C4S Upgrade Blown LED

[kgoss]

If the solder joints look good on the two wires behind the transistor on the left you should trim those wires just to be certain there is no chance of a short.
The joints themselves are hidden by the transistor so just make sure they are soldered properly.  And before you turn the amp on again you should go through the resistance checks again.

[JosephDuffy]

If the solder joints look good on the two wires behind the transistor on the left you should trim those wires just to be certain there is no chance of a short.
The joints themselves are hidden by the transistor so just make sure they are soldered properly.  And before you turn the amp on again you should go through the resistance checks again.

I have clipped the ends as much as I can, and checked underneath that the joints are good (sorry, I didn't realise that they were covered on the photo!)

I've just done the resistance checks and A5 isn't showing as anything. B5 is showing as 185kΩ. Neither B5 nor the red wire on the circuit board look to be bad joints, but I'll reflow them for the sake of it. Are then any other s I should look at reflowing to try and solve this problem, or any other areas I can look at?

Edit: I've reflowed A5 and the red connector on the board and I'm still seeing nothing on A5

[Paul Birkeland]

That is looking much better, nicely done!

[JosephDuffy]

That is looking much better, nicely done!

I'm still no soldering-guru, but I think I've learnt a lot from the S.E.X. kit Looking back at it, I shouldn't have been soldering on the day I completed the C4S upgrade; I was a bit too ill to be doing something like that, so I'm sure that didn't help.

[Grainger49]

Watch the solder liquify with the iron against both the PCB trace and the component.  After it liquifies hold it for 3s (it is a mental count).  This gives me good solder joints every time. 

[JosephDuffy]

Watch the solder liquify with the iron against both the PCB trace and the component.  After it liquifies hold it for 3s (it is a mental count).  This gives me good solder joints every time. 

As with most things, I did end up finding that spending more time and slowing down actions helped a lot. I think one of my main issues is when soldering on a board and the blob of solder is a little too large and it makes a sort of "flick" as I move the iron away. I'm assuming these are ok, but it'd be better if I used a little desoldering wick?

I've made sure to get good joints over A5, IA (on the circuit board), but there's still nothing showing when I do a resistance check over A5. Is this expected/safe to turn on/anything else I can be checking to try and find the dodgy joint/component (should one exist)

[JosephDuffy]

Ok, I found something interesting: My 150KΩ that's in R2 (B side) is coming up as ~30KΩ, whilst all the other resistors are showing up as their correct values.

I'm guessing I'm going to have to replace that resistor now?

[Paul Birkeland]

This would be a pretty unusual situation, and would really only result in very bright LED's.

[JosephDuffy]

This would be a pretty unusual situation, and would really only result in very bright LED's.

I just did a double check and after leaving the probes on either side it slowly climbed. In about 30 seconds it went for 30 to 65, and I only assume that it'll reach the 150 eventually. However, I can't get a reading over A5 or B5 now. Are there any readings I can check on the board itself so I can make sure it's safe to turn on again?

[JosephDuffy]

Right, scrap everything about the resistor; that seems to be ok-ish.

I've done some more tests:
From the red wire to the 3 prongs of the MJE5731A transistor on the B side I get a reading of 0 ohms. However, on the A side (the side that links to A5) I have a reading of 0 on the middle prong, but then no reading on the left nor right prongs. I've re-soldered all 3 prongs with no luck. Is it worth trying to swap them over?

[Doc B.]

The A side MJE is reading correctly. Sounds like the B side MJE is shorted. It is either blown or you have solder bridging the three pins.

[JosephDuffy]

The A side MJE is reading correctly. Sounds like the B side MJE is shorted. It is either blown or you have solder bridging the three pins.

I took careful precaution when resoldering yesterday and saw no bridges, so maybe it went when the LED blew originally. I'll take another look when I get home, though.

For now, the only options I've found have a "G" at the end (see: eBay). The model on the board has the "G" at the end, so I'm assuming these will be suitable replacement parts?

[mcandmar]

The G just means its a lead free version to keep the tree huggers happy, other than that its the same part.

[Doc B.]

If the 5731A is blown then the accompanying 2N2907 may also be blown. Do the same test, measure resistance across each possible pair combination of the three 2907 leads and see if you read 0s or very low resistance (under 100 ohms) on any pair. That will mean it is shorted.

[JosephDuffy]

If the 5731A is blown then the accompanying 2N2907 may also be blown. Do the same test, measure resistance across each possible pair combination of the three 2907 leads and see if you read 0s or very low resistance (under 100 ohms) on any pair. That will mean it is shorted.

I seem to be getting some off readings, so whilst I'm at, I might as well just replace those, too. I've ordered some of these. When I get the 5731A replaced I'll do another check and if I'm still getting odd readings then I'll replace the 2N2907 (I know the 5731A won't make a difference, but it will give me some time away")

Thanks for all the help, as usual!