Test equipment DC power supplies

Philips 6621A: troubleshooting and error codes

The ServiceForge knowledge base holds 32 documented symptoms for the Philips 6621A, with 49 ranked causes, 73 diagnostic checks with test points and expected values, and 10 error codes. Each symptom below shows its causes and the first check; the rest is one connection away. Connect ServiceForge to your AI

Symptoms

Output board component isolation and diagnosis high voltage

Flowchart-based troubleshooting to isolate faults to components on output board after fault has been isolated to particular board using board isolation procedures

Ranked causes

1. DAC/amplifier circuit defect on output board (unknown)
2. Analog Multiplexer U323 failure (unknown)
3. Signal Processor U327 failure (unknown)
4. Power Module U338/U339 failure (unknown)
5. Microcomputer U312 failure or software error (unknown)

First check

Disconnect line cord

Test point: Equipment AC input
Expected: Power disconnected safely

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

Incorrect analog multiplexer readback values

VMUX? command returns incorrect voltage readings from one or more of 8 input channels

Ranked causes

1. Analog multiplexer U323 defective (common)
2. Readback circuitry U321-U324 defective (common)
3. Input-specific circuitry defective (occasional)

First check

Read all 8 analog multiplexer input voltages using VMUX? command

Test point: inputs 1-8
Expected: all values match expected calibrated output

Incorrect status output reported patient safety

Power supply reports incorrect operating status (CV, CC, OV, OT, UNR) when queried

Ranked causes

1. Status monitor circuit section of U327 defective (occasional)
2. Microcomputer U312 defective (occasional)
3. Short on data bus lines between U327 and U312 (rare)

First check

Query output status and compare against front panel indication

Test point: front panel, data bus output
Expected: reported status matches front panel status indication

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

Signal Processor U327 malfunction high voltage

Incorrect output status, voltage levels outside specification, or failed self-test on power-on/start-up

Ranked causes

1. Defective overvoltage circuit in U327 (unknown)
2. Defective power-on/start-up circuit in U327 (unknown)
3. Defective status monitor circuit in U327 (unknown)

First check

Measure pin voltages on U327 against Table 4-16 thresholds

Test point: U327 pins 1-28
Expected: all voltages within specified low/high states per Table 4-16

3 more checks with pass and fail actions are in ServiceForge. Connect ServiceForge to your AI

Verification testing and setup high voltage

Performing operation verification, performance, extended, and temperature coefficient tests on 6621A-24A and 6627A DC power supplies to verify proper operation against specifications.

Ranked causes

1. Need to verify supply operation against specifications (common)
2. Need comprehensive performance test (common)
3. Need extended or temperature coefficient testing (occasional)

First check

Verify test equipment available and functioning

Test point: Test equipment per Table 3-1
Expected: All required test equipment present and operational

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

Calibration: Short term current drift test

Measure output current stability within first 30 minutes after line voltage or load changes

Ranked causes

1. Verify current drift within specification across line voltage changes (unknown)
2. Verify current is within specification for output type (unknown)

First check

Set line voltage to 13% below nominal

Test point: Variable voltage transformer
Expected: 13% below nominal line voltage

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

Calibration: Short term voltage drift test

Measure output voltage stability within first 30 minutes after line voltage or load changes

Ranked causes

1. Verify voltage drift within specification across line and load changes (unknown)
2. Verify voltage drift after load switch opening (unknown)

First check

Set line voltage to 13% below nominal

Test point: Variable voltage transformer
Expected: 13% below nominal line voltage

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

ERR message or power-on self test failure caution

Supply fails self test with ERR message displayed, indicating circuit problem or defective board

Ranked causes

1. GPIB board defective or circuit problem (occasional)
2. Output board failure (occasional)

First check

Check if output boards respond to GPIB commands

Test point: GPIB controller interface
Expected: Output boards respond and can be programmed from GPIB

Incorrect signature at IC output pin

Signature is incorrect at an IC output pin, indicating either a faulty output component or an input short circuit in a downstream component

