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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 23, 2018 8:32 pm 
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That would match with the chart I found in some pdf that showed no tap for the IFTs.

I'll download from your link and see if its a new file I haven't found previously.

EDIT: Its a new one. Thanks. But...The schematics starting at page 141 show taps on the 2nd IFT, the one in question. :shock: This is what has plagued my search for info from other manuals and pdfs.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Tue 23, 2018 10:18 pm 
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Yes, but there are notes above those transformers. Note 2 for the 2nd IF transformer and Note 3 for the 3rd IF transformer that are listed to the left of the schematic. Those notes say that the tap is at the top of the coil for the 6-9.1 Mc receiver which is what you should have. Since the circuits are almost identical for receivers R-25, R26, and R-27 they just included a generic schematic that covers all 3 models with notes of the differences. I doubt you will find a schematic that only pertains to the R-27 unless someone has taken one from a manual and modified it with the changes noted on the schematic.

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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Wed 24, 2018 1:54 am 
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Then I guess my only concern left is if the transformer I ordered with a 72xx part number is actually the same thing as the 96xx part number the manual calls for. They both are the same 2830khz IFT. I hope the number diff is only because one was for the ARC5 model and one for the SCR model.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Wed 24, 2018 4:04 am 
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The 7277, 7278, 7279 transformers are for the BC-455 receiver part of SCR-274N. Those transformers have only 1 trimmer capacitor on the primary winding and also have a coupling capacitor between the primary and secondary windings. 7277 and 7278 (1st and 2nd IF) are wired the same according to the manual. The capacitors within are also the same. Not certain about the coils but I would think they would be close enough. The connections at the base socket between 7277 and 9697 jive at least with regards to the coils. I think putting the 1st IF you purchased in place of the 2nd IF would be your best bet.

Another document I found and maybe you have also is this one written by Gordon White and gives details of the coils in the ARA, SCR-274N, and ARC-5 receivers.

http://www.radiomanual.info/schemi/Surplus_Handbooks/Command_set_receivers_for_all_frequencies_1967.pdf

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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Thu 25, 2018 6:21 pm 
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I know this mod will work with the SCR-274N receivers, but not sure about the ARC-5 receivers.

If this receiver does not have AGC it can be easily added by shorting to ground the line controlled by the volume control, installing a 2.2 meg resistor at the detector output like is done with any receiver using AGC then use a 1 meg control for the volume right after the detector connected to the audio tube. The other end of the 2.2 meg resistor goes to the nearly complete AGC circuit.

Also if you want to drive a speaker at more than a watt or two you can install an electrolytic cap from the 12A6 tube plate to ground then cap couple the signal from the cathode to whatever amplifier you desire to use.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Thu 25, 2018 10:04 pm 
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I haven't decided yet if I will stick with the usual RF/IF gain control for volume or to install an actual volume control. I changed out the cathode and grid resistance for a little more power out from the 12A6 amp tube. The existing audio output transformer is driving a speaker that has a small transformer (1k-8ohms) on it and the result is plenty of volume for me. The mailman just dropped off my latest order of caps. After recapping and playing with the radio a bit more I'll decide on what if any more mods I want to do. I like it a lot as is.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Thu 25, 2018 11:24 pm 
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Roger, thanks for the mention of the "universal IF transformer" ! I have seen a ton of them but never gave it a thought about what that meant. I'll worry less about particular IF cans, then!
BTW, about IF cans for 'Command Receivers'. There's a great YouTube video showing various ways of improving
selectivity. One that really, and I mean really, stands out, is putting a crystal in the can for a single-crystal filter.
I think he may have used an old 2837 kHz crystal from the old 2-MHz boat radio days. This gives true SSB
reception, one sideband is suppressed. I been saving 2837 rocks!
-Hue


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 2:56 am 
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To get more from the 12A6 install a cathode bypass cap of 25uF.

I too really liked mine as is, but I found that AGC made it so much better as in I no longer had to adjust the volume as the signal faded in and out.

Given mine was butchered to use what amounted to best I could tell an AA5 output transformer I first tried an Edcor 7.6K to 8 ohm transformer which is what the 12A6 wants to see and it wasn't quite loud enough no matter what I did so I made a power supply with push pull 6BQ5 tubes on it for around 10 watts of audio.

Also you'd be surprised at how good the audio can sound with at minimum a proper output transformer that doesn't have limited bandwidth like the original most likely does. If you should decide at some point that you want to try a better output transformer let me know as I still have that Edcor transformer I used.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 5:03 am 
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I forgot to mention that I do have that bypass cap on the 12A6. I also have a few output transformers with the right impedance range but they are too big to stuff under the chassis where the original is. But I'm happy with the sound of the original right now. This isn't going to be my main daily driver so max fidelity isn't a big concern.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 6:35 am 
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The 12A6 only puts out moderate power because
1) It was only designed to drive headphones, not a loudspeaker, and
2) It is inside a very cramped cabinet with zero air circulation, so no proper cooling.
Running it with higher output power will likely shorten its life.

For higher loudspeaker volume, use its output to drive an external amplifier.

- Leigh

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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 7:03 am 
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can you post a pic of one of the transformer. do yours plug in? i have some that plug in. and they say 1of3 2of3 3of3 on them.tnx randy


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 12:50 pm 
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forumuser wrote:
I forgot to mention that I do have that bypass cap on the 12A6. I also have a few output transformers with the right impedance range but they are too big to stuff under the chassis where the original is. But I'm happy with the sound of the original right now. This isn't going to be my main daily driver so max fidelity isn't a big concern.


