On Fri, 26 May 2017 18:19:52 +0100, R D S wrote:

On 20/05/17 16:07, R D S wrote:
Both tubes in a flourescent are flickering and won't light.

Christ, what a palaver I seem to have provoked!

Don't blame yourself. The "palaver" was largely the result of a "Rod
Speed baiting frenzy" by Fredxxx and tabbypurr most of which I swiftly
passed by unread (I did briefly consider killfiling both perpetrators
but, hey! it was a Rod Speed baiting exercise after all and he's already
in my killfile filter). :-)

Your post did give me cause to track down my "Lamps and Lighting" book
(1972 edition) to check out fluorescent ballast circuits but I didn't see
any twin tube switch start circuit examples (only one each of the
Quickstart , Rapid-start and cold-start transformer circuits, each sans
any 'starter switch').

I'm pretty certain the total of 14 fluorescent ballast circuits shown in
the book are far from an exhaustive list but I can't think where else I
may have seen a more comprehensive one.

Perhaps your twin tube fitting is simply two independently ballasted
switch started lamps wired to a common PFC capacitor? Does the fitting
use two plug in starters? Also, does removing one of the lamps prevent
the remaining one from even starting up or else modify the flickering
symptom if it does still light up?

It's quite possible that it's just a case of both lamps wearing out at
almost the same time. The second lamp to start flickering may well have
masked the flickering of the first lamp failure giving a false impression
that both lamps have either improbably failed simultaneously or else the
problem lies with another common component.

Having checked out what wikipedia has to say on the subject (not as much
as I'd hoped) and a google search, I did see a few examples of twin tube
fittings using switch starting where a couple of 2 foot 20W tubes are
connected in series via a single ballast choke to the mains supply using
two starter switches, one for each tube. To my mind, a rather improbable
circuit but since I saw at least three examples of this twin tube
circuit, I guess it must exist IRL.

If your fitting is using the same arrangement, it will be blessed (or
cursed) with *two* starter switches. Also, this means that removing any
one tube will cause the remaining tube to stop working (they are in
series). The presence of a starter across each tube must make for an
unusual starting sequence under normal working conditions (there's going
to be some interaction between the operation of each starter switch).

It seems that the flickering of *both* tubes only requires one of them
to go faulty, loss of emmissive material at a cathode, either from old
age or from premature failure. BTW, I can't figure out how a *single*
starter can be wired up to start a double tube fitting. If you've already
tried replacing "the starter switch", singular, I reckon you must have
missed one.

The problem could be as simple as a single tube failure. However, in
view of the liklihood of it being age related wear out, I'd suggest
replacing both tubes, keeping hold of the old but still working tube as a
spare for diagnostic testing purposes the next time one of the tubes
starts exhibiting such symptoms (often the result of being used way
beyond its useful 8,000 hours or so service life typical of use in switch
start circuits - 16000 hours being more typical with modern delay start
electronic HF ballast units).

--
Johnny B Good