Replacing Resistors and Capacitors
Any replacement components must follow (as close as possible) the path and position of the originals. In particular this applies to those close to the chassis and those in the RF stages. Connections to the chassis should be made to the original point if possible. The original layout would have been planned and optimised to avoid instability, so it is best not to deviate from this.
Any lead that is close to another component or the chassis should be sleeved with PVC sleeving (or a bit of wire insulation) to avoid any risk of short circuits.
Gary Tempest added the following suggestion:-
This tip was shown to me by a great engineer over 40 years ago. Say you want to replace an axial component. Often this can be tricky particularly if one end is soldered to chassis or under lots of other components on a valve base. This method goes like this.
First, cut the component out leaving a reasonable length of wire at each end - half an inch or more. Now cut the new component to size allowing about a quarter of an inch of overlap of its wires on the stubs of the original item. The next step is to take a small drill of the right diameter such that a small coil wound on its shank will encompass the stub wire and the new component wire. I make my coils out of 24 SWG tinned copper wire. Now finally tin the wires; slip the coils over the overlap and solder. Do not put too much on as otherwise you get an unsightly teardrop. The result is a sound and surprisingly neat job.
After reading this Nigel Hughes said:
A contributor mentions use of little coils of wire to make "floating" joints between components. My Philips 538A radiogram of 1934 uses this method for the entire component wiring under the chassis. Not only does it make quite a neat looking job; it is easy to dismantle. I found it paid to make a new coil rather than reuse the originals when replacing components. 10 turns of wire seems about right.
The Philips used paper capacitors with very strong varnished card sleeves sealed with pitch at each end. It proved possible to soften the pitch at each end with a soldering iron and then tug the guts out of the tube with pliers. Modern capacitors were then slipped back into the original tubes and sealed up with the pitch, leaving the under-chassis view largely original.
Another variation on the basic idea from Alex Warburton:
I've just finished my first restoration, a simple battery/mains portable - Marconiphone T36AB, and to avoid having to partly disconnect the wafer wavechange switch to reach a capacitor I wanted to replace, I was interested in the tip you include from Gary Tempest in the "Replacing Components" section of your site. Having read through it, I felt that the technique wasn't entirely right for the situation facing me because of the very limited space available. I came up with a simpler technique, which I found worked really well. It goes like this. Follow Gary's first step, ensuring you leave enough of the old leads to provide (say) a 1 cm overlap with each of the new component's leads. Now, use the remains of the leads still attached to the old component as a former to coil the ends of the new component's leads. Five or so turns should be enough. Slip the coiled ends of the new component's leads over the stubs of the old leads and solder up. As long as you solder sparingly, the result is very neat.
Faulty Valve Holders
A fairly common problem, particularly on cheaper sets, is poor contact between the valve and the valve holder. This could occur anywhere in the set, although it tends to occur more often with rectifier and output valves. "Murphy's Law" seems to come into play here too - it is invariably the most inaccessible valve holder with the most connections to it that fails. Which is often the mixer-oscillator valve.
Try applying some contact cleaner, then plugging and unplugging the valve a few times. This is often sufficient to clean the contacts.
The problem is sometimes caused by the contacts in the holder losing some of their spring tension. The contacts can often be tightened by pushing a small jeweller's screwdriver between the contact and the body to close the contact slightly.
Sometimes the contact will be broken or weakened by corrosion, in which case there is no option but to replace the valve holder. There are various "techniques" used by service engineers for overcoming valve holder problems, but these are more appropriate to quick repairs than serious restoration.
With some types of valve holder it is possible to extract the contacts from above once the connections have been desoldered and the tag straightened. In this case, it is often easier to replace the defective contacts with those from another valve holder of the same type, rather than replacing the complete holder. I generally use this method - particularly when only one or two contacts have failed. If you don't have a suitable spare valve holder to take the contacts from, you could remove one or two unused contacts from the rectifier valve holder in the set (rectifiers generally have no more than five used contacts) - it's not ideal but better than nothing.
Before disconnecting anything, make a detailed diagram of the connections. When desoldering the connections, be careful not to damage any components with the hot iron. The solder should be removed from the tags with a desoldering tool. If the existing solder does not seem to melt and flow very well, apply some new solder before removing the lot with the desoldering tool. The wires will probably still be held by a very small amount of solder, but can be unwrapped and removed with a pair of fine long nosed pliers.
If the wires are long enough, I prefer to cut then close to the tags. This is quicker, and reduces the chances of causing damage while desoldering. The wires are then soldered to the tag of the new valve holder.
Another alternative approach is to cut the tag off close to the body, then remove the top part and fit a replacement from another valve holder. The tag of the new contact can then simply be soldered to the remains of the old one. This can be done one contact at a time, removing the need for a diagram. It also reduces the chance of damaging components, but the result is a bit messier.
Valve holders are often fixed to the chassis by rivets, which will need to be drilled out to remove the holder. The new holder can then be fixed in place with small 6BA (or M3) screws and nuts. The wires can then be reconnected in accordance with your diagram. If any components have been damaged, they should be replaced.
The pins of the valves can be carefully cleaned with fine wet-and-dry paper, and then sprayed with a little contact cleaner. The contact cleaner may remove the markings, so be careful where it goes! It is better to spray the valve rather than the holder, as we do not want to get the cleaning fluid on the components under the chassis.