Agreed especially if yours has not been butchered by a previous owner.

Mine unfortunately was butchered so it wasn't much of an issue for me to use the larger output transformer which just fit underneath the cover someone had made for the rear where the dynamotor would have been.

With mine originally I think I paralleled the 1K cathode resistor with another 1K and used the bypass cap which got more output power.

Leigh wrote:
The 12A6 only puts out moderate power because
1) It was only designed to drive headphones, not a loudspeaker, and
2) It is inside a very cramped cabinet with zero air circulation, so no proper cooling.
Running it with higher output power will likely shorten its life.

For higher loudspeaker volume, use its output to drive an external amplifier.

- Leigh


Also it is driven directly by the detector which doesn't have enough audio voltage to drive the 12A6 to full output power. The easy thing to do is switch it to a 12SN7 or 12SL7 use the first triode as a voltage amp then the second one as a cathode follower that can then drive any amplifier solid state or tube.

The OP did say the audio was good enough to him as is though.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 2:35 pm 
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Leigh: I'm not too worried about overheating the output transformer. A number of articles mentioned these radios work with as little as 100VDC instead of the rated 250VDC. The bench supply I have been using so far for testing is about 155VDC under the 45ma load of the radio. When the time comes to build a dedicated supply I'll probably go a bit higher but still stay well below the 250VDC level. I guess it will depend on the value of the power transformer I end up using. I have many to choose from. So my guess is even with the resistor value changes on the 12A6 tube I'll be within specs. Its a guess because I didn't measure current draw before and after.

Radiocollector: My output transformer is not a plugin type. It looks basically like a larger version of the tub capacitors under the chassis.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 3:11 pm 
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For best sensitivity I'd suggest running it at around 220Vdc. I've ran mine from 220-260Vdc and it works great.

In order to reduce the chance of hum you will need a DC heater supply.

I've ran my tube heaters on AC before and got some hum. Has to do with how things are grounded in the receiver.

Of course if the original audio transformer cannot reproduce 60Hz at all then you might not even hear any hum.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 3:36 pm 
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When running at full or close to full voltage, how hot does it get under the cover. Probably scorching hot. Do you run the radio with the top cover off? On the bench I am running DC for filaments and plan to do so with the dedicated supply. It will be a separate outboard unit so trying to cram it in on a radio chassis is not anything I want to deal with. Plus with 2 radios I'd rather build one P/S to share amongst them.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 4:30 pm 
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forumuser wrote:
Leigh: I'm not too worried about overheating the output transformer.

I did not mention overheating the transformer.

I was talking about the 12A6 tube itself getting too hot.

- Leigh

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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 5:53 pm 
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forumuser wrote:
When running at full or close to full voltage, how hot does it get under the cover. Probably scorching hot. Do you run the radio with the top cover off? On the bench I am running DC for filaments and plan to do so with the dedicated supply. It will be a separate outboard unit so trying to cram it in on a radio chassis is not anything I want to deal with. Plus with 2 radios I'd rather build one P/S to share amongst them.


When I ran mine with the 12A6 with lower cathode resistance the tube itself didn't get too hot at least it seemed to work fine for the 2-3 years I ran it like that.

I too would never cram a power supply on the chassis.

With mine when I switched to AGC I also added a couple transistors and an indicator lamp to serve as as S meter with the light getting brighter as the signal strength increased.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Fri 26, 2018 7:02 pm 
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Ooops, my mistake Leigh. With the cover off the radio I'm still able to touch the tube without a sizzle and leaving a chunk of burning skin on it. :) IIRC the 12A6 was drawing somewhere around 10-12ma total.

On my other radio it was already switched to 6 volt tubes when I bought it and the audio tube was switched to a 6V6.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 4:35 am 
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SUCCESS. The IFT arrived in the mail this afternoon. It is the 3rd IFT but it plugged into the 2nd IF socket and works. It has only 1 tuning cap as opposed to 2 on the original. The little mounting ears don't line up but thats a minor issue. I could drill and tap a new hole to hold it in place.

While waiting for the IFT to arrive I started the recap. I had 11 replaced by the time I got the IFT plugged in. I fired up the radio and it seemed to be barely working. The front end seemed dead and the IF weak. I triple checked my recap and found no errors but yet I was still missing screen voltage for some tubes. I was 100 percent sure I made no wiring mistakes because it was pretty straighforward. Remove wires from 3 terminals of the old caps and place a new cap on each wire. If there was 2 wires on a terminal of the old cap I would solder them back together right away so as not to get confused later on. I did 3 of those triple caps plus 2 in the P/S filter area. Turns out that as I was stripping insulation from a wire I must have pulled it too hard because it moved the tube socket terminal just enough to touch another pin. I bent it out of the way and the IF section came back to life. But still nothing from the RF front end. That one was a little easier. I pulled out and re-seated the 3 coil assembly after cleaning the pins. The radio is loud and sensitive and I'm a happy camper. I'll finish the recap on this one then do the same for the other radio which is also working good.


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 Post subject: Re: ARC-5 radio IF transformer question
PostPosted: Jan Sat 27, 2018 6:50 am 
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VG Congrats.

- Leigh

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