front   back

  Unrestored front                                                Restored chassis

The wiring of this Sonora Sonorette matches Model 855D (schematic 70 kB).
The radio originates from 1933 and its housing is a wooden cabinet with the measures (hxwxd) 19x29x13.5 cm.
Model 855D is a superheterodyne receiver, employs 5 tubes (6A7, 78, 77, 43 and 25Z5), has an IF of 135 kHz and covers two bands.
The mains voltage is 110 Volt and rectified by the 25Z5 (tranformerless). The 110 Volt AC is also supplied, via a powerline cord of 160 ohm, to a series connection of the filaments of the 5 tubes.


chassis before   chassis after

Chassis before and after restoration (tubes not yet put back in place).
bottom before   bottom after

Bottom side of chassis before and after restoration (not all components put on place yet).


The radio (acquired on a radio flea market) had the following main defects:
1. open winding in filter inductor
2. volume potentiometer (50 kohm) burnt out
3. electrolytic capacitors in bad shape
4. various resistors and capacitors off-value
5. dirt, dust, rust everywhere .........

As the chassis was in a bad shape (a.o. rust), I dismantled the radio until the last screw and piece of wire. The chassis was cleaned thoroughly and thereafter zinc-plated in a local workshop.
The open-circuit in the filter inductor appeared to be caused by a corroded junction of the connecting wire at the last winding of the inductor. The inductor body was provided with complete new windings (total resistance 310 ohm; originally 350 ohm).
The volume potentiometer was replaced by a rather similar potmeter of 50 kohm out of my junk-box.
The IF-transformers were dismantled and cleaned and the tuning capacitor, trimmers and band switch got the same treatment.
As all electrolytic, paper and mica capacitors were lossy and/or leaky, they were replaced. This was accomplished by emptying the respective housings, placing new high quality components in the original housings and sealing the housings again with a dark grey, kneadable two-component epoxy. This material hardened after 5 to 10 minutes after mixing the two components by kneading.
Most resistors had increased resistance values. As I had no resistors available of the same type, I left the original resistors in their place and connected a new resistor of an appropriate value parallel to the original ones in those cases where the original resistors were too far off-value.
At this moment the radio is working fine on one band (PO). On the other band (GO), however, some kind of oscillation seems to occur, which needs further attention.

 


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Last update:  December 21, 2005