C4S Upgrade Blown LED

[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!

 

[Paul Birkeland]

Please do swap out both transistors at once, just to be on the safe side.

 

[JosephDuffy]

Right, I've replaced the 5731A and 2N2907 transistors and I've still having issues getting any sort of readings. Once I removed the 5731A it seemed to be working as expected when I performed my resistance checks, which seemed a little odd. I've tested the resistors and as I said previously they seem to be ok. I also cannot find any other areas that seem to raise concern.

I'm hesitant to do much more soldering on it since I'm worried that I might start damaging the board after removing/replacing/reflowing so much.

I'm not sure what my next steps could be other, other than getting a replacement kit or simply removing the C4S and getting hold of some of the old resistors and putting them back in place. I'm still hoping that there's something silly that's gone wrong, but after replacing so many components it's just getting more and more confusing.

 

[Paul Birkeland]

Right, I've replaced the 5731A and 2N2907 transistors and I've still having issues getting any sort of readings. [...] I performed my resistance checks, which seemed a little odd.

You can post what you're seeing.  "any sort of readings" and "odd" aren't really enough for us to be helpful. 

It would probably help to put the 150K resistors back in to be sure the amp is still functional.

 

[JosephDuffy]

Right, I've replaced the 5731A and 2N2907 transistors and I've still having issues getting any sort of readings. [...] I performed my resistance checks, which seemed a little odd.

You can post what you're seeing.  "any sort of readings" and "odd" aren't really enough for us to be helpful. 

It would probably help to put the 150K resistors back in to be sure the amp is still functional.

Sorry, maybe the "odd" comment wasn't well formatted. I meant that it was odd that when I removed the 5731A the 5731A itself seemed to be working as expected (i.e. the resistance checks were showing the same as the new one I'd just received)

By "any sort of readings" I mean that I'm unable to get a reading (0 or more) from the OB to either of the side connectors of the 5731A, which is problem that I was having before I replaced the 5731A.

Edit: Sorry, forgot to add: I was going to try and replace the original resistors, but I was struggling to get them out originally so I cut them. I've only had a quick chance to look but I was unable to find replacements that looks suitable. I will take another look, but right now, I cannot replace them. All the resistance checks, other than B5 (the one on the left when it's unsdie-down, not sure if it's A or B), show as they should be from the original S.E.X. manual.

 

[JosephDuffy]

Right, so here's where I'm at now:
I've taken the 150K resistors from the C4S board and put them back in the main circuit, leaving out the C4S board completely. I've done my checks, turned it on, and am having no issues, so I'm going to installed the Output Transformer Switched kit since I've got some speakers arriving (hopefully) pretty soon.
I can't find any replacements for the 150K resistors so I'm planning on tackling the C4S kit again in a few days.
I took at look at just buying another C4S kit because it feels like starting over again might help, but the postage to the UK is $40. Is there any chance of getting that price down? The kit's fairly light and small and $40 seems a little high?

 

[Paul Birkeland]

I think we could do a USPS Priority flat rate box at the small size, which is still around $25.

The 150K resistor can actually vary quite a bit in value.  The maximum resistance that I would recommend would be 330K, and the minimum would be 120K.  At 330K, a 1W resistor is OK, at 120K, a 2W resistor will be necessary. 

 

[JosephDuffy]

I think we could do a USPS Priority flat rate box at the small size, which is still around $25.

The 150K resistor can actually vary quite a bit in value.  The maximum resistance that I would recommend would be 330K, and the minimum would be 120K.  At 330K, a 1W resistor is OK, at 120K, a 2W resistor will be necessary.

If I can get the shipping for $25 then I'll just get a new C4S kit and not bother getting any replacement resistors. How do I go about ordering a new C4S kit with the $25 shipping?

Quick question regarding the Output Transformer Kit: On page 14 it states, when discussing the wires leading from the binding posts to the boards: "Carefully confirm that each black wire is aligned to its respective transformer". How I do I know which transformer each wire should go in to? My kit came with 4 black covers (rather than 2 black and 2 red) so I can't say "the red/black side goes in to the left/right board". It seems to me that it would make sense if the black wire from the left binding post goes in to the left board, and the right in to the right, but I just wanted to confirm that.
Whilst I'm double-checking things, the picture on page 8 has the resistor in the reverse order that the bands are noted as. Do I do that for the resistor that I use? I've got it going "brown, gold, yellow, black, blue" when the board is the right up, from left to right.

Thanks, again, for all the help you guys have provided!

 

[Paul Birkeland]

My kit came with 4 black covers

Your binding posts have colors...  Each set of binding post goes to the transformer it is closest to.

Whilst I'm double-checking things, the picture on page 8 has the resistor in the reverse order that the bands are noted as.

Resistors are not directional parts, this doesn't matter.  (But still good that you're paying attention)

 

[JosephDuffy]

My kit came with 4 black covers

Your binding posts have colors...  Each set of binding post goes to the transformer it is closest to.

Whilst I'm double-checking things, the picture on page 8 has the resistor in the reverse order that the bands are noted as.

Resistors are not directional parts, this doesn't matter.  (But still good that you're paying attention)

Yeh, the actual binding posts do, but the little plastic covers on either side of the plate are all black, so I couldn't mention the colours that they have. Thanks for the clarification. I'll go over and finish the job now so I can do all the tests.

I'd noticed that resistors seemed to be put in any direction, but thanks for confirming that that applies for all of them!

 

[JosephDuffy]

I got the Transformer Kit installed and working, and after the "practice" on the C4S board, it was a lot easier to solder!

I'm a bit more convinced that I should be able to (successfully) complete the C4S kit, so how do I go about getting the reduced postage costs?