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Speaking of audio mods




 
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Author Topic: Speaking of audio mods  (Read 5668 times)
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N9NEO
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« on: March 15, 2005, 08:41:34 PM »

I have a Racal RA17L receiver that I have audio problem with.  It is a little distorted.  Output tube is one 6AQ5.  

I'm thinking to pick off the detector and run into outboard audio anyway, as I don't think I can do a whole lot with one 6AQ5.  Is this a reasonable plan.

Also, I have a SP600 in the Que that I would consider similar audio.  I would like to hear any comments.


73
NEO
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Steve - WB3HUZ
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« Reply #1 on: March 15, 2005, 10:34:39 PM »

I've done this with every receiver I've owned. It works FB.
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K1JJ
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« Reply #2 on: March 16, 2005, 01:21:02 AM »

Bob,

Here's a short article on tapping off the RX detector written by a wonderful but ugly guy from the Bizzaro Whirl.

http://www.amfone.net/ECSound/K1JJ4.htm

Be sure to run some tones through the whole system and make sure that the sine wave is nice and clean. [See the article about sweeping your RX in the same "East Coast Sound" area]

Sometimes you might need a buffer between the detector and amplifier input to prevent loading it down, even slightly, causing distortion. I did on mine and used a FET buffer for my SP-600.  Some RX's already have one  [like the R-390?] and are a direct connection into a  1K audio amplifier without problems.

73,
Tom, K1JJ
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w3jn
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« Reply #3 on: March 16, 2005, 07:08:15 AM »

Obtaining distortion-free audio from the SX-100, SX101, and similar Hallicrafters radios directly from the detector is impossible due to the design of the ANL circuit.   A .05 uF capacitor in the ANL network causes flat-topping of audio if the signal is modulated >70% or so.  You can either remove this capacitor or disconnect the 180K resistor between the detector and the offending capacitor.  

Even with this mod, flat-topping of the audio occurs at about 100% modulation.  Doubling or tripling the value of the resistor from the detector diode to the R/C filtering network improves this quite a bit.

ER had a couple of articles last Sept/Oct(?) that outlined this problem and how to solve it.  Most likely other boatanchors with ANL circuits have similar difficulties.  However, if you're getting a nice clean sine wave at the detector load with a 100% mudulated signal, it's not something to worry about.

Last, you hafta make sure you capacitively couple to the detector or you run the risk of loading down the detector DC if your amp doesn't have a capacitor-coupled input.  Some amp's inputs go directly to a volume control... and even 1 meg will load down the detector considerably causing distortion.

Lemme take a peek at the RA-17 skizmatic to see if it might have similar ANL circuit problems.

73 John
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w3jn
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« Reply #4 on: March 16, 2005, 07:51:50 AM »

Bob, just downloaded the skizmatic from BAMA and it looks like the RA17 has a very similar ANL design to the afore-mentioned Hallicrafters radios.  The goal here is to minimize capacitance to ground from the detector.  This is what causes the clipping and distortion.

Feed the bad boy a 100% modulated signal, 400 cycles or so, and scope the center of the volume control.  If the waveform is clipped or distorted, here's what ya need ta do:

1.  Disconnect one end of C217, a .1 uF cap at the junction of  1 meg and 1.2 meg resistors at the 6AL5 det/ANL tube.  See if this improves matters.  Removal of the cap could kill the ANL action.  If this is a showstopper then leave the cap in.

2.  Disconnect one end of C209, a 330 pF RF filter off the last IF xformer and see if this further improves the distortion.  If there's marked RF getting thru to the audio (which I doubt, since it looks like there's plenty of filtering there) try a 100pF cap as a compromise.

This ought to give you FB audio from the detector then, as Steve and Tom noted, the easy way to FB sound is to feed the audio into an external amp.  There are other things you can do to improve the existing audio amp but I suspect the limiting factor is the output tranny.

73 John
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WA1HZK
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« Reply #5 on: March 16, 2005, 09:33:24 AM »

I took a 600 ohm to 600 ohm small iron and put the primary in the cathode of the audio driver of my NC-303 and fed the secondary to my amp. works pissa, does not load anything down and lets the original audio amp run into a resistor so the headphone jack still works.
Keith
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N9NEO
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« Reply #6 on: March 16, 2005, 09:48:07 PM »

Ok good info all the way around.  I did look up on AMwindow, but in mods section.  I hope to get to it this weekend between taxes.

73
Bob
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Steve - WB3HUZ
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« Reply #7 on: March 16, 2005, 11:13:27 PM »

Quote from: WA1HZK
I took a 600 ohm to 600 ohm small iron and put the primary in the cathode of the audio driver of my NC-303 and fed the secondary to my amp. works pissa, does not load anything down and lets the original audio amp run into a resistor so the headphone jack still works.
Keith


Neat idea. I don't have the 303 schematic in front of me, but I suspect a blocking cap somewhere between the detector and the first audio stage would need to be enlarged for best low end response.

I may have to try this on mine.
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