Re Capping

[andyjm]

Has anyone on here had a go at replacing surface mount electrolytic capacitors?

Probably the wrong forum, but you never know. My old Krell has finally cooked its electrolytics (its a very poor thermal design to be honest) , and it is uneconomic to have Krell fix it. I thought I would have a go at replacing the caps rather than throw it out.

The boards are surface mount, so are going to need a hot air gun to rework. Not done this before, so it could easily end badly. There are a few youtube videos of guys doing cap replacements, but half of them seem to be cowboys.

Anyway, please reply if you have had a go.

 

[drummerman]

Yes. One of my cyrus players developed a leaking small rated electrolytic.

It was a rather tricky undertaking and made harder work out of it than it should have been.

It was fiddly with hardly any space around the cap and I managed to lift a track in the process (removing the cap) despite having reasonably good soldering equipment.

I could'nt repair the track but managed to use a long legged/through hole rather than sfm cap and jump the damaged track.

Small price to pay for the alternative repair cost by Cyrus.

 

[Vladimir]

My advice would be to try and practice on something else (old PC mobo) and see if you can do it.

 

[MajorFubar]

I chickened out and got a pro to do it, despite the fact I'm usually handy with domestic soldering and usually I'm first to advocate people do the same. Considering the often cramped working conditions I think 90% of the battle is having good tools and I don't have a desoldering gun, just a soldering iron and pliers.

 

[drummerman]

Obviously not what the OP wanted to hear

Moral ... get a pro to do it.

The Krell is surely worth it.

Having said rhat, if it is the main electrolytics you probably have a little bit more space to work in.

Have fun.

 

[seemorebtts]

Send me them IL put them on.just buy the caps

 

[Vladimir]

 

[andyjm]

Thank you for the offers of help, but I want to have a go myself. If I feel keen, I will see if I can post a picture or two.

I have been ordering the parts. The amp is a Krell Showcase which was Krell's brief flirtation at making an 'affordable' amp. If I recall correctly, I paid £5,500 for the amp, and about the same again for the matching processor.

The amp has 5 channels, each rated at 125W into 8 ohms and 250W into 4. The amp is quite a neat design, each channel is self contained module - Krell use the same modules in some of their lower end monoblock amps. Given the overall price of the amp, thats about £1000 per module plus a bit left over for the case and power supply.

Now the point of the post. Each module has 10 decoupling caps, used to smooth the supply rails to the circuitry that drives the output stages. I have been able to identify the exact part (its printed on the side), Nichicon 33microfarad electrolytic cap, 35V. The complete amp has 10 caps per module x 5 modules = 50caps. Expensive huh?

A number of posters have suggested that the reason high end equipment is expensive is because of the high quality (and expensive) parts used. The Nichicon smoothing capacitors I need are normally found in computer switch mode supplies, and cost the princely sum of 12.5p each. Thats £6.25 in parts to replace all 50 smoothing capacitors. While the semiconductors may well be much more expensive, the caps are certainly not 'high quality and expensive'.

Slightly more worryingly, having read the spec sheet, the caps are only certified for 1000 hours of use. While that doesn't mean they are all guaranteed to fail at this point, it does give an indication of their likely life.

Given the price of the amp, and its 1000 hours life, that's £5.50 per hour.

 

[Vladimir]

1000h at a certain temp. Usually 85C or 105C. If they are at room temp they will outlive couple of generations of pets. If they are located near hot transistors and heatsinks, expect good 5-10 years before EOL. Chinese caps usually less.
Panasonic FM 105C are very good choice.

 

[andyjm]

1000h at a certain temp. Usually 85C or 105C. If they are at room temp they will outlive couple of generations of pets. If they are located near hot transistors and heatsinks, expect good 5-10 years before EOL. Chinese caps usually less. Panasonic FM 105C are very good choice.

Vladimir, I think this is the problem.

The Krell is class AB, with a high quiescent current through the output stages at no signal, and runs damn hot. The thermal design is (in my opinion) poor, with the caps in the convection hot air flow.

The amp is about 10 years old, and uses mainly Japanese Nichicon caps, though these days they are probably made in Shenzhen along with everything else.

I will see if I can find Panasonic FM 105C caps with the same form factor as the Nichicon.

