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Help with DC Lamp Wiring Head Scratcher....

Rob_Feature

Rob_Feature

Rob Feature
Hiya,
So I'm working on an old 16mm film projector (that part's probably not very important). In the projector there's a "exciter" bulb. This bulb lights up and through the magic of 'optical sound' provides a signal to the amplifier in the projector. Again, the specifics aren't real important.

Here's the weird thing I can't figure out..maybe someone has some experience that can help:

The bulb doesn't light up (I've tested the bulb and it's good). I turn on the projector which sends power to the bulb socket (about 4v DC). The weird thing is this: When I put my multimeter on the socket, I put my positive lead on the bottom pin, and my negative lead to ground (which is the socket base). When I do that I get a NEGATIVE DC reading! If I switch them around I get a proper positive reading around 4v.

But the bulbs this projector uses (GE BAK 4v exciter bulbs) wants power the opposite way (and the way I'd expect): Positive to the bottom 'pin', and negative to the body. My testing with that configuration makes the bulb light (hooking up a battery directly). So it's almost as if this projector is wired backwards!

More crazyness: When I first plugged this projector in, I was seeing an indicator the bulb was working. At some point during my cleaning, etc, it went out.

Is there something that can happen on a PCB (bad caps, something else) that could cause the DC pos/neg to switch in a way that I'm seeing when measuring voltage on the socket?

I can't work this one out...any help is appreciated. Thanks!
 
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Is there a diode or something in exciter bulb/lamp that allows it to work only one way?

If you use your multimeter to check the bulb/lamp, do you get the same/similar resistance readings either way the test leads are applied, or no?
 
petehall347 said:
what voltage does it measure with the lamp installed ?
Click to expand...
Hmm...this seems like something. Without the lamp i measure almost 4v. With the lamp it drops down to about 13mv (ie. nothing). What's that mean?

For the record I've tried 2 different bulbs (both known working) and have the same issue.
 
whoaru99 said:
Is there a diode or something in exciter bulb/lamp that allows it to work only one way?
Click to expand...
Sorry, I'm an idiot. I never considered actually testing the bulb the other way....both ways of wiring work. :) So, forget my noob question about 'backwards wiring'. I think petehall347 is onto something...I just don't know what yet. :)
 
Also, I sort of mentioned it in my original post: But the power for this light comes out of a PCB. The caps look original. I'm really hoping I don't need to recap it, but as I think about it, that seems like it could be logical (?)
 
If this lamp is run on DC, it is fed by a power supply somewhere. Not only caps but also rectifier diodes can fail. I'd be checking both on that power supply board.
 
voltage dropping is the clue here ..
you proved lamp should be ok so there is a bad connection in the circuit .maybe a simple corroded fuse holder or loose wire or poor solder connection ..
 
toxcrusadr said:
If this lamp is run on DC, it is fed by a power supply somewhere. Not only caps but also rectifier diodes can fail. I'd be checking both on that power supply board.
Click to expand...

If something was failing on the board, would I still get the initial 4v without a light bulb? It seems like that would be either present, or not...but it's there without a bulb.
 
Possibly
High resistance (impedance?) connection . The meters high impedance wouldn't show the voltage drop. Due to the low resistance in the filament it becomes apparent
 
Thanks for everyone's input...I'm frustrated enough for today so I may take a break on it for now. There was nothing obvious (loose/corroded wire, fuse, signs of burnt components, etc) so I'm kinda stuck until/unless I want to get really deep into tearing it apart. I may do that eventually...but for now I guess I'm stuck with watching silent films on it :)

Thanks everyone...the input is appreciated!
 
i am wondering whether there should be any voltage at lamp when its not producing any sound ? ..
i just read the patent for it . it doesn't go into much detail .just mentions deflectors .
have you got any manuals for it ?
 
Reading the service manual (and understanding none of it). See it here. It's a Bell and Howell 1592.

The exciter lamp should be on as soon as the power goes on. There really is no on/off for that circuit, it just has power or doesn't from what I'm reading.
 
page 38 should get you in the right places ...
looks like something around the main switch or pc board .or amplifier switch .
 
Now if you just could win the lottery and had a guy you could hand that to and say "Tom, check out this board. And have Jeeves bring the car around."

Alas, it's pretty much all 'yours truly' until you win the lotto. :biggrin:
 
How old is the projector? I have an old Devry which used high frequency AC to power the exciter lamp... electrolytic caps at the time were expensive and they didn't want poorly filtered DC to get through and modulate the sound track with a bunch of hum.
 
Fascinating. So they used AC at a frequency higher than would be audible? What would that be on a period projector sound system?
 
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