Basic Radio Restoration

General

Please be aware that vintage radio receivers often use high voltages with components operating at harmful temperatures. Special precautions are essential when dealing with ‘live’ equipment where the chassis has direct connection to the mains supply. Failure to take adequate safeguards could result in serious shock or death to yourself and others. If you are in any doubt about your abilities to work safely with vintage electronic equipment, you should refer repair to a qualified and/or experienced engineer.

 

Getting Started

Having obtained your restoration project, do not be immediately tempted to apply power as many vintage sets have been stored in poor, damp conditions such as sheds and outhouses. Put the set in a warm dry room for a week to expel moisture. If the receiver has not been powered for many years the high voltage supply electrolytic capacitors will at least need reforming. Applying power may cause irreparable component damage that with care could be avoided. If possible obtain the service sheet or manufacturers service data as this will provide at the very least a schematic, parts list, voltage tables and alignment procedure. Some may include a circuit description to help understand specific circuits.

 

Equipment

Essential equipment at a minimum includes side cutters, pliers and assorted screwdrivers. Use 60/40 lead/tin cored solder together with a soldering iron that should have a minimum rating of 25W to apply enough heat to obtain satisfactory joints. As many vintage radio receivers have live chassis construction always use a 1:1 mains isolating transformer while servicing or making adjustments. When first powering up an unknown receiver use a variac or lamp limiter (100 Watt light bulb) in series with the mains supply to bring the power up slowly.

 

Before Powering Up

Without delving into specifics there are a few components that will always need replacing before any power is applied.

  1. Mains filter capacitor. After many years idle the capacitor will usually fail spectacularly, and either explode or squirt hot wax everywhere. Probably best to carefully snip one end or just replace as a matter of course. Remember to use correct X class type rated for direct connection across the mains.
     
  2. Audio output valve G1 coupling capacitor. Please, please replace this first as only a slight leak will cause damage to the valve, output transformer or power supply. If it is a particularly rare set replacements are not easy to come by and transformer rewinds are expensive.
     
  3. Reservoir and smoothing electrolytic capacitors will need special care. Usually these will have started to leak electrolyte indicating replacement is required. If they look physically OK then careful application of supply voltage using a lamp limiter or variac will often help in bringing them back to life. DO NOT be tempted to apply full power straight away!
     
  4. Mains flex insulation will more than likely have perished if original fit. If it has been replaced in recent years check carefully for cuts and signs of damage, renew if in any doubt. Radio sets originating from the USA will often have line cord in place of normal flex to form part of the ballast. Do not shorten or replace with regular cable as this will damage receiver. Obtain if possible correct line cord or use a step down transformer. Note that line cord often contains asbestos so careful handling is needed.

Visually inspect the chassis for any signs of repair work, missing components and that correct valves are fitted. It may be necessary to remove 60 years of dust attached to everything, but use a soft bristled paint brush or slurry brush to carefully loosen dirt while keeping the vacuum nozzle about 2 inches away. Sets with live chassis should be checked for previous replacement of on/off switch and/or volume control where user isolation via the shaft and control knob may be impaired. Look also for the classic bodge where a failed switch section is linked out.

 

Stand Well Back

Once you are happy to proceed with powering up and your set appears to have all the bits in the right order, select correct mains voltage tap and connect lamp limiter or variac. Switch on. With a variac try starting off at 50 Volts and incrementing in steps of 10 to 20 Volts every 30 minutes.

 

Watch very carefully for signs of smoke or components looking distressed. Excessive hum will indicate reservoir or smoothing capacitor failure. Sometimes it may be useful to keep a finger poised to switch off quickly. Your set will often initially smell like a hot dusty electric fire. This is normal and will recede as droppers and big power resistors dispel dampness and burn off dust when they reach operating temperature.

 

Connect an aerial and if all is well you may be receiving stations, but more often the set will not be functioning properly. Narrow down the fault by connecting an audio input to the gram socket if fitted or check the tuning indicator is operating showing the receiver front end is working. Discern carefully how the receiver is behaving, does it hum, crackle or is there no sound at all? Do all the valve heaters light up?

 

Time To Roll Up Sleeves

Before delving in make sure you have a copy of the manufacturers or trade service sheet for the receiver as it will be essential to help pinpoint if a particular stage is working correctly. For the purpose of guidance I will use as reference a typical superhet design popular from mid 1930’s to 1960’s.

 

Heater Supply [1]
 
With AC sets all the heaters except rectifier in some cases are fed in parallel from a low voltage transformer winding. When one or more valve heater fails the remainder will still be lit. If no heaters are lit check transformer voltage is present. Check primary voltage tap adjuster is in place or if the screw type firmly tightened. AC/DC. Here all the heater filaments are wired in series with a ballast resistor and surge limiting device such as a thermistor or barretter. When one filament fails, all heaters will be out. Open circuit ballast resistors are a common failure. Watch out for leaky or short circuit capacitors decoupling the heater circuit. Also heater to cathode short circuits are more common and will be seen with top part of the chain glowing away brightly while the lower end stays cold. 
 
HT Supply [2]
 
In AC sets HT is obtained from a mains transformer winding together with a full wave rectifier. Common problems usually are  low HT and excessive hum. Most attention should be directed towards the reservoir and smoothing capacitors as these need to be up to specification. When replacing a failed rectifier it is good practice to replace the reservoir capacitor(s) too. If HT is low, check for low emission rectifier, leaky or low value capacitors and output valve bias is correct as this is the main supply current drain. Sometimes shorted turns on the transformer will cause low HT but this  is accompanied by overheating of the windings. Check correct mains tapping if voltage is unexpectedly high.
 
