How did smacking electron tube TVs help?

Question

In old movies (or new movies playing in these times), I often see people smacking the top of electron tube TVs or screens. Somehow it seems to help to stabilize/sharpen the picture. But why?

Are there any reasons an electronic circuit could go from not working as expected to working as expected from a sharp mechanical jolt? What are the conditions that cause such an issue in the electronics? It would appear that the Atari ST suffered from a similar problem that could be fixed by performing what became known as the "Atari drop" (as discussed in this section on wikipedia) although this is described as being due to loose connections, are there any other failure modes that could be remedied in a similar manner, and why?

Answer 1

This practice is generally known as 'Percussive Maintenance'.

Any touching contacts, for instance in connectors, valves and their bases, and between the wiper of a potentiometer and the track, have a tendency to build an insulating film between the contacts. This happens most readily at higher temperatures, in high humidity, and when there is airborne contamination, especially sulphides. This can introduce higher resistance, non-linear behaviour, or break contact altogether. It can produce intermittent behaviour, changing with humidity, or voltage across the junction.

Sending a mechanical shock through the equipment can move the contacts with respect to each other, disrupting the film, and restoring contact.

In the case of a TV with a loudspeaker in it, sometimes the vibrations from the audio will change the contact state.

High contact pressures, and gold plating, improve the reliability of contacts against this sort of problem. TV valves, because they got hot, were especially vulnerable, and of course they aren't used these days.

Answer 2

It's worth noting that older TV's were constructed with point-to-point electronics soldered by hand, lacking a firm place for the components to be anchored to, such as this image showing the underside of the chassis of a 1948 Motorola VT-71 7" television.

(image taken from the wikipedia page for Point-to-point construction)

From the image alone it is clear why a solid whack to the box could provide enough energy to move loose components and bring them to rest in such a state they go back to working correctly. It could be down to exposed conductors touching, or a poor solder joint being loose and the movement provided from a solid hit moves the offending components back into a position that they work again - by either breaking connections that shouldn't be connected, or by doing the opposite to the components that should be connected.

Answer 3

Bad connections, corroded vacuum tube sockets making poor contact, cold solder joints and so on could sometimes be temporarily mitigated by the judicious application of 'percussive maintenance' techniques.

Answer 4

This was not just a tube-era practice. In transistorized-era displays (ie, where the only remaining vacum tube was the CRT itself) the target of percussive maintenance that could occasionally get more use out of a failing monitor was most likely magnetic components such as transformers, chokes, the flyback, etc. These tended to be physically large and massive so could develop intermittent connection to the PCB as a result of temperature cycling or shock, and also to be composed of parts such as core laminations which could loose mechanical fixation if their cementing compounds failed and audibly vibrate, often to the detriment of image stability.

Answer 5

The contacts in the drum/turret tuners were a big contributor. [

Answer 6

I am no expert, but I remember when I was a kid, doing this to my PC 486's 14" monitor (brand was Leading Edge). at some points of time, first appeared to be randomly, the display was going bananas, showing something like C64's loading screen colors but with much higher frequency (it did not matter whether I was in the black DOS prompt or a colorful game!). The only solution I knew as a kid was to punch the damn thing, and after few punches everything was back to normal.

Later in time, even kicking the damn thing was not helpful, so I called my dad to have a look, he found out that the 15pin RGB at the monitors end is lose from inside the cabinet, soldered it back again and it was fine and dandy again.

I have no experience with old CRT TV's, but my guess would be the punching momentary fixed the position of some lose passive components e.g. caps/resistors/inductors or wires. not sure even if a calculated punch could tell electrons whether or not show clean or distorted picture on the screen.