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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 7:50 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18257
Location: Warner Robins, GA
When I recapped mine I was the second one to do so and I just soldered ceramic disc caps directly to the terminals the wiring from the original caps were connected to.

Eliminated several wires.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 10:14 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
Glad to hear your new IF can arrived. You have noticed that the shape of the 1st and 3rd IF can mounting plates are the same. Remove the 4 screws on the bottom of your 1st can and use the mounting plate as a template to fab one for your new #3 can. While you have the cover off you will see how easy it would be to move the wires around to different pins inside the can if necessary to make your new can work in the 3rd socket.

I have restored two of these receivers in the last couple of years. My 3-6mc one has an external power supply and I have restuffed all the original canned capacitors with new caps keeping the original look. This receiver has 6 volt tubes running off a 6vac transformer winding. I did put a .22uf cap between each lead and ground. There is NO ac hum detected in this receiver.

My second one is a .5-1.5mc BCB receiver. It has an onboard power supply which becomes very crowded. I removed the CW Oscillator coil and related circuit. You don't need it for AM radio stations and also removed the original output transformer to make room at the rear of the chassis for filter capacitors and diodes etc. The power transformer had a 24 volt winding so I left the filament circuit as original. One side to chassis ground and the and the other side to the tubes per the original schematic. I did not use .22uf caps to ground in this receiver and there is NO ac hum detected here either.

Yes, these receivers seem to run hot, but you have to remember, these were in bombers that were flying over 10,000 ft. where it was freezing inside the aircraft. I just lift the lid off the top when I operate mine at sea level and let the heat escape. Don't worry about it.

Here's an easy mod that can be reversed in a couple of seconds that I do to all ARC-5 receivers. Lift the opposite end of the resistor that is connected to the cathode (pin5) on the RF amp and connect it to ground. This removes the RF amplifier from the gain control line and will improve the signal to noise ratio. I have tried running this tube a little "hotter" by changing the value of this resistor, but have noticed very little, if any, difference.

If you feel that when you're finished tuning it up the volume level is not as high as you would like it is very easy to rewire V7 socket to accept a dual triode like a 12SN7 or 12SL7 depending on the gain you desire. These are also available in the 6 volt variety. The detector portion of the original 12SR7 can be replaced with a diode. Check with the crystal set guys as to which is the "hot ticket" this week. A 1N34A is always the good reliable choice. Just go form the output of the 3rd IF transformer to ground, keeping in mind the orientation of the diode. Half of the new dual triode will be wired as per the original CW oscillator and the other half can now be configured as an audio preamp for the 12A6. This is a pretty easy mod that is easily reversible also. Something to think about. Best wishes with your receiver. ARC-5's are fun.

PS: I would just use the IF gain to control the volume level as it was designed and allow the AF amp to run wide open. Now if you choose to do the preamp mod, you might consider placing a volume control pot in front of the preamp and have two controls like the more advanced designed receivers.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 1:27 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
I already applied the RF stage mod you mentioned. The front end seems to already have lots of gain and like you I didn't really notice a difference after lower the cathode resistor. So I'll leave the resistor grounded but put it back to original value.

I actually looked forward to not restuffing those cap cans. Its tougher to troubleshoot those radios with them in the way of almost every tube socket.

I have more than enough volume and won't be modding for an extra stage of audio. Using the existing high impedance output transformer driving an external speaker that has its own 1K-8 ohm transformer on it is plenty loud. I thought about removing the not really needed power supply chokes and caps to make room for a single new OP transformer. Maybe in the future but not right now. I want to play around with the radios for a while.

Thanks for reminding me about the IF can and switching wires if I need to. I remember now that I supposedly needed to switch pins 5 and 6 since I used the new 3rd IFT in the 2nd IFT socket. I didn't switch them yet the radio seems to be working well. I'll open up the can and see whats needed or not.

Right now I'm planning to keep the IF gain as the volume control. The previous owner didn't space controls on the front panel plate to leave room for anything other than the BFO on/off switch and the IF gain pot.

I'm wondering if I should mess with the BFO coupling level. Right now it is more than enough to listen to even the strongest SSB signal. The reason I might want to adjust it is because of the hiss it adds to the audio when switched on. It seems like its overcoupled. But since we're talking such a small value of coupling, less than 3pf according to schematic, it might be difficult to change. IIRC it was done using the usual method of a wire twisted around another. So if I can decouple it a tiny bit to reduce the hiss I'm willing to adjust the IF gain control on the strongest signals for clear reception.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 5:54 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
An update to this morning's post.

I finished the recap.

I decided to leave the RF tube cathode resistor value for the "hotter" receive.

I'm not going to mess with the BFO injection level. The reason the hiss was more annoying than before is because I forgot to solder in a new cap across the output transformer primary to cut down on the treble a bit. It sounds acceptable now.

I played musical chairs with the 3 IFTs. Any combo will work in a pinch but at lesser performance.

My other radio, BC-454 has only the RF and Audio cathode mod and the 5mfd P/S cap replaced. The rest is original and I'm going to hold off on the recap for that one. Call me lazy. :)


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sun 28, 2018 5:02 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
It seems that you have a really good understanding of how this receiver works. It's hard to tell the knowledge level of the person you're talking with when you first meet. I know how you feel about restuffing the original cans. I used ceramic disks on my BCB receiver and they work just fine giving you a lot more room to work in the chassis.

Before you button up the bottom plate give this Idea a try. I liked it so well I left it in place on my 80m receiver. Find a really small air variable cap. with a shaft that will accept a knob that is under 10pf. I made mine by breaking plates off one that had plates about the size of a dime until I had one stator and one rotor remaining. I have also seen this mod made by using a second cap like the ANTENNA TRIM. Anyway, ground one side and connect the other to the plate (pin 6 ) of the det-cw osc. tube.

