Useful Zenith Royal 500 Transistor Radio Repair Tips

    After repairing many Zenith Transistor Radios, I have found these to be the most common component failures. Also, several useful troubleshooting tips and a pictorial webpage showing how to recap a Royal 500 set are listed.

    While not a component failure, the NUMBER ONE additional problem with most non-working or poorly working Zeniths is the fact that owners cannot resist the temptation to turn adjustment screws when a radio's performance starts to fail. The RF and IF sections can only be aligned by using a signal generator that can be set the proper alignment frequencies. Also, on the hand-wired early radios, the transformers were sealed with wax after alignment. If one tries to turn the adjustment core, chances are very good that you will rotate the coil and sheer off the the micro-fine wires that connect internally with the terminals. If this happens, it is the end of the radio since replacements can only come from another donor radio which are unobtainium these days.

    Also, while battery manufacturers have made great strides in quality; there is no such thing as a completely leak-proof dry cell battery!!


    MOST COMMON COMPONENT FAILURES
    COMMON SYMPTOM(S)
    1. audio coupling capacitor from volume control to driver transistor low or no audio
    2. audio driver transistor emitter bypass capacitor low audio
    3. battery supply bypass capacitor "motorboating" audio
    4. AVC filter cap distorted, low/high audio levels
    5. worn volume control (missing all or part or carbon substrate) (see information below) no volume until volume knob is well advanced ; or audio too loud or "scratchy audio" at low volume level
    6. open audio interstage transformer (usually the primary) fairly common to the Royal 500E models clicking sound when turned on—no audio
    7. bad RF transistors clicking sound when turned on—no audio
    8. bad audio transistors clicking sound when turned on—no audio
    9. Royal 500H—break(s) in PC foil (various locations). This is very commom problem with the thin, fragile lands on the 500H PC board. No audio; intermittent audio, pressing on the volume control knob or tuning knob makes sound come or go.
    10. mis-adjusted/broken I.F. transformer tuning slugs low and/or distorted audio, oscillations
    11. open speaker voice coil no sound
    12. defective earphone jack no sound

     

    Here is a table that gives some electrolytic capacitor values found in the old radios and the modern equivalent capacitor values. Most electrolytic capacitors used in Zenith radios will work with values + or - 30% or more. Also, most modern electrolytic capacitors are well-made and will last for many years. There is no need to pay extra for close-tolerance or high temperature rated electrolytic capacitors. Again, the cheaper ones work just fine.

    ZENITH RADIO VALUE
    MODERN EQUIVALENT
    5 MFD
    4.7MFD
    16MFD
    15MFD OR 22MFD
    30MFD

    33MFD

    50MFD
    47MFD
    500MFD
    470MFD



    This picture shows the carbon wiper material found inside the Zenith 5K volume control. As one can see, the carbon is completely worn away from the "low volume" end from frequent radio usage. This volume control is totally shot and beyond repair. A good volume control should measure 1-5 ohms between the wiper and the ground side with the on-off switch just advanced to the "on" position. Once a volume control is worn, it can measure is high as 200 ohms. If your radio plays a normal listening levels or louder when just turned to the "on" position, this is a symtom of a worn volume control. This is very common in these 50 year-old transistor radios. Unfortunately, this is a specialty component, and is available only from another Zenith "donor parts" set.

    REPAIR TIPS

    Repair Tip #1 Having the schematic and making voltage measurements on every transistor is important. This troubleshooting tip is helpful to quickly indentify problem area(s).

    Repair Tip #2 Replace all eletrolytic capacitors while the radio is removed from its cabinet.

    Repair Tip #3 The ferrite antenna on the Royal 500 has a mounting system where the waxed ends are pressed down into mounting ears. After the chassis has been removed one or more times, the antenna, sometimes, will not remain in the ears. This can be remedied by dripping fresh candle wax onto the ends of the ferrite antenna rod before reinserting the chassis into the cabinet.

    Repair Tip #4 If you a collector and only play your radio(s) only occasionally, leaving the volume control/on-off switch in the "on" position maintains metal-to-metal contact and reduces switch oxidation problems. Of course, you need to have the batteries removed from your radio.