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Model 18 Scope Troubleshooting

Fhamre

Fhamre

Super Member
Planning on digging out a Model 18 I have had waiting for restoration for awhile.Before I put it away I determined that all functions appeared to be working except the scope. So now I am hoping to troubleshoot the scope to determine if it is the circuitry ,a dead scope tube or the transformer winding.
Besides the warning about standing in a puddle while probing the 500v circuitry I would welcome any advice on the things to check. I will eventually restore the unit and try to remove all signs of smoke and nicotine from the faceplate(yuck) .Right now I just want to determine if I am looking for a new NOS tube or?
 
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I would start by measuring the power supply voltages, DC level first then the AC component. Make sure you don't exceed what the meter can handle voltagewise!
 
Here are a few pics.
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It looks stock. Judging by the screws that are missing it has been opened up a few times. But I can't see any signs of molestation.
The chassis is stamped 7025. Does that mean it was made in 1970. If so I would imagine that all the 1969 service bulletin updates should have been done at the factory.
 
With the unit plugged in to my dbt I get -350V at 12L on the scope board. With my DMM set to AC where do I put the pos and neg to measure the AC?12K and 12L?
 
Good voltage, that's what it's supposed to be. In the dark with the scope brightness set to the highest setting power it on for 30 seconds and power it off. Do you see the scope swing to the center? Also can you see the heaters on the tube light up, also important.
 
Yes I see the light! On power down the trace went by. And yes I could see a very bright glow from the heaters.
Fixed a 4400 with the same symptom and it was the FETS. I have TIP47's.
 
Those should work, just make sure you get the pinout correct. IIRC the originals were connected with wires, I think the to220's can be installed straight down into the board and fastened to the heatsink but don't quote me, been a while on the pinout.
 
There was something strange about the pinout , I think. looking down at the board it's ECB but the tip's are BCE. Like I said, it's been a while. You may want to go with something with a ECB pinout or get into some interesting lead bending- don't forget to insulate the case.
 
Couple of questions. To get at that board it appears I need to pull the scope.So I have to remove the faceplate,correct?
Looks like a couple of those big 10K resistors are cooking the board. I guess I need to replace all four?
I will have to dig out some square mica insulators,quite likely.There are just wires soldered to the leads so I just have to pay attention, use pictures and my Peak Analyzer and a couple of brain cells.
 
Nope, you can remove the socket from the tube, remove the rubber strap and the tube will slide backward then remove the screws holding the driver board assembly to the chassis. Don't forget about the rubber insulator around the tube to protect it from cooking the dial string.
 
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Thx Brad,good tips.I plan to tackle it in the morning. Hopefully the rubber strap comes off without breaking.
 
Having any tips for cleaning this face plate? It is yellowed from a smoker. Have made some progress but would like to do even better while keeping the lettering.
 
Fhamre said:
Having any tips for cleaning this face plate? It is yellowed from a smoker. Have made some progress but would like to do even better while keeping the lettering.
Click to expand...

Be very carefull! The silkscreening on the 18's is not that great. I have found out the hard way. I have our cad guy at work drawing up new inserts for the 18's face plate. I am going to get a few done in the original finish or as close as I can and some in black with white lettering, just for fun.

As for the scope.. I had a similar issue and it turned out to be adjustment of the scope, I know it sounds stupid.... When I picked up one of my 18's it was advertized with a bad scope. It would not display any trace but I could see it deflect when the power was removed. I followed the procedure in the manual and if I remember I needed to adjust the horizontal and/or vertical gain pots, it's all in the manual. Of course it could be something else......
 
The four scope transistors test fine on my Peak. So far I have left one still installed as the screw does not want to cooperate.I could fight it some more and get it off or just put the originals back in and start looking for other possibly failed components on the board.There has been some heat behind those 3 watt 10K's.11.JPG
 
Those 3 watts are pretty close to spec.
If I put the TO220's in will I have to insulate the screw?
Will be a bit tricky.
IMG_4431[1].JPG
 
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