Screen Printers Open - A Forum for Screen Printing and Embroidery

Hello folks!

I'm an old-time participant from the old USSPI boards while the Fresner's were running things...

Question.. Does anyone else have some experience with exposure units?  I made mine about 10 years ago.  The parts were from an electrical supplier who had done a retro-fit of old street lamps for the city, and had the:  110v-480v Transformer, Starter capacitor, mogul (bulb base), and the bulb (1000W Metal Halide).

Last week - my unit would not light up.  Symptoms:  When I plug it in, it does it's "normal hum" for about 1/2 second, then dies.  The bulb does light up for that 1/2 second, then goes out.

Is it more likely to be that starter capacitor? or the 480V Transformer?

I have an el-cheapo multimeter here at the shop - HOWEVER - it does NOT read capacitance - so I have no way to tell whether it's within it's labeled range of 24uF +-3%.

I've checked online with Grainger (since they are located pretty close to my office), to see if they carry a replacement starter capacitor and/or transformer...  They do..   Capacitor is about $45, and the transformer (kit with another capacitor, base, etc) is upwards of $250.  Needless to say - I'd rather it be the capacitor... But need to see if anyone here is experienced with it enough to say whether it's for sure the capacitor or transformer.

Luckily I have a backup screen burner... but it's OLD, uses flourescent tube bulbs, and takes 12 minutes per screen.  I'm REALLY missing my 40-seconds per screen burn times with the metal halide exposure unit.

Thanks,
Rex Smith

Generally speaking transformers don't go bad nearly as often as capacitors.

How old is the lamp? You did not say.

In my years of experience as and electrician anytime a MH light went out we would replace the ballast and the cap if a new lamp did not make it work again. When all that is installed in a light fixture the components get extremely hot and the varnish on the ballast windings can break down over time. We never had any long term luck just changing the cap out so we would also just change it all.

Now with all that said if you know the lamp is good you can just change the cap out and it should start working again. The question is for how long. Being as you are using it for an exposure unit and not to light up the outside all night long you may get a lot of time just by changing the cap.

Ain't it the darndest thing...

I went out to check the Xformer output voltages - and the darned thing worked!  Made the hot Texas sun look like a grey winter day in comparison. ha!

110v in, and 440V out.  The only real thing that I did, was clip the wire-nut leads for the input, and re-exposed & re-twisted the connectors together, and the darned thing fired up after I plugged it back in to check the voltages.

I'm gonna button it back up, knowing that I may need to replace the transformer & cappy later.

Thanks all
Rex

That means you have a connection that is starting to fail from heat as you found out. Check all the connections before you button it back up.