Applying Power for the First Time

This can be a nerve-wracking experience! Hopefully nothing too dramatic will happen, but there is always the risk that you have missed something. You are about to apply high voltages to components that have been dormant for years and may not have been in great condition before then. The tests outlined previously should have found those that are likely to cause us the most problems, but you can't be too sure and should take great care at this stage.

Series Lamp Limiter

The use of a Series Lamp Limiter to limit the current in the event of a fault is very strongly recommended. If the set is drawing around the correct current the lamp will scarcely glow and will only drop a few volts, so the set will be working on close to the normal voltage. If the set draws too much current the lamp will glow brighter and drop more voltage, leaving less for the set thus reducing the risk of further damage being caused by whatever fault is causing the excess current consumption. It gives you enough time to do a couple of checks then switch it off.

Preparing to Apply the Mains

Assuming you have worked your way through the suggestions on the Initial Checks and Tests page, you are now nearly ready to apply the mains! However, there are a couple more things to check first.

Check that the dial lamps are OK and the correct type. In some AC/DC sets, the lamps are in the heater chain or the mains neutral line and if they are faulty or missing the set will not work at all. Working dial lamps are a good visible reminder that the chassis is powered, however remember that non-working lamps do not indicate that the chassis is safe to touch!

Reconnect the output transformer and speaker if you disconnected them previously. Make sure any speaker switching arrangements are set so that the internal speaker is connected.

Check that the mains voltage setting matches the supply voltage in your area. Normally you would set it to the highest setting, because our new European compromise 230V supply is still nearer 245V in many areas of the country.

Finally, fit the valves if you removed them previously, making sure they are in the correct positions and firmly in the sockets. Remember to connect leads to the top caps.

Applying the Mains for the First Time

Connect a test meter between the positive terminal of the main HT smoothing capacitor and the chassis. It is worth making up a set of leads with 4mm plugs to fit the meter on one end and insulated croc-clips on the other end. Set this to a DC voltage range of at least 400V.

Turn the power switch on the set to the on position, set the volume to about a third, the waveband to MW and the tuning towards the low wavelength (high frequency) end where there are likely to be some stations. If the set does not have an internal aerial, connect a few feet of wire to the external aerial socket. The set probably will not work straightaway, but you can hope!

Now arrange some sort of safe mains connection that you can switch on and off easily. This should be protected by a low value fuse, ideally 1A. Keep your finger near the switch when the set is on - so it can be switched off quickly. Try to arrange things so you do not have to lean over the set to reach the switch; indeed you may want to be a few feet away until you are confident that nothing dramatic will happen!

If you have built a Series Lamp Limiter, connect this in the mains feed to the set, and switch the lamp into the circuit.

Valve Heaters

Switch the power on for just a few seconds. Hopefully all the valve heaters will start to glow, and the dial lamps should illuminate. The valve heater is in the centre of most valves and can be usually be viewed from the top.

If any valves heaters do not glow, or some are brighter than others, the reason should be investigated. Valves are fairly robust, but the heaters must not be overrun for any length of time or the cathode will be damaged.

If one valve does not glow it may have lost it's vacuum, not be making proper contact in the socket, or have an open-circuit heater. In an AC/DC set, this valve or the one before it may have a heater to cathode short circuit. This fault could also cause one or more valves to be excessively bright. If in doubt, get all the valves tested.

Note that the valves in a battery set will probably not glow visibly. The same situation will also occur if battery type valves are used in a mains or mains/battery set. Also, some valves are coated or have metal cases such that the innards cannot be seen. In these cases, one has to assume that the valve heater is functioning correctly unless further tests prove otherwise.

On an AC/DC set, the dial lamps may initially be very bright or very dim, depending on the circuit arrangement. When the set has warmed up they should be reasonably bright, but probably not full brightness.

If nothing happens, the power switch may not be switching reliably. It is fairly common for the internal contacts to become tarnished. Often, operating the switch a few times with the power applied is enough to burn through the tarnishing and get the switch working. Because the chassis is still essentially in unknown condition, this MUST be done using some sort of insulation between your hand and the switch shaft. Either fit a plastic knob and use that, or operate the switch with pliers having well insulated handles.


