On Monday, April 7, 2014 3:03:26 PM UTC-4, wrote:
Believe it or not I obtained one ! However it is from another scope so I am kinda back where I started becasue two scopes are better than one.



Turns out the first specimen had been modified but I didn't really notice it. Beside my eyesight going to the bats (not dogs), I was totally unfamiliar with the unit and the modification was under the aluminum plate on which the vertical output amp lives.



The first one had option 034, which is alot more useful to us than the option 101 on the parts donor. Don't look it up, 034 is the multimeter on top and 101 is an interface for a data acquisition unit.



So, upon installing the IC along with the whole board because I figured there would be the least amount of alignment needed that way, plus who knows what else they did to it, it worked, but not quite fine. Here's what channel A looked like on the internal calibrator with a 1X probe :



https://dl.dropboxusercontent.com/u/...725/shch01.jpg



At first I thought the output amp was all whacked out for some reason, can the plate capacity be that damn different ? Nope, this is channel B :



https://dl.dropboxusercontent.com/u/...725/shch02.jpg



Turns out the brainiac who came up with the modification decided to tamper with the vertical preamp instead of what had been changed. I am a mad modifier from way back so I know better. I also know what DC coupling is, unlike that guy, who came up with this :



https://dl.dropboxusercontent.com/u/.../wingnut01.jpg



All nicely put on a perf (which is OK actually) :



https://dl.dropboxusercontent.com/u/...725/this01.jpg



And mounted well : (or is that mounted, well ?)



https://dl.dropboxusercontent.com/u/.../1725/aa01.jpg



And totally disfunctional. It had no vertical deflection except in chop mode where you could see a haze of a waveform when you adjust the position. Not quite the accuracy I was looking for...



So someone here turned me on to a Romanian website with a possible modification thus :



https://dl.dropboxusercontent.com/u/...725/romo01.jpg



Looked pretty good. The MOSFETs probably didn't exist when the thing was built, and it saves components. So I used it on specimen number two. Of course it was supposed to be this :



https://dl.dropboxusercontent.com/u/...725/orig01.jpg



After looking around, I found a local supplier with some surplus VN88AFs laying around. Looked like pretty low capacitance so I took them.



At this point, other than the usual other problems like it won't trigger half the time and the sweep time knob takes a pipe wrench to turn, the amplitude is low. About 33% low. It is the same on both channels. I am pretty sure I can adjust it there, but I would rather get the gain where it is supposed to be, especially since the position controls just barely take the trace off screen. It should have a bit more range than that.



On this unit, the 1Khz calibrator signal looks pretty good. However a 1Mhz square wave, even though I can't get it to lock right now, appears to have a hell of alot of ringing in it. Also in chop mode, you can see a bit too much that you are not supposed to see.



I figure I should tackle the gain problem first because without that the thing is a toy. What I am thinking at this point is to change the 1,000 ohm resistors to about 1,500. Or should I go higher ?



In the orginal IC, the two 500 ohms add up to 1K, that constitutes the feedback obviously. I find it hard to believe the gain is lower using that MOSFET. I do realize that it takes more voltage to turn it on, but that doesn't necessarily cause that.



Also, the Romanians have their 47 ohm to ground, I tried that and the plate voltages were too low. I tied it to the original land for the IC pin on the amp board and it is alot closer. The print calls for 40 volts and I got 38 and change. Of course I tried jumping out the resistor but as expected it did nothing for the differential gain, just lowered both plate voltages some.



So at this point I have set it aside and figured I'd see if anyone has any ideas. I'll find the right resistors sometime later and see about getting that gain up. Then I want to do something about the ringing but first mayb I'll get it to trigger. I suspect that as I've read on a bunch of these units, the switches are just dirty. I'll give them a good bath of course. Hopefully that does it.



There is another problem that might be best fielded by someone familiar with these units. I have my little board under the aluminum, which is right where part of option 101 goes. I have no room for both. I could try to make the thing smaller so it can go on the top, but since the original was on the heatsink, I figure the modification should be as well. It would be damn tough to get these parts to fit down there.



So since I have no need for option 101, any ideas on how I can remove it and have the thing still work ? It would do nothing without it connected. Not just not a trace, not even a dot at the left side of the screen.



Any ideas, thanks. If you want to comment and tell me I am crazy, fine. I'll just go back on my meds - Budweiser. (probably do that anyway...)


I did like the square wave idea and was thinking about trying it but suddenly work got very busy, so as long as duty calls I have to leave this for a Saturday afternoon. Lenny.