Stuck! Tank Light Stopped Working And I'm Baffled!

[ChrisSwan]

Hi all,

Just found this forum that looks like the best for help with aquariums, so I'm really hoping someone can help me with this one.

I have a smallish tropical fluval tank that takes 2 x 18" flourescent tubes. I've had it for over a year, and have only had to replace one bulb when it started flickering.

I went on hols for a week and came back to find no lights working (and noticed there'd been a powercut while away).

So I started going through the list of things to check and replace from lowest price up. The current state is:

• The fuse was checked and I eventually replaced the plug
• Both starter units have been replaced
• Both tubes have been replaced (with the correct wattage)
• The ballast unit has been replaced with a new version so the same type (and it's definitely wired in correctly)

So that's everything! The connections seem fine, and my last resort tonight was to use a multimeter on it: it turns out that their is an oscillating voltage going across the pins on the tubes when it's turned on, so it seems like everything should be working. Only it isn't of course.

Can anyone advise on what's left for me to check? It seems like a pretty simply circuit and my last assumption had been a lose wire somewhere, so I'm really confused now that I've found a voltage going across the tubes.

Many, many thanks in advance for any help!

Chris.

 

[Mikaila31]

how much voltage are you reading?

 

[ChrisSwan]

Good question - I hadn't thought about trying to do some maths on it! It seems to osciallate between 0 and 2.7 volts, which is too low right? Since it has a 3 amp fuse I guess it must need 5+ volts to get to 15 watts?

I think you've already got me onto something: my local shop only sells one kind of starter unit (in the battery-style kind), so I hadn't considered that they may have different power ratings. The new ones I bought say they are ~4.65 watts but I'm guessing it should be matching the tube wattage?

 

[Mikaila31]

Is that AC or DC. If you are measuring the current across the pins you should be using AC. I just tested my 15 watt fixture and without the bulb it had an osculating voltage from 30-45V which seems normal for bulb start up. Once a bulb is lit voltage should drop.

 

[ChrisSwan]

Ah - interesting... I did have that wrong and was meauring DC. Measuring AC voltage with the bulb removed I get up 36V on one end of the pins, and about 4V on the other end.

So that doesn't sound too far out from what you get with your bulbs. I would be shocked if I'd managed to buy two faulty bulbs but maybe that's the answer for me? Or do you think there could be some other problem?

Many thanks for your help so far!

 

[ChrisSwan]

Just an update on more findings, although I'm still baffled

I took the tank hood to a nearby aquarium shop, and they:

1. Tested both of my new tubes, which work fine
2. Checked the plug - all good
3. Tried new starter units - still nothing
   

They were very helpful and seemed to have a lot of experience with flourescents playing up, but couldn't work out what the problem was for me. They suggested that it could be a loose wire on one of the connectors, which could cause the voltage to not go high enough.

So I took it home and tested EVERY wire connection and every length or wire. It's all completely intact.

Still nothing turns on, and the voltage still seems low. My last thought is that somehow my new ballast was dead on arrival, but any other suggestions would be massively appreciated: this is two weeks now that my fish have had no light (although they seem happy)!!