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Modifying the NTN4633B or
NTN4634B Rapid Battery Charger

by Robert W. Meister - WA1MIK
Additional information from Mike Burgess G7HID
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The NTN4633B is the 120V, 50 / 60 Hz rapid charger designed for the HT600, HT800, MT1000 and similar series radios. The NTN4634 is the same charger factory wired for 220 / 240V, 50/60 Hz (the only difference is the power transformer and the AC line cord). Both can charge several sizes and capacities of Nickel-Cadmium (NiCd) batteries ranging fom 600mAh to 1.2Ah or even higher. A resistor inside the battery pack controls the charging current. One characteristic of NiCd batteries is that after a pack is fully charged any additional charging current gets dissipated as heat. The charger takes advantage of that characteristic with a thermistor in the pack sensing the rising temperature and switches the charger into a trickle-charge mode. The batteries should be removed within a few hours of going to this low charge rate. If left in too long, the trickle current can cause the batteries to heat up and shorten their life.

A Motorola official field modification is presented here, along with pictures showing the locations of the components that need to be removed or changed. This procedure brings a "B" version charger up to the current production "C" version. For those of you who need to know why something is being done, small schematic sections are also provided.

Note from Mike Burgess G7HID via an email to WA6ILQ:
This modification is also valid (and works well) on the model ELN1040B (240V, 50 / 60 Hz with Euro plug) and the ELN1041B (240V, 50 / 60 Hz with UK plug) chargers.

The "Current" Problems:

  1. The unit overcharges batteries with about 72 milliamps when in the trickle-charge mode.
  2. If power fails and returns with a battery in the charger, it will restart the rapid-charge cycle.

The Benefits of Doing This Modification:

  1. The trickle-charge rate will be reduced to a safe 25 milliamps.
  2. If a battery is already in the charger, it will power up in the trickle-charge mode.
  3. Battery life will be extended due to lower trickle-charge current and fewer rapid-charge cycles.
  4. If you remove a fully charged battery and want to trickle charge it only, you can insert it, pull the power plug, and plug it back in.

Step-by-step Procedure:

  1. Remove any radio or battery and unplug the charger.
  2. With the unit upside down, remove the five large Phillips screws from the bottom as shown in the photo below.

  3. Gently but firmly remove the plastic cover from the chassis.
  4. Now remove the mounting hardware holding Q1 (on the back of the circuit board) to the chassis. Don't lose the insulators. See the photo under step 13 below.
  5. Remove the small Phillips screw holding the circuit board at its bottom edge near Q1 to the chassis bracket. The board and charger contact assembly may now be removed together.
  6. Unsolder R14, a big 7-watt, 2 ohm ceramic resistor on the back of the circuit board. This gives you access to R10 in the next step. See photo below for component locations.

  7. Remove R10, a 22-ohm resistor. Refer to schematic below for the electrical location.

  8. Install a new 62 ohm, 1/2 or 1 watt resistor in place of R10. This changes the trickle charge rate. See photo below for the component hole locations on the back of the circuit board.

  9. Reinstall R14 on the back of the circuit board. Position it as far away as possible from the board, or anything else, to improve ventilation.
  10. Unclip the top of the circuit board from the charger contact assembly. Remove C14, the upper 10 microFarad capacitor near the left edge of the circuit board. Removing this capacitor prevents the unit from powering up in the rapid charge mode. See photo below for the completed modifications.

    Refer to schematic below for the electrical location.

    Make sure there are no solder bridges or other residue remaining. Clip the charger contact assembly back onto the circuit board.
  11. Clean Q1 and the chassis area where it mounts, and apply new heat sink compound. Make sure there's an insulator between Q1 and the chassis.
  12. Remount the circuit board to the chassis bracket with the small Phillips screw.
  13. Make sure Q1 is lying flat on the chassis. Then reinstall its mounting hardware. Make sure there's an insulator between the screw and the tab of Q1. See photo below of the complete unit.

  14. Carefully line up the four charger contact fingers into their slots and replace the cover onto the charger base.
  15. Install the five large Phillips screws into the bottom of the charger.
  16. Note on the bottom of the charger, perhaps with a Sharpie pen "Field upgraded to version C" and the date for the next guy that works on it.

Checking The Results:

  1. Verify that the charger is unplugged from the AC and insert a battery into the charger.
  2. Plug the charger into an energized AC power source.
  3. Verify that the COMPLETE (green) LED lights up, indicating that the unit is in its trickle-charge mode. You should measure about 50-60 millivolts across R14, which equals 25-30 milliamps.
  4. Remove and reinsert the battery.
  5. Verify that the CHARGING (red) LED lights up, indicating that the unit is in its rapid-charge mode. You should measure about 1.5-2.4 volts across R14, which equals 750-1200 milliamps. A resistor in the battery pack ultimately controls this current. Larger packs can be charged with more current.
  6. If power is interrupted with a battery in the charger, it will come back on in the trickle-charge mode. Batteries may be left in the charger for up to 24 hours in the COMPLETE mode without harm.

Additional Information:

Here's a typical HT600 / MT1000 battery schematic. RR (Sense Resistor) is used to select the rapid charge current. It has typical values of 3k or 5k. The thermistor on pin 1 is around 10k at room temperature and drops to about 5k when the pack is fully charged.

Notes from Mike WA6ILQ: Above is the schematic of a typical HT600 / HT800 / MT1000 battery. The charger connects to the bottom four contacts, and the radio to the upper two. The circuit breaker at the top (not present in all battery packs) prevents a defective (shorted) radio from damaging the battery, or against the time when you put a spare battery nose down into your pocket on top of your keyring. The "charge diode" at the bottom prevents damaging or overheating the battery if you accidentally short the charging contacts, perhaps by inserting the radio in your pants pocket on top of your keyring... been there, done that on an older HT200 radio that didn't have the diode (or if it did, it was shorted)... The "sense resistor" connected to pin 3 (labeled "Rr") is the one mentioned in the "Background" text above that determines the actual charging current. The thermistor conected to pin 1 is the sensor that controls the switchover from full charge current to trickle charge, with the thermostat on pin 4 used as backup. The thermostat is an actual mechanical switching device that is normally closed when not activated (i.e. when cold). The thermistor and the thermostat are carefully positioned inside the battery pack so that they are physically touching the battery cells and can therefore get an accurate reading of cell temperature.

Acknowledgements and Credits:

Circuit information for the NTN4633 / NTN4634 charger was obtained from Motorola's official service manuals and service bulletins (6881079E24 dated 9/1988). Manual 6881108C47 dated 1991 also covers this charger.

The battery schematic came from the HT600 manual, 6881065C75, which contains user instructions, maintenance, schematics, etc.

All Motorola part or model numbers (NTN..., NLN..., etc) are the property (possibly copyrights/trademarks) of Motorola, Inc.

Mike Morris WA6ILQ of the Repeater-Builder web site staff deserves credit for converting my original Microsoft Word file to clean HTML and resizing all the pictures and images to make it all look real nice.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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Article text and all photos Copyright © 2005 by Robert W. Meister, WA1MIK.
Artistic layout and hand-coded HTML © Copyright 2005 and date of last update by Mike Morris WA6ILQ.

This web page, this web site, and the information presented in and on its pages is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors.