Resistor Failures on PCB - Mk4/Mk5 GHD's Completely Dead

This problem only affects Mk4 and Mk5 GHDs. Mk3 GHDs do not have this problem and their PCBs almost never fail.

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 connection to make sure there is good reliable connection. 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 mains socket.

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:

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, and it is very very 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 50 or 100 Ohms as explained above. If it's significantly more than 50/100Ohms 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:

For the electronic engineers out there... when I say 50ohms I mean 51ohms, but I try to keep things simple. 51ohm MELF resistors are impossible to come by (the distributers don't even have the part on their system!) so in the shop we stock 47ohm replacements (but call them 50ohm to keep things simple again!).

Replacing the resistors

For a guide to replacing the R8 resistor, please see the R8 Replacement how-to.

Here is a photo of a 4.0B PCB where R8 has been replaced with a MELF resistor (which is less likely to fail compared to a "normal" surface mount resistor):

We can supply replacement resistors via our spares shop:

If you don't fancy resoldering the resistor yourself then we supply complete second hand PCBs which have had these resistors replaced as a precaution:

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:


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