Although your set should be basically working by the time you get to this section, that's not the end of the job. There is further work and repairs required to make the set safe and reliable.
Some faults may not be immediately apparent by the operation of the set, but could cause premature ageing of the valves and damage to other components.
Reliability - Capacitors
The output valve grid coupling capacitor has already been mentioned, and as I have indicated I would always change this. There are a couple of other capacitors in stressful positions, and serious consideration should be given to changing these too.
The first one is on the anode of the output valve, either across the output transformer or between the anode and chassis (or occasionally cathode). It may be combined with fixed resistors or a tone control pot or switch. There may be more than one capacitor. Because the the high voltage audio frequency signals this capacitor is subjected to, it leads a hard life, and failure could cause further damage. Unless the original capacitor is in very good condition I would replace it. The replacement must be rated at no less than twice the HT supply voltage, and preferably more. For an AC/DC set a 630V DC (400V AC) capacitor is OK, but for an AC only set (which generally has a higher HT voltage) I would always go for a 1000V DC (630V AC) component.
The other one is any capacitor connected across the mains input or across the rectifier. Again these are subjected to large AC voltages and live a hard life. A capacitor across the mains must be replaced with a Class X suppresser capacitor, and it is a good idea to use a similar component (or Class Y) to replace any capacitors across the rectifier.
I tend to replace all wax-paper capacitors on sight, because I know from experience that they will be badly leaky. I am also very suspicious of the Hunts Mouldseal types. However you may prefer to retain as many of the original components as possible. Look at the Capacitors page for more information.
Safety - Mains Wiring
Check the state of the mains flex. If it is not double insulated PVC flex it should be replaced with modern cable. The only exception to this is if the cable contains ballast resistance (this cable is normally thicker than average, and can be confirmed by measuring the resistance of both cores).
If the set is an AC/DC type with no mains transformer, use two-core 3A cable. One side of the mains will be connected directly to the chassis, or via a low resistance. Make sure this is neutral (blue wire) - this way the chassis will be near earth potential, which is safer.
If the set is an AC only type, with neither side of the mains connected to the chassis, I prefer to use three core 3A mains flex, and earth the chassis. I feel this is much safer, and in some cases it can improve the reception and eliminate the need for a separate earth connection when using an external aerial system. Other restorers prefer to use two-core cable, on the basis that the set was made that way.
The cable used must use the current colour code of brown (live), blue (neutral) and green/yellow (earth), in order to comply with electrical safety regulations. It should not be more than 2.5 metres long. Once the set is assembled the free end should be fitted with a current type 13A plug having shrouded live and neutral pins. The plug should be fitted with a 1A fuse (available from Farnell) if you can get one, otherwise 3A. If the mains switch is a single pole type, make sure it is switching the live (brown) connection. If it is a double-pole switch, it should be switching live and neutral.
Make sure the mains cable is securely fixed to the chassis so that there is no strain on the connections if it is pulled or twisted. In many sets the original cable passes through a grommet and then has a knot tied in it to stop it from pulling out. However a knot won't stop the connections being flexed if the cable is twisted. Depending on the size of the hole, you may be able to replace the grommet with a snap-in cable clip (it is generally better to fit these from the inside because they are less likely to pull out). Alternatively you may be able to use a P-clip under a suitable screw on the chassis (such as one of the transformer fixing screws). Or you may be able to secure it with a couple of cable ties to something solid. If you can't do any of this, then tie a knot - it's better than nothing. Make sure the grommet is in good order too - if not replace it.
You also need to ensure that if one of the wires should become detached from the tag it is soldered to, it cannot float about and touch something else (this is a safety precaution in the event that the main strain relief fails). If the set has a double-pole switch this is easy - fit a cable tie around the two cores as close as possible to the connections (you may have seen this in commercial equipment and wondered why - now you know). If the wires go off in different directions, tie them to the closest wire to achieve the same result
Check the condition of all mains wiring in the set (from switch to voltage selector to transformer or dropper resistor etc.). If the insulation is crumbling or the wire has become rigid, replace it if possible. If it cannot be replaced (for example, fixed flying leads from transformers) sleeve it with suitable diameter heatshrink sleeving. The wiring to dropper resistor is a problem because of the heat. Normally it will have heat-resisting insulation and will probably be sound even if it is rigid, in which case leave it alone.
Check in particular the voltage selector - particularly if it is accessible with the set fully assembled. The voltage selectors used could not comply with current safety regulations unless they are fully enclosed and inaccessible without the use of a tool. If it is accessible externally you will either have to relocate it or disconnect it. It is often easier to disconnect it and wire the set permanently to the highest voltage setting (225V-250V or whatever). Any flying leads for the lower voltages that cannot be removed from their origin (such as fixed voltage tapping wires from the transformer) must be carefully insulated. Cut off the bare end, double over about 10mm or insulated wire and cover with two layers of heatshrink sleeving or similar, then fix them out of the way with cable ties.
