for initialization (common)

First check

Replace A2 Isolator board with A2 GPIB board

Test point: Board slot A2
Expected: A2 GPIB board installed and recognized

4 more checks with pass and fail actions, 3 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Calibration: Output voltage measurement setup high voltage

Procedure for measuring DC output voltage at sense terminals with proper lead configuration and noise mitigation

Ranked causes

1. Need to establish correct measurement configuration for voltage verification (common)

First check

Verify sense terminal connection type

Test point: +S and -S terminals
Expected: Connected for either remote or local sensing as required

2 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

CC Load Effect test

Measure change in output current from full-load voltage to short circuit condition

Ranked causes

1. Load change from full-load voltage to short circuit (common)

First check

Turn off power supply and connect DVM across current monitoring resistor

Test point: Current monitoring resistor (see Fig. 2-3)
Expected: DVM connected and ready for measurement

5 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

CC noise (PARD) measurement

Measure residual noise on DVM to verify current noise within specified limits

Ranked causes

1. Noise from other equipment affecting measurement (common)

First check

Measure residual noise on DVM with power supply off

Test point: DVM measurement points
Expected: Baseline noise reading established

3 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

CC Source Effect test

Measure change in output current from minimum to maximum ac line voltage within specifications

Ranked causes

1. AC input voltage variation (low-line to high-line condition) (common)

First check

Turn off power supply and connect through variable-voltage transformer

Test point: AC power input
Expected: Transformer connected in ac power line

8 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

GPIB interface failure

GPIB port cannot communicate with external controller

Ranked causes

1. GPIB transceiver failure (occasional)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Output voltage held high high voltage

Output voltage cannot be reduced and remains at high level

Ranked causes

1. Output control circuit failure (common)

Output voltage held low

Output voltage cannot reach programmed value and remains at low level

Ranked causes

1. Output pass transistor damaged (common)

Overvoltage circuit activated at power-on high voltage

OV protection engages immediately when unit is powered on

Ranked causes

1. Output sense line shorted (occasional)

PASSWD ERROR or error code 2 during calibration

PASSWD ERROR appears on front panel display or error code 2 occurs during GPIB calibration when attempting to enter calibration mode.

Ranked causes

1. Incorrect calibration password entered (common)

1 more check with pass and fail actions, 2 corrective actions, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Serial data line failure

Serial communication line on A2 board or main board broken or shorted

Ranked causes

1. A2 board serial line broken (occasional)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Signature analysis testing for front panel microprocessor

Technician needs to verify front panel microprocessor A1U3 using signature analyzer after disassembly

Ranked causes

1. Front panel microprocessor or related circuit fault (unknown)

First check

Verify power supply voltage signature

Test point: 5V
Expected: 3395

21 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Signature analysis testing for primary interface

Technician needs to verify primary interface circuits on Systems Supply A2 GPIB Board using signature analyzer

Ranked causes

1. Primary interface circuit fault or component failure (unknown)

First check

Verify power supply voltage signature

Test point: 5V
Expected: 9FFP

11 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

Signature analysis testing for secondary interface

Technician needs to verify secondary microprocessor A5U504 using signature analyzer with special activation procedure

Ranked causes

1. Secondary interface microprocessor or related circuit fault (unknown)

First check

Verify power supply voltage signature

Test point: 5V
Expected: lC4C

40 more checks with pass and fail actions, 1 corrective action, and part numbers are in ServiceForge. Connect ServiceForge to your AI

UART communication error

Serial port unable to transmit or receive data correctly

Ranked causes

1. UART chip defective (common)

1 corrective action and part numbers are in ServiceForge. Connect ServiceForge to your AI

Error codes

Code	Meaning	Trigger	Severity
`1`	Calibration inhibited or denied	INHIBIT CAL jumper installed; GPIB calibration attempted on Systems Supply with inhibit active	warning
`2`	Incorrect calibration password	Wrong password entered during calibration mode entry attempt	warning
`CAL:STATE ON passcode error`	Calibration mode enable failed due to incorrect passcode	Passcode does not match [withheld] location in EEPROM	error
`E1`	Front panel RAM test failed	Power-on selftest or *TST? query	critical
`E10`	Secondary interface 5 volt readback test failed	Power-on selftest or *TST? query	error
`E11`	Ambient temperature readback test failed	Power-on selftest or *TST? query	warning
`E12`	CV or CC DAC tests failed	Power-on selftest	critical
`E2`	Front panel ROM test failed	Power-on selftest or *TST? query	critical
`E3`	Front panel EEPROM checksum test failed	Power-on selftest or *TST? query	critical
`E4`	Primary interface external RAM test failed	Power-on selftest	critical
`E5`	Primary interface internal RAM test failed	Power-on selftest	critical
`E6`	Primary interface ROM test failed	Power-on selftest or *TST? query	critical
`E7`	GPIB interface test failed	Power-on selftest	error
`E8`	Secondary interface RAM test failed	Power-on selftest	critical
`E9`	Secondary interface ROM test failed	Power-on selftest or *TST? query	critical
`EE WRITE ERR`	EEPROM write failure	Error during EEPROM write operation	error
`SBUF FULL`	Serial buffer failure	Buffer full condition	error
`SBUF OVERRUN`	Serial buffer failure	Buffer overflow during serial operation	error
`SECONDARY DN`	Serial data line failure on main board	Communication failure during operation or selftest	error
`SERIAL DOWN`	Serial data line failure on A2 board	Occurs during operation or selftest	error
`SERIAL TIMOUT`	Serial data line failure on A2 board	Communication timeout during operation or selftest	error
`UART FRAMING`	UART framing error	Serial data framing error	error
`UART OVERRUN`	UART buffer overrun error	Serial data buffer overflow	error
`UART PARITY`	UART parity error	Data reception error	error

Likely causes and actions for each code are in ServiceForge. Connect ServiceForge to your AI

Community

0 fixes logged by technicians through their AI, with outcomes, for this model.

ServiceForge knowledge base record. Field-unverified unless technicians confirmed it. A teaching summary of the record, not a service manual. Export 2026-10-14.