Elmo GP-E lamp not getting enough voltage to turn on?

[James Culbertson]

Trying to restore an Elmo GP-E projector. Seems to run well but when you pull the knob for the lamp to go on, it wont. Checked the bulb (100w, 21v). It has continuity with about .4 ohms resistance. The plug inside the projector was only putting out about 5-6 volts sometimes up to 8. Put a different bulb in (NOS). It had an ohm resistance of about 1.3...but still neither will turn on. So I'm guessing that the bulb plug should put out 21 volts right? One fuse inside, its fine. Any idea what to check next?

[John Burgess]

Hi James, have you checked the voltage at the transformer supplying the lamp, you should read 21/22 volts. If that’s the case it sounds like a faulty lamp holder. If not the transformer will be at fault . Could luck .
John

[Shane C. Collins]

Have you also checked the contacts to make there isn't a build up of corrosion? I've had this problem before with certain machines. Some contact cleaner usually fixes the issue.

[Steve Klare]

Here's something you can try:

Unplug the machine and put an ohmmeter in both pins of the lamp socket. The resistance looking back through the lamp socket should be very low with the switch on (<<1 Ohm). If it's not, repeatedly operate the control knob through all the positions and see if it starts to fall.

Was the 5-6V with the lamp in or out?

[James Culbertson]

The 5-6 volts was with the lamp out and the multimeter leads in the empty socket and the lamp switch on. Lamp switch off I get nothing. If I unplug the machine and unplug the lamp and use the multimeter (on resistance) I get nothing. Even if I repeatedly pull the lamp knob on and off. Either the resistance is so low my multimeter won't read the number...which could be possible (I'm pretty amateur at this stuff). So I'm wondering if its a switch issue? but then again the voltage does go up and down a bit. Or a transformer? But the machine runs....Thanks for the advice though. Its good to have people give me some ideas to play around with.

[Steve Klare]

A voltage that low indicates either the wrong voltage is coming out of the transformer or there is some huge resistance there that shouldn't be.

An ohmmeter will read somewhere between Zero (a really good connection) and infinity (open circuit). On a digital meter open circuit will often be something like "OL" shown on the display. On an analog meter the needle will be down at the left in its resting position for an open, but as the resistance measured gets lower and lower, the needle will swing further and further to the right. Analog meters will usually read ohms on the topmost scale, usually in red and starting at infinity and swinging rightwards towards zero.

You should be able to see this by taking the two probes and touching them together and taking them apart. That's the basic test for an ohmmeter.

There are also resistance ranges, but we'll take that one step at a time.

[James Culbertson]

Thanks Steve for giving me some advice. Using a Southwire Digital Mulitmeter. So the projector is unplugged, bulb removed and testing the plug I get an open circut (OL) on either way I put in the black and red probes. I did do a bit of cleaning to the interior metal parts in the ceramic plug base. Did test the multimeter just in case by touching the probes together as you said. I get something like 000.3 I don't think this projector has ever been touched on the inside. Very clean.

[Steve Klare]

OK, so now we're talking "huge resistance".

Let's look at the transformer. Keep the machine unplugged. Trace the lamp wiring back to the transformer: try to identify the terminals where lamp power comes from. You may find the terminals are marked "Lamp" and/or "21V". Measure the resistance between those two terminals. We're hoping to see some very low number. All that's going on inside there is heavy wire wrapped around an iron core a bunch of times: it should be close to zero ohms on an Ohmmeter.

Here's the "be really careful" part. Bearing in mind that there will be 100VAC on some other pair of terminals on that same transformer and even in the best case scenario it can throw you across the room (...been there!), plug the machine in and measure the lamp transformer voltage on an AC voltage scale.

When you are done, please unplug the machine.

[James Culbertson]

Steve, First off let me say thanks for giving very simplified information to a newbie like me. Most appreciative. So I have traced the wires from the plug back into the machine. Kinda complex as there are quite a few. As far as I can tell, One wire goes to the lamp switch, another wire come out of that and I think goes into the transformer. The other wire goes into this black, plastic square thing. Two wires come out of that and one of those goes to the transformer.
This is the transformer. No labels. I'm guessing I have to peel off the yellow plastic.
This is the black plastic thing. As you can see, the blue wire (from the plug) goes into this little box and two green wires come off of it. ONE of the green wires goes to the transformer.
So not completely sure how to test this...Does this sound right? Take off the yellow plastic and find out where those wires eventually come in and test for resistance? Or....just test from where the wires go into this little black box thing and the switch?
Again, thanks for being helpful!

[Steve Klare]

OK, this isn't what I expected: my Elmos have a board with terminals the wiring is soldered to.

-we need a new plan.

the lamp switch should be simple: closed when the lamp is on, open when it is not. The full 21V should be across it when it is turned off, and basically zero when it is turned on.

If you unplug the machine, find exposed terminals on either end of the switch, put a meter (AC Volts) on them and install a good bulb, you can power up and hopefully we will see the transformer voltage and see how well the switch works.

This may all fall apart if there is an open circuit somewhere else, but it's worth a try.

[Maurice Leakey]

I suggest that the 21 volts will only show on one of the lamp switch terminals when the switch is turned off.
And when turned on, will show 21 volts at each terminal.
I assume it is a single pole, single throw switch.
Or, did I not read it correctly?

[Steve Klare]

The switch should have all of the transformer voltage across it when open, but (almost) none when closed. I'm hoping the terminals are easy to access.

There are other games we can play with these same two terminals: if James powers down and measures the resistance of the switch, he should see the resistance of the bulb plus transformer secondary (both low) when open, but basically a short circuit when closed. If he removes the lamp and puts a wire jumper from pin to pin on the socket with the switch open, it should just be the transformer resistance.

[Nantawat Kittiwarakul]

If this isn't too different from my GP-E Deluxe I currently have...

I remembered there's (at least) one interlock switch in the film path to prevent lamp on before threading complete, sort of that. The black box in above picture MIGHT be one of that, and might be the culprit.

To be honest since my projector is still fully functional, I dare not touching any of its internal component. This is one of the most complicated dual8 projector I've ever seen - about millions of parts inside. Anything fails, I may have to throw it away due to its (un)serviceability.

[James Culbertson]

Thanks again to everyone with an idea or suggestion. Very useful to join this group. So, I managed to get access to the switch and did the following while checking resistance. Switch turned off...OL. Switch turned on about .4 or .5. Switch on (piece of wire with either end going into the ceramic lamp plug)...same...4 or .5 Next I turned on the projector and tested the switch for voltage. This is kinda weird ...but OFF I get about 7.2 volts. Switched on I get something like. 0.004 volts. So is it a switch? Or do I need to go further back down the line? You can barely see the two wire connectors on the far let at the edge of the paper.

[Steve Klare]

To me it sounds like your switch is doing its job.

This interlock Nantawat talks about is intriguing. I've never heard of this before.

Have you tried running the machine with film? Maybe that would satisfy the interlock and put you in business!