Preamp/Floating Ground Question

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Mark01

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Question for anybody knowledgeable in electronics .
I am using a GaAsFET preamp on a beam antenna mounted in an attic my power supply to the preamp is a 12vdc regulated walwart with the positive wire soldered to the external + positive pin and the negative wire soldered to the external - negative pin , about 30 ft of rg6 connects to my scanners,the scanners are powered by walwart power supplies.​


The problem is the GaAsFET in the preamp tends to blow out ,what i've been told is the reason this happens is that this is a "floating ground type setup".I popped the preamp case open and sure enough it's only grounded to the metal preamp case,so it does not have a real ground.I've been advised to take the ground wire from the walwart and connect that elsewhere and not on the preamp metal case. I am soliciting any help or ideas where to run the ground from the walwart?​
 

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prcguy

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Check the output voltage of the wall wart feeding the preamp. Many are unregulated and will put out 17v or more with no load or a light load. It should not matter how you power the preamp as long as you don't exceed the voltage rating and grounding should not be an issue with the setup you described. The preamp should not care if the power supply is grounded or floating or run from batteries. If the antenna were outdoors in a high static environment like blowing sand or snow with very low humidity, the preamp can be damaged from static electricity.
prcguy
 

fineshot1

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You state the "preamp is a 12vdc regulated". How do you know this is so? In my experience very few if any wall wart power supplies are " well regulated ". I doubt its a grounding problem. The case of the ARR preamp is metal. The circuit board inside is usually supported by metal standoffs attached to the metal case. The circuit boards usually have 3 or 4 screws holding them down to the metal standoffs inside the case. The ground path is threw the case and threw the standoffs to the circuit board ground foils which the standoffs are held down to on the circuit board. With the info in your post it seems to me I would suspect a power source
problem first or possibly a static discharge around the area of the antenna and preamp.
Good luck & post back if you find the problem....
 

Mark01

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The walwart is a regulated 12vdc 300ma type, the preamp specs state "preamps are designed to be powered by a 10-16 volt dc source with a current consumption of 25 mA " so it seems there is some leeway if it was excess voltage. The ground looks like I described I can not see anywhere it's grounded to the circuit board,in talking with the guy who makes these he flat out stated that there is no ground on this preamp.I run a power strip with a few other electrical devices,including a extension cord that powers the walwart power supply that is connected to the preamp ,maybe it is interference from other electronics?, but that is only a guess​
 

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prcguy

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There is an internal regulator, probably 5v or 8v, so the FET should be reasonably protected from over voltage or transients on the DC line. My thinking is the antenna input is more susceptible to damage from high static or possibly high power transmitters on the same property. Do you have any VHF/UHF transmitting antennas on the roof? Looks like somebody replaced BNC connectors with F type.
prcguy
 

fineshot1

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Mark01 said:
The walwart is a regulated 12vdc 300ma type, the preamp specs state "preamps are designed to be powered by a 10-16 volt dc source with a current consumption of 25 mA " so it seems there is some leeway if it was excess voltage. The ground looks like I described I can not see anywhere it's grounded to the circuit board,in talking with the guy who makes these he flat out stated that there is no ground on this preamp.I run a power strip with a few other electrical devices,including a extension cord that powers the walwart power supply that is connected to the preamp ,maybe it is interference from other electronics?, but that is only a guess​
You can not see the dc ground on the circuit board because its on the bottom side. You would have to remove the circuit board to see it on each one of the standoff holes on the other side. If there was no dc ground the preamp would not get any dc voltage and not function.

Have you measured the dc voltage under load of this " well regulated walwart " ? Hopefully you have a decent DVM to check that. Also - if possible try to measure the current consumption while you are at it and make sure its at the 25mAh spec. You may also want to check the AC source the walwart is plugged into.
 
