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Marantz 18 Scope Troubleshooting

I swung the pots, went from 0 to 40V nothing higher. Desoldered the resistor, getting 220V out of it.
 
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Are you checking Q1202 collector? 30v steady?
Check Q1205 collector voltage and vary horiz centering pot. Does it change voltage?
check base of Q1202 and vary vertical centering pot. Does it show +\- 0.2v?
camman
 
After more test- q1205 is getting 120v off the collector so that just leaves q1202 that's seems like it's a dud, probably worth replacing at this point, everything else checks out,
 
Base is -0.6, emitter is -1.2V collector is 21.5v, I was moving the wrong pots on my previous post, the swing went from 0-40V
 
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That is good idea to start with.
Because Q1205 is operating the only other supplies are the 12H +12v
and 12D -12v must be ok.
camman
 
This unit is going to see the local Marantz doctor, I now have almost no sound out of the left channel and I didn't even touch that side????
 
Still at the service depot, so far I've got the red power switch instead of the black and I picked up a spare working transformer as they are hens teeth. This arrived in the mail today, a reproduction dial glass that is spot on and looks sweet, can't wait to install it.

img2011122000271.jpg
 
I took this unit back as it had been sitting on the bench for 4 weeks (seemed longer) so I figured I'd go round 2. Well I just felt what 500V feels like , yikes, i'm still feeling it as I type this. But for all who thought 500V was fed from the transistors or diodes, well it isn't, it's from the transformer. I bought a replacement power switch and the seller happened to be parting out his 18 with a working scope and since the old school experts that I had phoned and spoke to said I should have 500V I took a chance and bought the transformer. I now have a working scope, it won't center mind you- I now have to figure this one out but when I power off the unit the dot appears!!! The second channel is also back in business, I took the dc pot off, cleaned it and reinstalled.

Transistor 1202 is the culprit, the highest it will feed out is 30V where all the rest are as high as 200V on the high adjustment. I have 200V going into the 10w resistors and the measure within spec so i'm thinking this is the issue.
 
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Almost there, this is with one of the 10K resistors disconnected, I swapped 1202 but i'm thinking now it's 1203, 1203 is pinned @ 200V and won't adjust.

img2012011400316.jpg
 
bktheking said:
Still at the service depot, so far I've got the red power switch instead of the black and I picked up a spare working transformer as they are hens teeth. This arrived in the mail today, a reproduction dial glass that is spot on and looks sweet, can't wait to install it.

img2011122000271.jpg
Click to expand...

Hi,

sorry, did not see this thread before. Just would like to ask you, where you got this replacement front glass from?
Is it a remake? Made of glass or plastic?

Just asking, because mine has a bad replacement installed with bad new print.

Thank you!
 
It's glass and yes it's a remake, I got it from soundsclassic.com in the states. As far as I can tell it's high quality, I will install and post pics at some point, still working out the scope.
 
Here is the dial glass installed, makes a huge difference as far as i'm concerned.

So- I pulled the two driver transistors for the vertical adjustment and stuck in 2 from a television. It pulled the scope center with no ability to adjust. I have a lead on a scope driver board, i'm going to just replace the whole board. It seems I don't have the "cotton candy" effect that i've seen on other 18's, just a line that expands with the sound, which means mono according to the manual. It does however tune in stereo and the stereo light lights up. Is this the MPX or the scope driver board?

img4743i.jpg
 
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Went and had a shower, guess thats where I do my best thinking, thought the transistors might be reversed so i flipped them around, I now have stereo display and vertical adjustment, scope fixed.

img2012011500324.jpg


im a happy man :banana:
 
Hey thanks, i've been on cloud 9 all day, I love the scope action- can't wait to get the distortion fixed so I can listen to this beast. Any thought on what would be causing the distortion on the right channel??? Thanks for the help btw. I swapped the output from right to left and same issue. Before i sent it out i had the scope and signal generator on it, I believe it's before the driver board.
 
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Well this is a closed case, it's now back in business. First pic is the scope on the left channel with a reading taken off of a cap.

img2012011500325.jpg


Second pic, same cap on the right channel, that is the distortion i'm seeing.

img2012011500327.jpg


So I used the scope to find the issue, traced it to the first transistor on the right driver board that the signal is fed into, swapped it out with a new one I subbed and the distortion is gone-sounds amazing!!!

img2012011500328.jpg


Here is the final product:

img4751i.jpg


I guess the scope and signal generator is a real time saver to find issues like this, took me 5 minutes to track it down.
 
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Excellent job. You save the girl.

Agree with you on the sound. I have an 18,19 and a 2270 that I rotate in every so often to keep the caps happy (and me). The 18 and 19 are clearly better than the 70 especially listening to FM but I still enjoy the 70.
 
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