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 Post subject: Atwater-Kent 20C 7960 - T4 audio transformer, open primary
PostPosted: Dec Fri 22, 2017 5:25 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Hello Group!

I am a new to the forum and hope someone can help.

Firstly, I am trained in electronics. Honestly I don't know much about tube radios.

That why I am here :-)

I recently acquired a beautiful Atwater-Kent 20C 7960 which I would like to get into working condition.

I've identified all of the power cables and done continuity tests while waiting for a speaker and a missing tube.

I've found that the T4 transformer from the detector has an open primary. T5, the output transformer appears to be OK.

Does anyone have or can someone direct me to who may have this transformer or its equivalent.

I'd like to keep the radio appearance consistent with the period if possible.

So far I only have info on the audio output transformer T5 which has different specs than the interstage transformer T4

Thanks in advance to whomever can help.

All the best

Ralph C.
AA3VV


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 3:44 am 
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Joined: May Fri 01, 2015 2:33 am
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Location: Roslyn,Pa 19001
Hi Ralph and welcome,
https://www.tubesandmore.com/products/t ... tage-10-ma
Pretty sure if you unpot the old one you can fit this inside the old cover. Abt
175-200 deg should do it for abt 10-15min. Much hotter you'll burn the paint.

GL
Terry N3GTE


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 3:56 am 
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Location: Gretna, Nebraska
Another replacement option that I have used in my A-K 20.
The standard transformer shown on this page. http://www.arbeiii.com/Transformers/

It also will fit in the original housing.

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Paul
...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 4:08 am 
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Posts: 33178
Location: Maryland 20709, USA
RSC wrote:
I've found that the T4 transformer from the detector has an open primary.
T5, the output transformer appears to be OK.
Hi Ralph, and elcome aboard.

The 20C #7960 does not have an audio output transformer.
B+ current for the output tube flows directly through the speaker.
You need a horn speaker or headphones with about 2000 ohms DC resistance.

Here's a modern schematic that might help:
http://atwaterkent.info/TechData/Drawin ... 960sch.pdf

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 2:59 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Thanks to everyone for your inputs!

I think I know how to proceed although I admit to being nervous
about taking apart a 90 year old radio :-)

New to the group so I'm still getting used to how the forum works.
I'll review the bylaws and rules to ensure that I communicate with the group
in the right topic and protocols in the future.

So gad to find such an excellent forum

Merry Christmas to all

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 8:14 pm 
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Posts: 12573
Location: S. Dartmouth MA 02748-1225 USA
A bit of a heads up,:

The audio filament resistor #8303 may be a length of resistance wire within a sleeve of varnished cambric. Looks like a length of ordinary wire. If the cambric is ratty do not replace with a fresh length of insulated copper wire, oops...

The OEM transformer is potted in asphalt within the can. Some folks freeze the transformer and smack the case with a soft blow hammer. The asphalt crumbles and the windings can be lifted out. Some folks heat the can and extract the old transformer from the goo, while hot, plunge in the new windings.

Caveat, the crackle finish on the radio and the transformer cans is soluble in alcohol, in fact most any solvent or heat may soften it.

GL

Chas


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Dec Sat 23, 2017 8:21 pm 
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I'm glad Chas brought this up. As I recall, I have had good success with the freezer method of removing the old transformer guts.
After inserting the new transformer in the can, align one screw hole or the new transformer base with one hole in the housing and insert a screw during reassembly to lock it all together.

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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Fri 12, 2018 2:55 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Hi to all in the Group,

Thanks gain for all of your inputs to my original questions.

I finally have the time to begin repairing my radio!

I've now double-checked the measurements that I have made so far without having removed the chassis
and the transformers read as follows.

-The first transformer T4 (7:1) has an OPEN primary (from DET plate-22.5V), Secondary = 7160

-The second transformer T5 (3:1) is GOOD at 1766/3500 Ohms.

However I'm still not clear on which transformer I should use for T4 replacement.

The AES P-T156 is a 3:1 transformer and seems appropriate for the transformer driving the final audio tube (T5).

- My first question is: "will the AES P-T156 also work to replace T4 (a 7:1 transformer) ?"

-My second question is: "should I go ahead and replace the good transformer T5 while I replace
the defective one T4 since all of these transformers seem to fail a lot?"

- My final question for now is: "How can I remove the 8 front panel screws with damaging them"

Thanks in advance to all who respond. As a newby, I truly appreciate the support as I get started.

all the best

Ralph C.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Fri 12, 2018 3:49 pm 
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Posts: 12573
Location: S. Dartmouth MA 02748-1225 USA
A 3:1 transformer will work, there will be some loss of volume, however. MANY 20's battery radios use 3:1 transformers in both locations.

You can shop around for an original transformer or a 6:1 modern if the laminations will fit.

Take a good images that can be posted to ARF of the transformer island circling the bad transformer. Post dimensions too.

Look to PTOP Playthings of Past may have one or set search on ebay.

Opps forgot your other questions. Leave the good transformer alone :shock:

For the front wood screws, use a good fitting screwdriver, even if you need to buy a new one. The knurled buttons are non-functional decoration. Most, if not all A-K trim is gold plated, even the round head wood screws, common brass polish will wipe the gold away use a Jewelers cloth to clean.

GL

Chas


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 2:42 am 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Thanks for the reply

I just realized something odd about the AK 20C 7960 detector output transformer T4.

In the AK 20C 7960-

T4 has a 1K-Pri, 8K-Sec

T5 has a 1.7K-pri, 3.25K-Sec

However, the AK 20C models 7570 and 4640 models both
use a 1.7K-pri, 3.25K-sec transformer for both T4 AND T5.

