Just desoldered main pc board in EQ, how did I do?

[z-adamson]

This is the back of the main board in my hk EQ7.

I just finished desoldering and removing all the aluminum e caps.

I cleaned up each joint with solder wick and on the first few joints I removed some of the green foil off of the board....inadvertently of course.

So, how does my desoldering look? Any signs that I went about it the wrong way?

 

[z-adamson]

I have another question, when I go to resolder the new caps into place, am I soldering to the green foil or to the circular silver pads that become exposed after desoldering? Where exactly do I need a good electrical connection?

 

[kaplang]

The tabs

 

[z-adamson]

The tabs

To make sure we are on the same page, how do you define "the tabs"?

 

[audiodummy]

Not "tabs" but rather the "pads" or the silver circles of metal (actually, solder or tin covered copper) that connect to the light green (copper covered by solder mask). The dark green spots are just solder mask covering the FR4 or whatever base material your PCB is. The solder mask is deliberately impossible to solder to, you'll need to scrape it of if you must connect to it.

Also when replacing capacitors, I'd suggest replacing them one at a time, carefully, so you don't mix them up. Watch polarity carefully!

 

[z-adamson]

Double checking.....solder to the pads and ignore the green layer and pay no attention to the fact that I removed some of the green layer?

Right?

Polarity is clear here....the board is marked for positive and the caps are marked for negative with a stripe.

 

[jheu02]

As above, the solder will only stick to the pads or other metal and yes, make sure polarity is correct unless using film caps as they are non-polar.

Also, just a minor request for future postings/projects: keep them all under one title that will make sense for search engines and to help others in the future. You've now got at least these two threads going about your EQ7, but neither have that in the title, so someone in the future might not find it. It would be better for your future H/K system work to have a separate thread for each piece and leave all your questions/comments/photos related to that piece in the one thread. These two could have been titled "H/K EQ7 rebuild" or "Harman Kardon EQ7" or something of that nature.

As to your question...looks like a couple pads may be starting to lift from the board. Be very careful when soldering the new components.

 

[JP]

Polarity is clear here....the board is marked for positive

Hope you checked before pulling them. Mismarkings are common.

 

[z-adamson]

I did check markings and I made a drawing and took pics. I'm confident about part.

Which pads appear to be lifting?

Another thing.....I hooked my esr meter to each cap that I removed....I got 0.2 ohms, 2 ohms, 5 ohms, 7 ohms, 4 ohms. Each of the 16 caps was one of the above. The lower the uf rating the higher the esr it seemed. The lower the uf rating, the further away from if spec it was.

 

[Dale Beaver]

The green is actually solder resist. It's for wave soldering on modern pcb's

 

[z-adamson]

Also, just a minor request for future postings/projects: keep them all under one title that will make sense for search engines and to help others in the future. You've now got at least these two threads going about your EQ7, but neither have that in the title, so someone in the future might not find it. It would be better for your future H/K system work to have a separate thread for each piece and leave all your questions/comments/photos related to that piece in the one thread. These two could have been titled "H/K EQ7 rebuild" or "Harman Kardon EQ7" or something of that nature.

I see your point. I felt that I was asking more about my desoldering and what I need to do to improve rather than EQ7 specific questions. Hopefully this project will be done soon. My next project will be the hk725 preamp. I will maintain a dedicated thread for it if that works best.

 

[nedseg]

Also, it looks like the thru holes aren't totally clear (lower right, first pic), solderwick sometimes doesn't get those clean (or takes overheating the pads, which can lift them), and makes it a hard or impossible fit for the new leads.
Solder suckers (Soldapullt, or 'lectric/heated ones) work great for that...but also takes a bit to master.
I usually suck first, then wick for cleanup, if needed - avoid overheating the pads/traces (duh).
Practice, practice, practice. Get some old junk boards and practice on them first??