AC/DC sets obtain HT from the mains input via a half wave rectifier and series resistors. In many other respects these have the same failure modes as for the AC receiver. 
 
AF Output [3]
 
Replace the control grid (g1) coupling capacitor as part of normal service as these pesky blighters are responsible for a number of faults such as low HT through being leaky. NO LEAKAGE at all is permissible as any positive voltage from the previous stage  will upset bias conditions to allow the output valve to draw far more current than specified. If left unchecked, catastrophic failure will occur of the output valve, output transformer and in some cases power supply. Check capacitor insulation is good for at least 500 Volts.
 
No output fault may be an open circuit transformer primary. Low output could be caused by tone correction capacitor on the transformer primary being leaky or near short circuit. For distortion look at cathode bias, G1 coupling capacitor, high value G1 grid leak, shorted turns in the output transformer winding and speaker cone or voice coil problem.
 

 

Demodulator [4] and AGC [4a]
 
Nearly all faults here tend to be leaky capacitors and high value resistors assuming the valve is in good order. The AGC circuitry is often very high impedance >1MΩ and decoupling capacitors must have very low leakage.
 
IF Stage(s) [5] and [5a] and RF [7]
 
Most of the receiver gain and selectivity is determined by the IF stage(s). Extra gain before the mixer is provided by the RF stage. Leaky capacitors and high value resistors are main failures. Occasionally an open circuit screen (g2) or anode decoupling capacitor may cause parasitic oscillation giving an apparent low gain fault. Dabbing a capacitor across the existing one will prove if this is the case.
 
Frequency Changer [6]
 
Same problems occur with leaky or open capacitors and high value resistors, as for IF Stage.
 
Tuning Indicator [8]
 
Dim or no display due to phosphor decay. Quite a few types are now very difficult to find as New Old Stock (NOS).
 
Basic Fault Finding Guide



Symptoms

Observations

Check

Check Specifically

No Power No dial lights
No valve heaters
Power supply
  • Mains lead
  • Fuse(s)
  • On-off switch
  • Mains dropper or power transformer
       
    Valve heaters
  • Valve heater o.c. (series chain)
  • Dial lamp (series chain) o.c.
  • Thermistor/barretter (series) o.c.
  • Mains dropper or power transformer o.c.
No Sound No background hum, hiss or crackle
Dial and valves light up
No clicks when touching screwdriver on volume control centre pin
 
HT supply
  • Rectifier low emission
  • Reservoir and smoothing capacitors
  • HT feed resistors o.c.
  • Speaker energising coil o.c.
  • HT short circuit or high current load
  • HT fuse if fitted
       
    Output stage
  • Valve(s) faulty
  • Transformer windings o.c.
  • Speech coil o.c.
  • External/internal speaker switch
       
  Background hum, hiss or crackle IF/FC/RF stage
  • Valves in detector, IF, FC and RF
  Hum when touching screwdriver on volume control centre pin  
  • Semiconductor detector diode open or short
  • Screen grid decoupling capacitors leaky
  • o.c.screen grid decoupling capacitors (oscillation)
       
Distortion All volume levels Output stage
  • Valve(s) faulty
  • Transformer windings shorted turns
  • Leaky capacitor across primary
  • G1 coupling capacitor leaky
  • Cathode bias capacitor and resistor

 

  Speaker
  • Speech coil rubbing against magnet
  • Insufficient magnetisation (energised type)
  • Incorrect adjustment of speech coil ‘spider’

 

 

 

    Detector
  • Incorrect detector diode load resistor
  • Detector diode faulty
  • AGC capacitors leaky
  • AGC resistors high value
  • AGC Delay diode (if fitted) not working
  High volume levels HT supply
  • Low HT
       
Low Output Undistorted low volume Output stage
  • Valve(s) faulty
  • Transformer windings shorted turns
  • Leaky capacitor across primary
  • Cathode bias capacitor

 

    Speaker
  • Insufficient magnetisation (energised type)
  • Weak magnet (PM type)
       
Weak signals All bands IF/FC/RF stage
  • Valves in detector, IF, FC and RF
  • Screen grid decoupling capacitors leaky
  • High value screen grid resistors
  • o.c.screen grid decoupling capacitors (oscillation)
  • IF or RF alignment suspect
  • Local oscillator fault

 

     

 

One or more bands Tuning coils
  • Coils damaged or presence of green spot
  • Padding/trimming capacitors missing/maladjusted

 

     

 

Weak at one end of dial FC or RF stage
  • Oscillator valve low emission
  • Incorrect oscillator or RF ganging
  • Low HT
       
LF Oscillation All bands HT supply
  • HT decoupling electrolytic capacitors

 

     

 

 
Finally
 
This basic guide is a starter for anyone considering repair of vintage radio receivers, however there are many excellent texts available to read on this subject. Many of those considered essential are now out of print and although rather old cover the subject well.
 
Radio Servicing by G.N.Patchett Foundation of Wireless 4th
Ed. By M.G.Scroggie BSc AMIEE
 
Radio Upkeep and Repair 6th Ed. 1947 by Alfred T Witts CEng AMIEE Pitman
 
Wireless Servicing Manual 6th Ed. W.T Cocking AMIEE Iliffe
 
Electronic & Electrical principles 4/5 1982 S.A.KNIGHT BSc Hons Butterworth (tuned circuits and degrees of coupling)
 
The Admiralty Handbook 1938 Volumes I and II
 
This guide was prepared by VRAT forum member Marconi_MPT4