Initial Checks and Tests

Before you even consider applying power to your new set, it is essential to give it a general check over to assess the condition of the major components. This should prevent any nasty (and expensive or dangerous) surprises when you do get to showing it some electricity.

What happens if I just plug it in?

This depends on luck and the condition of the set. There are several possible outcomes:

As you can see, just switching it on without carrying out some basic checks and tests is asking for trouble.

Suppose you obtained a vintage car that hadn't been run for years. Would you just put in some petrol, hook up a set of jump leads and try to start it? No - you would do a few basic checks, change the oil and water, try to charge the battery with a charger, probably remove the rocker cover and check the valves aren't jammed, try to turn the engine manually to make sure it isn't seized... I'm no car expert so I don't know what else you'd check, but I know you wouldn't just try to start it and see what happened. If this situation actually arose I would seek the relevant information and take it carefully, knowing that I could very easily make things a lot worse by being too hasty.

This is why I am giving the relevant information for vintage radios here. You could find and fix a small fault, and thereby prevent it causing expensive or irreparable damage to other parts of the set when power is applied.

Assessing the Condition of the Chassis

Depending on the layout of the set you may have to partially disassemble it before you can get to the chassis sufficiently to make some meaningful checks. See the Disassembly page for more information.

Remove the back. If the set has a removable plate in the bottom of the cabinet remove this too, to get reasonable access to the underside of the chassis.

Start by give it a thorough look over. You may be able to spot likely problems at this stage, and should be able to get a general idea of the work necessary.

You can generally tell whether the set is largely original and untouched, or has had a couple of previous repairs that look tidy and professional, or has been seriously got-at. The untouched example will probably be the easiest one to repair because you do not have previous repairs and bodges to add confusion. A couple of professional repairs are not generally a problem.


Check that the valves are the correct types. If the type numbers do not match the service sheet or internal label, do not assume they are wrong. Different valve manufacturers use different numbers for the same component, so the valve fitted may be a direct equivalent or a viable alternative (this is where a valve data book or CD-ROM is useful).

Having said that, I have come across cases where the previous owner has swapped the valves around, presumably in an attempt to get the set working! I have also seen sets that are fitted with totally incorrect valves, possibly because the originals were valuable or missing and the seller wanted the set to look complete. These impostors may have had their markings deliberately removed. Such situations are not that common fortunately.

Many sets use similar valve line-ups - so by comparison with the details of similar sets you can get some idea as to whether the included valves seem reasonable. There is much information about this approach and other methods of establishing the type numbers of missing or unmarked valves on Gerard Tel's website (see my Links page for his details).

If the valve markings are missing, there are a couple of tricks you can try to make them show up. Try breathing on the glass to make it steam up, this will sometimes cause some sign of the markings to appear briefly as the steaming clears. Also try rubbing it gently through your hair - some grease may attach itself to the residue where the markings used to be. With experience, you will gradually be able to recognise many common valves just by looking at the innards through the glass.

If any of the valves have what looks like a milky white deposit on the inside of the glass, the vacuum has been lost and the valve must be replaced.

If you have a valve tester you may wish to remove and test the valves at this stage. I don't normally bother unless the set looks like it's had a hard life, has other signs of problems or has been got-at. The main ones to test are the rectifier and output valves; this is where the current, heat and stresses are. Faults in the other valves (with the exception of heater-cathode shorts in AC/DC sets) are unlikely to cause further damage. Don't worry too much if the tester shows the valves as having fairly low emission, chances are they will still work in the set and if not you can change them later. What you are looking for is more serious faults such as inter-electrode shorts, excessive grid leakage current and open-circuit heaters.

Visual and resistance checks

Look closely at any signs of previous repair work or modifications. It is worth comparing the values and positions of any replaced parts with the service sheet.

Look for signs of excessive heat build-up. The high power resistors obviously run very hot, but charred or browned low power resistors should be noted. Check any high power resistors with a test meter. If you have the service information or can read the markings, you can measure the actual resistance, otherwise just check they are not open circuit.

By far the most common causes of problems are capacitors. Many of the types used in valve radios are prone to failure, normally current leakage. On many AC/DC sets, a capacitor is connected directly across the mains after the power switch, and this capacitor will often be found to have blown itself to pieces.

While you have the meter out, check the windings of the mains and output transformers. The actual resistance's are given in the service sheet, but are not important at this stage. We are simply checking that the windings are not open circuit. If the speaker and output transformer are OK, you will probably hear a feint click from the speaker when you measure the resistance of the output transformer primary.

With all the valves in place, connect the test meter between the live and neutral wires of the mains flex. Operate the power switch on the set and note the resistance reading. For an AC/DC set it will probably be somewhere between 500 ohms and 3 k-ohms. For an AC only set using a mains transformer it will be a couple of hundred ohms (the exact figure will be on the service sheet). Anywhere around this area is OK for now. If it is very low (tens of ohms) or short circuit, you have a problem that needs further investigation and correcting before power is applied. See the Power Supplies page for more information.

