Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Question

[nobbnobb1]

Hello,

I have a Velodyne DPS-12 subwoofer that was making weird noises. Upon taking it apart, I noticed there was yellow glue all over the circuit board which had turned brown, so I scraped it all off and that fixed the problem. I used hot glue instead around the various components.

There was also one bulging capacitor, and another that must've been in a really hot location because the labelling looked like it was scorched. You can see in the picture the circuit board has discolored over time from this heat. Could either be from the MOSFET (with heatsink), or the resistor. I decided to just go ahead and re-cap the entire board since it was only $20 in capacitors.

I have two general questions:

1. Looking at the discoloration of the circuit board (picture 1), should I be concerned with heat? Like I said, everything does work fine. This picture is before I re-capped it. When I re-capped, I did somewhat try to angle & position the capacitors away from the heat sources as best as I reasonably could.

2. On the main amp backplate, there are several transistors screwed onto it, along with a thermal pad. When I assembled this, I removed the old thermal pad and used heatsink grease instead as I figure this would help with cooling. However, now that I think about it, the thermal pad would be an electrical isolator. Do you guys think I should re-use this thermal pad, or is it ok for the transistors to be attached directly to the amplifier plate like so? The plate appears to be anodized aluminum so it's not conductive on the surface, but of course I don't want to be taking any chances if this is not good practice.

Picture 2 is of how I re-installed these and picture 3 is the thermal pad that came off.

Love to hear your thoughts. Thanks!

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Hello,

I have a Velodyne DPS-12 subwoofer that was making weird noises. Upon taking it apart, I noticed there was yellow glue all over the circuit board which had turned brown, so I scraped it all off and that fixed the problem. I used hot glue instead around the various components.

There was also one bulging capacitor, and another that must've been in a really hot location because the labelling looked like it was scorched. You can see in the picture the circuit board has discolored over time from this heat. Could either be from the MOSFET (with heatsink), or the resistor. I decided to just go ahead and re-cap the entire board since it was only $20 in capacitors.

I have two general questions:

1. Looking at the discoloration of the circuit board (picture 1), should I be concerned with heat? Like I said, everything does work fine. This picture is before I re-capped it. When I re-capped, I did somewhat try to angle & position the capacitors away from the heat sources as best as I reasonably could.

2. On the main amp backplate, there are several transistors screwed onto it, along with a thermal pad. When I assembled this, I removed the old thermal pad and used heatsink grease instead as I figure this would help with cooling. However, now that I think about it, the thermal pad would be an electrical isolator. Do you guys think I should re-use this thermal pad, or is it ok for the transistors to be attached directly to the amplifier plate like so? The plate appears to be anodized aluminum so it's not conductive on the surface, but of course I don't want to be taking any chances if this is not good practice.

Picture 2 is of how I re-installed these and picture 3 is the thermal pad that came off.

Love to hear your thoughts. Thanks!

So, if the original screws are metal connecting the transistors to the heatsink you will not have to worry about isolating the transistor with a thermal pad. In my day they use to like to use thermal pads because the heatsink grease would dry out. I do not believe that is a major issue today. Perhaps younger techs could elaborate on this more. The yellow glue may not be a glue at all but an old conformal coating when exposed to heat turned yellow over time. Again, a younger technician should be up of all the modern-day coats that help isolate the board from the elements.

[nobbnobb1]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

So, if the original screws are metal connecting the transistors to the heatsink you will not have to worry about isolating the transistor with a thermal pad. In my day they use to like to use thermal pads because the heatsink grease would dry out. I do not believe that is a major issue today. Perhaps younger techs could elaborate on this more. The yellow glue may not be a glue at all but an old conformal coating when exposed to heat turned yellow over time. Again, a younger technician should be up of all the modern-day coats that help isolate the board from the elements.

On the screws, two of them had a plastic isolator and the others were just a screw. That's why I'm having my doubts and think I should just use the thermal pad instead of grease.

As for the glue, it's a well known problem with electronics, similar to bad capacitors. It should be yellow but with time and heat turns dark and conductive. On older equipment (20+ years), I make it a habit to take things apart to check for both bad caps and the yellow glue plague. If you catch it early you can prevent premature failures.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

On the screws, two of them had a plastic isolator and the others were just a screw. That's why I'm having my doubts and think I should just use the thermal pad instead of grease.

As for the glue, it's a well known problem with electronics, similar to bad capacitors. It should be yellow but with time and heat turns dark and conductive. On older equipment (20+ years), I make it a habit to take things apart to check for both bad caps and the yellow glue plague. If you catch it early you can prevent premature failures.

