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Author Topic: SG500 Noise  (Read 2330 times)
KA4KOE
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« on: January 24, 2013, 09:43:35 PM »

I am running AM. Due to the duty cycle, I typically don't run more than 100-125W. However, after the amp starts warming up, it will "whine" inside, almost like microphonics.

I'm using this inside my house. Sound like cooling issue? In any even, a muffin fan is in my future, I think. I've not run the amp long enough to shut it off via the thermal protection.

Thanks

Philip
KA4KOE
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KC4MOP
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« Reply #1 on: January 25, 2013, 03:29:08 AM »

There is a fan kit offered for the amp. If you are running AM then the proper cooling is needed. 10 minutes seems like max time for full duty cycle mode.
Be a little gentler with it in AM and run only 100W un-modulated carrier.
SS amps sound like crap when they are pushed to the limit.
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KE3WD
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« Reply #2 on: January 25, 2013, 07:14:03 AM »

When setting up the modern rig with any type of linear amp for AM work, don't get into the notion that the Carrier max. is what is to be measured by the output rf power meter. 

Turning the Carrier DOWN to the point where your mic modulation causes the power meter to rise on the voice is the goal here.  Use of a Peak Reading wattmeter can make finding that carrier setting a bit easier, but not mandatory. 

Turning the carrier power down will not only put a lot less pain and strain on the amplifier, it will sound much better at the other end.  Not "tight" in the audio. 

Max. power out is not always the best thing for communication.  In AM mode, the modulation sidebands are where the information is. 

When you get the hang of where to set the carrier, which is done at the rig, you will know by the fact that old timers will give you signal reports about how good you sound.  Don't be afraid to keep on lowering the carrier power until you find that point. 


73
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G3RZP
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« Reply #3 on: January 25, 2013, 07:47:45 AM »

A good start is to tune up with a carrier to the full rated PEP. Then back off the drive until the output power drops to one quarter, and even better, to one fifth of the full PEP. So if you can tune for 1 kW out on carrier, back off drive until you get 200 watts of carrier - and DON'T touch the tuning!
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KC4MOP
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« Reply #4 on: January 25, 2013, 08:25:24 AM »

His amp is a no tune SS rig 500W PEP. SG-500

http://www.sgcworld.com/ampProductPage.html

So he should set the carrier from the transceiver for 100W carrier output to the SG-500 and adjust the mic on the rig ( transceiver...he did not mention what he's using) so there is NO ALC action on the metering.
I was suggesting to be gentle on the amp and not tromp on it for every watt it can put out.
100W carrier with no modulation will result in 400W PEP.
Fred
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G3RZP
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« Reply #5 on: January 25, 2013, 09:31:06 AM »

Not set the mic gain using a monitor scope? (100 watt carrier - adjust mic gain for about 90%  modulation on a whistle)

Not much point in exceeding about 90% as diode detectors don't like it.
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KC4MOP
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« Reply #6 on: January 25, 2013, 04:06:16 PM »

Getting a 'scope involved will complicate things until he gets a good grip on the transceiver / amp.
It takes a little while to adjust the 'scope and get the proper pattern and know what to look for.
I could not find any videos on YouTube.
The ALC reading on his transceiver will be a starting point.
Advice to join AMFONE forum. It's all about the AM mode on the Ham bands.

Fred
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KA4KOE
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« Reply #7 on: January 30, 2013, 07:29:07 AM »

I went from 150W to 100W carrier, and the folks on 3885 said it measurably increased the audio quality. Just received a high CFM muffin fan which will be located below the SG500. I set up the audio per W1AEX'x procedure on his webpage for the Flex 5000a.

I do have a good oscilloscope. Where can you direct me for how to sample my transmitted signal to see if I'm clipping. I have a dual channel 100 MHz Tek 465.

Tnx.

Philip
KA4KOE
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N4ATS
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« Reply #8 on: January 30, 2013, 09:38:23 AM »

Just as an FYI , you DO have both positive and negative wires (very heavy guage) from the battery straight to the amplifier correct? NEVER ground the amp to the frame and run a single positive wire or you will be asking for trouble...Been there done that MANY times over and over with people and usually corrects the problem
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KD0REQ
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« Reply #9 on: January 30, 2013, 09:49:29 AM »

here's how they do it at the dollar-a-holler commercial AMers

http://www.thebdr.net/support/xmtr/maint-ammod.pdf

and a little video from w2aew which shows attenuations needed, etc

http://www.youtube.com/watch?v=y4Zt_LJX1Tc
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KA4KOE
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« Reply #10 on: January 31, 2013, 12:23:17 PM »

Yessir, heavy gauge stuff, straight to the battery.
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KC4MOP
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« Reply #11 on: February 02, 2013, 11:31:53 AM »

I went from 150W to 100W carrier, and the folks on 3885 said it measurably increased the audio quality. Just received a high CFM muffin fan which will be located below the SG500. I set up the audio per W1AEX'x procedure on his webpage for the Flex 5000a.

I do have a good oscilloscope. Where can you direct me for how to sample my transmitted signal to see if I'm clipping. I have a dual channel 100 MHz Tek 465.

Tnx.

Philip
KA4KOE

Congrats to you Philip.
You are the man!!! The guys on 75 are very helpful. They want you to have success.
The guidance on a 'scope starts here:

http://www.icycolors.com/nu9n/scope_your_audio.html

this guy sells RF couplers to get a little bit of RF to feed the 'Scope. There are adjustment procedures to get the 'scope patterns shown in his pictures.
There are ways to build a sampler. You can google if you like to build /experiment. You do not want to overload the input to the 'Scope. The equipment you mention will be more than enough for AM modulation.

I'll keep checking back.
Fred
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