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Two weeks ago, I found and began testing a pageant AV-12E6 sound projector. The projection mechanism works, and the amplifier also works when using an external audio source. However I noticed that the exciter lamp wasn't lit and there was no voltage getting to the lamp socket. I found a wirewound resister (R-26) on the schematic was completely dead and I replaced it. However, when I tested with a multimeter I appear to be receiving almost 30 volts instead of 6v at the lamp socket. This is an improvement over having nothing before but I don't know where to go from here. Any ideas what would be causing this to be so much higher than expected?
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Hi Bobby,
If I interpret your schematic correctly, DS-2 is the exciter lamp. It has a resistor in series with it. The lamp has a low resistance, so when it's in-circuit, it will load down the voltage because of the current through that series resistor. A voltmeter has very high resistance, so the current through the series resistor will be tiny, so the loading effect will be tiny as well and the voltage measured will be higher than with the bulb installed.
-so it's possible you will get your 6V once you put the bulb back. (Then again if my theory is wrong, you may create the first projector with a flash-bulb! -normally such a camera-thing!)
Questions: Did you match the resistance of the new wire-wound with the old (dead) one? Was it the one between the lamp and CR-5 and CR-6 in the schematic? (I can't read the reference designators very well.)
Welcome! -
Hey!
Here's a little estimation, (Strictly for laughs...You get your laughs your way, I'll get mine my way!)
R26 in the parts list is 25 Ohms
A 6V, 6W exciter lamp passes 1Amp. 6V/1A=6 Ohms
The voltage on the lamp will be the ratio of the lamp resistance to the total resistance times the voltage up at the top.
SO: Vbulb=6 Ohms/(6 Ohms+25 Ohms) x 30V= 6/31x30=5.80V
-certainly in the neighborhood!
The sharp-eyed will notice R18 in the schematic and ask if that resistor in parallel with the lamp will load down the voltage even further. The answer is "yes", but in the parts list it's 500 Ohms and in parallel with a 6 Ohm lamp it's going to lower the lamp voltage something less than 2%.
It's a lightbulb, not an atomic clock: we can live with it!Comment
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Isn't R18 also connected to the ground, not just the bulb? Also, I tested R18 and it came back with a resistance of 498 ohms so it is functioning perfectly.Originally posted by Steve Klare View PostComment
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Yes, R18 is grounded on one side, and connected to the bulb terminals on both sides.
To me, it's as if they are providing a discharge path for those capacitors that are also in parallel with the bulb. Since it's under 30Volts I'm not clear on exactly why, but it's there anyway.Comment
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Sir, I was skeptical, but it appears as though you were correct! The bulb is lit solid and the current draw did in fact cause a sustainable voltage drop! I got a new bulb in today after I accidentally blew the old one up a bit ago trying to get it working in the first place.Originally posted by Steve Klare View PostLast edited by Bobby Gunnels; January 04, 2024, 04:13 PM.Comment
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I'm glad to hear that!
As I've grown older, I may have become skeptical about some things I used to be pretty sure of, but at least I can still count on Ohm's Law!Comment
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