Plastic isolator. Exactly what type. Was the plastic isolator a piece of plastic on the back of the transistor heat sink? Was the plastic isolator an insert into the mounding hole of the transistor. Is the screw and the nut plastic? Not that it matters much. If it is electrical isolated, I am just curious of how.

Have you measured the resistance of the heat sink pad to find out what the electrical conductance of the heat sink pad is and what characteristic you will have to match? Also, one needs to measure the capacitance of the heat sink pad to determine how isolated it is in capacitance. I am sure there are thermal grease that is also non- conductive with capacitive listed on the data sheets.

When you talk about glue I think of Elmers. There are many different types of coatings with different characteristics. I would think the best thing to do is to match the same coating unless you know exactly what the parameters the engineer was trying to accomplish with the coating.

[diif]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

If the designers put in an insulation pad then you should put it back IMO.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

If the designers put in an insulation pad then you should put it back IMO.

As I say it is not that simple. In the 20+ years since this unit was made there could be advancements. Also, the part is old and worn and may warrant needing replacing. Lastly, the design engineer can have preferences that may not be the best practice. In last case one has to know exactly why it was used before substitutions can be made.

Normally, I would agree with you if the unit was new as that would be the simple solution.

[diif]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

We can see it in the photo, why would it wear ? it doesn't need substituting, just fitting back.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

We can see it in the photo, why would it wear ? it doesn't need substituting, just fitting back.

Things off all kinds wear with age. The property that originally made up the material degrades with age. Just like people degrade with age. It is really simple to replace a part once a person knows what it is so why would one want to take a risk of future failure when repairing equipment. If the pad degrades to not allow maximum heat transfer to the heat sink it will put the transistor under thermal stress. One cannot simply look at a material by eye and determine if the molecular structure has degraded and by how much.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Here is a good video of the property of the pads for transistors that may help you. I only watch about 3 minutes of the video. That told me the guy has done his research.

https://www.youtube.com/watch?v=AK4SNhHEgmY

As with all explanation also get back up information to confirm what was said.

[lti]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

In the second picture, it looks like there is a plastic insert in the screw hole for the TIP31 on the right. The two larger parts are in a package that usually has plastic molded inside the screw hole, so they don't need the extra plastic insulator.

I would say that the pads and plastic insulators under the screws are there for a reason. You can replace them with new insulating thermal pads if you want (there are also pads that are electrically conductive, which you don't want for this).

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

In the second picture, it looks like there is a plastic insert in the screw hole for the TIP31 on the right. The two larger parts are in a package that usually has plastic molded inside the screw hole, so they don't need the extra plastic insulator.

I would say that the pads and plastic insulators under the screws are there for a reason. You can replace them with new insulating thermal pads if you want (there are also pads that are electrically conductive, which you don't want for this).

Thanks for the information.

[nobbnobb1]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Thanks for the info. Just to be safe, I decided to not take any chances and apply the insulator along with some heat sink grease to improve cooling. I've re-capped this unit with better than OE quality caps to keep it going for a long time. It's a really excellent sub!

[eccerr0r]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

The only case where it's clearly a superfluous insertion of a insulator is if the package is something like a TO-220F or TO-3PF where the body is completely plastic. Heatsink compound is still recommended but the insulator sheet is not needed or recommended. It's *possible* to not need it, but almost always, if there's more than one metal tab and two dissimilar devices, the insulator is needed.

Heatsink compound/grease is there for a different purpose than the insulator sheet. The grease is for thermal conductivity. The sheet is solely there for electrical isolation. You may have gotten lucky that your device didn't short out - anodized aluminum surface is not conductive and the plastic shoulder washer helps against the screw shorting, and most transistors are collector/drain to metal case, and this is does not help any of the transistors as for class AB drivers, none of the transistor cases are at ground level.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

The only case where it's clearly a superfluous insertion of a insulator is if the package is something like a TO-220F or TO-3PF where the body is completely plastic. Heatsink compound is still recommended but the insulator sheet is not needed or recommended. It's *possible* to not need it, but almost always, if there's more than one metal tab and two dissimilar devices, the insulator is needed.

Heatsink compound/grease is there for a different purpose than the insulator sheet. The grease is for thermal conductivity. The sheet is solely there for electrical isolation. You may have gotten lucky that your device didn't short out - anodized aluminum surface is not conductive and the plastic shoulder washer helps against the screw shorting, and most transistors are collector/drain to metal case, and this is does not help any of the transistors as for class AB drivers, none of the transistor cases are at ground level.

Depending on the type of thermal grease he used may have also help him a great deal. Silicon thermal grease is an electrical non- conductive grease. However, there are also conductive thermal greases.

Aluminum that oxidizes whether through anodized or by nature does lower it conductivity. I am sure there must be some anodized aluminum that are also treated with a non-conductive coating when the anodized process is done to further the anodized protection.