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Mark01

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Do you have any VHF/UHF transmitting antennas on the roof? Looks like somebody replaced BNC connectors with F type
No VHF/UHF transmitters on the roof, my antennas are all recieve only 3 beams and 1 discone attic mounted ,theres another AR2 preamp hooked up to another beam,no problems with that though.The F type connectors were put on by Advanced Reciever at the factory at the customers request.


Have you measured the dc voltage under load of this " well regulated walwart " ? Hopefully you have a decent DVM to check that
No don't have a DVM ,but I have used a different walwart power supply before with the same results,(the GaAsFET has blown about three different times) .
 

zz0468

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The bottom line here is that SOMETHING is exceeding the preamp's ratings. The most likely culprit is the wallwart, and lacking the ability to measure what the actual voltage is, I would refrain from simply hooking it back up the same way.

The simple lack of a ground is not likely to be blowing the preamp. Putting an unknown voltage across it is.

Does it blow instantly, or does it operate for a while and then just randomly fail?
 

viper21

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Floating ground by definition is a system where the system ground is not actually connected to another circuit or to earth ground. Most if not all preamps are designed this way. Same is used on marine ships to keep from shocking crew personnel, in this case your feedline/scanners

On this preamp your board should not have a earth ground, if you take a ohm meter and check some solder points or "ground" points none should show a 0 to the metal chassis. if there is then your floating ground is broken and I'd say you're seeing a voltage and amperage increases at the GaAsFET( gallium arsenide field-effect transistor).. More than likely any one of those 4 large isolators are shot... You might want to make sure it's installed right, in should be the antenna , out should be to your scanners, if it's not this way already...

The negative terminal is there for a purpose I suggest not to attach it anywhere else..
 

Mark01

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Does it blow instantly, or does it operate for a while and then just randomly fail
It's worked fine for about two years with no problems.Over the 3 years I've owned the amp i've observed it blow after I have manually adjusted the beam antenna direction.I do this by hooking a handheld scanner running on batteries and a short run of rg6 cable and adjust until I recieve a strong signal on the system I want to recieve. In fact when I hooked this amp up when it was new a couple years ago it blew out at that time when I installed it to the antenna and made my adjustsments.Possibly the antenna could be gettting overloaded and causing something to blow while adjusting the antenna.

I am definately going to rework my power supply setup when I hook it up next and cross my fingers.Thanks for all the feedback​
 

prcguy

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If its blowing the FET and not the internal regulator I would not suspect a power supply problem. It's got to be a static problem or it could be oscillating and burning itself up. Some preamps are not very stable when the input or output match is not perfect, maybe the short cable you connect for peaking is a particular length to cause it to oscillate? I have had many preamps oscillate and some were not cheap but never with my Angle Linear preamps.
prcguy
 

Mark01

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If its blowing the FET and not the internal regulator I would not suspect a power supply problem. It's got to be a static problem or it could be oscillating and burning itself up
Yes the FET blows everytime,I do hear the osccillating sound when I adjust the antenna,if I listen with headphones and sit on a frequencie I can hear the signal desense than switch back to clear and it will go back and forth like that.If there is a mismatch ,would that be on the antenna output to preamp input? The antenna has a 75-300 Ohm Balun antenna twin lead connector.
 

prcguy

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If it's actually oscillating and thats what's damaging the preamp, you need to find a better preamp. It would take a spectrum analyzer to prove this and changing the cable lengths between the antenna or scanner may help, but the preamp should not oscillate under normal conditions If this has been the problem and you have been paying for FET replacement then ARR owes you some money back for repairs and a defective preamp. Just my opinion.
prcguy
Mark01 said:
Yes the FET blows everytime,I do hear the osccillating sound when I adjust the antenna,if I listen with headphones and sit on a frequencie I can hear the signal desense than switch back to clear and it will go back and forth like that.If there is a mismatch ,would that be on the antenna output to preamp input? The antenna has a 75-300 Ohm Balun antenna twin lead connector.
 
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