 

[loudnoises]

Also, it looks like the thru holes aren't totally clear (lower right, first pic), solderwick sometimes doesn't get those clean (or takes overheating the pads, which can lift them), and makes it a hard or impossible fit for the new leads.
Solder suckers (Soldapullt, or 'lectric/heated ones) work great for that...but also takes a bit to master.
I usually suck first, then wick for cleanup, if needed - avoid overheating the pads/traces (duh).
Practice, practice, practice. Get some old junk boards and practice on them first??

Another vote for practice, not that you're doing badly by any means. small work takes time and there's no replacement for due caution.

Sometimes the solder won't clear the holes and you'll find yourself heating one pad too much just to clear out a wee bit of solder. Suckers are handy for this, but if you don't have one a very tiny controlled blast of air will push the heated solder out. watch where it goes as it'll splash, but it works.

 

[nedseg]

What he said. Another technique is to slap the board hard against something...which results in splashes of solder all over the floor (easy to tell which assembly people did that!)...but may not be appropriate for PCBs with heavy components on them. Fast & cheap, tho.
And, yes, production desolderers are too expensive for 'home use'.
I have a corded heated ssucker I got from RS (I think) decades ago that works suprisingly well - all metal, too - though easy to overheat with. Still tend to prefer the soldapullt, tho...the nylon tips risk less pad damage than the heated metal one...choice depends on the PCB.
Darned if I could not find anything similar today (but only checked 'zon).
Keep up the practice, and asking questions!

 

[audiodummy]

Reminds me of this one guy on another forum that replaced a capacitor...he insisted that he put the capacitor in correctly, but wondered why the capacitor exploded when he turned it on... white silkscreened is negative right? It turned out white was positive on this board, explaining everything. Luckily he only replaced one capacitor and I spotted that cap he replaced was indeed backwards compared to the others on the board, solving that mystery.

Removing the green soldermask on top of the tinned copper trace is not a big deal.

Also at a fixed testing frequency, yes ESR "increases" as the rated capacitance "decreases"; you need to increase testing frequency to test smaller capacitors. Most ESR meters will give fairly "high" numbers for capacitors 47µF and smaller, but again it depends on the tester. However it tends to be "good enough" and if you measure a very high resistance on a 10µF capacitor, it's probably bad anyway, and they're cheap, just replace.

 

[z-adamson]

Reminds me of this one guy on another forum that replaced a capacitor...he insisted that he put the capacitor in correctly, but wondered why the capacitor exploded when he turned it on... white silkscreened is negative right?

Do capacitors explode everytime that they are installed backwards?

Secondly, let's say you know where + and - are on the board, but there is confusion as to + and - on the cap. Is there a simple error proof test that will confirm which lead is + and which lead is -?

 

[jheu02]

The cap is typically marked well, unless it's a bipolar cap in which there is no polarity. Common convention also has the positive leg the longer of the two, the negative the shorter.

 

[Eastham]

A little soldering advice from me is to be quick, you don't want to be heating the pad's for too long. When you're soldering in the new components, apply just enough solder to get a concaved looking blob of solder, leave the iron on the joint for a second or too and then pull the tip up the lead of the component not all the way up but enough to pull the solder up it. A good joint will be shiny and concaved. A bad joint typically will look very dull and ball shaped. I'd also reccomend you get some 60/40 lead solder I also like to use rosin core lead solder, perfect joints every time.

 

[petehall347]

i like the 60/40 lower melting point solder . get in the quick and out quick .0.7mm or thinner .
touch my iron to the lead of component an rest it on pad then apply solder .then cut the excess leg off .

 

[audiodummy]

63/37 is eutectic and has the lowest melting point of all, though I've found 60/40 to be close enough.

Typically all polarized electrolytic capacitors will explode if charged in the opposite polarity or go over voltage. And probably 95% of all polarized electrolytic capacitors have the line/bar drawn on it, have a "-" symbol on that line/bar, and points to the negative terminal. The ones that don't, well, it still could be either way and you'll have to go to the data sheet to be sure.

This makes it tougher as usually the line on a tantalum electrolytic is usually positive, whereas aluminum is typically negative...