Marantz 18 scope not Working how to check if scope is faulty

Audioo

Active Member
how do i check if scope is faulty or the scope board is faulty on a marantz model 18 receiver. I looked for these scopes online and they are a bugger to locate .Thanks
 
Turn it on for a minute in the dark. If the x or y is out when you shut it off the dot should center at power off. If it doesn't you'll have to check to see if the transformer is feeding the tube its 500 volts.
You may also have to crank the brightness via the pot on the scope driver board. May also be a weak tube.
 
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The power supply electrolytic caps love to go bad on scope models. I'd start with measuring the power supply voltages against the schematic.
 
Measure -500 dc at 12L of the scope driver board- that will tell you if your HV winding is still good. High voltage so be aware.
 
This is the measurement for all the windings on the transformer:

Red to Red 26 VAC CT (GND) 26 VAC support + 36 VDC - 36 VDC
Blue to Blue Estimate 38 VAC CT(GND) 38VAC support - 36VREG after R14 R17 430 Ohm
Grn to Grn Estimate 150 VAC CT (GND) 150 VAC support 200 VDC Stereo Neon lamp.
Red/Grn to CT(GND) 460 VAC support -500VDC for osillocope.
Yel to Yel 6.3 VAC heater oscillocope
Brn to Brn 6.3 VAC for lamps
 
So i understsnd i mesure it from here ( picture 1) to here ( picture nr 2)

Whats 12L?
 

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No sir, download the service manual and you’ll find where 12L is on the scope driver board. Chassis ground is your black wire dmm reference point, red goes to 12L
 
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You posted a question and got the answer and now you won't post what you got only that you found it- how about we do this one step at a time? Did you get the -500 that's supposed to be there?
 
Thats perfect one step at the time,please check photo
 

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Also use the chassis as the ground point rather than a potentiometer shaft. If something for whatever reason went wrong and something shorted those pots are very hard to find.
 
So for now i can not located red to red on the transformer ( there are two burgundy) when i was trying to messure them the fuse blowed up, also i cant faind R14 and R17
 
Your 12L dc is too low, perhaps you have a shorted cap on the -500v dc line. That is your focus , I own a 18 and -500 or close should be the reading. A low reading like 139v and your tube won’t work.
 
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I know it's been mentioned but be very careful taking measurements in and around the power supply and scope. If you can use some grabbers with your volt meter, that would be the best. Shut the power off, make the connections carefully (the caps hold a charge) and then turn it back on and read the voltage.

If you don't have grabbers put tape around all of the exposed metal on the volt meter lead except the tip. This will prevent accidental contact with something you don't want to touch.

Be careful! The 18 can bite..
 
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