[rfeenstra]

I started working on an Atwater Kent 55F table radio.  The F is a radio designed for 25 cycle AC rather than 60 cycle.  Hopefully it will also work on 60 cycle.

I got the Filter Capacitor can open and was quite impressed with the workmanship. It was potted in some sort of white wax that was quite easy to remove. Unfortunately, I'm not as impressed with the potted transformers.  Once again I'm faced with an open winding in one of the transformers.  This time it's the secondary of the audio driver transformer.  Both the driver and output transformers are potted in the same can with a very hard tar like substance.  It is going to be a very great pain in the behind!

Since there is a fair amount of mouse urine damage to the chassis,  I'm going to have to use paint.  I'll probably remove all of the transformers so I can do a better job of cleanup.  I think this will be the most work yet as I learn this restoration hobby.

There is a loose, very heavy cup shaped device with a small hole in the top. From what I've been able to gather, this is a tub shield for either the detector or 1st audio driver. It would appear to just sit on top of the tube. Any knowledge out there as to it's purpose and it's weight? Is the mass there to help dampen vibrations on the tube ?

[morzh]

A 25Hz radios work fine with 60Hz.
One thing to keep in mind: as the Mains frequency rises, the B+ voltage also does. So the very first capacitor after the rectifier needs to go down in value some to keep the voltage down or the radio will have to work at the increased B+ and this will make tubes run hotter.

[rfeenstra]

I was wondering about that.  I was kind of planning to put a dropping resistor in the line for that reason.

On the 55 early, the first cap is 2 mfd, the second is 2.3, and the third is 2.3.  On the 55F, the first is 1.7 mfd, the second is 4 mfd and the third is 1.  From your experience, what would be good?  I might put 1 as the first, and 2.3 as the next 2.

[Radioroslyn]

> a very hard tar like substance.  It is going to be a very great pain in the behind!
Methinks it IS tar! Throw this contraption in the to the oven to soften the tar to get it apart. Pretty sure there is a suitable replacement available.

> Any knowledge out there as to it's purpose and it's weight?
It to help prevent microphonics in hi gain circuits when using the old globe style tubes. These tube don't have much support to the element so they can rattle. The newer shoulder type have mica supports at the top and bottom of the element so there much more stable.

[rfeenstra]

If I can get it apart and cleaned up, I can rewind it.  But, a replacement would be much easier.

I kind of figured that that was the purpose of the funny cap/shield.  It makes sense for the globe tubes.

[Arran]

I would not worry about the values of the filter caps, the originals were all paper and of low values compared to electrolytics, the difference in output voltage will be negligible, if there is any difference. I'm not sure about the 55 and 55F chassis but it may also have a choke input off the rectifier tube rather then a cap, if this is the case then the size of the input cap would never be an issue. Also most of these early radios used a series of filter chokes to filter the ripple out rather then relying on the capacitors, since large paper caps were bulky and expensive. After wet electrolytics came out the capacity of filter caps went from 1.5 to 2 uf to 8 uf to 12 uf, so the cost of large value capacitors went down, and so they could reduce the size and number of filter chokes they used, usually reduced down the field coil in the speaker.
Regards
Arran

[rfeenstra]

The schematics that I have for both the 55 and the 55f show 2 chokes plus the field coil, and 3 caps in the filtering section. One of the caps is shown to be a 4 mfd on the schematics, but physical examination of the 3 larger caps in the can are all about the same size so I doubt that one of them is 4 mfd.  Also, I found it interesting that the field coil in the 55f is in the filtering circuit. On the 55, one end goes to ground.  It seems they were working hard to reduce ripple on the 25 cycle sets.  I think I'll just use all 2 mfd caps in the filtering section and hope for the best.  My bigger concern right now is the stupid audio driver transformer.  I saw some AK transformers on ebay, but not what I needed - Rats!

[Radioroslyn]

https://www.tubesandmore.com/products/tr...tage-10-ma

[tbone]

Depending on the Schematic, I would definitely match the values as close as possible. As mentioned before, they are part of a resonant circuit with the coils to form a filter. In this case probably lower in frequency than 120 Hz because the ripple in a 25 Cycle set is lower.

When I did my 55, I used all 630 film caps in the power supply since originals were paper. They will hold up better and last longer than electrolytics. Just a recommendation.

https://www.tubesandmore.com/products/ca...ylene-fast

http://www.atwaterkent.info/TechData/akS...ex.html#55

[rfeenstra]

Ordered one of the interstage audio transformers.  Thanks, radioroslyn.

[rfeenstra]

tbone, been using film caps for all filter caps except in my 14.  I'll see how that one holds up. thanks for the AK link!

[Arran]

It's only logical that a 25 cycle set would have more filtering, instead of a 120 cycle ripple you would have a 50 cycle ripple. In Canada 25 cycle power grids were quite common, including in the area I reside now, with most manufacturers they had 60 cycle and 25 cycle versions, except for Rogers which seems to have used 25 cycle capable chassis in everything, unless the set had a clock or had motorized tuning. That was typically what they did, they either added larger, or extra filter caps, or added extra filter chokes, I guess Rogers dispensed with that to reduce parts inventories and worked via economies of scale since they knew that 25 cycle radios would work fine on 60 anyhow. There is one advantage to having a 25 cycle transformer, they have larger cores, and more iron, so they run cool even on 120 volts.
Regards
Arran

[tbone]

Glad to send the link. I find that site helpful.

As for the 25 cycle sets, I have seen a few collectors on the other forum with some really nice Rogers radios.
While we have nice sets in the states, I would make room for one of the Rogers consoles should one show up at the right time.

[rfeenstra]

I can't win with transformers!  I took all or the RF transofromers out so that i could clean up the chassis.  Measurements show that the 1st RF transformer (the one after the antenna transformer) has an open primary winding.  it should still work, but just not on the distant setting.  This coil is on the inside of the main tube so a bit more difficult to get out to rewind.  I may leave it until I know that the radio works good.

I got all of the rust cleaned up from the mouse urine.  took everything off and bead blasted it and painted.  It seems a shame to have to do that, but i guess you have to protect bare metal after rust removal.  It seems the plating is different on the AK than the Philco chassis.  More like some sort of nickel rather than cadmium.  Very thin plating that will peal off with light bead blasting.  I don't know if that is an excepted procedure to use, but it did clean it up nicely and quickly, making a nice base for painting.

[rfeenstra]

So far, not too impressed with AK.  the radio is up and running, but am having problems with the tuning caps.  The shafts for the movable gangs are worn and so will not stay centered between the stationary plates.  As a result, the 3 tuning caps do not track at all accross the dial and will not tune anything in on the lower end unless I physically push on the shaft to center the plates.  I presume the wear is a result of the side tension placed on the bearings by the belts that tie the 3 caps together.  Not sure what I'm going to do.  I'll dissassemble the caps to see if I can make new bushings to thighten things up a bit.

I haven't found any alignment instructions but it looks like the only way to adjust the 3 stages is to physically adjust the pulleys on each shaft for maximum signal.  Any ideas from experience would be appreciated!