Silencing The ALS-1306 Fans

Transcription

1 Silencing The ALS-1306 Fans This document describes measures I've taken to reduce the noise from the power supply and amplifier fans in the Ameritron ALS-1306 amplifier. The major modifications are mechanical in nature, with the only electronic modification being one supplied by W8JI on the Ameritron Amplifiers Yahoo group. While the ALS-1306 cooling fans aren't loud compared to the typical tube amp, I always thought it odd that the four fans in the amplifier cabinet ran at full speed all the time, whether the amp was cold or warm. It turns out that the amp design includes thermistor control of the fan speed based on heat sink temperature, but, even when cold, the circuit applies around 9.5 VDC to the fans causing them to run near full speed. NG7M described how to add a thermistor controlled fan speed controller to the ALS1300 amplifier, which allows the fans to stop when the heat sinks are cool, but rise to maximum speed as the heat sink temperature increases. After much discussion on the subject in the Yahoo group, W8JI provided a modification to the existing fan speed control circuit that eliminated the need for any added circuitry. While that mod results in a higher fan idle speed, Tom explained that it was necessary to have a minimum air flow in the amp to cool other areas than the heat sinks. The Amplifier Fans Electronic Modifications The amplifier has two separate but identical PA units, each with two 80mm fans. At least one 1306 owner has reported that his fans do change speed as the amp temperature rises, but mine come on at what sounds like full-speed and there is no discernable speed change as things heat up. The amplifier control board includes circuitry to control the fan speed with temperature, but in at least some instances, that circuit doesn't do the job properly. While my fans start reliably at 3.2 VDC and are relatively quiet even up to 6V, the existing circuit applies 9.5V to the fans when cold and with that voltage my fans sound like they're running at full speed. The modification provided by W8JI is to the control board in the That board is accessible by removing the 1306 cabinet cover and the board is located on top in the front of the amplifier. The mod involves removal of two surface mount resistors, R136 and R138 and I was able to remove them easily by using solder wick. It's not necessary to remove the board from the amp to do this. Here's a link to W8JI's photo on the Yahoo group showing the location of the resistors: Page 1

2 orderby=ordinal&sortorder=asc&photofilter=all#zax/ If your operation is primarily casual CW or SSB, then you can further slow the low fan speed by adding two resistors to the control board as shown in the photo. To do that, you'll need to remove the screws holding the control board to the chassis so the board can be raised enough to access the nuts on the heatsink screws where you'll fasten the ground end of the resistors (you can use a solder lug under each screw for that). Mechanical Modifications Over the past few years, increased cooling fan requirements for desktop PCs led builders to look for ways to reduce the noise created by the multiple fans required. Fan manufacturers have developed much quieter blade and bearing designs, but higher speed fans like those used in the 1306 still produce a bit of noise. That noise is exacerbated in the 1306 by the fan mounting technique, where metal screws are used to secure the four fans to an aluminum panel that then bolts to other aluminum chassis panels. I decided to replace the metal screws with rubber mounting screws or pins, which serve to prevent transmission of fan noise to chassis panels. A total of 16 screws attach the fans to the aluminum panel, and you need to remove the fan panel in order to access all the screws. A total of 8 screws (4 on top, 4 on bottom) are used to fasten the fan panel to the other chassis panels. To get to the bottom four screws, you have to remove the bottom plate of the cabinet. To do that, turn the amp upside down and remove all the visible screws, saving the three in the front to last. Those three screws fasten the bottom plate to the front panel and once they're removed, the front panel is no longer attached to anything. With the bottom panel removed, you'll be able to get to the four screws that fasten the bottom of the fan panel to the chassis. Getting to the top four screws is a bit more difficult as access to those is blocked by the low pass filter and control boards on top of the chassis. It's not necessary to remove the boards, but you do need to be able to move them aside slightly and you'll be able to do that without removing any connectors. To do that, you first have to remove all the screws holding the boards to the chassis. Next, remove the large nut that secures the output coax connector to the rear panel, and remove the two screws below the connector to the right and left, thus freeing up the small SWR board and the coax connector. You should now be able to slide the low pass filter board and control board aside just enough to access the fan panel screws. Page 2

3 After all eight fan panel screws are removed, you'll be able to slide the fan panel out as shown in this illustration: Illustration 1: 1306 fan panel removed from the chassis. Use the notch in the lip of the rear panel to aid in removal. When removing the panel, be extremely careful not to damage the two wires connected to the bottom two pins of the power connector. You can see the corner of the panel here just touching one of the leads. These two leads provide the 14 volt power to the amplifier and they're very small gauge and can easily be broken off the connector pins. In this photo, the screws for the first fan have already been replaced by the rubber mounts. I suggest you do the fans one at a time. Now a word about the rubber fan mounts. These are sometimes referred to as rubber pins and sometimes as rubber screws. It's important that you use the right pins they're very soft and stretchy Page 3

4 and are often described as ultra soft or soft silicone pins. Those typically sold on ebay are junk they're too hard and not pliable and are easily broken during installation. Here's a photo showing how stretchy good silicone pins are. Illustration 2: You can see that a proper rubber pin is easily stretched quite a bit without breaking. Don't use any that aren't this soft. They are available from a number of sources on the Web, including the sites that concentrate on devices for quieting PCs. Those sites typically charge as much for shipping as for the product (one site wanted $17 for the pins and $17 for shipping!). At the time of this writing (2/25/17), the pins I used were available from Amazon for just $8.14 for 20 pins. Search on this string: uxcell PC Case Fan Mouting Pin Anti Noise Vibration Silicone Screws 20 Pcs. Page 4

