Tektronix 2225 checkout and inspection

One of the things that an oscilloscope can do is "freeze" a signal, so that you can look it over and see what the device you are testing is doing.    You can spot normal and abnormal signal patterns if you can visualize the signal.

I took about an hour to perform some very cursory testing of the new oscilloscopes that I scored a couple of weeks ago.  I put sticky notes on the CRT screens for each scope about what I found.  None of them are working perfectly.  Fortunately I have the excellent 2465A scope that I bought off Ebay from an electronics recycling company that works perfectly, below.

The lower-end scopes, with sticky notes.

And the mid-range scopes.

I picked one of the simplest low-end scopes to test today, one of the 2225 units.  I put a signal simultaneously on both the good scope and on the 2225.  Below is what the good scope shows.

Below is what the 2225 shows.  The image didn't capture very well, but there is a lot of noise on the signal. In fact there are 2-3 different wave forms being displayed that overlap one another. 

The traces above all seem to have about the same amplitude (vertical height), but are shifted horizontally from one another, so possibly something is amiss in the trigger circuit.  The trigger is  what causes the trace to begin to re-draw each time it reaches the right side of the screen. So maybe the trigger is not consistently starting the trace at the same point.

When I grounded each channel, there was still quite a bit of noise being displayed on the screen.  This should be two flat lines.

So I took it apart.  Sliding the chassis out of the case took nearly 15 minutes and a lot of 3-in-1 oil.  It was really stuck.

I heard some buzzing coming from the power supply section (bottom right), that also caused the screen to display noise.  I was tapping around the area and the buzzing quit, but that did not correct the noise on each channel.

Next I checked the DC board voltage values at each test point, and they were all pretty close.  I did not check for DC ripple using the good 2465A oscilloscope.  That's certainly a possibility.

Right now because each channel shares the same symptoms, and the buzzing that I heard, I'm leaning toward an issue with the power supply.  This thing is old enough that we might be looking at a bad capacitor allowing a bit of DC ripple to affect the stability of the displayed signal.  I need to do some reading before moving forward.

Below is an image of the power supply for this scope.  I just had a thought.  It's certainly possible that the electrical noise is not passing through each channel.  The noise could be due to a problem with the power going to the screen itself.    

 A while back I purchased a 25 MHz signal generator, and was really annoyed by the interface.  You have to break out the manual to make even simple changes.  However it's really designed to be run from a laptop.  I bought a 64bit laptop late last year, and finally have the software loaded to run the signal generator.  I'm going to try to locate the data cable and see if I can run it through the virtual control panel.  Screen shot below.

Between having a working signal generator and a good schematic, this should make the entire project a lot easier to tackle.

Edit:
Went out to the shop again today to fiddle with the Tektronix 2225 a little more.  There is quite a bit of noise on each channel - even without an input signal.

Below, I took this image of the scope with no signal.  Once again, this should be flat lines, indicating no voltage to the input.  But clearly each channel is showing fluctuating voltage.

Then the power supply started buzzing again, and some additional noise showed up.

I have a lot of learning to do to figure out the source of these issues! 

Edit 2:
Another hour or so of testing.  I put a saw-tooth wave on both scopes.  Not a very good pattern on the 2225 (top scope).  If I can figure out what is causing that distortion, I'll be pretty much out of the woods.  No big deal if I can't, because the scope only cost me $5, but I really want to find the problem as a personal challenge!

Edit 3:
After a lot of difficulty, I was able to get the drivers for the signal generator installed on my laptop, and operate it from the software interface.  What a huge improvement!  What they won't tell you is that you cannot drive both channels at the same time!  What point is it having a two-channel signal generator if you can only use one at a time???  Fail!