Ranked causes

1. Faulty component producing the output (common)
2. Input short circuit in downstream component (occasional)

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

Output board hardware error on power-on caution

Errors 11, 12, 13, or 14 (HDW ERR CH < ch >) displayed during power-on self-test; indicates output board RAM or ROM failure

Ranked causes

1. Output board RAM failure (rare)
2. Output board ROM failure (rare)

First check

Identify failing output channel from error code

Test point: Front panel display
Expected: Error code format HDW ERR CH <ch> where ch is channel number

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

Analog multiplexer readback validation

Verify analog multiplexer U323 and readback circuit operation using VMUX? command

Ranked causes

1. Need to test multiplexer and readback circuit (unknown)

First check

Set power supply to SKIP SELF-TEST mode if needed

Test point: Power supply control panel or program
Expected: SKIP SELF-TEST mode activated to allow output programming

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

Calibration locked

System displays error 18 (CAL LOCKED) when calibration mode is attempted to be enabled

Ranked causes

1. W201 jumper in CAL. LOCKOUT position (common)

First check

Verify W201 jumper position

Test point: Connector P201, W201 jumper
Expected: +5 V NORM RUN position

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

Calibration: CC RMS noise measurement

Verify that constant current RMS noise is within specification after programming output current and voltage

Ranked causes

1. Need to verify CC mode RMS noise specification (unknown)

First check

Program output current to 1.9 amps

Test point: Selected output channel
Expected: 1.9 A set via ISET <ch>, 1.9 command

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

Calibration: Output after MODEL command

After sending MODEL command to reset to default firmware values, all output calibration constants are removed and must be recalibrated

Ranked causes

1. Firmware reset via MODEL command removes calibration data (common)

First check

Wait for supply initialization after program execution

Test point: Supply front panel
Expected: 3 seconds minimum before AC power cycle

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

Calibration: Output channel calibration after board or component replacement

Recalibration procedure required after output board replacement or repair of DAC, voltage control, current control, readback buffers, amplifiers, or signal comparator components

Ranked causes

1. Output board or control/readback circuit components replaced (unknown)

First check

Refer to Appendix A Operating Manual calibration procedure

Test point: Output channel
Expected: Calibration procedure steps completed per manual

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

CC Load Effect test high voltage

Verify constant current regulation performance under load variation.

Ranked causes

1. Verify CC load regulation (common)

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

Current Programming and Readback Accuracy test high voltage

Verify supply can program and read back output current within specification.

Ranked causes

1. Verify current programming capability (common)

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

CV Load Effect test high voltage

Verify constant voltage regulation performance under load variation.

Ranked causes

1. Verify CV load regulation (common)

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

CV Noise test high voltage

Measure output voltage noise in constant voltage mode.

Ranked causes

1. Verify CV output noise performance (common)

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

Electrostatic damage to components caution

Semiconductors suffer performance degradation or failure from static charge exposure during maintenance

Ranked causes

1. Handling static-sensitive devices without proper ESD precautions (common)

First check

Power off before handling

Test point: Power supply
Expected: Power off

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

External verification of multiplexer output voltage

Use external voltmeter to confirm VMUX readback accuracy by comparing to actual pin voltage

Ranked causes

1. Verify multiplexer operation matches VMUX readback value (unknown)

First check

Run verification program

Test point: GPIB interface and computer
Expected: Program sends VMUX? command; readback value displays

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

Firmware revision A_02 vs. 1.01 signature differences

Signature tables for ROM chip differ between firmware revisions; A_02 (2629) tables differ from previous 1.01 (2605) revision

Ranked causes

1. Using wrong firmware signature tables leads to misdiagnosis (common)

First check

Confirm firmware revision installed in unit

Test point: ROM chip U206 label or via ROM? command
Expected: Revision clearly identified

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

Firmware revision verification

Query ROM firmware revision date to confirm installed firmware version for proper signature analysis reference table selection

