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.
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 50 Ohm "Melf" resistors called R8 and R11 which are connected in series to form 100Ohms. One of these resistors will often 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 reistors 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:
- 0.4W 100 Ohm MELF resistors - suitable for repairing 4.0B irons
- 1W 100 Ohm MELF resistors - higher power for better robustness. Suitable for 4.0B, 4.1B and type 1 4.2Bs
- 1W 50 Ohm MELF resistors - higher power for better robustness. Suitable for type 2 4.2Bs
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:
- 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 contact 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.
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