You can just run the wire out onto the table next to the receiver to see if you like how it works. What you have created is a pitch control for your BFO. It basically parallels the little CW Osc adjustment screw in the right side of the chassis. I have found this to be extremely helpful in making SSB signals more clear. You wouldn't need this if these receivers had a band spread cap. Give it a try. You'll probably want to find a place to mount it permanently. If you do, set it in the center of the range before you adjust the little screw for the CW Osc. If you are willing to relocate capacitor C5 there's room there.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sun 28, 2018 2:51 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
Has anyone successfully added a stable "fine tune" cap in the 12K8 LO section? Or has the BFO mod mentioned above been the better solution?


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 30, 2018 8:08 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
These ARC-5s were modified to death back in the day. I doubt if anyone is seriously looking into making them better any longer. I had one of these when I was 12 back in the 50s and when I had an opportunity to get my 80m receiver it was a chance to relive my childhood and make one of these actually work. Back then it was all magic and I'm sure mine ended up in the trash like so many others.

I had thought about the Band Spread idea but was hesitant to disturb the front end alignment. I think the BFO mod is more sensitive anyhow and basically does the same thing. Like I said, it works very well.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 30, 2018 12:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18257
Location: Warner Robins, GA
I got my BC-455A maybe nearly 20 years ago heavily modded (mostly cosmetic mods) for $10 at a flea market. The seller didn't know what it was.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 30, 2018 3:32 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
I bought mine for $18 each about 3 weeks ago. One worked and the other was missing the antenna trim cap and 2nd IFT. I can see why they were so popular and fun to use for decades. After I get these finished and running with a power supply I'll move on to another radio that won't be as much fun. A BC-923 tank receiver that has sat on a wet concrete floor along side a leaking car battery for years. With much effort I pryed the chassis out of the heavily rusted case yesterday and was surprised to see how well it was preserved by the yellow coating the military used. I'll probably start a new thread when I dive into that one. Other than a small section in a larger manual of radios, there isn't much I could find on it. I suppose that has to do with it's 29-36mhz FM range.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 30, 2018 4:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
Yes those ARC-5 receivers are indeed fun.

Mine with a few feet wire antenna (maybe 20') will pull in the stations.

The push pull amp I use gives the receiver plenty of volume.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Wed 31, 2018 3:57 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
Are you looking for an Antenna Trip cap ?


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Wed 31, 2018 4:29 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
I planned on installing a non original cap from my stash but if you have the real deal send a PM.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 20, 2018 3:54 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
Has anyone here added this noise limiter mod to their radio? Did it work OK? It looks simple enough to try.


Attachments:
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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 20, 2018 5:04 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18257
Location: Warner Robins, GA
Didn't know such a mod existed, although I would not need it given I use AGC which seems to keep the noise level down to what one would expect from a typical high quality AM radio.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 20, 2018 6:46 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
I think I've seen that circuit along with a couple more that require switches and pots, but neither of us want to drill holes in our receivers. You know the saying "Ya get what ya pay for" ? this circuit looks way too simple. Let us all know how you like it and I might give it a try. My 80m rig isn't all that noisy. Best wishes my friend.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 20, 2018 5:55 pm 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
The circuit is very similar to whats found in a lot of tube type CB radios. The unshielded long wire from my garage to my basement workroom picks up some impulse noise from about 3-20mhz. Not real bad but enough that if i can easily cut it down some I'll be happy.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Tue 20, 2018 7:02 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 900
Location: Champaign IL 61822
forumuser wrote:

I'm wondering if I should mess with the BFO coupling level. Right now it is more than enough to listen to even the strongest SSB signal. The reason I might want to adjust it is because of the hiss it adds to the audio when switched on. It seems like its overcoupled. But since we're talking such a small value of coupling, less than 3pf according to schematic, it might be difficult to change. IIRC it was done using the usual method of a wire twisted around another. So if I can decouple it a tiny bit to reduce the hiss I'm willing to adjust the IF gain control on the strongest signals for clear reception.


Me too on my BC-453. The hiss overpowers the signal unless I turn the RF gain control up
so much that my ears hurt, on CW only, as AM level is just fine. Its odd that they would feed
the BFO into the IF amp rather than direct to the detector. Perhaps this is done to reduce pulling.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Thu 22, 2018 3:51 am 
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Joined: Apr Thu 12, 2007 3:24 am
Posts: 1194
Location: Milwaukee,WI
I looked into the BFO coupling this afternoon. I was expecting to find a twisted wire gimmick from the BFO tube to the IF tube but there was none. The wires coming off the tube sockets run perpendicular to each other and get within about 1/2 inch from each other at the closest point. Evidently that must be enough coupling to simulate the 3pf shown in the schematic. Both wires were cut to length and very stiff like the rest of the radio so there was no easy way to adjust anything. I'm not sure if its worth the effort to cut and reroute wires in hopes of lessening the coupling by a small fraction. The lack of gimmick coupling also makes me wonder if the coupling is going on inside the OSC/DET tube itself. It gets confusing when there are so many different schematics and slight differences in the radios. I mention this because the latest schematic I just looked at does not show the 3pf gimmick coupling. So it must be all coupled in the tube itself. Either way you look at it, there isn't really much of a way to reduce it.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Feb Thu 22, 2018 8:46 am 
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Joined: Aug Sat 13, 2016 6:03 am
Posts: 89
You've made me querious. The schematic for my BC-454 states regarding the coupling "Note: No capacitor used-capacitance of the wiring only" It's been 18 months since I've restored mine and I don't remember what it looks like now. I like your idea about the coupling taking place inside the tube. Couldn't you confirm this by detaching the coupling from the plate of the dectector/oscillator? Then you'd know for sure. Looking forward to hearing your results.


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