Cleaning the Chassis
Remove the valves, and carefully clean the glass envelopes with a dry tissue (such as kitchen towels). Breathing on the glass - as though you were cleaning spectacles - may help. Take great care not to remove the markings, which are often very soft. If the valves are in a filthy state, you may need to use a little Isopropyl Alcohol. This is available in small quantities from TV shops and Tandy as Tape Head Cleaner, and can be obtained in more sensible quantities from electronic component suppliers such as RS Components and CPC.
Over the years, the chassis will accumulate a layer of dust and grime. The aim is to clean this away without damaging the components. Servisol Foam Cleanser is good for this, but it should be sprayed onto the tissue and not the chassis. A toothbrush or cotton buds are useful for getting into the awkward gaps. It takes time and patience to achieve good results, but it is worth the effort.
The foam cleanser can also be used to clean other metal components, such as control shafts, transformer mounting brackets and loudspeaker frames. Take care to avoid getting it on transformer cores, speaker cones and electronic components.
Aluminium RF cans sometimes become ingrained with dirt, which can be difficult to shift. SolvoAutosol Paste (available from car accessory shops) is good for cleaning these. Take great care not to let it get inside the cans or under the chassis.
The aim is to remove surface dirt and grime, and make the chassis look presentable. We are not trying to polish it or make it look like new. It should look it's age.
Nigel Hughes added:
Radios can be covered in a thick layer of dust. To avoid inhaling vast quantities, it is a good idea to wear a dust mask. If you have a vacuum cleaner with one of those suction attachments like a flat tube used for cleaning upholstery, it is a good idea to use this as a start to suck up the loose dust. However, you must be careful not to damage any of the components with the hard end of the dusting tool. The next step is to loosen the obvious dust with a brush while keeping up the vacuum cleaner suction going.
I find that the most useful tool for dusting is a soft haired varnish brush. Sometimes I use small artist's paint brushes as well.
Grease can be removed from large open areas of the chassis with a cotton cloth moistened with white spirit. Cotton buds, as sold for cleaning into babies awkward little places, are very useful for getting into awkward corners, and a toothbrush can be good for removing the thick layers of grease. It is a good idea to wash the toothbrush before you use it again on your teeth!
Having removed the grease, you will be left with some areas that are corroded or rusty. I have not so far met a radio whose chassis was so rusty as to require complete dismantling. However I can imagine that restorers who wish to achieve a brand new look may feel justified in dismantling of the radio totally. I personally prefer to leave as much original as possible. If rust is only a superficial, it can sometimes be removed by rubbing with a cloth moistened with white spirit. Where rusting is more severe, use of a rust remover such as Jenolite, will be necessary. You have to be careful with this because you really only want to put it on the steel parts. You may need to protect some components such as aluminium IF cans with masking tape. Apply a thin coat of Jenolite, leave for a few minutes and wipe off with a rag. In severe cases you may need to use steel wool with the Jenolite. Finally, clean off the rust remover with a damp cloth, moistened with methylated spirit.
It is a good idea to use a cotton bud with white spirit to clean the insulation on the tuning capacitor and presets. With presets, the white spirit will get under the mica, so go easy, but they dry out soon enough.
Painting the Chassis
If the chassis is badly rusty or corroded, the only option may be to paint over it. Smooth Finish Hammerite is good for this, as it is designed for painting over rust. I would suggest a grey colour, as the more obvious choice of silver looks artificial. You will need to remove any loose rust with a small wire brush or by scraping with a screwdriver.
For more valuable sets it is worth considering completely dismantling the chassis then having it professionally cleaned and plated. This is an expensive and time-consuming job, but for valuable sets it is really the only option.
Printed Circuit Boards
PCBs are more of a problem to clean, because the cleaning products can cause damage to the components. Initially try using a dry toothbrush to remove the dust and grime. Patches of wax can be carefully scraped away with a small screwdriver, but if it is not doing any harm you could just leave it there!
If the PCB is particularly filthy, you can try using foam cleaner. Take care to avoid rubbing the components, since it will remove the markings from some resistors. Cotton buds are useful for this job. The SRBP material is slightly absorbent, so allow it to dry out thoroughly afterwards.
Darren Stewart has this suggestion:
As a test technician for a local electronics company we manufacture PCBs from design upwards we also populate the boards test and repair them. The product we use which confirms to ISO 9002 is available from Farnell it comes in five different forms. The one we use is called Ultrasolve and comes in 400ml aerobrush tins costing £6.12 per tin (it lasts a long time).
A bit of advice when using it spray some on the board and use the brush on the tin to scrub it in. When this is dry (about 20-30 seconds) spray it on again and use a nail scrubbing brush and really give it some stick. This will bring off about 100% of the dirt on the PCB.
To titivate it afterwards use an ordinary paint brush and just rub the excess off.
We have no trouble whatsoever with this product. It takes the muck of the dirtiest of boards, even some we have repaired that have been down mineshafts for about 10-15 years. Try this and you will be surprised at the results even on the dirtiest radio.
I use it myself with the restoration of my radios and believe me it cleans off everything including exploded capacitor residue, transformer wax and so on even metal chassis and Bakelite/wooden cabinets.
The only thing I would add to Darren's suggestion would be to try a small amount of this product in a hidden area or on a scrap PCB first. What you don't want is for it to lift off all the components markings with the dirt. Don't let it get into transformers etc.
If the original back is missing you should arrange an alternative. This is particularly important if the set is to be used, to prevent little fingers finding their way onto live terminals. If you have a supply of scrap sets, you may have a suitable back that can be modified to suit.
A suitable replacement back can be made from hardboard or thin plywood. Once it has been cut to size, drill a large number of 5/16" (8mm) holes for ventilation. In particular, there should be holes near the output and rectifier valves, and any high power resistors. Drilling hardboard gives a rather tatty finish, which can be tidied somewhat with medium grade sandpaper. The back can then be sprayed with black aerosol paint if desired. The result is functional and makes the set safe, but that's about all.