[diif]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Things off all kinds wear with age. The property that originally made up the material degrades with age. Just like people degrade with age. It is really simple to replace a part once a person knows what it is so why would one want to take a risk of future failure when repairing equipment. If the pad degrades to not allow maximum heat transfer to the heat sink it will put the transistor under thermal stress. One cannot simply look at a material by eye and determine if the molecular structure has degraded and by how much.

What nonsense.
Please explain how the insulation material has worn with age?
There's no heat marks and no friction to wear it down. The device failed due to conductive glue.

[CapLeaker]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Agreed, diff. Haven’t seen that kind of wear of thermal pads. I did see that they do get backed on to things and get ripped during removal process, but that’s about it. Did I substitute white heat transfer compound to replace the ripped thermal pad? Absolutely, yes!

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

What nonsense.
Please explain how the insulation material has worn with age?
There's no heat marks and no friction to wear it down. The device failed due to conductive glue.

Instead of saying nonsense please do your homework as I am tired of doing it for you. Research it with an open mind instead of just trying to look for slanted views that might support your statements. This information is out on the internet and available so do your own research.

Also, when making a nonsense statement and disagreeing with something technician please provide an Electrical research engineer's statement, who has a background in Physics or Chemistry, to back up your opinion and give his credentials. Would be good if it was on a video.

In the above explanation provide the type of insulator pad that is used. That is the type of material and the material doping that is used to create the heat transfer through an insulated material. We will need things like thickness, dielectric strength when the material was made and by what manufacturing company. Then ask the starter of this thread to inform you of exactly what his pad is made of so we can do a complete comparative analysis to make sure that we are talking about exactly the same thing. If you can do this and prove my statement to be not correct, along with the research engineer statement and his credentials, then I would only be able to agree with you. But if you cannot do this then please to don't assume you are right, and I am wrong.

There is heat on a heat pad to transfer heat even when no heat marks are present. Environmental factors also wear materials down. Friction is not the only thing that wears a material down. Understand water passing over rock will wear rock down overtime. This is not just done because of the friction but of the water makeup. Cement pools will get etched just because the Ph in the water is too low. Batteries are made of this principle of wearing down materials. These are things in our environment.

If the coating fails over years that should also tell you that there are reasons for the failure. With a heat pad we cannot tell what it might looked like when new so it may have discolored. However, failures in materials are not just limited to visual eyesight. This is why we have electron microscopes. Metals can fail internal and look perfect outside. In this last statement I have made if you did agree with me then fine. Disagree. However, that will only be an opinion of only yours until you actually back that up with technical fact from technical research people with advance degrees who actual test these things.

[diif]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Instead of saying nonsense please do your homework as I am tired of doing it for you. Research it with an open mind instead of just trying to look for slanted views that might support your statements. This information is out on the internet and available so do your own research.

Also, when making a nonsense statement and disagreeing with something technician please provide an Electrical research engineer's statement, who has a background in Physics or Chemistry, to back up your opinion and give his credentials. Would be good if it was on a video.

You haven't done anything for me, please provide some facts to back up what you write otherwise it's just nonsense. You write lots of words but you always fail to back them up.

The inert insulator was not discolouored from heat and had been doing it's job for years. It did not need anything doing to it. I used my brain not some bullshit you think i need to provide.

[eccerr0r]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

I think the OP's decision to remove the thermal pad is because they "learned" from CPU "delidding" and other ridiculous ideas as to reduce thermal impedance. This is not acceptable for amplifier design because CPU heatsinks tend to be isolated from any electrical potentials (for desktops at least; laptops usually do have some constraints) and even if the CPU shorts to the heatsink, it may not matter as the heatsink is completely shrouded by the computer case.

Amplifiers do typically require electrical isolation between heatsinks and semiconductor devices due to sharing, if not also for electrical potential requirements since the heatsink may be exposed to the outside of the chassis or even *is* the chassis, as running the case at high voltage potential with respect to ground typically is not a good idea for UL approval.

[keeney123]

Re: Fixed a Velodyne DPS-12 Subwoofer. Thermal Management + Heatsink Grounding Questi

Agreed, diff. Haven't seen that kind of wear of thermal pads. I did see that they do get backed on to things and get ripped during removal process, but that's about it. Did I substitute white heat transfer compound to replace the ripped thermal pad? Absolutely, yes!

Let me ask you this why do the thermal pads rip during removal? Is it just because of neglect by the user when removing it or has the pad also started to fail and allow it to be easily torn? I can tell you when I was an electrical/ electronic technician working in production that the mica film pads are robust when new. One can bend them every which way without failure.