Restoration Of the Standardyne 1928 AC Model Receiver

Standardyne AC-29 radio repair

Author: Edgardo Castro Bruse
Author: YS1ECB
Calle Loma Linda
Residencial Loma Linda casa No. 4
Col. San Benito.
San Salvador. El Salvador.

San Salvador January 15 2016

Standardyne, Model AC-29


It 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 circuit elements

The circuit

This 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.
The original diagram that came with the receiver diagram with a few modifications Standardyne original diagram

Power Supply Before Restoration

To 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!
Power Supply Before Restoration Power supply wiring condenser block Elements removed from a metal box Filter and condensers

Restoration of Power Supply DC Filter

Plastic lace to hold the Choques Filter choques The two choques are connected in series Power Supply DC Filter

Restoration of Power Supply-Voltage Divider

Detector's elements Detector's voltage Restoration of Power Supply-Voltage Divider

Under Chassis Repairs and Data

The 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.
Filament bypass condenser Jumpers to restore continuity to the filament of a 226 tube RF by pass Capacitor
Chassis elements Chassis

Putting the radio receiver back into the cabinet

AC-29 Restoration Standardyne AC-29 cabinet Putting the radio into the cabinet Receiver in case Vernier

DC Voltages and AC voltages of a 1928 Standardyne AC Model Receiver.

Jan 25-2016 at 4.50 PM
B+ RF&1st Audio Amp. Bias 112-A Bias 226 Detector's voltage AC filament Voltages
187.8 V 105.7 V 13.9 V 3.0 V 26.4 V 5.17--2.28--1.49 V
Feb 14-2016 at 9.30 AM
186.1 V 101.2 V 13.9 V 3.0 V 26.0 V 5.15--2.26--1.48 V
March 6-2016 at 4.05 PM Line voltage 112.9 AC
175.6 V 101.7 V 13.1 V 2.86 V 24.8 V 4.96--2.16--1.42 V
March 11-2016 at 415 PM Line voltage 118.6 AC
189.0 V 103.6 V 14.3 V 3.28 V 26.7 V 5.26--2.3--1.5 V
Feb. 19-2016 at 11.20 AM Using a 5CZ5 output tube. Unfortunately I did not measure the line voltage.
175.0 V 96.8 V 12.0 V 2.80 V 24.4 V 5.07--2.28--1.49 V

Adapter To substitute the 112-A for a 5CZ5

On 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 Remarks

The 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.

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