I remember reading somewhere about a more sensitive detector tube 200A

Could the different transformer be used to make the new tube work properly?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 3:35 am 
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Posts: 33178
Location: Maryland 20709, USA
RSC wrote:
In the AK 20C 7960-
...
However, the AK 20C models 7570 and 4640 models

You cannot compare components between models.
The winding resistances are given only so a tech can determine if the part is open or shorted.
They tell you nothing about the transformer's operation.

Kent did a lot of experimenting with transformers.
The wire that was used is very fine (for unknown reasons).
Consequently they experienced a high failure rate when the sets were new.

The 200A detector with its 1/4-amp filament came out after the Model 20 variants went out of production. The sets were designed to work properly with them since their development was anticipated. I'm not aware of any improved detector operation with the 200A.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 3:46 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK!

I've removed the chassis from the cabinet

The chassis is pristine; the wires are in great shape and the resistors are still shiny.
I can see no corrosion anywhere.

I have ordered a replacement transformer from AES.

Next step is to measure all continuity from the power cable on
and check the resistors and caps. I may have a few questions about
that :-)

My next question has to do with how to remove the transformer without damage.

It appears that the transformers are bolted to the chassis with brass bolts
and fastened with two nuts.

It seems like I should be able to use a nutdriver to remove them.

Should I do anything else?

should I use penetrating oil first?

Thanks
Ralph C


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 4:17 pm 
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Posts: 12573
Location: S. Dartmouth MA 02748-1225 USA
Unless brass hardware has been exposed to corrosion, no penetrating oil is needed. I have never had a problem disassembling an A-K, even one that I found buried in the sands of Chatham, Cape Cod :) To keep the hardware in a "new" like condition, use tools that fit properly.

If the battery cable sounds "crackley" when bent then the rubber insulation is failing. It has been my experience with failing cable that they will leak current. A small amount indeed but is sets up an environment for electrical noises. Replacement (sorry, not exact) battery cables are available. Advantage is one can get the length needed to get any power source out of site.

The use of a gas detector depends on both the radio being designed for it and the users skill at tweaking the external circuit (adjustments) to achieve the best results. Results are fleeting and not permanent and will vary with signal strength and atmospheric conditions. A single static crash could shift the operating point and require another round of adjustment.

GL

Chas


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 6:40 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Wow- I guess my luck is still with me. Should have bought a lottery ticket!

The power cord is still in great shape. Not crackly at all.
Could be only 20 years old instead of nearly 100!

Thanks for your info on the brass nuts holding the transformers into place.
It gives me some confidence moving forward.

I'll try the freezer method first to remove the defective transformer from the can.

In the meantime, I've ordered the replacement transformer from AES.

This radio is in such good shape I admit that I am a little intimidated.

I feel almost like a caretaker of a museum and don't want to make any mistakes or damage it.

Never done hands-on radio servicing before. All digital background but finding RF rocks. :-)

Thanks to all in the ARF group I've met so far for helping move forward.

I'm treating this as a radio version of eating an elephant, one bite at a time.

Depending on my schedule, I'll pull the transformer
and measure the resistors and look for caps shorts.

Thanks again for the help.

Ralph C.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sat 13, 2018 11:12 pm 
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Location: Gretna, Nebraska
Here is a restoration point to ponder.
When replacing the audio transformer, see if you can carefully remove the leads from the old winding and attach them to the new one up inside the can.
When done successfully, you can scarcely tell that the transformer was replaced, even from the underside of the chassis.

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...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 1:41 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK

I suppose that means I should trace the transformers lead to where they are soldered
and de-solder them? Not cut the leads and splice them?

---------------------

Also, if anyone has replaces this transformer on the AK 20c, I'd appreciate and tips and cautions on
how to avoid "doing it the hard way" ;-)

Seems sometimes you don't know how to do a task until you've already made a mistake.


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 2:00 pm 
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Location: Gretna, Nebraska
Sometimes it works better to cut the old leads close to where they enter the old transformer and splice them to the new leads.

Then cover the splice with a fabric insulated sleeve which will match the appearance of the old insulation.
Old insulation may not tolerate much flexing and can crumble during disconnection and movement.

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Paul
...... how hard can it be?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 8:17 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
Thanks for the reply on keeping part of the lead and splicing.

Can you recommend a supplier for the cloth sleeve?


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 8:31 pm 
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Joined: Dec Sat 09, 2017 11:58 pm
Posts: 23
OK Folks

I am making some progress!

I've checked all of the resistors, caps, and coils.
-The grid leak is open but all the other resistor are good.
-All caps check OK
-The RF coils are fine.
-The tube socket pins are shiny, no corrosion.
-The first audio interstage transformer primary is open (already discussed)

So my next steps seem to be:
1. Replace the audio interstage transformer as recommended (on order)
2. Solder a 2.2 M Ohm resistor under the grid leak resistor ( should I use a carbon-composition? )

Thanks to all who have given me suggestions. Its been a GREAT help

Any further suggestions are welcomed as this is my first radio repair of any kind :-)

Thanks

Ralph


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 Post subject: Re: Atwater-Kent 20C 7960 - T4 audio transformer, open prima
PostPosted: Jan Sun 14, 2018 8:54 pm 
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Location: Gretna, Nebraska
I would strongly suggest replacing the RF bypass condensor in the metal can. While it may test good now, they are prone to failing while in use.
If you heat the can, the old cap will slip out and you can stuff a modern replacement .33 uF in the can, using the original wire leads.

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Paul
...... how hard can it be?


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