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RAC HP027  &  RING RCB322

Battery Charger Engine Starter

RAC Heavy Duty Battery Charger Engine Starter

Fault diagnosis and repair

RING RCB322 has same components inside.

On our Le Tonkinois Varnish show demonstrations we use 12 volt batteries and inverters to power electric drills and occasionally lights.  The batteries need to be recharged before the next show.

We purchased 2 RAC Heavy Duty Battery Charger Engine Starters in 2007.

In 2010 one was used for charging with the rotary switch accidentally left in the Engine Start position.

It expired emitting smoke signals!.

As there were no new ones advertised we bought a second hand replacement.

When we eventually got round to using it we found it was also faulty, too late to return it.

The details of the repairs could help others with the same problem, if so a thank you email would be appreciated. It is mainly intended for people with a basic understanding of electronics. I have added a few bits to help non technical users but as the number of users is low I do not intend to spend more time on the RAC Charger section of the site.

Most of my time is taken by the Eberspacher advice section which has a great deal more users.

Thanks to Georges Pradon for the information that the Ring RCB322 charger by Ring Automotive UK has the same internal design.

Thanks to Zoltán Hódos Sr for sending a copy of the full circuit diagram, it would have saved much time if it had been available when I did the repairs.

This page contains details of some simple diagnostic tests and solutions for people with limited electronics experience.

Pages 2 to 6 contents are primarily aimed at people with at least a basic knowledge of electronics and soldering.

Since publishing the main repair pages several people have contacted me, all with the same burnt out transistors so I have quickly added this page giving a little more information on the testing.

It is not possible to cover how to diagnose and cure every fault, experience like that is gained over a lifetime as an electronics engineer.

Health and safety warnings apply to everything these days and working inside the charger is no exception. If you cannot work safely on electrical equipment leave it alone.

If the charger has failed, before even taking the case off do the simple preliminary checks.

If the green On indicator is not lit:-

Test or replace the 13A fuse, check the fuse fixing clips are tight and the mains cable wires are tightly secured in the 13 amp plug.

Check for damage to the cable and that the mains socket is working, extension leads and garage sockets can be faulty.

If these are not the cause you will need someone who is qualified to safely work on 240v circuits to localise the fault.

If the green On indicator is lit power is reaching at least as far as the transformer output.

These diagnostic tests need only be done if the charger is faulty, i.e. giving no charge.

The three tests on this page are deliberately kept simple and require no electrical knowledge.

The main test can be done with only screwdrivers to remove the case and change wiring connections.

The other two tests are optional and require a multimeter.

Multimeters are cheap, on ebay March 2015 new ones virtually the same as the one in the photos were just £2.30 including postage from China. I use these cheap meters for most of my general testing in preference to my expensive Fluke model as they are smaller and lighter and usually survive if dropped.

Note that no charge current does not always mean the unit is faulty.

It is an automatic charger so only charges the battery if it is discharged.

In my tests I found the battery had to be almost half discharged for any charging with the high charge setting.


The most important test is the transistor bypass test, if you do not have a multimeter go directly to it.

If you have a multimeter do the other two tests first, they can locate several serious problems.


Click on images for better quality.

Remove the cover.   The fixing screws are marked in green


This test checks the power rectifier diodes for major faults. The diodes are probably the second most likely component to have been damaged due to switch position errors. The circuit diagrams only show 2 diodes, both consist of 2 diodes connected in parallel to share the load. It is not possible to fully check the four diodes individually without removing them but it is still a useful test.

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Test leads reversed.

Disconnect the charger from the mains and the battery.

The front panel switch positions are unimportant.

Switch the multimeter to the diode test position, most if not all digital multimeters have one, usually marked with a diode symbol. In the diode test position the multimeter applies a small current through the probes and measures its voltage.

Connect the probes as shown making sure the connection breaks through any oxidation layer on the aluminium. As diodes pass current in only one direction the readings should change when the probe positions are reversed.

The 1 reading on my meter indicates the reading is 'out of range' which is correct for a diode measured in its non conducting direction. The other reading is the voltage when it conducts, this will vary from diode to diode, if it indicates from 500 to 700 mV it should be ok. 500 mV is the same as 0.5V so it is possible some meters could read 0.500 or .500    If both the readings are 'out of range' the probes are not making good contact or all diodes are open circuit.  If both the readings are very low one of the diodes is short circuit. This could cause more damage during the bypass test. The short circuit should also have caused two more diodes to be blown.

They can be removed if needed but fitting them back is difficult, see final page for details.

The four diodes are MIC RA501. These are 50 volt 50 amp devices. Thanks to Georges Pradon who found AMP 1502 50A alternatives on Ebay April 2015. These are higher voltage rated 200v.


This test checks both halves of the transformer secondary winding are ok.

If one winding is faulty the charger might partially charge but not with full output.

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             Voltage across both windings                                   Voltage across one winding

If anyone is wondering why the voltage in the single winding photo is not half the 21.0 v  reading, the mains voltage has changed.

Set the switches to 12v Trickle and Charge.

With no battery connected connect the charger to the mains, making sure the battery connectors do not short circuit. Switch the multimeter to ac volts position, usually marked ACV or with a ~ symbol.

