R8 or R8 and R11 Resistor Failures on PCB - Mk4/Mk5 GHD's Completely Dead or Bleeping.
This problem only affects Mk4 and Mk5 GHDs. Mk3 GHDs do not have this problem and their PCBs almost never fail.
This problem doesn't affect the GHD S7N261 Gold or S7N421 Max irons.
The Mk4 and Mk5 low voltage power supply design relies on a resistor in series with a capacitor to generate 5V for the microprocessor. This resistor is susceptible to power surges caused by intermittent cable connections which causes the resistor to go open circuit. Therefore if you find a resistor has failed, we'd advise inspecting the cable and the cable socket to make sure there is good reliable connection with no visible damage or signs of arcing.. Also make sure the mains switch contacts are clean and that you are turning the irons off before plugging them in or out of the main wall socket. A faulty mains wall socket or extension lead can also cause sparking and resistor failure.
It has become apparent over the past 5 years that arcing contacts in the mains switch is a significant cause of resistor failure, always strip and clean the contacts in switches fitted to Mk4 type 2 irons and all Mk5 irons.
Do not attempt to clean switches fitted to the Mk4 type 1 range of irons! They work differently and touching the contacts will damage the switch.
The different generations have had different resistor values and types, but they all fail:
- GHD 4.0B - These have a single 100 Ohm "thick film" (i.e. flat) resistor called the R8 resistor. This style of resistor was particularly prone to failure and often you can see a crack across the middle of the resistor
- GHD 4.1B and Type 1 4.2B - These have a single 100 Ohm "Melf" (i.e. cylindrical) resistor called the R8 resistor. These resistors are more suited for the application however they still fail and go open circuit.
- Type 2 GHD 4.2B and GHD Mk5 Gold Series - These have a pair of 51 Ohm "Melf" resistors called R8 and R11 which are connected in series to form 102 Ohms. One of these resistors will often fail. Change both!
If you are not sure which type of 4.2B irons you have then read the type 1 vs type 2 page.
The only resistors on the PCB that fail are R8 and (if fitted) R11. The other resistors all have values larger than 1Mohm which makes them difficult to measure in circuit, and it is very unlikely they will have failed.
Testing the R8 / R11 resistor
To find out if your GHDs have this problem simply use a multimeter to measure the resistance of the R8 and (if it exists) R11 resistor It should be 51 or 100 Ohms as explained above. If it's significantly more than 51/100Ohms then it probably needs replacing.
There are no other resistors on the Mk4/Mk5 boards that fail. I often get asked about the other resistors which are in the range of 1 MegOhm so if you wish to test them you will need to put your meter on a higher range to get a reading from them.
Here is a short video on testing the R8 and R11 resistors on a pair of Type 2 4.2Bs:
The resistors we supply are rated at 350 volts, 1 watt and are metal film types. These have been 100% reliable in our experience if the cause of failure of the originals is rectified.
Replacing the resistors
Here is a video showing showing three different PCB's that you may encounter. One has just R8 fitted, the other two have both R8 and R11 fitted, although notice the slightly different orientations of the R8 and R11 resistors.
We can supply replacement resistors via our spares shop:
- 1W 100 Ohm MELF resistors - higher power and voltage than the originals for better robustness. Suitable for 4.0B, 4.1B and type 1 4.2Bs
- 1W 51 Ohm MELF resistors - higher power and voltage rating than the originals for better robustness. Suitable for type 2 4.2Bs and 5.0 Gold
If you don't fancy soldering the resistors yourself then we supply complete second hand PCBs which have had these resistors replaced as a precaution:
- 4.0 PCB
- 4.2 PCB - We do not differentiate between type 1 and type 2 PCB designs as there is no practical difference in reliability, despite the change from 1 resistor to 2 resistors!
Repeated R8 and/or R11 Failures
A number of times I've been asked why a new R8 resistor might fail soon after it's been replaced. Possible reasons are as follows:
- Poor connection in the cable socket. This often happens on the type 2 cable socket design. See the type 2 cable socket page for more information. It can also happen in other socket designs and we would recommend some Electrolube CG60 high voltage grease on the moving socket parts.
- Intermittent connection inside the cable causing power surges to the PCB. This would cause R8/R11 to fail prematurely. Try fitting a new cable.
- Damaged switch contacts - arcing in the switch can cause problems for the resistors. Try taking the switch apart (without loosing the little springs) and cleaning the contacts up.
- Intermittent connection elsewhere - mains plug, mains socket, poor electrical power supply (a generator for example) etc.
- Make sure the irons are turned off before plugging them in or out of the mains socket.
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