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Marantz 18 Scope Troubleshooting

bktheking

bktheking

Gitter Done!
Heater is working, full voltage is coming out of the transformer. Looking at the tube diagram there is a grid at the end of the tube made up of 4 sides, pin 9,7,11 and 6. These are collector voltages coming off of 4 transistors, q1205,1206,1202 and 1203. 3 of the transistor collector V's measure up around 200v with the tube removed. The 4th transistor, q1202 measured 30v. I'm assuming that this grid needs the full voltage around the tube. I did a transistor check on q 1202 on diode check I got .535 and .696, I measured the same transistor beside it and got 0 and .535. Am I to assume the transistor is the issue? I also measured the large 10K resistors coming off he 200V line, they measured at 10K however the board behind looks darkened. Should I replace those resistors while i'm at it, could this be the reason why i'm getting around 200V when measuring or is this because the tube is out of the unit. The transistor is 34-1006, a marantz part #, does anyone know a replacement?

Thanks
 
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Update to the one channel for distortion, checked dc offset and it's bouncing around between 4 and 7 VOLTS eek, the relay which this has one per channel is kicking in and out like crazy- All transistors are good but one, Q1004 transistor in the middle of the board, appears to be shorted. Test calls for -40v at the base, -38 at the emitter and 4 volts at the collector, this thing is 38V's on all pins. I tested it power off and it is measuring in both directions on the diode setting, the other channel is not.

Edit- Just tested the other channel's board, confirmed 1.9V on the collector, pretty sure this is it.
 
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Well since i'm talking to myself on this one I may as well post some pics, maybe that will peak some interest on this Marantz 18 troubleshooting exercise.

This is this inside of chassis after a thorough blow out with an air gun.

img4589ol.jpg


This is a pic of the heater fired up in the tube, pretty cool stuff.

img4593w.jpg
 
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This is a pic of the front of the unit with the face off. Apart from the 22 series, this is one heavy front face, it's all steel. Each knob is metal as well and is held on with an allen key screw. Apart from what every one says, the front glass is either decals or stenciled, it ain't hand painted and yes it does rub off easily. No LEDS or fuse light here baby, it's side lit with a pair of push and turn automotive type bulbs.


img4596za.jpg



This is half the value of the receiver right here, the holy grail of tubes, telefunken goodness that's common between the 18,19 and 20 receivers.

img4597rz.jpg



It's one small tube!
 
This is the measurement of q1202 being taken on the middle pin.

img4604yh.jpg


Here is the reading-

img4603q.jpg


Here is the reading on the same transistor right beside it on the same pin-

img4605yn.jpg
 
Here is the transistor on the amp board that's in question, notice how they are marantz part #'s.

img4616y.jpg


This is the transistor on the scope driver board, same thing, marantz part #'s.

img4618v.jpg
 
Hey, you have not been alone here. Talking to yourself is probably going to help someone out with these types of units someday. Pretty nice looking receiver btw. :thmbsp:
 
I just think nobody knows about them. I spoke to Terry D last night and he doesn't even know what parts he used to sub to get these back up and running, mentioned something about transistors out of television sets. I've been researching most of the night- looking at schematics from the first solid state oscopes from Heathkit to schematic from ultra rare Pioneer scope receivers. I'm sick as a dog right now and the flu kept me up half the night. I believe the 2sc1507 will do for the scope driver, it's the same one used in the high voltage supply supply of the scoped 4400. Every other early model such as the 19, 20, 115 used a completely different design. The 4400 is close. Terry said that the part that I found that was dead would cause the scope trace to go way off center, the lines that are represented in the diagram are 2 vertical and 2 horizontal traces within the tube. If you don't have the ~150V going to all 4 spots in the tube it will never be centered. Make sense?
 
Thanks Tom, i've been in contact with Sounds Classic out of Chicago, the tech has 30 years of notes on the 18 and has several parts spares he will look at. As for the transistor in the amplifier, can they test good out of circuit but still be bad? Since i don't know the specs on these i pulled it and connected it to my sencore cricket. The dial went off the scale, i've never had one do that so i thought it might be bad. I put it back in the board and retested it on the diode check. Appears to be good again so i fire the receiver back up and again I get 35v at the collector when i should have 4. I test it again in circuit and it appears to be open in both directions again.....
 
I subbed the 2sc828 and replaced it with one I have, the DC issue is back to normal and I still have distortion. As for the scope, I spoke to the tech at Sounds Classic and he pointed me in the right direction, the -500v side of the transformer is shot.
 
I believe the 500v power is developed from the horizontal and vertical voltage through cr1201 & cr1202. There is only a return connection to the transfomer at the 6.3vac CRT heater winding.
camman
 
If you look at the diagram there is a -500V dc run coming from one of the yellow wires of the transformer shown below, mine has 0 V coming out.

500vg.jpg


I was told this was a common issue with these 18's. I can either ship it out and have it rewound or put in a second transformer with a rectifier to drive the -500V which is what I think i'm going to have to do.
 
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So here's my idea. Find a 120V primary and 250-0-250 secondary center tapped transformer. Grab a full wave bridge rectifier, wire it up, ground the positive side to the chassis and run the -500V out to the tube supply where is originates on the rail.
Of course I'll run a fuse and a seperate switch so I can turn the scope on and off.

Will grounding the positive side smoke anything? I need -500 V is the issue.
 
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I hate trouble shooting scopes... and they hate me.... they usually shock me at some point... so the feeling is mutual ....


jk
 
If you are getting the CRT to light the transformer windings are good.
Does it light?
You can use your volt meter and see if you get 6-7vac with unit on or,
check for continuity between these CRT filament windings (with unit off.)
Hopefully someone else will chime in. The transformer doesn't have a 500v winding.
Camman
 
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