R8 Resistor Failure - Mk4 GHD's Completely Dead

I first discovered this problem on a pair of GHD 4.0Bs. Since then I've heard of it happening to 4.1Bs as well, so it seems to be a common problem with all Mk4 GHD's. A similar, but slightly different problem occurs on the 4.2B and 4.2P models (see section further down this page) The symptoms are that the GHD's are completely dead. This is because resistor R8 (the resistor on the input of the microprocessor's power supply) has gone open circuit. Replacing this resistor will magically bring them back to life :-)

To find out if your GHDs have this problem simply use a multimeter to measure the resistor of R8 (I think it's got the same reference designator in all Mk4 versions, although I could be wrong!). It should be 100Ohms. If it's significantly more than 100Ohms then it probably needs replacing.

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

I can supply you with a suitable replacement resistors for £4 (inc P&P) - you can order online in the spares shop.

I have seen some Mk4 GHD's fitted with a MELF resistor from new, so it looks like GHD have fixed this problem on later PCB revisions.

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

GHD 4.2B and 4.2Ps

There are two versions of PCB used in the GHD 4.2B and 4.2P model. One of them is the same as previous models and only has a single resistor on the input marked "R8" (see above!). The other is fitted with two 50 ohm MELF resistors (R11 and R8) in place of the single R8 used on previous models. We have seen these fail due to intermittent cable socket connections as well. Replacement resistors are available on the shop here.

Here is a short video on testing the R8 and R11 resistors:

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!).

Repeated R8 (and R11) Failures

A number of times I've been asked why a new R8 resistor might fail soon after it's been replaced. I don't have an answer for this, however my theories are:

Intermittent connection inside cable causing power surges to the PCB. This would cause R8 to fail prematurely. Try fitting a new cable.
Intermittent connection elsewhere - mains plug, mains socket, cable socket etc.
Some other fault on the PCB causing the R8 resistor to have to cope with a higher current than expected

If any of these theories solve this problem, please do let me know!

We now stock a high power version of this resistor for situations where the standard resistor has failed repeatedly. These parts are more expensive, but will hopefully solve the problem.

FAQ

How to replace the R8 resistor

This page was last updated at 8:19pm on 3rd June 2011