Tektronix 2225 troubleshooting update

Update:  I set the noisy Tektronix 2225 oscilloscope aside for a while.

Here it is with both channels grounded, showing plenty of garbage on the screen. Both traces should be completely flat.

The reason I set the first 2225 aside is because I couldn't safely use a separate oscilloscope to probe it.  There are a number of points on the circuit board that you can check for signals to help determine where the problem lies.

 I did not want to start poking around inside this thing using a good oscilloscope without first taking some electrical precautions. 

I needed a way to prevent a possible screw-up from blowing up the good scope.   If you watch the video below, this fellow explains how a minor error with a grounded oscilloscope and a grounded item under test can lead to big trouble.

I do not want a brain fart or inadvertent electrical contact destroying my oscilloscope, so I ordered an Isolation Transformer.  This will add quite a bit of safety, both for the oscilloscope, and for my own safety.  Personal safety will be particularly improved while working on older radio sets.

Below is what I purchased.  The left two outlets electrically isolate the hot wire through a 1:1 transformer.  Due to electrical code requirements, however, the ground connections are not isolated - they pass through from the wall outlet to the isolated outlets.   This means that anything plugged into this is NOT ground isolated.  To prevent a ground fault path from anything that is plugged into the Isolation Transformer, I will need to lift the ground lead to the isolated outlets. 

To be clear, it's normally a good thing for electrical equipment to be grounded.  That way if there is an electrical fault that causes the hot wire to contact metal on the case, you don't get shocked.  Instead large amounts of current goes to ground, and the fault then trips the feeder breaker or blows a fuse due to excessive current.

For the purpose of testing electrically faulty equipment though, the above scenario is not desirable.

Anyway, since I couldn't do much troubleshooting with the first 2225 scope, I moved to the second one.  I used electrical contact cleaner in all the potentiometers and rotary and toggle switches - all the moveable electrical stuff behind the faceplate knobs.  The knobs went from really stiff and gritty to being quite smooth and light to the touch.  This stuff is NOT cheap - you don't want to spray it around like WD-40 onto a rusty bolt.  This little 5 oz can of DeoxIT cost over $20. 

DeoxIT works amazingly well though!  I had thought that this second 2225 scope was as messed up as the first one, but apparently, the variable resistors (AKA: "potentiometers" or "pots") and/or switch contacts merely had poor connections.

Below, the second Tektronix 2225 scope, showing really clean calibration square waves - looking great on both channels!

I'll calibrate this one, and then use this $5 oscilloscope to troubleshoot and repair the others. 

The vertical separation of the top and bottom of each wave corresponds to the voltage, while the horizontal separation corresponds to the frequency.  Calibration is what ensures the displayed wave is the correct height and width for the input voltage and frequency.

The 2225 is an entry-level scope, but it's not too bad.  It can resolve frequencies up to 50 Megahertz - which is still only about halfway up the FM radio band.  But it's a Tektronix, so probably a much better build than the competitor's entry level scopes.


 Anyway I'd prefer that if I make a mistake, I'd much rather blow up a $5 scope, rather the $200 one below.