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Marantz 18 Scope Troubleshooting

Only the heater filament lights up otherwise nadda. i'm getting the 6vac for sure. I was told that the reference point 12L which is visable in the diagram is a direct -500V from the transfomer. I tested it at the rail where it connects to the wire going to 12L, 0Volts, the other yellow wire read 12V, I believe this is split for the heater voltage.

Just spoke to terry, he verified it is 500 AC coming out of the transformer then goes through a diode to convert to DC.
 
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Camman said:
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
Click to expand...

If this were the case, why does the diagram show the arrow going away from the transformer at -500V instead of going to it. The filament in the diagram for the scope light is the orange wire - pin 3, it ties into the 500V supply which doesn't even go near the other side of the board, comes in right off the transformer. I appreciate your input.

Now looking at the diagram the diodes are pointed in the direction of 500V, if it were the feed they'd be the other way around, maybe you are on to something!

Remember I said in the beginning Q1202 didn't show proper voltage and tested bad, could this be the drive that i'm missing?
 
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Highest it went was 180V, It was connected to 2 other caps, the two top silver ones.


Here is a pic, you'll see the 2 yellow wires going to the rail with the diode for the green wire that feeds to the board where it's supposed to have -500. Is the diagram wrong or am I just envisioning things. 2 different tech's, one who knows the 18's and has been servicing them since the 70's told me -500V out of that one wire, how could he be wrong?




img4623k.jpg
 
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i am not too clever with power supplies to be honest ...next i would check diode cr5 ...
and the voltages on the other caps .. maybe measure from earth to other side of cr5 ..
 
I checked all diodes when I was in there and they were all good. The diagram looks like the 200V supplied to the board is to power the transistors and fets, -500 would power the tube.
 
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The diodes together create the 500v.
So, with the lack of proper voltage two each diode in there isn't enough juice.
Hang in there and get the transistor changed and we will see if things improve.
Don't think a new transformer needed:yes:
camman
 
Camman said:
The diodes together create the 500v.
So, with the lack of proper voltage two each diode in there isn't enough juice.
Hang in there and get the transistor changed and we will see if things improve.
Don't think a new transformer needed:yes:
camman
Click to expand...

Camman, if you are correct then you are the man. I was always intending on changing it before even looking at putting in a transformer , I even have a sub # that I can use. I just don't understand why the diagram shows 500v out from the transformer, arrow is pointing the wrong way!
 
i would have expected 500v across the cap . i don't know how much current is involved here ..if its very little then a cap might be shorting and taking it down to earth ..i only say this as i had a similar problem with a guitar amp ..it was a bad silver foil cap that apparently don't go wrong .... i would check the rest of the circuit that the 500v should come through .. sorry if i got this all wrong ..its not easy without having something in front of me and studying it in detail ..
 
I agree Pete, but like Camman said, with that transistor smoked i'm not getting the voltage if this is the theory that`s correct.


Other good news, just found a 5.6k 2 watt resistor that's smoked , the one that connected to the collector of the transistor I replaced. That should take care of the distortion on the right channel. I also ordered new dial glass today, this unit is gonna look slick.
 
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What number is that resistor?
I don't see it on my schematic. Mine shows 10k 3watt connected to the collectors.
If C1201& 1202 are paper caps, I would replace them before proceeding. Might be the reason the transistor and resistor smoked as petehall 347 mentioned.
camman
 
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Camman said:
What number is that resistor?
I don't see it on my schematic. Mine shows 10k 3watt connected to the collectors.
If C1201& 1202 are paper caps, I would replace them before proceeding. Might be the reason the transistor and resistor smoked as petehall 347 mentioned.
camman
Click to expand...

C1201 and 1202 are ceramic from what I see.


Moving right along with the resistor theory, that 5.6k is on the amp board, the reason why i`ve still got distortion on the right channel, it`s feeding 32V to the transistor instead of 4V. I checked the other channel and it`s a normal 4 volts coming out so this leads me to believe the resistor is toast.

Now get this- I just went down and unsoldered that transistor on the scope driver board and tested it to be sure, turns out good. You bring up the 10k 3 watt resistors, each one of those are connected to the collector as you say, i`ve got 200+ volts going into each resistor, 2 of them are feeding the correct voltage to the 2 transistors that are working (approx 130V), as it turns out, the inner ones R1203 and R1220 are feeding only 30volts to the inner two transistors, maybe this is why she ain`t workin! The board underneath those resistors is a cooked brown, `something tell me this receiver was used ALOT. The reason these 2 failed and the other 2 didn't, these 2 are not only responsible for driving 1 hor and 1 ver in the tube, they actually power the tube, way more stress on these 2 guys.

img4625bh.jpg
 
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PC board is browish due to heat from the resistors over time. The resistors don't look bad. Best to go ahead and check them. When they installed them originally they should have left an air gap to allow better cooling.
Can you post a photo of C1201& C1202 .01uf 1kv?
If they are ceramic, probably not the problem but, if paper, they are most likely leaky.
camman
 
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I believe that cap is in the photo. If they don't look bad then why low voltage. The diagram shows 200V going in to all 4 of these resistors, one would think a high voltage would come out the other side to match the collectors they are feeding.
 
Ohm them out and see what you get.
You are right, looks can be deceiving!
The cap in photo is ceramic. They very reliable.
 
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The voltage will very according to the verticle/horizontal pots position.
Try adusting them and see if the voltage varies at the collector.
The transistor conduction / voltage varies with the pots.
Camman
 
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