Marantz 18 Scope troubleshooting part deux

[bktheking]

This time it isn't mine, it's Ed Dantes. There's a story behind it, he'll share if he wants to. Basically, things after purchase aren't always what they seem. It was said in the ad that there was an issue with one channel and that it was cosmetically perfect. The pic in the ad showed a working scope, as it should with full stereo seperation.

After hooking it up, 1 channel was dead and 1 channel severe distortion, the scope display showed a line in audio mode tuned to FM and the display dial was missing some of the numbers- clearly the seller was a liar.

Anyways moving right along- the dead channel consisted of 2 smoked resistors and 2 dead outputs. I have marantz 2235 outputs in the one channel for now and it sounds fine, the true 1000 and 1001 motorola outputs are on there way from NY. The issue with the other channel was a blown transistor- Q1002 which was replaced with NTE159. After it was replaced I set all the audio adjustments and it acted normal again.

On to the scope- if you have a receiver that will move horizontal and vertical and displays something properly, the scope driver board is not the issue. On these units the MPX Matrix board is reponsible for sending a right and left audio signal to the FM selector switch which in turn goes through your display selector switches to the scope driver board then to the tube. If one of these is missing you will have issue. The issue with this thing- someone had already attempted to repair it, here's where the puzzle begins-

 

[bktheking]

Once I got the marantz 2235 outputs installed I noticed something odd. The left channel was lower than the right for sound. This lead me to believe that it may have something to do with the fact that I stuck outputs in this thing that weren`t meant for it. Prior to doing so I checked to see what the specs were for them, they were high enough voltage so they should work. I then checked audio with AUX1 through my ipod, perfect balanced audio- it has to be on the FM side!


If you take your oscilloscope and connect to 6h and 6j on the mpx matrix board and look at the signal you should see a right and left channel stereo signal feeding out. When I connected my scope this is what I saw:

Left Channel

Right Channel

 

[bktheking]

Then I moved in for a closer look, somebody stuck a random JFET in where one of the old school germaniums were. One thing about these units- if you see a newer style to-92 transistor installed it`s not factory. These units take the old school space ship looking transistors (as shown on the left). So the sub was no good. While it worked, it didn`t put out a proper stereo signal nor did it produce volume of any sort. After messing around for an hour subbing 4 different p channel jfets I finally found an equivalent- 2N5460 did the job perfectly. The trouble with these units- the part numbers in the manual are marantz part numbers so a lot of the repair going into these units is trial and error.

 

[bktheking]

Now there`s perfect audio display. Once I get a new dial glass for it and the proper transistors installed it`s going back to Mr.Ed to be enjoyed.

 

[Robisme]

Nice job. I was thinking of picking up an 18 that has a nonfunctional scope.:scratch2:

Rob

 

[bktheking]

So long as it's got voltage to the tube, a driver board isn't too bad to repair, transformers are a pita to replace and find.

 

[eddantes]

Once again bktheking proves to be a Marantz fixing maniac! Thanks man!

 

[Overundr1]

Nice work yet again. Inquiring minds do wish to know where you sourced those motorola outputs
-Lee

 

[bktheking]

An ak'r who doesn't post much but he knows everything that goes on. I bought my transformer and spare scope tube and driver board from him. I knew he still had the chassis so I emailed him.

If you need a pair there is a heatsink with them on ebay.

 

[PB_Audio]

As always, nice work, bk.
I love to 'watch' you work.:thmbsp:
Jay

 

[rBuckner]

Great info to share for posterity, thanks BK!

 

[PooPoo]

Then I moved in for a closer look, somebody stuck a random JFET in where one of the old school germaniums were. One thing about these units- if you see a newer style to-92 transistor installed it`s not factory. These units take the old school space ship looking transistors (as shown on the left). So the sub was no good. While it worked, it didn`t put out a proper stereo signal nor did it produce volume of any sort. After messing around for an hour subbing 4 different p channel jfets I finally found an equivalent- 2N5460 did the job perfectly. The trouble with these units- the part numbers in the manual are marantz part numbers so a lot of the repair going into these units is trial and error.

Then I moved in for a closer look, somebody stuck a random JFET in where one of the old school germaniums were. One thing about these units- if you see a newer style to-92 transistor installed it`s not factory. These units take the old school space ship looking transistors (as shown on the left). So the sub was no good. While it worked, it didn`t put out a proper stereo signal nor did it produce volume of any sort. After messing around for an hour subbing 4 different p channel jfets I finally found an equivalent- 2N5460 did the job perfectly. The trouble with these units- the part numbers in the manual are marantz part numbers so a lot of the repair going into these units is trial and error.

Hey BK will 2N5460 work on Q1201/Q1204 on the scope board? I still can’t get my darn scope to come on.