 

[Vladimir]

Even if you don't find FM in the specific format, the guideline remains, aim for 2000h+ at 105C min. and made in Japan. Unless in the signal path, don't waste money on exotica brands for filtering application.

EDIT: and solid state, not electrolythic if possible at the required min capac. and voltage. I find Mouser's advanced search filters practical for looking up components.

 

[Vladimir]

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

 

[insider9]

Whatever you decide to do, good luck. I wouldn't dare myself but understand the temptation.

 

[drummerman]

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

That was my plan. Even bought a nice clip on illuminated magnifying glass

 

[andyjm]

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

Thanks. Very nice quality work.

He does have the advantage that everything is spread waaaaay out, in my case everthing is crammed in next to each other. The SMD electrolytic caps have their legs partly bent underneath, so it looks like it is going to be a pain to unsolder.

I did pick up a dead Showcase amp for a few £hundred on eBay, so I am going to practice on the blown modules first.

 

[Vladimir]

This guy does it without a hot air gun. Just fine tip, steady hands and a zoom lense. Crammed like downtown Tokyo...

Panasonic solid state OS-CONs.

 

[andyjm]

This guy does it without a hot air gun. Just fine tip, steady hands and a zoom lense. Crammed like downtown Tokyo...

Panasonic solid state OS-CONs.

Great pictures. This is the 5 channel version, the same case had two more channels for the 7 channel model. If you look in the pictures you can see the mounting holes for the extra channels on the base. The 5 modules can each be removed separately which makes access easier, but the component density on the boards is high, so there isn't much leeway for a misplaced soldering iron.

Each module has two PCBs, a main board with the power electronics bolted to the heatsink and a piggyback control board mounted on spacers that has signal preconditioning, bias control and the DC servo circuitry (amp is DC coupled).

Again, for those fretting about their 'interconnects' it is worth noting that Krell decided that computer grade ribbon cable is OK to carry the low level input signal to the amp modules.

It is actually this ribbon cable that seems to contribute to the thermal problem. The amp has 6 transistors in parallel in the output circuit, 3 pushing, 3 pulling. To share the load between these transistors, low value 'emitter resitors' are used - these are the 0.68ohm wire wound ceramic white oblong blocks mounted along the bottom of the power PCB. The amp is already hot, but these resistors add to the problem. The way the ribbon cable is folded directs the heat over the piggyback board and fries the components - in particular the three surface mount caps in the centre.

If you look at the first picture, you can see how the middle of the piggy back board has discoloured - I think because of the heat. I had originally assumed this was due to manufacture, but my amp was always very well ventilated and only shows very slight discolouration at this point. The blown amp I bought has piggyback boards that are almost brown in their middle section.

Frankly, not a great thermal design, it would have been far better if the modules had been mounted the other way up. Given that the signal input socket seems to be at the wrong end of the module, I do wonder whether they were designed to be mounted this way originally. It wouldn't be the first time that the electronic design team didn't communicate properly the the team doing the case and mounting design and things got flipped.

 

[drummerman]

Talking about thermal efficiency my cyrus amp, PSX-R ... in fact most cyrus components have/had their circuit boards mounted upside down ala certain Perreaux. However, there are no vents ( the newer models have a few small slants on the front underside.

So the only way heat is regulated is through the the heatsinks. The chassis is aluminium so it too acts as heat sink but it's not the most thermally efficient design.

I have since long ago used small rubber rings to act as spacer on the Base plates of the amplifier and Power Regulation. It leaves a tiny gap around the Base plate, less than a millimeter which helps venting.

The amp is a good ten years old, on for perhaps 3 hours a day. That's around 10000 hours on the components with no problems. They won't last indefinitely but I give them a helpful hand and would expect the amp to last a few more years.

 

[Vladimir]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

 

[shadders]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink.

So - why is this an issue ?.

Thanks and regards,

Shadders.

 

[drummerman]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink. 

So - why is this an issue ?.

Thanks and regards,

Shadders. 

It's not an issue. High bias/voltage circuits are under much more stress.

 

[shadders]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink.

So - why is this an issue ?.

Thanks and regards,

Shadders.

It's not an issue. High bias/voltage circuits are under much more stress.

