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Model 18 Scope Troubleshooting

Crap, Scrap that last request..... You need to measure the bottom side of R515.. Just above and to the right of 5L to R519 and R522..
 
Bottom of R515 neg to ground is 114V.Pushing mute sometimes sees the voltage drop very briefly but not always.
 
The bottom of R515 (just above and to the left of 5L) and the connection between R519 and R522. No measurement to ground.

If you trace it back you can see that it's at the same potential of the lamp in the LDR.
 
I am going to sack in in for the night..

In the morning I'll check mine and we can see what bktheking's reads since it works.
 
I measure -56v with muting on and tuning dial turned so that the lamp shines bright.
With mute off I read -1v
 
I will measure the circuit this evening... What would be great to understand is:

Voltage (DC) rating of the neon lamp.
Current rating of the lamp

On and off resistance of the photoresistor along with it's current rating.

It may be impossible to get this information because the unit I am working with is no good....

I would also like to see if I have a light leakage issue..

I am going to order some photo resistors and neon lamps and try and make a new one... The parts are dirt cheap..
 
Fhamre said:
I measure -56v with muting on and tuning dial turned so that the lamp shines bright.
With mute off I read -1v
Click to expand...

I get basically the same thing....

In my 18 the mute function has become really weak. The sound is lower when it mutes but I can always hear some interstation noise. How is yours?

I am ordering a bunch of neon tubes and a couple of cells to test out. Hopefully I can get the right combination to work... If we are super lucky it will solve the popping problem.

Should have some parts tomorrow.
 
Last edited:
txturbo said:
In my 18 the mute function has become really weak. The sound is lower when it mutes but I can always hear some interstation noise. How is yours?
Click to expand...

My 18 (which pops when selectors is turned to FM) is the same - the mute reduces interstation noise only somewhat, but I can always hear the interstation noise.
 
I suspect that this is the root cause of the pop...

I am reading a bit more about the photocells now... I am thinking that they may not wear out.. It's the neon tube. This is actually good because the hard part to spec would be the resistive element, it's on and off resistance value.
 
since bktheking has noted his 18 doesn't pop when the selector is turned to FM, it would be interesting to understand how well the muting function on his unit suppresses interstation noise. is muting working properly on his unit?
 
txturbo said:
I suspect that this is the root cause of the pop...

I am reading a bit more about the photocells now... I am thinking that they may not wear out.. It's the neon tube. This is actually good because the hard part to spec would be the resistive element, it's on and off resistance value.
Click to expand...

Can LEDs be substituted for neon to implement a more lasting fix?
 
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