I’ve just watched a guy spend more than a week fighting with an old valve radio, trying to pin down a fault that kept appearing and disappearing while worked, yet failed to manifest a definite symptom that could be traced and tackled.
When it appeared, it killed the radio completely, making it impossible to carry out any sort of fault-finding procedure.
After all his efforts, I think he was seriously considering abandoning the effort, as nothing he tried worked. In fact, the only thing he seemed to be able to reproduce was provoke it by poking around near the base of an octal valve, but despite testing the valve, and even replacing it (with another tested valve), the problem still appeared and disappeared at random.
It took a THIRD replacement valve to restore operation of the radio, and suggest the cause.
The valve was similar to the one shown below, and metal cased.
Incidentally, did you know you have to search for pics like this using the term ‘tube’? When I started with ‘valve’, all I got was bases!
His final analysis, NOT verified by dismantling, was that one of the wires from the glass envelope inside the metal casing had either not been properly connected at the factory, or had broken over the years, and was only making intermittent contact inside the big pin in the octal base – which accounts for the sporadic operation as he was poking around in the area of the base.
My hidden fault
Ironically, I just suffered a similar fault, when I couldn’t work out why an old mechanical time switch I just picked up in a charity shop had been working fine, then appeared to stop – even though I could hear the motor running inside.
There’s really almost nothing in these things, just a synchronous AC motor driving a short gear train, rotating a ring carrying pins that operate a switch which turn the mains on and off to a connected device.
I recently started using one of these to make sure hazardous items (with their own time switches) actually did turn off when expected, in case the internal switch failed (it’s Chinese and cheap).
The problem was easy to spot – with the worm gear on the AC motor sometimes failing to engage sufficiently with the worm wheel. However, this was hard to see, since the worm gear was still INSIDE the worm wheel, just not enough to actually engage with, and drive it.
Initially, I could find or see any reason for this issue, as the degree of movement between the gears meshing and failing to mesh was almost imperceptible. The motor is tiny, being about a cube of about 1 cm on each side, not counting the stator pieces and coil, mounted off to one side.
Like the guy with the radio, I could reassemble the timer and find it was working just fine, then it would just stop driving if I tapped it – and vice versa.
The motor was held in place by a single tiny screw, which tested tight and secure, a finding that initially stopped further investigation here.
Eventually, I decided it would be worth the effort of unsoldering the motor from its connections and removing it for more detailed inspection.
There was a circular mounting boss moulded into the casing, with the motor secured in place by a screw which was threaded into the centre.
This boss had cracked and split along its length, AND cracked at its middle, so more than half it was separate, and was stuck inside the corresponding opening in the motor.
While there was still enough of this boss left on the case to hold the motor, and the remains expanded and locked the screw when it was tightened, the reality was that the motor was NOT secure, and could rock very slightly on the remaining stump of the broken boss – just enough to allow the worm gears fail to mesh, and not drive.
Fix
I couldn’t see a quick fix for this.
Broken original moulded features are hard to repair at the best of time, as the homogenous structure is impossible to reproduce with any strength. Adhesive has very little area to work on, so bonds are usually very weak, and parts usually just break as soon as any load is applied.
I could just have epoxied the motor in place, which would have worked, but I was concerned about where it might settle. Given the fact that just rocking on the original broken boss was enough to lose drive, I didn’t want the motor bonded into place, unable to be adjusted.
Eventually I drilled a clearance hole through the remaining part of the boss, and used a tiny screw, washer, and nut to clamp the motor in place, leaving the option of tweaking the location if the worm drive failed to mesh – but it was fine, so this was a win! 😊
Although the screw was metal and the head was outside the case, I wasn’t worried as the shape of the case means it cannot be touched when the time switch is plugged in and live. Apart from that, there is no wiring to come loose inside, as all the connections are made via solid metal bus bars. Plus, I dropped a spot of glue on the nut securing the motor to the screw, to stop it coming loose, and acting as an insulator from anything conductive that might come loose inside the case – although there isn’t anything.
‘New’ version
Surprisingly, I could not find a single pic of this timer online, just this modern clone, which is almost the same (looks as if all they did was rotate all the insides 180°), and probably copied from the decades old Smiths original.
Still on sale from Amazon for about £10.