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Model 18 Scope Troubleshooting

Fhamre said:
I am going to try and narrow down my pop today. Before I start I would like to know what the switch at the back inside of the 18 is for ans what setting should it be at?
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If you want to turn the unit on while the top cover is off, you have to insert the top plate rear middle screw just to the left of the interlock switch so it depresses the screw down.
 
It's spring loaded. If the screw is out, then the unit won't power up. It could be the person who last serviced your unit disabled the spring so it would stay on without having to put the screw in. Or maybe the spring is just broken. Either way, good luck and let us know if you solve the popping issue.
 
Some units didn't have the interlock switch just as an FYI. If your unit powers up with the switch in the up position then it's not doing its job.
 
Took a few voltage measurements on the MPX OSC 11-1004 board. Get 220V at 5L which should be 200V. The others seem to match the schem.
However measuring voltage on the muting neon lamp I get 155V when I switch to FM dropping immediately to 126V. I am maybe ready the schematic wrong but it looks like it should be around 55V?
 
I would consider rebuilding one but I need to confirm that it is no longer working properly.
Yes, that is a good read.
 
Ok so let's put the theory to test- In a light-dependent resistor the resistance decreases as the light intensity (brightness of light) increases.

You should see 55v, which I suspect would make a neon lamp glow a lot dimmer than one getting 155v applied to it. If this is the case , one of some of the resistors in that MPX circuit has drifted enough to near triple the voltage to that neon lamp- the resistance has dropped too much causing the pop when the FM is switched on- this is my theory. The resistors that make up the circuit are in the 100's of K and one is a Meg, not small values by any means. All it takes is one value to drift and the entire circuit is out of spec.
 
I like the theory. But I need to take a few more measurements. The lamp does not glow very bright so I am concerned about the accuracy of my measurements.
 
I don't know enough about neon lamps to see if there is a difference in brightness with an increase in voltage, i'm reading now.
 
I think you need to measure accross the resistive part of the LDR.

The voltage accross it should drop when the lamp is on.
 
Let's see if we can figure out the function of each.

LDR 501 is clearly for the mute function.

LDR 602 is for the stereo switchover.. The service builiten states to jumper it out.

That leaves LDR 601...

To light it it looks like Q602 needs to be on.
 
Is it possible that the 55 should have a 1 in front of it as in 155vDC? It says neon lamps need at least 90v to fire up. My 18 doesn't pop. If you give me a bit I can bench mine for readings.
 
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