Ranked causes

1. Need to confirm firmware revision for correct S.A. signature tables (common)

First check

Execute ROM query program to read firmware revision

Test point: GPIB port 705
Expected: Revision date displayed (e.g., 2629 for A_02)

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

Incorrect signature at input pin but correct at source

Signature is incorrect at an IC input pin but correct at the output of the driving IC, indicating a connection problem rather than a component fault

Ranked causes

1. Printed circuit trace break or poor solder joint (common)

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

Incorrect signature indicates shorted IC pin

Signature is incorrect and matches Vcc or ground voltage signature, indicating the test point is likely shorted to power or ground

Ranked causes

1. IC pin shorted to Vcc or ground (common)

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

No power or blank display at startup caution

Supply fails power-on self test with blank display, indicating AC input failure or defective GPIB/output board

Ranked causes

1. AC input failure (common)

First check

Verify power-on self test completion

Test point: Display output
Expected: Normal power on self test indications as described in Section III of Operating Manual

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

Overvoltage Protection test high voltage

Verify overvoltage protection circuit functions correctly.

Ranked causes

1. Verify overvoltage protection operational (common)

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

Self-exercise routine: DAC and microcomputer functional test high voltage

Automated test mode that exercises microcomputer U312 and all DACs (U313, U314, U321) by repetitively programming output voltage and current limit to full scale; requires U312 pin 27 tied to common before power-on; display shows HDW ERR CH format during routine

Ranked causes

1. Microcomputer U312 or DAC failure suspected (unknown)

First check

Tie U312 pin 27 to common before power-on

Test point: Microcomputer U312 pin 27
Expected: Pin 27 connected to common ground

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

Self-test skipped caution

Error 22 (SKIP SLF TST) displayed on power-on; unit bypasses power-on self-tests

Ranked causes

1. W201 jumper in SKIP SELF TEST position (common)

First check

Verify W201 jumper position

Test point: Connector P201, W201 jumper
Expected: +5 V NORM RUN position

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

Signature Analysis Testing

Microprocessor-based troubleshooting method using signature analyzer to compress bit streams into four-character signatures for isolating faults to specific components

Ranked causes

1. Unknown fault requiring component isolation via signature analysis (unknown)

First check

Verify correct test setup used

Test point: Figure 4-8 reference
Expected: Test setup matches specified procedure

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

Two IC pins have identical signatures

Two pins produce the same signature value, indicating they may be shorted together or to a common node

Ranked causes

1. Two IC pins shorted together (common)

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

Voltage Programming and Readback Accuracy test high voltage

Verify supply can program and read back output voltage within specification limits.

Ranked causes

1. Verify voltage programming capability (common)

First check

Program supply to test voltage setting

Test point: Supply programming interface or front panel
Expected: Supply accepts and confirms programmed voltage value

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

Error codes

Code	Meaning	Trigger	Severity
`11`	HDW ERR CH < ch >	Output board fails RAM or ROM tests	error
`11`	HDW ERR CH 1 - Channel 1 output board hardware error	Output board 1 fails RAM or ROM self-test on power-on	critical
`12`	HDW ERR CH < ch >	Output board fails RAM or ROM tests	error
`12`	HDW ERR CH 2 - Channel 2 output board hardware error	Output board 2 fails RAM or ROM self-test on power-on	critical
`13`	HDW ERR CH 3 - Channel 3 output board hardware error	Output board 3 fails RAM or ROM self-test on power-on	critical
`13`	HDW ERR CH < ch >	Output board fails RAM or ROM tests	error
`14`	HDW ERR CH < ch >	Output board fails RAM or ROM tests	error
`14`	HDW ERR CH 4 - Channel 4 output board hardware error	Output board 4 fails RAM or ROM self-test on power-on	critical
`18`	CAL LOCKED - Calibration locked	Calibration mode enable attempted with W201 jumper in CAL. LOCKOUT position	warning
`22`	SKIP SLF TST - Skip self-test	Power-on with W201 jumper in SKIP SELF TEST position	warning

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.