On this page I will go through the operation of the GHD Mk4 to Mk5 pcb to help people with fault finding and repair of the pcb.

What the pcb components do

Ghd pcb component functions

Components	Function
D2, R1	Trigger used to synchronize triac switching with mains sinewave.
R5, R6	Measure input voltage used to switch between 117v mode(100v - 150v)and 240v mode(200v-250v).
R10, C1, D9, C7 , C6	Power supply and smoothing
R7, R3	Pull up or pull down resistors
R8,R11	Inrush protection

 

Fault finding tips

Fault symptoms	Possible solutions and things to check
Heating up to fast but not over heating.	Check R5, R6
Nothing happening when PCB is powered on.	Check R8 and R11 are not open circuit. Check U2 IC has around 4.5volt across pin 1 and 8. If it is less than 4 volt, then most likely U2 IC is faulty. Try removing U2 IC and recheck across pins 1 and 8. If you still don't have around 4.5v then D9 may be short. If U2 has correct voltage but PCB is not running. Check D2 is not faulty. To do so, set your meter to the diode test mode and test across D2. You should get 0.2volt drop in 1 direction and an open circuit in the other direction. If you get around 1.7volt drop across the D2 diode in the other direction then you need to replace the D2 diode. This is a common fault. If D2 diode is fine and the voltage across U2 is correct then U2 is faulty.

 

Safely testing PCBs

To test PCBs you should not directly connect them to the mains. You should use a isolating transformer to limit current and lower your chance of getting a shock. I just use a transformer from a shaver socket to power the PCB but do not try to run the PCB with the heaters connected.  

I will continue to update this page when i think of more faults to add.