By Member: Marconi_MPT4
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.
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.
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.
- 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.
- 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.
- 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!
- 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.
Basic Fault Finding Guide
|No Power||No dial lights
No valve heaters
|No Sound||No background hum, hiss or crackle
Dial and valves light up
No clicks when touching screwdriver on volume control centre pin
|Background hum, hiss or crackle||IF/FC/RF stage||
|Hum when touching screwdriver on volume control centre pin||
|Distortion||All volume levels||Output stage||
|High volume levels||HT supply||
|Low Output||Undistorted low volume||Output stage||
|Weak signals||All bands||IF/FC/RF stage||
|One or more bands||Tuning coils||
|Weak at one end of dial||FC or RF stage||
|LF Oscillation||All bands||HT supply||