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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: 1579
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
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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: 1579
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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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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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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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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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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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: 120
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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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Mon 06, 2018 9:03 pm 
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Restarting my old thread here instead of starting a new one. Has anyone here performed the mixer mod of replacing the 12K8 with a 12AT7 and a couple of resistor value changes for lower noise? Was it worth it? Did you happen to make a recording of the before and after difference?


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Mon 06, 2018 9:25 pm 
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I haven't done it as I didn't know such a mod existed.

It is a very interesting mod.

http://www.w7ekb.com/glowbugs/Military/ARC5MixerMod.pdf

Personally with the way I run mine with AGC I don't notice any extra noise, although I probably should short he antenna connector to ground and see if I do hear any internally generated noise.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Mon 06, 2018 10:14 pm 
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Location: Milwaukee,WI
If I'm bored some weekend I might try it. With all the man-made and natural noise out there these days I'm wondering if I'll notice a difference.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Tue 07, 2018 6:26 am 
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I did read about the 12AT7 modification, but I kinda remember someone saying it didn't make that much difference. If you want to give it a try maybe there would be someway to bread board the circuit before deciding to install it permanently.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Tue 07, 2018 6:32 am 
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When I first restored my ARC-5 a few years ago I was concerned about the noise also. I used a large solder lug mounted under an IF transformer dust cap and connected an 8pf capacitor between it and the grid cap on the 6K8 mixer. You just push the lead down between the cap and the connector. You'll be surprised what a difference this will make. I have never removed mine.

I found the loop antenna I have since made is far less noisy than my 70ft longwire.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Wed 08, 2018 12:14 am 
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michael w wrote:
When I first restored my ARC-5 a few years ago I was concerned about the noise also. I used a large solder lug mounted under an IF transformer dust cap and connected an 8pf capacitor between it and the grid cap on the 6K8 mixer. You just push the lead down between the cap and the connector. You'll be surprised what a difference this will make. I have never removed mine.

I found the loop antenna I have since made is far less noisy than my 70ft longwire.


Will that work for one that receives the 6-9.1MHz band along with an AGC mod and is any realignment necessary?

An explanation of the AGC mod.

The AGC mod consists of a 2.2 meg resistor connected between the ungrounded end of the 510K resistor in the detector circuit and the nearly already complete AGC line. The original volume control connections are just shorted to each other and a 1 meg audio taper pot for the new volume control is coupled to the same point on the detector by a .01uF capacitor. The wiper of the volume control can be cap coupled to the output tube if so desired which negates the control grid resistor on the output tube or it can be capacitor coupled by a .001uF capacitor. Now it is possible to use a 500K pot in place of the 510K resistor provided the pot is capacitor coupled to the output tube, but I don't like DC on a volume control as that can cause noise as the control is rotated.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Wed 08, 2018 6:21 am 
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Joined: Aug Sat 13, 2016 6:03 am
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There's no reason this mod won't work on your 40 meter ARC-5. This does not affect the alignment at all and the 8pf value is not chiseled in stone. Try anything in that range. As for your question regarding the AVC circuit. I don't know, because I gutted the AVC completely out of my set. To me they are not worth the space they take up, but that's my personal opinion.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Wed 08, 2018 3:07 pm 
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Location: Beaver Falls, PA. USA
This brings back memories; I reworked a 3-6 Mhz Command Set as a teenager, following an article in the June, 1963 issue of Popular Electronics. Mine was the same model as described in the article, and I had no problems getting it to work. However, a shorted original electrolytic capacitor blew out my 6X4 in the new power supply, so I had to fix that. I still have the little rig!

_________________
Tim KA3JRT


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Aug Wed 08, 2018 4:20 pm 
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michael w wrote:
There's no reason this mod won't work on your 40 meter ARC-5. This does not affect the alignment at all and the 8pf value is not chiseled in stone. Try anything in that range. As for your question regarding the AVC circuit. I don't know, because I gutted the AVC completely out of my set. To me they are not worth the space they take up, but that's my personal opinion.


I used mine several years without AGC and i find that I am not adjustimg the volume control nearly as much with AGC as i was without AGC.

Once tuned to a station I usually don't have to adjust the volume.


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