Fluorescent fault

The fluorescent tube in our garage has started playing up.
It fires up and is OK for a few seconds, then goes off, and re starts within a second or two.
The starter was replaced around 3 months ago as the tube struggled to start, so I thought it would now be the tube which must be around 20 years old. Exact same problem with a new tube.
That kinda suggests it might be the ballast, but could a faulty starter cause this sort of problem or is it effectively out of circuit once it has done its job?
TIA, Midge
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PS - forgot to add, it carries on with this going out and back on randomly every few seconds.

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First - just try a new starter.
A couple of years ago, I bought two 60W fluorescents (complete units) from a large DIY shed, and installed them in the garage. Within a couple of days, both did exactly as you describe. As I happened to have a pack of three starters (from a pound shop), I swapped the originals, and they've been fine since.
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Ian

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Just buy a new fitting, they're only a tenner or so, from the above it sounds like the ballast as you say but once you've changed the bits you have changed it's hardly worth expending more time on it.
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Chris Green

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What tube length and rating is it? Does the starter match?
Faulty starter, faulty tube, bad connection somewhere, and for 8' tubes, mains voltage too low or temperature too low can all be the cause. Ballast is most unlikely to be the cause.
If the ballast is old enough to have a voltage selection jumper, try reducing the voltage setting by 10V.
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Andrew Gabriel
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Hi Andrew - it's a 70W 6ft tube. Garage is integral to the house and draft proofed too so doesn't get cold enough to be a problem I don't think.
Seems unlikely the new tube has the same "fault" as the old one, so my assumption is that at least is OK. Starter is 4-80W rated.
I did notice just now that the old tube has something loose inside it. Looks like an oval metal band as far as I can see. Wonder if that could have shorted something and blown starter or ballast? I might get another starter first and try that.
I'm thinking it isn't loose wiring as it seems to get less frequent the warmer the fitting gets, but I'll check that.
Cheers, Midge
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Hmmmm tried a new starter I bought this morning and that behaves exactly the same. The odd thing is if I remove the starter in the brief period the light is on, it goes stable - and likewise when I plug it back in, it will start going off/on again. So I'd have concluded the starter was at fault if it didn't behave exactly the same with the new one.
This also suggests we can probably rule out any wiring fault.
I've been testing solely with the new tube, and I've established that the piece of loose metal in the old tube is some kind of shield which is normally fixed around the filament.
Any other thoughts before I get a replacement ballast/fitting?
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