Often the first stage of any repair and restoration is to remove the chassis and establish what work is needed. With some sets, reasonable access can be obtained without disassembly, but if you are planning to do anything more than a basic repair the chassis will have to come out sooner or later anyway, so you might as well do it before you start. Then you can put the cabinet to one side to minimise the risk of damaging it.

Disassembly should be carried out with care, so as not to cause further damage. It is worth sorting the various screws and small parts into separate containers (or a box or tray with several compartments), and making notes or sketches so that you can remember how it all goes back together.

Access Panels

Some sets have access panels in the bottom of the case, to allow access to the underside of the chassis. If the repairs required are relatively simple and you have no other reason to remove the chassis, removing this access panel and the back may be the only disassembly required. However, if the repairs are major or you also intend to clean or restore the chassis or cabinet, you will find it more convenient to remove the chassis straight away.

Removing the Knobs

It will often be necessary to remove the knobs before the chassis can be withdrawn. This is usually easy, but a little corrosion in the wrong place can cause real problems.

There are three usual methods of fixing the control knobs. Probably the most common method is grub screws, accessible through small holes in the side of the knobs. Sometimes the screw passes through a hole in the shaft so complete removal is needed. The grub screw hole in the knob will sometimes be filled with wax or a plastic screw, this is a safety precaution on live chassis sets and it is imperative to replace the filling.

Julian Weston added:

I solved the problem of the un-removable knobs on some Philips receivers. Trick is to remove the short insulating grub screw and then look in the hole and remove the long steel grub screw that goes right through the shaft! Obviously used on live chassis models but fooled me for a while after returning to repair work after a break of 20 years.

This is one of those things that's obvious when you know, but it can clearly cause confusion if you haven't encountered it before. Julian was lucky to find the plastic insulating screws still in place. In my experience these are usually missing!

If the grub screw will not shift relatively easily, squirt a SMALL AMOUNT of WD40 into the hole and leave it for an hour or so. Often this will free the grub screw sufficiently to allow it to be removed. If you can grip the shaft securely with a pair of grips and twist the knob back and forth, you might be able to loosen it sufficiently to enable it to be removed. Alternatively, twist the knob firmly against the end-stop of the control and see if you can loosen it that way (there is a risk of breaking the control or shaft).

If the grub screw still refuses to budge, or the screwdriver slot is damaged, you may have no choice but to drill it out. This is a last resort however, and will probably result in a badly damaged knob and possibly some cabinet damage too. A couple of visitors have told me about flexible drill chuck extensions (basically a small chuck on the end of a flexible bowden cable) which can be used with a low speed power drill and which allows the grub screws to be drilled out without causing any other damage.

If there is no grub screw hole, the knob is either a push-on type or is retained by an internal screw. Internal screws may be accessible either from inside the cabinet or through holes in the base.

Push-on knobs can be difficult to remove. One old method is to lay the set so that the knobs are uppermost. Wrap a length of strong thin wire around the base of the knob two or three times to form a loop and gently pull the knob off. NEVER use a screwdriver to lever the knobs off, you will end up damaging the case or breaking the knob.

In some cases, the control knobs will be on the glass tuning scale. If so, loosen the chassis mounting screws and move the chassis slightly. If the tuning glass moves too, it is attached to the chassis so it is not necessary to remove the knobs at this stage.

Ian D Edwards adds this observation:

The better designed sets always had the tuning dial attached to the chassis. Cheaper variants had the dial fixed to the inside of the cabinet. Bad news when you had got accustomed to unbolting the chassis and sliding the whole lot out, knobs and dial still attached, only to be greeted with sound of breaking glass. You'll never guess how I found this one out - fortunately I had a spare dial from a scrap set.

An ice-cube tray is ideal for separating screws when dismantling a set.

Removing the Chassis

In most wooden and heavier Bakelite cased sets, four screws or bolts on the underside of the cabinet retain the chassis. The cabinet feet may cover the screw heads. On some lighter and more modern sets, particularly AC/DC sets, the chassis fixing screws may be internal. They may be on the back of the chassis, at either side, with the front of the chassis being supported by appropriate mouldings in the cabinet. Sometimes screws will be found towards the front also.

The previous comments assume that the chassis is in the base of the cabinet. In some cases (again particularly later AC/DC sets), the chassis is mounted vertically towards the front, with the valves facing the back. Screws near the corners and sometimes in other positions normally retain the chassis.

If the set has a PCB (Printed Circuit Board), this may form the main part of the chassis, with often just a metal plate at the front holding the controls. The chassis mounting screws will probably pass through this plate into bushes in the front of the cabinet.

The chassis screws sometimes pass through rubber grommets or similar to isolate the chassis from the speaker vibrations. Like everything with valve radios, there are numerous variations and I can only comment on common arrangements that I have encountered.

I have one set (Murphy U504) where the chassis is simply sandwiched between the plastic front and rear sections of the cabinet, with rubber support pieces. The cabinet is held together with just one screw!

You may have to disconnect the leads from the loudspeaker or output transformer, and possibly remove the dial lamps. Before disconnecting any wires, note their positions carefully. You will need to reconnect the wires to the output transformer before the set can be tested, so it may be better to remove the transformer (and possibly the speaker), leaving all the connections intact. Even in this case it is worth noting the connections in case one of the wires comes away. Sometimes the leads are fitted with plugs or terminals to enable easy disconnection, but more usually, they will be soldered.

With soldered leads, I normally cut them leaving about 1/4" (6mm) of insulated wire on the tag. This acts as an easy reminder as to which wire goes where when reassembling (assuming they are all different colours of course). It is also quicker and easier than poking around inside the cabinet trying to desolder things. Once everything is dismantled it is an easier job to desolder the bits (having made notes first), then clean and tin the tags ready for reassembly.

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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.