Hi,

Thanks, I thought so. I can see how a PCB placed upside sown in the horizontal position could be an issue, if heat is trapped in the enclosure.

Regards,

Shadders.

 

[Vladimir]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink.

So - why is this an issue ?.

Thanks and regards,

Shadders.

It's not an issue. High bias/voltage circuits are under much more stress.

Hi,

Thanks, I thought so. I can see how a PCB placed upside sown in the horizontal position could be an issue, if heat is trapped in the enclosure.

Regards,

Shadders.

Heat radiates spherically from the heatsink, not just the fins, so effectively the whole PCB is baked constantly at higher ambient temp than if it was placed at a 90 degree angle. With lower biased amps this doesn't really matter. You go over 100mA and things get tropical.

Also my friend was refering to the plastic clips that hook harnesses from preamps to these power amp modules. As they get old, brittle and corroded. Some issues coming from those too, now that I think of his rants.

 

[shadders]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink.

So - why is this an issue ?.

Thanks and regards,

Shadders.

It's not an issue. High bias/voltage circuits are under much more stress.

Hi,

Thanks, I thought so. I can see how a PCB placed upside sown in the horizontal position could be an issue, if heat is trapped in the enclosure.

Regards,

Shadders.

Heat radiates spherically from the heatsink, not just the fins, so effectively the whole PCB is baked constantly at higher ambient temp than if it was placed at a 90 degree angle. With lower biased amps this doesn't really matter. You go over 100mA and things get tropical.

Also my friend was also refer to the plastic clips that hook harnesses from preamps to these power amp modules. As they get old, brittle and corroded. Some issues coming from those too, now that I think of his rants.

Hi,

Thanks. If the transistors are not directly behind the PCB, then this will reduce the heat directed to the PCB. If you design the amplifier to run cool, this should help too, and, a cool amplifier means cooler transistors, and the conductivity of doped silicon is a peak at room temperature - so added benefits.

Plastic - the nylon type plastics seem to be worse from memory.

On a side note, everyone complains about plastic bags from the supermarket not being biodegradable, but I have some notes in the loft in plastic bags, and the bags have disintegrated into very small pieces. I mean, they are everywhere, and they are really difficult to clear up.

Regards,

Shadders.

 

[Vladimir]

A service tech that I know calls these vertical boards mounted on the heatsinks 'suicide boards'. Regardless of amp make or model, they have bad thermal properties, lytics cook, predrivers bake.

Hi,

I am intrigued as to why the vertical alignment of the PCB will cause heat issues. The heat in the picture is dissipated by the heatsink, the PCB component side is not facing the heat sink - and usually, pre-drivers are also on the heat sink.

So - why is this an issue ?.

Thanks and regards,

Shadders.

It's not an issue. High bias/voltage circuits are under much more stress.

Hi,

Thanks, I thought so. I can see how a PCB placed upside sown in the horizontal position could be an issue, if heat is trapped in the enclosure.

Regards,

Shadders.

Heat radiates spherically from the heatsink, not just the fins, so effectively the whole PCB is baked constantly at higher ambient temp than if it was placed at a 90 degree angle. With lower biased amps this doesn't really matter. You go over 100mA and things get tropical.

Also my friend was also refer to the plastic clips that hook harnesses from preamps to these power amp modules. As they get old, brittle and corroded. Some issues coming from those too, now that I think of his rants.

Hi,

Thanks. If the transistors are not directly behind the PCB, then this will reduce the heat directed to the PCB. If you design the amplifier to run cool, this should help too, and, a cool amplifier means cooler transistors, and the conductivity of doped silicon is a peak at room temperature - so added benefits.

Plastic - the nylon type plastics seem to be worse from memory.

On a side note, everyone complains about plastic bags from the supermarket not being biodegradable, but I have some notes in the loft in plastic bags, and the bags have disintegrated into very small pieces. I mean, they are everywhere, and they are really difficult to clear up.

Regards,

Shadders.

Of course they are near the board, in way worse layout than at 90 angle. With veritcal orientation you have a sandwich of [ PCB / transistors / heatsink ][/b], all few mm appart. The modular design is done with manufacturing and servicing in mind, not so much for heat dissipation efficiency.