Restoration Of the Standardyne 1928 AC Model Receiver
Standardyne AC-29 radio repairAuthor: Edgardo Castro Bruse
San Salvador January 15 2016
IntroductionIt seems to me that my model is an earlier version of the Model AC-29. My model comes with a complete diagram, but it does not show the values for the resistors and condensers of its power supply. There is also a difference in the volume control circuit. Otherwise it is almost identical to the Standardyne AC-29 model.
The outstanding feature of this radio is its wiring that resembles a primitive printed circuit! It uses flat strips of cupper interconnecting the circuit elements. These are held in place by means of rivets with special concave washers that add extra pressure.
The circuitThis is a regular TRF receiver. Its diagram is self explanatory. I am posting the original diagram that came with the receiver and the diagram for the Standardyne AC-29. You will also find the original diagram with a few modifications and the actual values for voltages and parts. Please click on images to enlarge.
Power Supply Before RestorationTo renovate the power supply, it was necessary to remove the two filter choques and the condenser block from the metal box filled with Tar! I made a small fire by using a compacted piece of cloth impregnated with thinner, and then I wrapped the box with stiff wire making a kind of a handle. I moved the metal box about the flame keeping it slanted until the tar melted. All of the sudden the contents fell out by gravity!
Restoration of Power Supply DC Filter
Restoration of Power Supply-Voltage Divider
Under Chassis Repairs and DataThe wiring is made by using flat copper strips held in place by means of rivets with a special concave washer. This wiring resembles a primitive printed circuit board. Should the need of changing tube sockets, coils and transformers as well as any of the resistors arise. The task to do this job would be very difficult. Luckily I did not have to do any of this.
Putting the radio receiver back into the cabinet
DC Voltages and AC voltages of a 1928 Standardyne AC Model Receiver.
Adapter To substitute the 112-A for a 5CZ5On February 19 I built an adapter to substitute the 112-A output tube, for a 5CZ5 audio output tube. I Used it in triode connection. The result is just great. The sound was excellent and much louder than the 112-A. I used a cathode resistance of 750 ohms, this causes a cathode current of 17 mA. which does not load too much the Power Supply, since the 112-A uses 7,7 mA. This adds only 9,3 mA more. I noticed that the AC 29 uses a 171-A instead of the 112-A and has the same Power Transformer. This means that the 5CZ5 should not overload it. The 5CZ5 works just great. I am including a couple of pictures of the adapter. You will also find a picture comparing the 5CZ5 with the 112-A and pictures of the 5CZ5 in operation. 5AQ5 and 5V6 could be used too!
Since I do not have a moving armature speaker to connect directly to the radio I used a small output transformer to couple a modern speaker to the output tube.
Final RemarksThe operation of this radio is somewhat complicated, because above 900 kHz it auto-oscillates, when the volume control is nearly fully open.
I think it auto-oscillates, because the antenna and the RF grid circuits, lack the anti-parasitics 500 ohms coiled wire resistors that most receivers of this type have.
Because of the copper strips type of connections I did not try to add these resistors. Nevertheless, with some practice, the auto-oscillation may prove to be an advantage. One most tune this receiver as a regenerative receiver.
With the tracking condenser (selectivity control) and the volume control, a station may be re-tuned until the radio is brought near the oscillating point. At this point excellent selectivity and very good RF and audio gains are achieved. It is important to note that the volume control is a 1670 ohms potentiometer shunting the primary of the first RF stage. This means that the volume is controlled by controlling the RF gain of this stage. This is why the volume control position also determines the oscillating point of this RF stage!
Finally I want to thank my friend German for posting this article in his page.