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Back On the Farm-Set    AK 768Q

6/8/2017

3 Comments

 
While waiting for the arrival of the NIB replacement tubes for the Stratosphere project I have had time to work on the Atwater Kent 768Q first mentioned two posts below.

I had been lucky enough to have completed the AK 559N project a short time ago. This radio is also of that vintage with similar cabinet design as well as chassis design along with the associated common issues.

The  768Q is a 8 tube "farm" radio meaning that it was battery powered. In most circumstances, this was an effort to provide radio receivers to a large portion of the country that had yet to acquire commercial electric service in the mid 1930s.

This radio shares a cabinet with other AK models such as the AK 447. Other than the power source, significant differences between these models include  - tube count, the lack of a shadow meter and the use of a pin-type speaker using a natural magnet rather than a field coil. Most of these differences reflect the considerations that must be made to improve battery use economy. The addition of an additional tube also reflects this consideration. The 8th tube is in the audio output, the 768Q having push-pull 30s driving the speaker directly through a center tapped "magnetic" coil. This coil energizes the speaker accomplishing cone movement as compared to the voice coil in a dynamic speaker such as in the 447.


Picture
I had to use 3 separate power supplies to generate the necessary voltages. 2V for the tube filaments, 180V for the plate voltages and -22V for a C voltage (bias). I did experiment with lower plate voltages. Except for a slight decrease in volume the radio performed well down to about 135V. (decreasing the C voltage accordingly).
Picture
Above, the before and after view, noting that the cabinet had been stripped by one of the previous owners and had incurred damage including the loss of veneer as well as some structural damage.
Picture
Besides restuffing all of the capacitors and building 3 replacement resistors (note the metallic end caps of a slightly different grey/silver color) most of the rubber insulated wire was also replaced. See pile at bottom of picture above.

This chassis had survived storage very well, maybe better than any other AK of the period that I have worked on. Most of the original resistors were OK and the rubber wire was remarkably well preserved - estimating that only around 1/3 was cracked or had hardened insulation. None the less, I replaced all of the wire on the IF and RF transformers and the interstage transformer even if it appeared ok. Having the radio apart for service made replacement of the wire more practical now rather than waiting for it to fail and I did not want to take the transformers off again later. So all in all , I replace about 90% of the original rubber insulated wire with new neoprene rubber insulated wire of the same colors - or at least close.

As is often the case, the rubber insulation on the black wire was always bad and failure of other colors also tended to be similar in all examples. It seems that the pigment has something to do with the failure rate. Also degradation by mold/fungus seems to be a factor so wire that is isolated in a can or elsewhere  usually is in better shape than the mass under the chassis.
Picture
The original speaker had been removed - unplugged. It probably went to some hi-fi project, for which it would not work (well) or maybe another radio since the original owner had some interest in restoring this radio and probably had other projects as well.

Other than the cabinet being stripped/sanded, it looked like somebody had done a bit of poking around under the chassis, cutting a few wires, but nothing else.
Above: The radio in operation on the bench. Note the dial lamp. Dial lamps were often omitted from farm radios to reduce A battery consumption. It is nice that AK decided to include this option probably because it was a high end set, even for a farm radio. If the user wanted to conserve power he could have just removed the bulb.

The pin type speaker is of the same size as the original (there were two different models of speaker used in this radio). It is similar in appearance to one of the original speakers. It is not a dynamic speaker rather one that uses the lever action attached to a pin that exerts force on the cone. The "permanent" magnets are not permanent and often need repolarization. It can reproduce midrange frequencies pretty good but is very poor at reproducing low frequencies.

To make this speaker work  ,I had to remove the winding (coil) and rewind it with a center tap which goes to B+ with both ends going to the plate of one of the push-pull number 30 tubes. A number 30 is a triode similar to a 01A but with a 2 volt common cathode filament. They are efficient but don't produce a lot of audio, so using 2 in PP was a good option.
Picture
I had never noticed before that 2 of the 5 knobs have serrated edges, probably for better grip.
Above is a demonstration of the cool band selector. The 2-speed tuning is on the upper right. The knob on the lower left is a multiple position tone control and the one in the lower center is a local/distance switch.

The local distance switch is useful since it is easy to overdrive the front end increasing distortion. Testing using my AM transmitter required the switch to be in the local setting for best reproduction.

The radio has 3 SW bands with the upper band going to about 22.5 mhz. Reception on the shortwave bands is good.
3 Comments
Robert Lozier
6/8/2017 05:42:24 pm

Great looking website! The 768 looks great. Where in the heck have you found neoprene wire??? Back before retirement, the contract engineering firm I worked for got a 500 ft. roll of silicone jacketed 18 Gauge stranded 12 conductor cable. It was very expensive cable designed to go in a deep mining bore hole camera control circuitry. It was sent to us by mistake. It looked just like vintage wire. I told my employer I REALLY REALLY wanted a length of that wire but they could not do it because of questions of ownership. One Saturday I come into work over time to see two guys finishing up clean-out of our storage room. I asked about the cable, they told me that a scrap metal dealer had carted it off a couple of hours before!!!! Boy! was I pissed... Rant Over...

Reply
Russ
6/8/2017 05:59:44 pm

The wire is specifically called:

◦Premium grade, color coded 90°C EPDM
It is ethylene propylene rubber.

It is a components of a cable called SOOW

Reply
Arran
7/6/2017 09:36:54 pm

It looks like this set uses what was called a "balanced armature" magnetic speaker, which seems to have quite popular for use in 1930s farm battery radios prior to P.M dynamic speakers becoming common. I remember owning a DeForest-Crosley tombstone battery set that had one, about 1935-36 vintage, but I also had a Rogers battery console of the same year that used the same chassis but it used an 8'' PM dynamic. The balanced armature speakers actually sound pretty decent when they are set up properly, there is some sort of adjustment you can make to the armature but I've never had the need to try it.
Speaking of dial lamps both of these sets had a pilot bulb actuated by a push button on the side of the cabinet, when you wanted to view the dial you would press it. It's curious that A.K chose to use a pair of #30s in push pull, whilst I have seen this before it was much more common to use a #19 tube with a #30 as a driver, in class B. The audio output power of such setups is quite good, probably comparable to an AC/DC radio with a 50L6.
Regards
Arran

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CUSTOM DESIGN BY SUE WEBB  06/2013       Redesigned by Russ Webb     Approved by Fuzzy   Updated:  Pretty much all the time, but I forget to change this date
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