In message , Mike Tomlinson
writes
In article , Mike Tomlinson
writes

Following up to myself:

I took the RHS cover off the boiler. The buzzing noise is coming from
the gas valve. Took the PCB out and found several dry joints on the
connector pins and the relays. Resoldered those and refitted. The
problem still exists.

Found the Potterton troubleshooting flowchart which says to check for
300v DC on the red and black wires to the gas valve. There's 215V dc on
these wires,

215 DC sounds about right - the 300 volts quoted in the manual is (IMO)
incorrect (when under load - i.e. with the gas valve solenoid not open
circuit)

and using the AC scale, it reads 100v,

measured using a
multimeter. This doesn't seem right to me. I'm wondering if one of the
rectifier diodes has gone on the PCB - does this sound likely?

Quite possibly


I'd welcome some reassurance that the problem is the PCB and not the gas
valve; if it's the PCB, I'll order a re-con one from Geoff; if it's the
valve, I'll have to call in an engineer.

Measure the resistance of the valve, somewhere around 2k I would expect
(i forget), but what's important is that its not open circuit, which,
given the voltage you quote above, doesn't sound to be the case

i.e. probability is ... pcb

--
geoff