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Marantz Model 18

.2 and 69 volt at muting lamp. It goes to 73 and 80 volts when muting off is pressed.
 
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I see tuner signal at 5B on the MPX Oscillator board using an oscilloscope. I tracked the signal to the gate on Q510. I am not sure where to go next.
 
If you look at the scope signals with your o-scope or a volt meter do you see movement when you tune the dial?

When you check 5B what do you see when you move the dial?
 
txturbo said:
If you look at the scope signals with your o-scope or a volt meter do you see movement when you tune the dial?

When you check 5B what do you see when you move the dial?
Click to expand...

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I have not repaired the scope driver board yet.

Tony
 
There are signs of life. If i turn the volume all the way up I have FM in the right and left. I think it's back to MPX Matrix post 15

Tony.
 
There are two signals that run to the scope board that represent the center tuning and the signal strength when the scope switch is in the tuning mode. You can look at the signals one at a time with your scope or even a volt meter and you should see them move when you tune the dial. If you can see the signal strength go up and down (12E) and the center tuning (12C) go from 0 to plus or minus of zero then that's a good sign. If you have two scope probes and put your scope in X Y mode you can display exactly what the scope on the receiver would show.

What happens when you engage the tape monitor switch?
 
I pick up a pretty badly abused parts unit. I was hoping the transformer would be good but I have no 500volts. Could there be something other than the transformer at issue?

Tony
 
You need to get 360Vac between the transformer output and ground and -500Vdc across C11.
If not, the transformer HV wiring is dead. A recurrent issue on Model 18.
 
How common is it for the HV transformer wiring to fail? Is it a question of "when" not "if"? Is there anything that can be done to prevent the failure? What is the root cause? Are all transformers afflicted or did Marantz correct the issue in later production units?
 
RV59 said:
You need to get 360Vac between the transformer output and ground and -500Vdc across C11.
If not, the transformer HV wiring is dead. A recurrent issue on Model 18.
Click to expand...

I just want to be sure it the Red/Green at CR5 that should be 360Vac.
 
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It's hard to believe but the parts unit tuner is working. So as a last resort I have boards to use. I am still going to try and fix the first tuner.

Tony
 
tonebells said:
I just want to be sure it the Red/Green at CR5 that should be 360Vac.
Click to expand...

Yes. Exactly here:

5a75127dd735a5f19e896521da157e5b.jpg
 
monkboughtlunch said:
How common is it for the HV transformer wiring to fail? Is it a question of "when" not "if"? Is there anything that can be done to prevent the failure? What is the root cause? Are all transformers afflicted or did Marantz correct the issue in later production units?
Click to expand...

Difficult to say from where comes the problem because when it happened, it's too late.
But I think it comes from the transformer itself.
It is possible to add a small transformer to overcome this defect.
 
I think as a precaution replacing C11 and CR5 might be a good idea. I'll try and take a current measurement of the circuit and see how much it flows. An appropriate fuse could then be added to protect the winding. Hopefully it's not just an aging thing that kills it.

RV59 said:
It is possible to add a small transformer to overcome this defect.
Click to expand...

Where did you source the transformer?
 
Now that you have a good tuner you can use it to trace down the problem. If you take the detector output of the parts unit and connect it to your good receiver you can rule out the front end, IF, limiter and detector. You can do it in both directions. If you use the input to the detector you can literally do it with an RCA extension cable. I have done this a few times on my stash of 18's. I have had bad RF transistors in the limiter and IF strip that can really kill the reception.

One last thing to check.. I vaguely remember a connection around the stereo threshold pot on the MPX OSC board getting shorted out. If you look at the bottom center of the board there are a couple of soldered connections. One of them was shorted to the pot and it killed the reception or activated the muting or something bad :(. It's a slim possibility.

35914892942_7f29d74afd_b.jpg
 
txturbo said:
So... Did someone scrap this parts 18 because of the HV winding? Sad....
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A quote from the guy I got it from "-The previous owner said: "the unit was working fine a few months ago, I didn´t use it too much, so, i put it in a room. When i tried to test it again, a few days ago, it didn't power on, so i don´t like it, i wont repair it and i will send it to the waste if you dont want it, i have a pionner elite 7.1 amplifier, so, this old piece of **** is not welcome for me and my wife in the house" This is all the history i could have for the unit.".

It was blown up badly. Burn mark from the output stage and all.
 
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