If the set has a two-pin reversible mains inlet connector (such as the DAC90 etc.) this is not acceptable. Not only can it be reversed making the chassis live, but also the pins can be touched with the connector partly mated. The correct solution would be to remove the connector, replace it with a piece of metal plate through which is drilled the hole to take a fixed cable and clamp. This rather spoils the originality though.
This lot might seem excessive. However we need to ensure that our restored sets comply, as far as possible, with current safety regulations. We will never fully achieve this because of the design of the sets, but we should still do the best we can. If someone is electrocuted by the set you repaired, it would help your case to show that you did everything you reasonably could to make the set safe. Of course making the set as safe as possible reduces the risk of accidents anyway.
This particularly applies if you are selling sets or repairing sets for third parties. Not everyone who comes into contact with the set is as aware of the risks as you - most people will assume it can be treated with the same disregard to safety as modern consumer electrical goods. If you choose to leave these aspects in their original state then that's your decision. It's your set and you are doing the work! I can only advise on the correct way to go about things.
Safety - Preventing access to live parts
This is obviously something you need to consider not only when finishing the electrical repairs, but also when reassembling the set. When the set was made, electrical safety standards were not as strict as they are now, and indeed the public were credited with more common sense than they are now!
There are some areas of the set which would have been adequately safe when the set was made, but due to deterioration of the parts is is no longer so. One area is the back. When new it was flat and probably reasonably rigid, and with it in place the chassis could not be touched unless somebody really tried. Now, with the effects of heat, damp and age, it is probably somewhat flimsy and twisted, and does not afford the protection it once did. It may even have sections missing. Of course we know that to poke fingers in the gaps when the set is on is not a good idea, but a young child who is intrigued by the strange looking object doesn't.
A poor condition back is a problem because there isn't that much you can do about it. You may be able to flatten it by stacking some heavy books on it for a few weeks. You may be able to stiffen it by coating the inside with varnish or a watered-down PVA glue. Certainly any missing sections should be repaired by fixing pieces from the back of a scrap set behind with a suitable adhesive (I use Evo-Stik). Make sure all the screws are in place, and perhaps devise extra fixings if necessary (fairly easy with wooden cabinets but not so with Bakelite and plastic. Any other ideas are welcomed!
While round the back, consider the various input and output sockets (aerials, gram and speaker). On an AC only set with an earthed chassis these should all be at safe potentials anyway. But with AC/DC sets they are often coupled into the circuit via capacitors. The capacitors are intended to block the dangerous voltage that may exist (particularly if the mains wiring is reversed and the chassis is live) while still allowing the signals to pass. Any such capacitors should be replaced, preferably with Class Y suppression capacitors, but any good quality rated at 630V DC (400V AC) or higher should be OK. The value of the capacitors should be as low as you can get away with, and certainly no more than 0.01uF. The extension speaker sockets on AC/DC sets are normally connected to the output transformer and the internal speaker only - there should be no connection between this circuit and the rest of the set.
Around the front we have the control knobs to consider. Again on an AC only set with an earthed chassis the shafts, grub-screws etc., are at earth potential so there isn't too much to worry about. But with an AC/DC set the shafts could be at mains potential so great care has to be taken to ensure the user cannot come into contact with them, even if operated with wet hands. Grub screw holes would originally have been filled with wax or a plastic screw, but this has probably long since been lost. I normally fill the hole with silicone sealant (the proper silicone rubber, not the synthetic stuff sold as bathroom sealant). It is available in white, black and translucent from DIY stores (I prefer the translucent), and can be picked out of the hole fairly easily with a pointed tool should access be required in the future. Make sure the knobs are secure, so they cannot come off. Those fitted with grub screws should be fine if the screw is tight, but push-on knobs are a problem. If we were sticking to the regulations these could not be used (live parts cannot be accessible without the use of a tool) but if they are tight enough that they cannot be pulled off easily or accidentally they should be OK. If they are too lose try a layer of masking tape or a piece of thin card to pack out the flat part of the shaft.
While on the subject of knobs on AC/DC sets, the Bush DAC90 and DAC90A sets (and probably some others) have holes in the bottom of the cabinet to allow access to the knob fixing screws. These holes are large enough to poke a finger into so they must be blocked or covered somehow. Some sort of push-in bung would be a good idea such as the top cover of a collet knob or a blind grommet? Any ideas?