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 Post subject: R-390 Audio mod question
PostPosted: May Tue 22, 2018 1:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20280
Location: Warner Robins, GA
I have modded my R-390 to where the local audio control feeds the local audio out terminals directly for driving an external push pull 45 amp that drives a 12" field coil speaker which works quite nicely, but the bass response is slightly lacking.

Is there an audio transformer that has better specs for a reasonable price that will work?

I can go larger in size as I use an external regulated 180Vdc power supply which leaves the area where the 6082 tubes went free aside from the two tube sockets.

The idea is to get the bass response to where it starts rolling off around 40-50Hz.

The audio transformer drives the audio filters and is responsible for the lacking bass response.

The transformer will have to handle some current in the primary winding though.

Had thought about an Edcor WSM series transformer which would fit, but those are not designed for current in the primary winding and I am not spending the money required for the design fee just to get a proper transformer made.

Primary is 10K.
Secondary is 600 ohms.


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 Post subject: Re: R-390 Audio mod question
PostPosted: May Sun 27, 2018 7:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Mt. Airy, Maryland
Tap the audio from the detector load, terminal strip at the back of the radio. No transformer in that circuit. You need a series capacitor as the AVC voltage is present there.

Downside is that neither volume control on the R-390 will adjust the audio at that point.

Still not sure why you're running this with an external supply instead of just using the 6082s. :?


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 Post subject: Re: R-390 Audio mod question
PostPosted: May Mon 28, 2018 5:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
To reduce the heat inside the R-390.

The circuit is so that I can reconnect a few wires and install the tubes and be back on the rectifiers.

With mine the 18K high watt resistor cracked so I got a 25K variable to replace it with which enabled me to set the regulated B+ voltage which compensates for weaker 6082 tubes.

At the time I wanted to replace them I read where 6082 tubes weren't easy to find and a little expensive, but I was later proven wrong and given how I am (very stubborn at times) I continued with the desire for a solid state regulator or external power supply.

I thought of the diode load, but I wouldn't have the audio filters either and I do find the MED and WIDE useful depending on what I'm listening to.

Also I use the line audio amp to drive a small pair of headphones through a resistor and would lose that feature, although I could use the external amp to power the headphones.

I could use the diode load to feed the push pull 45 amp I have on my R-390.

BUT

That is too simple and is what a lot of people do and in some instances I prefer to do some things differently or rather I prefer to get every bit of performance I can (within reason) out of original circuitry. It in some cases provides an alternate way of doing something that I can share with people.

In other cases it doesn't work and I know what not to do and can share that info as well so that others don't have to waste their time trying it.

I'm one where any mods done must improve performance or fix some deficiency.

The receiver as it is now, playing through the external amp and 12" electrodynamic speaker in a proper open back speaker box sounds almost like a good quality console radio. It's just that the bass is slightly more rolled off than I prefer.

What I prefer for console radios and communications receivers whenever practical is to have the bass start rolling off around 40-50Hz which I achieve by careful selection of coupling caps and cathode bypass caps after first ensuring any audio transformers in the signal chain can handle bass down to 40Hz. I tweak each coupling cap and cathode bypass cap value so that the roll off isn't all in one stage.


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 Post subject: Re: R-390 Audio mod question
PostPosted: Aug Sun 19, 2018 3:27 am 
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Joined: Mar Sat 23, 2013 1:08 pm
Posts: 177
Location: Lunar Station 4
Confirmed... Mine (Collins) always had very good Hi-Fi audio at those points. I used to run it into an amp with 4 real 6L6's, back when there was something of good quality to listen to. It sits in the rack and hasn't been used in some 4 years now. :(

w3jn wrote:
Tap the audio from the detector load, terminal strip at the back of the radio. No transformer in that circuit. You need a series capacitor as the AVC voltage is present there.

Downside is that neither volume control on the R-390 will adjust the audio at that point.


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