4610M - Adjusting Back Panel Bias Current

It may be necessary to adjust the bias current on the back panel of the 4610M, typically after replacing the back panel assembly after repair.

Two methods can be used to adjust the bias current on the back panel assembly, with the first method requiring less disassembly and fewer tools.

IMPORTANT NOTE - DO NOT SHORT THE SCREWS IN THE TWO SETS OF FETS TOGETHER. THIS WILL DAMAGE THE BACK PANEL ASSEMBLY

Method 1 :

Tools Required :

- EA002 Coil or similar
- Oscilloscope (low bandwidth requirements, < 5MHz)
- Handheld Meter (optional)

- Screwdriver set for removal of case

- POT adjuster

Remove the top cover of the 4610M (Requires removal of handles and the 6 screws that hold the top case on), and slide towards the rear. Power the unit back on, making sure not to short the back panel assembly with tools.

Configure the 4610M to output 2.1A AC, at a frequency of 60Hz into the 50 Turn coil from the 30A Terminals. Leave the unit in standby. The oscilloscope leads should be connected with the ground clip/connection to the negative terminal, and the tip of the probe to the positive terminal. The voltage will be quite small, less than 1V, when performing this test

On the back panel assembly, confirm if R61,R62 and R63 are fitted or not. These resistors limit the amount of adjustment available from the POT's. With these resistors removed, there is a much wider adjustment range, so adjustment must be performed carefully.

Turn the hightled POT's ANTI CLOCKWISE to the far extent. This is the minimum position, and all measurements are performed relative to this position.

Turn the output of the calibrator on and look at the waveform on the oscilloscope. There is likely oscillation on the waveform.

Starting from the RIGHT-HAND SIDE (with the large capacitors and transformer to the right, and the front of the calibrator towards you) of the back panel assembly, turn the right hand most POT clockwise until the output stops oscillating and return to a sinewave. Then turn the POT anticlockwise by approximately 1/2 a turn.

Proceed to the next POT, turning clockwise until the distortion disappears. Then turn this POT back by 1/2 a turn. 

Finally, perform the same on the last POT, but do not turn it back by 1/2 turn.

To check that each POT is balanced, each should cause distortion when turned back by approximately 1/4 turn when completed. You may need to adjust by turning a POT clockwise as part of this final procedure.

Note - The FETS will warm up and draw less current as the output is left on, so you may need to re-check for distortion after setting the output to 10A

If there is not enough adjustment available on a POT, turn the pot fully ANTICLOCKWISE and turn the 4610M off. Using a soldering iron, remove the associated resistor (R61, R62 or R63).

Method 2

Tools Required :
- Handheld multimeter with 10A AC Range - Requires resolution of at least 10mA AC. Keysight U1252B or similar
- Gunson Automotive Fuse Adaptor Test Leads P/N 77068 (https://www.gunson.co.uk/product/77068/Automotive-Fuse-Adaptor-Test-Leads) or equivalent. Fitted with 5A blade fuse.

- Screwdriver set for removal of case

- POT adjuster

Remove the top cover of the 4610M (requires removal of handles and 6 screws along the edges). In addition, remove the 3 screws at the rear of the unit underneath the back panel assembly.

Slide the cover towards the rear of the unit, and then remove the 8 screws holding the back panel assembly in place. You may need to loosen the side screws in the case of the unit (the Cross head, non-hexagonal screws) to loosen the metalwork to allow the back panel to move more easily.

It may be possible to reach the automotive fuse holders on the back panel without removing the back panel in its entirety; otherwise, remove the Ground cable, which goes to the earth stud on the bottom of the 4610M Case and then move the back panel out of the enclosure.

Remove the two 30A Blade fuses from the black holders on the back panel, and then place the Gunson (or equivalent) test lead in either of the fuse holders. Connect the output leads of the test lead to the 10A range of a multimeter, set to AC measurement.

On the back panel assembly, confirm if R61,R62 and R63 are fitted or not. These resistors limit the amount of adjustment available from the POT's. With these resistors removed, there is a much wider adjustment range, so adjustment must be performed carefully.

Power the 4610M on, and turn the POT's fully anticlockwise and note the current level on the multimeter. This varies but make a note of this reading as the baseline.


Going along the back panel, from RIGHT to LEFT, adjust each POT to add 300mA to the measurement. Adjusting clockwise increases the current; anticlockwise decreases the current.

For example, if the base reading is 0.45A, adjust the first POT so the reading is 0.75A, and then adjust the second pot so the reading is 1.05A, and then the third for a reading of 1.35A. Each stage should add approximately 0.3A to this measurement. There is noise associated with this measurement, so the adjustment can be +/- 20mA for each POT.

If there is not enough adjustment available on a POT, turn the pot fully ANTICLOCKWISE and turn the 4610M off. Using a soldering iron, remove the associated resistor (R61, R62 or R63).

Edited 16th February 2024