If all the heaters are OK, leave the set on for a little longer and watch the HT voltage reading closely. I normally switch off as soon as the HT has started rising (a few tens of volts) and watch how quickly it drops. If it returns to zero as fast as the meter can respond then the smoothing capacitor may be leaky, whereas if it takes a couple of seconds to drop the capacitor is more likely to be OK.

With the power on, after maybe ten or fifteen seconds the HT should start to rise, and will reach a maximum of perhaps 250V to 350V after a further five or ten seconds. The voltage will then begin to drop again, by between ten and forty volts as the output valve warms up.

Check the service information for the correct voltage on the cathode of the rectifier valve. The actual voltage can vary by about 20V either way, but any greater discrepancies should be investigated. It will often be a bit higher than indicated in the service information, particularly if you are using a digital meter. If it is too low, the output stage may well be the problem area. However if you are running the set through the Series Lamp Limiter all the voltages will be a bit low (around 10% to 20%) because the supply voltage is a bit lower due to the drop across the lamp.

If you have a digital meter with a high input impedance, you should measure the voltage directly across the control grid resistor of the output valve. It should be virtually zero. If there is a positive voltage here, the coupling capacitor is probably leaky, or the valve may be faulty.

If you are really lucky you may hear something from the speaker - but do not worry too much if you cannot. If you can tune in a station and the sound quality is OK, there probably is not too much wrong!

Once you are reasonably happy that nothing dramatic is going to happen, switch the lamp limiter out of the circuit so that the set is working from the full supply voltage. Some sets will not work correctly from the reduced voltage supply via the lamp limiter (typically they will work on part of each waveband only because the local oscillator doesn't like the lower voltage). Also, valves should not be run with incorrect heater voltages for an extended period of time. However the purpose of the unit is to help us confirm there are no major problems in the power supply and output stage. Once this is done the unit is no longer needed.

Quick Checks

If the set does not show signs of life, all is not lost. A few simple tests and observations may help to narrow down the faulty section.

If you get a loud humming noise from the speaker, or the stations sound like they are broadcasting from under water, one of the main electrolytic smoothing capacitors has probably failed.

Listen closely to the speaker for signs of life. If you can hear some sort of hum or noise, albeit faintly, the power supply and amplifier are probably doing something. If it is completely silent, check the connections between the output transformer and speaker, in particular the switch or whatever is used to disable the internal speaker. The primary of the output transformer may be open-circuit.

Turn the volume right up and touch a screwdriver blade on the centre tag of the volume control pot. If the amplifier is working you should hear a buzzing from the speaker. This would confirm that the amplifier and power supply are alive.

If there is background hum but no buzzing when the volume control tag is touched, the fault may be in the audio preamplifier stage (the stage between the volume control and the output valve). The anode load resistor for this is generally 220K and will often be found to be open-circuit.

With the volume turned up, operate the wavechange switch. If there are healthy crackles from the speaker, the IF and detector sections are probably alive. Confirm that the set is definitely dead on all wavebands. If some wavebands are working, the fault is narrowed down to those components or circuits that are used only on the faulty bands.

On VHF sets the VHF band will often be dead while MW and LW work OK. The usual cause is low emission valves on the VHF tuner assembly (either a UCC85 or ECC85 dual triode in later sets, or two separate pentodes such as EF80 or UF80 or triodes in earlier sets).

If the IF seems OK, try connecting an aerial or a length of wire to the control grid of the mixer-oscillator valve. If this produces some sort of noise or even stations, the connections and coupling between the valve and the aerial socket or ferrite rod aerial may be suspect.

If noise is heard which alters in note and volume as the set is tuned across the band, this may indicate that the local oscillator is not working.

What Next?

If your set still doesn't work, hopefully you have some idea from the tests mentioned above as to where the fault lies. You can then read the relevant section (either Power Supplies, Output Stages or RF and IF Stages) to help you track down the problem.

Once it is basically working, refer to the Further Electrical Work and Repairs section for details on how to make your set safe and reliable.

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.