How to go about checking this PCB

[david74]

Hi, I have this ignition PCB, it opens a valve but does not ignite. I bought a primitive tester for £7.99 and what I checked was the fuse and the output terminals.

The fuse is ok and output terminals show 0.05 (My Ohm meter goes from 1 to 0 and says 200kO on the selector knob dial). But I can't read anything on the input of the grey little box the output terminals are.

I went to the local phone shop and the person there poked around with the meter and spotted a blown capacitor.

He said he would be happy to swap out any faulty part but also said that he had no means of finding which one was which.

Can I find anything conclusive with a multimeter on the board?

I understand that capacitor appears as an open circuit to the 3V multimeter.

If I wanted to order the capacitor, how would I figure out its specs?

Please could anybody check out the photo?

There is a primitive diagram on the box.

 

[Spoon]

Ok, correct me if I'm wrong, but the component that you have circled is a resistor. (The one with the 4 coloured stripes)
To get the value of the resistor, you use the coloured stripes.
The stripes to me look like brown, black, brown. and the end one looks Gold. That will give you a value of 100 ohms and a tolerance of plus/minus 5%
The wattage of the resistor, you may find out by measuring the length and comparing it with the catalogue lengths.
Also the value of the resistor may be printed on the PCB when you take the damaged on off.

 

[david74]

Ok, correct me if I'm wrong, but the component that you have circled is a resistor. (The one with the 4 coloured stripes)
To get the value of the resistor, you use the coloured stripes.
The stripes to me look like brown, black, brown. and the end one looks Gold. That will give you a value of 100 ohms and a tolerance of plus/minus 5%
The wattage of the resistor, you may find out by measuring the length and comparing it with the catalogue lengths.
Also the value of the resistor may be printed on the PCB when you take the damaged on off.

Oh, thank you, it just says R42 on the board probably referring to a parts list on some schematics.

What about my other question. Should that grey box read anything on inputs? Actually!!! I pressed harder and it shows 0.01!!!

Do you think swapping that resistor out would sort the PCB? What potential other, bigger problem it could indicate to check first?

BTW, I checked the 24V power supply, and it does show 25V. I did not check it under load.

I also checked the 100 Ohm resistors on eBay but they also specify wattage e.g. 1/4 or 1.8. And the look small, mine is at least half an inch wide.

How do I check other resistors. My tester has diode continuity knob position. When I check a nearby component that looks like a diode, it gives reading jumping around upper 19 hundreds. And it does it both ways.

The other blue large resistor (R4) shows 0.05 but the yellow (R21) says 36 which means that my tester not just goes from 1 to 0 but also gives larger numbers than one.

 

[westward10]

I believe the terminals on the grey box are the 20kv output for the ignitor.

 

[david74]

I believe the terminals on the grey box are the 20kv output for the ignitor.

Yes, they are and I consulted AI and it says that the 0.5 reading on output and fluctuating reading between 0.0 and 0.1 on input is really low. Does it mean they have melted and then fried the resistor?

I ordered a pair of 3W 150 Ohm resistors on eBay because the black line Spoon took for black is actually blue.

There is a little bit of soot on the inside of the box around the 20kV terminals.

 

[mainline]

Oh, thank you, it just says R42 on the board probably referring to a parts list on some schematics.

What about my other question. Should that grey box read anything on inputs? Actually!!! I pressed harder and it shows 0.01!!!

Do you think swapping that resistor out would sort the PCB? What potential other, bigger problem it could indicate to check first?

BTW, I checked the 24V power supply, and it does show 25V. I did not check it under load.

I also checked the 100 Ohm resistors on eBay but they also specify wattage e.g. 1/4 or 1.8. And the look small, mine is at least half an inch wide.

How do I check other resistors. My tester has diode continuity knob position. When I check a nearby component that looks like a diode, it gives reading jumping around upper 19 hundreds. And it does it both ways.

The other blue large resistor (R4) shows 0.05 but the yellow (R21) says 36 which means that my tester not just goes from 1 to 0 but also gives larger numbers than one.

To be able to check most components properly you would need to take them off the pcb or at least one leg or more.

 

[david74]

I checked the other resistors and compared the readings with the stripes and they were all a match except one where AI said it should be 33K Ohm but my tester just shows that it was 32.7 without specifying that it was in K.

The problem was that the protective coating the PCB and parts are covered in are difficult to scrape through.

I could not get any readings from the relay but the board was opening the valve so if that was what relay was supposed to do, then that is ok.

 

[mainline]

I checked the other resistors and compared the readings with the stripes and they were all a match except one where AI said it should be 33K Ohm but my tester just shows that it was 32.7 without specifying that it was in K.

The problem was that the protective coating the PCB and parts are covered in are difficult to scrape through.

I could not get any readings from the relay but the board was opening the valve so if that was what relay was supposed to do, then that is ok.

You maybe lucky and it’s just that resistor gone open.

If not it could be a number of things.

 

[david74]

You maybe lucky and it’s just that resistor gone open.

If not it could be a number of things.

Well, before I cash out £83 on a new board, it is worth trying a resistor for £2.25.