If it is open circuit you may also have a problem that requires further work. However it is also possible that contacts in the mains switch are tarnished, in which case switching on and off a few times with the mains applied may clear the problem (the low test voltage from the meter cannot burn through the tarnishing but 240V mains can). Check the voltage selector arrangements are working, and on AC/DC sets check the dial lamps. If you can get to the underside of the chassis, a few quick resistance checks should show up where the problem area is. Again the Power Supplies page has more information.

Check the resistance across the HT smoothing capacitor. The meter should initially indicate a low resistance then rapidly increase as the capacitor charges. If it reads short-circuit or remains at a low resistance (anything below about 10 k-ohms) the capacitor is suspect or there is some other fault loading the HT line - again do not apply power until you have investigated further.

Check the state of the mains lead. Unless it is dangerous or has been cut off, I don't bother to change it at this stage (although of course it is replaced later). On AC/DC sets, make sure the neutral wire (black on old UK cables, blue on newer European cables) is connected to the chassis. It is possible that someone has changed the cable and got it the wrong way round.

Assuming the power supply and output transformer seem OK, the most likely cause of a set giving poor results, drawing excessive current and overheating is a leaky capacitor on the grid of the output valve. Some people might like to give this capacitor the benefit of the doubt and initially test the set with it in place, I know from experience that there is about a 95% chance that it will be leaky. So generally I will disconnect one end of it (by cutting the lead) and temporarily solder a replacement in position.

Also if there is a anti-modulation-hum capacitor connected directly across the mains input after the switch (common on AC/DC sets) that hasn't already blown itself to pieces, I will always cut this out of the circuit now because it almost certainly will go bang when the mains is applied. Later on I will fit a Class X2 suppresser capacitor as a replacement, whether mod-hum is a problem or not.

Applying a Test HT Supply

This section could be regarded as optional but advisable. If you have the equipment (such as a High Voltage Electrolytic Capacitor Reformer) you can do some meaningful checks in a few minutes, and if you repair sets fairly often it is worth building one.

Having checked over the chassis, and put right any glaring problems, it is time to apply some sort of power to it. Rather than applying the mains at this stage, which could cause problems, it is preferable to apply a high impedance supply of about 250V DC to the HT rail. This will show up some leaky capacitors, and will hopefully reform the electrolytics.

My High Voltage Electrolytic Capacitor Reformer unit is ideal for this purpose. Connect the negative lead to a convenient point on the chassis and the positive lead to the positive terminal of the electrolytic capacitor that is connected directly to the cathode of the rectifier valve. This is often the red tag on the main smoothing can.

Nigel Hughes provided the following useful tip:

Of course, once you have a working radio, you have access to a DC supply for reforming electrolytics. Just put a current limiting resistor in series with a multimeter and the electrolytic and monitor the current.

Switch the HT supply on, and watch the current reading. It will probably start high (maybe 30mA), and will hopefully drop after a few seconds as the smoothing capacitors charge up. If you are lucky it will drop to maybe 1mA, which is acceptable leakage for the electrolytics. However it is more likely to remain at a higher level and there can be several reasons for this.

Check for potential divider circuits across the HT supply. Most sets do not have one, but a few do. For this test to be meaningful, the lower one should ideally be temporarily disconnected. However this is generally inconvenient so instead calculate the current this circuit will consume (using Ohms law) and subtract this from the meter reading.

If the current reading is still over 1mA once the potential dividers have been accounted for, we need to establish where it is going. The most likely explanation is leaky capacitors.

Leaky Capacitors

Elderly electrolytics often have high leakage currents. These will sometimes improve if they are left powered by the capacitor reformer for a few hours. I prefer to remove the electrolytics from the chassis (if this can be done fairly easily) for reforming if it is likely to take more than a few minutes. This allows each section of a can to be reformed separately, without the effects of other components. (It also allows me to get on with some other work on the chassis). If the leakage is fairly bad the electrolytic could get warm after a while, in which case switch off the power and allow it to cool down again. The maximum acceptable leakage current is about 1mA for each 30uF.

My attitude is if the capacitor doesn't show signs of reforming within 10-15 minutes, or becomes warm, then it is probably not worth the effort of persisting further.

Further checks

With the reformer connected to the chassis you carry out further checks if you feel so inclined. Personally I generally leave this until I have some real power applied to the set, but this depends on your level of experience and confidence, and the value and general condition of the set in question.

Look for signs of resistors or capacitors getting warm (such as molten wax). However, do not poke your fingers into the chassis with the power on! Any capacitors that are getting warm will need to be replaced. If a resistor is getting warm, work out which capacitor(s) it is supplying.

Even if nothing is getting warm, we can use the same logic to establish the cause of the current consumption. Measure the voltage drop across any resistor (or resistive component such as a transformer winding) that is feeding one or more capacitors. If the capacitor is OK there will be no voltage drop.

You can also check other potentially leaky capacitors by this method. In particular I would urge you to check the coupling capacitor to the grid of the output valve (if you haven't already replaced it as suggested earlier). Measure the voltage between the grid and the chassis. If it is even slightly positive, change the coupling capacitor.

If there are several leaky capacitors in a set, the chances are that other components of the same type will be in a similar state. In this case, I would change all of that type, to save problems later.

This website, including all text and images not otherwise credited, is copyright © 1997 - 2006 Paul Stenning.
No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.