Glowworm PCB Failure - Resistor Banding

[Edward Durrant]

Hi all,

I have read many threads on glowworm pcb failures on this forum and others and these have provided some help but I was hoping someone could help me further.

Long story short I have had replacement PCB's sent out but these are for the newer model than I have and I have been struggling to locate my exact PCB for a reasonable price.

Anyway, I have read many people have had success replacing individual resistors and capacitors themselves, I have a history with electronics and soldering experience so I am fairly confident in my ability to do this, my issue is - I want to double check the value of two resistors before I order them. It is the R80 and the R111 I want checked.

Any guidance is greatly appreciated,

Edd

 

[Aquaheat]

Modification of a critical safety device is not permitted.

Jon

 

[Gerrydelasel]

R80 = 2.7k (are those scorch marks I see?)
R111 = 1Meg (this is not likely faulty)

 

[Agile]

Why do you want to change R80 and R111 ?

What resistance have you measured them at?

Tony

 

[DP]

E D, what is the fault and why do you think these resistors are the problem

Do you have a circuit diagram of the pcb and do you, based on what you say, can describe circuit operation?

 

[snes]

Why not try Geoff at CET ltd. His company reconditions pcbs on an exchange basis. A short chat with him would ascertain which board you need and at a reasonable price.

 

[ianmcd]

+1 for CET Ltd

 

[Edward Durrant]

R80 = 2.7k (are those scorch marks I see?)
R111 = 1Meg (this is not likely faulty)

Thank you for your help - yes they are minor scorch marks, perhaps if I replace just the R80 this would be successful. Does the degree of scorch marks look likely enough to damage the circuit?

E D, what is the fault and why do you think these resistors are the problem

Do you have a circuit diagram of the pcb and do you, based on what you say, can describe circuit operation?

Unfortunatley no I do not have a circuit diagram, I am going purely off what many others have recommended that have done this repair (may seem foolish but it is worth a try as changing the resistor is worth a shot before spending on a new board.

Why not try Geoff at CET ltd. His company reconditions pcbs on an exchange basis. A short chat with him would ascertain which board you need and at a reasonable price.

Thanks, I got a quote elsewhere for £150 for a repair of £175 new, so I will try the resistor fix and if this fails I will get in touch with Geoff. Hopefully he can help but at the above prices a repair is not really worth whilst. Thanks all for your help and will update you.

 

[bernardgreen]

Why do so many PCB's produced for gas boilers have over sized holes for component leads. ?

 

[AGAS]

 

[AdrianUK]

I'd be surprised if the resistors had changed in value......however, the electrolytic capacitors, that's a different matter all together. I think you may well have success by changing the caps.

 

[DP]

Crack on lad. Whatever sails your boat. This episode is like a rower sets out to go around the world without a map, or a compass.

 

[Gerrydelasel]

Thank you for your help - yes they are minor scorch marks, perhaps if I replace just the R80 this would be successful. Does the degree of scorch marks look likely enough to damage the circuit?

TBH, no, it look pretty minor, but that doesn't mean the resistor is definitely not toast. I would be tempted to replace it with a higher-power device, like 1W or something.

 

[Agile]

So you want to replace these resistors when you have not measured their values?

Tony

 

[bernardgreen]

I am going purely off what many others have recommended that have done this repair

So you want to replace these resistors when you have not measured their values?

If you really must try and fault find the PCB then with through holes so much larger than the component leads you should look for fractured solder joints.

The most common fracture when the hole is too large is an annular ring fracture. The solder bridging the gap between the pad and the lead is subjected mechanical stress due to vibration and expansion contraction caused by thermal cycling.

Oxidisation of the solder in the fracture creates an insulator in the fracture. To repair it is necessary to remove the solder and oxides and replace with new solder. Reinforcing the joint with a strand of copper wire is sometimes necessary to prevent a repeat of the failure.