Zenith H724

 


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Zenith AM FM Model H724

This radio was a challenge.  The chassis looks like it went through two separate events.  These events had a component short resulting in arching and or a small burn.  This is evident by scorch and smoke marks under the chassis.  The FM antenna standoff and associated capacitor to ground were totally charred. 

This is truly a Hot Chassis radio.  There is no power transformer to provide isolation from the line (Schematic).  And one side of the power cord is directly connected to the chassis through the power switch.  Zenith took great pains to insure there are no metal parts that electrically touch the Hot Chassis that lead directly to the out side world.  All the control shafts are mounted on insulating board, the tone control metal shaft is not connected to any electrical lead and the power cord is disconnected when the back is removed. 

The classic argument is to install a polarized plug so the chassis side goes to neutral.  However this effort is absolutely no guarantee of safety by today's standards.  Two ways this can be an ineffective solution are:

1) If the home has no polarized outlets or the polarize outlet is miswired defeats the polarized plug effort.

2) Once the power switch is turned off the no load (infinite resistance) appears on the chassis as measured to ground.  This will light up a neon test lamp, confuse and frighten the layperson.

This radio was tested for leakage using a 10,000 ohm load.  The voltage was measured across a 10k ohm resistor connected from the G antenna terminal to earth ground.  The maximum worst case voltage is 4.34 volts with 117 vac input.  This translates to 0.424 ma.  All other available exposed metal points measured less with all combinations of power on/off and the with the plug reversed. A neon lamp tester typically uses a 100,000 series resistor.  Therefore requiring 1/10 the measured leakage current.  So a neon test lamp will probably light. 

An external Isolation transformer may be purchased and used on this type of transformer less radio. 
im000019.jpg (1092192 bytes) The tubular white cap in the lower left is a replacement across the line cap.  The chassis underside shows signs of arcing. im000020.jpg (986996 bytes) FM antenna connections charred. im000024.jpg (1096904 bytes) Restored chassis. im000025.jpg (1070892 bytes) There is a new Y2 safety cap hidden under the antenna lead wires. im000022.jpg (323462 bytes) Clorox Clean up loosens the brown dirt.  Probably cigarette smoke.
im000023.jpg (231244 bytes) White paper towels turn brown. im000021.jpg (383554 bytes) Exposed areas of the speaker cloth under the metal grill has faded to yellos..  im000027.jpg (312590 bytes) The original under the unexposed areas appears to be wine color. im000031.jpg (359174 bytes) New sheer fabric. im000035.jpg (332257 bytes)Contact cement. 
im000036.jpg (77500 bytes)Applied the first layer of fabric. im000038.jpg (320378 bytes) Trim. im000039.jpg (272697 bytes) Cement again. im000040.jpg (289594 bytes)Second layer of fabric.  im000042.jpg (248722 bytes) Create control shaft holes with out creating fabric runs.
im000043.jpg (248076 bytes) im000045.jpg (257988 bytes) Trim. im000046.jpg (249830 bytes) More holes for retaining studs. im000050.jpg (266654 bytes) Touch up Zenith Emblem.  
im000108.jpg (946017 bytes)The yellow faded speaker cloth.  But I did not know this until disassembly. im000109.jpg (673261 bytes) im000110.jpg (793948 bytes) im000113.jpg (747519 bytes)  im000122.jpg (952942 bytes)
im000052.jpg (241882 bytes) Vanity Shot.  im000053.jpg (272377 bytes)  im000054.jpg (224390 bytes) im000055.jpg (259907 bytes) Tone Control on side. im000051.jpg (290695 bytes) Clean.
im000056.jpg (295304 bytes)Rear panel.  Use these contact for an isolated Dipole.  Not recommended to be connected to ground.  The G screw connects to the hot chassis through a capacitor.  That capacitor shorted arced and charred the area as mentioned second picture above. The whole area was rebuilt. All burned components replaced.     

 

 

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