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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Sun 25, 2018 3:36 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1390
Location: Milwaukee,WI
I have a feeling that the noise limiter circuit from the previous page was for use in the BC-453 and 454. Not for my R-27 which also has avc circuit. So I will not be attempting the mod in that radio. I might give it a try in my BC-454 if it doesn't require too much butchering.

I also decided not to replace the original audio output transformer with it's high impedance output for headphones. Instead, my external speaker will have a second transformer in it to convert down to 8 ohms. I brought out some jumper leads and started the search through my stash for good candidates. I was surprised at how many audio transformers of all shapes, sizes and values performed well. After reading many posts about using low voltage power transformers for audio I gave some of those a try too. Seeing as how there is no DC flowing anywhere may have contributed to how many transformers worked so well. Even tiny output transformers from transistor radios worked with an almost undetectable ( to the ear ) loss of low frequency.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 27, 2018 9:07 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1017
Location: Champaign IL 61822
About the cap (C33) connecting the BFO.

I have a BC453. It goes BFO plate to 2nd IF grid. Over the weekend I tried disconnecting it,
or both disconnecting it and attaching a 10 pF cap from BFO plate to detector
diode.

Disconnecting it resulted in about a 6 dB reduction in hiss, but a worse effect
on BFO operation. The other test did essentially nothing different.

So the circuit as shown is optimal. The best cure for the hiss would
be an attenuator in the headphone line. With my 2000 ohm phones the thing is just too loud anyway.
It has 600 ohm output.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 27, 2018 9:24 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1390
Location: Milwaukee,WI
Since I changed a cap value to remove excess highs in the audio section, the BFO hiss is less annoying now.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Wed 28, 2018 12:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18689
Location: Warner Robins, GA
dtvmcdonald wrote:
So the circuit as shown is optimal. The best cure for the hiss would
be an attenuator in the headphone line. With my 2000 ohm phones the thing is just too loud anyway.
It has 600 ohm output.


Why not try connectig a 866 ohm resistor in parallel with the phones which will give yoiu an impedance of 604 ohms

https://www.mouser.com/Passive-Componen ... 66&FS=True

That will provide a better impedance match.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Mar Sun 11, 2018 4:50 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1390
Location: Milwaukee,WI
I need to put the finishing touches on my external speaker/power supply for the pair of radios. Its a metal case with a 5x7 speaker which gives good volume and quality. The power supply has a small 35 watt isolation transformer being used for the B+ voltage of about 160vdc. The second transformer is 12vac with a center tap for the 6 volts needed for the 1st radio and 12 volts for the 2nd radio. I noticed no hum with AC for the filaments so I didn't bother with converting to DC. The 3rd transformer is the audio output matching to the speaker.

I ran a 5 pin harness out the back of each radio and one from the power supply. Only 4 pins are needed for each radio. But to remove the chance of accidently plugging in the 6 volt radio and having the supply set to 12volts I used pin 3 for 6 volts and pin 4 for 12 volts. Each radio is wired to use only the pin it needs and the other is not connected. So now I can't blow out the filaments.

It has a solid state bridge for instant B+ but I haven't decided yet if I will install a standby switch or not. I'll post a pic or 2 when I finish the supply/speaker.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Mar Mon 12, 2018 4:49 pm 
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Location: Warner Robins, GA
The standby switch would be a nice addition.

A possibly better option if the power transformer will support the heater load of both radios it is to have them both plugged into the supply then have a switch which will select which radio gets B+ and feeds audio to the speaker and still have a standby switch. Could just use a three position two pole rotary switch with one for standby and the other two positions selecting which radio gets B+ and feeds audio to the speaker.

I had actually thought of doing something like that if I ever got any more of these radios besides my BC-455A.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Mar Mon 12, 2018 7:05 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1390
Location: Milwaukee,WI
The filament trans is only 2 amps so not enough to power both radios. I thought about using separate filament transformers to be able to run both radios at the same time and have them mounted on a single board with the Speaker/PS cabinet. Then I decided against having another large footprint radio.

I have been running tube type CB radios for a long time. Most don't have a standby switch and I never noticed any increase in tube failure rate. But those radios use 7 and 9 pin miniatures. I don't know if they are less succeptable to instant on B+ than the older metal tubes in the ARC 5s


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Mar Mon 12, 2018 9:13 pm 
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Location: Warner Robins, GA
In this instance the standby switch could be used to silence the receiver for brief periods when necessary.

Instant B+ shouldn't hurt the tubes as mine uses diodes for the rectifier with no problem at all.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Mar Sat 17, 2018 8:45 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 100
I like the way you wired your power supply output. I did the same thing for the 4 receivers I run only I used an 8 pin plug. Two of my pins require a jumper to activate the primary of a special filament transformer. Eliminating that switch is nice, just one less thing to remember. Looking forward to the pictures.

One other thing I did on my P.S. was to install a single wall type outlet in the B+ line. You need to insert a plug that has been shorted out to complete the circuit. Whenever I go in to a receiver for repairs or mods the filaments remain on while I am assured the B+ has been disconnected by removing the plug. Much safer than a B+ on/off switch.


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