Re Capping

[Vladimir]

Neat trick to remove the old caps.

 

[andyjm]

Neat trick to remove the old caps.

Vladimir, thanks. Very helpful.

Just a quick update. I have bought all the caps (F series 105C Panasonic for the SMD caps) - again, thanks Vladimir. I used Nichicon for the standard through-the-hole decoupling caps as Panasonic didn't have an exact dimensional match. Total cost for approximately 100 caps about £40 including delivery. The Panasonic FC caps look to have a better spec than the Nichicon I am replacing, so I haven't scrimped on parts.

A temperature controlled hot air rework gun, magnifying illuminated lamp, solder paste, flux pen, desoldering braid, extra fine solder - less than £100 in total from eBay. Most of this stuff I will use again, so not really a cost for this project. This has to be compared with the last repair on the amp to fix a similar problem on a single channel (not all 5) which cost nearly £500 from the UK importer.

I had a go at removing some of the SMD caps with the hot air gun on my blown amp. Pretty straight forward when the cap is isolated, but when the cap is tightly surrounded by other components, I ended up melting the solder on those as well. Not the end of the world, but not ideal.

I have seen other videos of the 'twist to remove' technique you linked to, and it does work (I have tried it). The leads on the SMD caps are very fine, and it doesn't need much twisting to pull them off. This avoids frying nearby components when using the gun. There are plenty of 'the guy is a cowboy' comments on Youtube, but given a solid board with good traces and dense packing of components, perhaps using the twist technique instead of hot air is the least of two evils.

 

[Vladimir]

Maybe improvise some metal tip to get a smaller gauge on the heat gun. Alu foil or thin sheet metal.... metal tip from old pencile... etc.

 

[andyjm]

Thanks.

That is the kit I bought off eBay, about £60 if I recall correctly. Not really a professional standard of product, but great for the price. Haven't quite figured out what to do with the handle with the two wires sticking out of it. Any ideas?

Only caveat is that one or two comments on the web suggested that if you use the really thin nozzle on the airgun then the heating element burns out.

Even with a very thin air jet, you still have to warm the board up to melt the solder under the caps. On the tightly packed section, there are transistors on the board less than 3mm from the base of the caps, so it is almost impossible to avoid heating those up as well.

 

[Vladimir]

Absolutely no idea what that dual wire thingy is.

I'd be tempted to just go with the fine tip solder iron. Pre-tin cap leads on the side, pre-tin the iron, use lots of flux on the pads, just touch-solder. If you solder directly in a classical manner by heating the pad, the leads, the solder and the flux at the same time, you will lift traces like drummerman did.

 

[Andrewjvt]

This guy does it without a hot air gun. Just fine tip, steady hands and a zoom lense. https://youtu.be/gO7RkSnuGys

photograghy forum lol

 

[andyjm]

So, its going to be the twist technique to remove the caps. I had 100% success removing all the surface mount caps from a old blown board. Its quick, easy and doesn't seem to create undue stress on the tracks. The caps require very little effort to twist them off.

The circuit pads under the caps are quite large (relatively speaking) and the leads from the caps themselves very fine. Twisting the cap applies a shear force to the pad, not a lifting force, and the pads seem fine with this. The cap leads either pull out from the cap or break off.

I have cleaned up the pads on the board with solder wick braid and then washed the board in isopropyl alchohol. I am going to have a go today at soldering some new caps on. If all goes well with this, I will have a go at the real amp this afternoon.

- You may be wondering why I am bothering to try this. I have Martin Logan electrostatic speakers, and they have an impedance curve that looks like my dog's back leg, dipping down to under two ohms at one point. They need an amp with plenty of muscle, and Krell is often partnered with ML for this reason. The model of amp I have is no longer made by Krell, and its replacement, the Chorus 5200 is £10,000. Being semi-retired, I can't really justify spending £10,000 on a power amp - particularly if £40 worth of caps will bring my existing amp back to life.

 

[drummerman]

So, its going to be the twist technique to remove the caps.  I had 100% success removing all the surface mount caps from a old blown board. Its quick, easy and doesn't seem to create undue stress on the tracks. The caps require very little effort to twist them off.

The circuit pads under the caps are quite large (relatively speaking) and the leads from the caps themselves very fine. Twisting the cap applies a shear force to the pad, not a lifting force, and the pads seem fine with this.  The cap leads either pull out from the cap or break off.

I have cleaned up the pads on the board with solder wick braid and then washed the board in isopropyl alchohol.  I am going to have a go today at soldering some new caps on.  If all goes well with this, I will have a go at the real amp this afternoon. 

- You may be wondering why I am bothering to try this.  I have Martin Logan electrostatic speakers, and they have an impedance curve that looks like my dog's back leg, dipping down to under two ohms at one point.  They need an amp with plenty of muscle, and Krell is often partnered with ML for this reason.  The model of amp I have is no longer made by Krell, and its replacement, the Chorus 5200 is £10,000.  Being semi-retired, I can't really justify spending £10,000 on a power amp - particularly if £40 worth of caps will bring my existing amp back to life.

Fantastic

I will rely on your acquired knowledge next time.

 

[Vladimir]

I didn't wonder why you're doing this for a second. Great amp will work again, with added satisfaction of your own effort to get it back to life. Sort of a Dr. Frankenstein and his monster father son relationship.

 

[shadders]

Hi,

Don't forget to get the capacitor polarity right, else there may be a November the 5th moment.

Regards,

Shadders.