Some multimeters have more than one ac range, select one greater than 100v, the choice is not critical.

Connect the probes to the transformer connections.

My reading here was 21 volts but it varies as the mains supply voltage changes.

Any reading between about 19 and 23 volts ac is reasonable so you can proceed to the Transistor bypass test.

If the green On indicator is lit and the reading is very low:-  

Check you have selected an ac voltage range and not dc voltage and that the probes are making good contact. Carefully scrape off some of the lacquer insulation on the twisted wire(s) that go from the transformer to the printed circuit board. Then connect one of the meter probes to that point and the other to both the previous connection points one at a time. A reading of about 10 volts indicates that section of the transformer is ok. If both readings are very low I would suspect the test meter as there has to be some voltage to light the On indicator. If one reading is low and the other ok after rechecking for poor probe connections the charger is beyond economic repair. If both readings are about 10 volts the first part of the test should have been ok.

If the green On indicator is not lit and both the readings are very low:-

 1 No mains supply is getting to the transformer primary (see top of this page) or

 2 The transformer is faulty.

 3 Some of the rectifiers are short circuit which will cause the transformer to get extremely hot very quickly.

Major faults on the transformer, usually blowing fuses or no output, mean the charger is probably beyond economic repair.


A simple wiring change bypasses the Power Mosfets together with their associated circuitry, allowing the expensive part of the unit to be tested at high currents. Ideally complete the previous tests before starting this test but if they cannot be done read the advice on disconnecting quickly. The only tools required for the test are screwdrivers.

There are at least two versions of the RAC charger. The second hand charger we bought is older. It did not have the large resistor fitted on the printed circuit board and the wiring differed slightly. The additional resistor is shown on page 3 near the burnt out transistors.

The wiring arrangement may differ very slightly for each version.

a) The later version wiring changes


Original wiring connections on the ammeter.

I have identified the 3 cables here with colour spots.

Disconnect all 3 cables from the terminals.

If a connector is tight lever it with a screwdriver.

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Red spot cable moved from the right to left terminal.

Other 2 cables go back to the original positions. New cable arrangement is ready for the test.

Remove the case if not already done, the fixing screws are shown earlier on page.

Swap the cable identified here with red spot, from the right meter terminal to the left meter terminal.

The other 2 cables remain connected to their original positions.

Ie when viewed from rear of case

Original wiring

Left meter terminal  cable shown marked here with green spot.

Right meter terminal 2 cables shown marked here with green and yellow spots

Wiring for test

Left meter terminal  2 cables shown marked with red and green spots.

Right meter terminal cable shown marked with yellow spot

The test can now  be done.

The test

After completing the temporary wiring modifications double check you have done everything before proceeding with the test. When connecting the battery and mains supply it is always sensible to make sure you can disconnect quickly in case of problems.

Set the front panel rotary switch to 12v Trickle and the toggle switch to Charge.

Connect the charger to a battery.  Connect the mains and switch on.

If the transistors or other major parts have blown there will be no charge current on the ammeter at this point. Change the toggle switch to the Start / Boost position, this bypasses the transistors, switch back or switch off immediately if there are any serious problems.

If no current is shown try changing to the 12v Fast position. If there is still no charge in the Start / Boost position replacing the transistors will not repair the charger.

If the charger does show a charge current, even if the full repair does not work or you decide that the full repair is too difficult to do, passing the test means you can at least use the charger as a simple non automatic charger. (brief details of modification below).

Remember to replace the cables to their original positions after the test.

Simplified tests like this work correctly most of the time but there is one component fault that can give the wrong results. The meter can fail so it passes current ok but does not move the meter indicator.

Thus it would show no charge even though the charger is working ok.

Don't try measuring the charge current directly with a digital multimeter, it is not designed to measure pulsating currents and the results will be meaningless. Instead switch the multimeter to 20 volts dc range and connect it across the battery terminals. If the charger is then switched on you should see the battery voltage rising if any current is flowing. It wont tell you how much, even a small current will do this.

A better way if the battery is reasonably discharged a hydrometer could be used to check before and after a period of charging.

These circuit diagrams are included for people with technical knowledge only,

there is no need to understand them.

Above is the circuit before modification (updated 2014).

Below shows the temporary circuit changes for the Transistor bypass test.

The toggle switch short circuits the switching transistors when it is in the Start position.

After completing testing reconnect the wires to their original positions.

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If the automatic control circuitry is the cause of the problem but cannot be repaired the charger can be converted to a simple non automatic unit by adding one wire as shown above in green.

The charger will not switch off automatically but the charge and engine start functions should still operate.

The bypass connections must be good quality and it needs to be made with reasonably thick wire as it carries all the charger output current.

The Amber / Green LED is controlled by the now redundant automatic switching circuitry and the indications given will depend upon which part of the control circuit is faulty. It may or may not show the charge state.

Add a label that the charger automatic switching circuit is inoperative to remind anyone using the charger that it has to be manually switched off to prevent overcharging the battery.

Obliterate any wording on the case that refers to Automatic switching.

I have run out of time so am publishing this page "as is".

The following pages describe the repairs to my two units.

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