Tektronix 2225 - Sweep length below minimum

[airlomba]

Hello all,

This is my first post in this great forum because I am too rookie to be of any assistance in the topics I have ben following and because I have in hands a Tektronix 2225 oscilloscope that I want to repair.

So, the scope's problem is: the sweep length is below minimum. The scope's service manual states that the Sweep Length should be between 10.2 and 12 divisions. Well, at cold start, the length is just about 9.4 divisions, and after a run of about 4 hours at room temperature, the sweep length reaches almost 10 divisions (9,9), yet below the 10.2 divisions that correspond to the CRT width.

To show you how it looks like, here is a picture taken after a cold start, where one can clearly see that the traces don't fulfill the screen:



Anyone has already seen something like this? In accordance to the Service Manual, I need a Time-Mark Generator to be able to make adjustments in the Horizontal section of the scope. But I don't have such equipment. So I thank anyone who can point me some direction, make some comment or be of any help.

Best regards,
Emmanuel.

[PA0PBZ]

The fact that it changes while warming up points to one or more capacitors reaching end of life. However, you can probably simply adjust R775 to correct the problem for now. If you have any source with a known precise output check if the timebase is still correct.

[airlomba]

Hello PA0PBZ,

Thank you for your answer.
Before I open the unit, I checked the Timebase using a function generator able to supply a 10 MHz reference signal. This signal's frequency was confirmed with another oscilloscope and it is correct.

The timebase looks uncalibrated. Introducing the 10 MHz square wave in the CH1 scope input, one period of the signal spans across 7.8 (seven point eight) divisions only, using a 0.1us/div and x10 magnification in the Timebase.

If I repeat the test applying 1 MHz signal to the scope, in order to disable the x10 magnification while maintaining a 0.1us/div, the period representation now spans across 8.8 (eight point eight) divisions.

Unfortunately, I don't have the tools nor the time right now, to open the scope and tweak the R775 as you suggested. I'll have to do this next week. Meanwhile I'll read again the Horizontal chapter of the Service Manual and get familiar with the schematics.

I'll post the results as I get them. 

Thank you for your help.

Best regards,
Emmanuel.