5 Illustration 3: In the event you've never used the rubber pins before: It's a bit easier to insert the pin in the hole if you first wet the fat end of the pin with spittle or a detergent solution. Pull the pin through the hole until just the fat end piece is on the other side of the panel. When the fan is secured, you can trim off the long rubber pin, but leave some in case you have to remove the fan later. At the second fan, one of the existing fan mounting screws is used to provide a ground connection for a solder lug and there you'll need to drill a new hole in the fan panel for a screw to secure the solder lug. Just use the original fan mounting screw for the lug. Page 5

6 Illustration 4: The new hole is drilled between the fans for a screw to secure the solder lug. The terminal strip shown is secured to the top hole on the fan to the right just as it was originally (no ground required). The rubber mounts move the fans about 1/16 inch close to the heat sinks, but there's plenty of spare room behind the fans. Just make sure that the terminals on the terminal strip shown in illustration #4 do not contact the heat sink. In my case, they didn't and I didn't have to bend the terminal strip to avoid that. Page 6

7 Illustration 5: Here's how it looks when the fan is mounted on the new pins. I left the pin shafts fairly long in case I ever have to remove and remount a fan using the same rubber pins. Using a sound level meter, I measured an improvement of 4.5 db in noise level using the rubber mounts (at the lowest fan speed) and an improvement of 5.5 db at the highest fan speed. After quieting the amplifier fans, I discovered that the power supply fan runs all the time and the 3,000 RPM fan used generates a whine just loud enough to be irritating. So, I decided to see what could be done to tame that noise. Page 7

8 The Power Supply Fan The power supply has an 80mm muffin fan, secured inside the rear of the cabinet with four screws. The fan is a NMB model 3110KL-04W-B40, which is capable of 35 CFM at full speed while spinning at 3,000 rpm. This fan comes on when the amplifier is turned on and remains on until the amplifier is turned off. It emits an annoying whining sound, not at all unusual for a high volume 80mm fan using ball bearings. One solution to the problem would be to use a larger diameter fan that runs at a lower rpm. The problem with replacing the fan in the power supply with a larger one is that the rear panel space is completely used up and the line fuse holders and power cable passthrus are directly below the muffin fan opening. I first tried just replacing the metal screws holding the fan to the panel with rubber fasteners designed to minimize the transfer of fan noise to the panel, and while that reduced the fan whine a little, it was still far too loud. I happened to have an unused 120mm silent-running muffin fan in my parts bin and set out to see how I could adapt that to the power supply. As luck would have it, you can buy a converter or adapter which allows you to mount a 120mm fan where an 80mm fan is currently used. The adapter won't fit the power supply, however, due to the proximity of one of the line fuse holders to the fan opening, and the slight overhang of the cabinet top. Those problems were solved by removing the fan mechanism from an old 80mm muffin fan and using just the fan body as a spacer between the panel and the fan size adapter. (see Illustration 6 for details). I had some of the rubber mounts used in place of screws for fan mounting and I used those along with a rubber gasket to mount the spacer to the power supply panel. With the silent-running fan I used, that was probably overkill as there is very little vibration from the fan. For this fan modification, there's no need to drill any holes or make any circuit modifications. You will of course have to unsolder the existing fan leads and solder the new leads to the same points. Make sure you have the 120mm fan mounted to blow into the cabinet and that you observe the polarity of the new fan leads. The fan won't run at all if you have the leads reversed. Since my 120mm fan had a connector on it, I used a mating connector to make the connection to the power supply board to make future fan changes easier. With a silent running 120mm fan, all you'll hear when you turn the amplifier on is a very low level sound of air rushing through the power supply. There is NO whine from the fan. The fan I used is a Page 8

9 Nanoxia DF SELN, which spins at 1300 rpm and is capable of 60.1 CFM. The static pressure of this fan is 1.64 mm H2O compared with 3.27 mm H2O for the stock fan. At this point, I don't know if that static pressure is adequate to produce enough air flow, but during the recent CW SS the supply ran cool. You can find silent 120mm fans at any of the websites dedicated to silent-running PCs, such as or Noctua makes a 120mm fan that runs at 2,000 rpm with a static pressure higher than the original 80mm fan (NF-F12 industrialppc2000) and that one may be worth trying. The 120mm to 80mm fan adapter I used is from Nexus ( and similar adapters are available elsewhere. Illustration 6: This spacer is an old 80mm fan with the motor and supports removed. It's used to maintain adequate clearance for the cabinet top and for removal of the upper line fuse. Page 9

10 Illustration 7: This photo shows the fan size adapter and 120mm fan mounted on the 80mm spacer. Note that there are no permanent modifications made to the power supply circuit or cabinet and the original fan can be reinstalled if necessary. Measuring Air Flow It's pretty difficult to judge air flow from fans just by the sound produced. There are a number of anemometers available on Amazon and ebay that can be used to measure air flow and give you accurate comparisons of fans and fan speeds. Here's the one I use and the rather crude way I mount it for measurement of air flow from the 1306 heat sinks. Page 10

11 Illustration 8: This inexpensive anenometer (around $21 from Amazon) can be used to measure heat sink air flow and judge effects of fan control modifications. Floyd Sense K8AC February 25, 2017 Page 11

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