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 and 4.2Ps as well, so it seems to be a common problem with all Mk4 GHD's. 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) - just drop me an email or buy 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.

Repeated R8 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!

FAQ

How to replace the R8 resistor

This page was last updated at 1:05pm on 15th November 2009