Ebay Tektronix 2205 experience and confusions

[---dan]

Hello everybody,

I've been watching Dave's show for some years now and finally followed his advice in getting an oscilloscope. Now, because of financial constraints, I got this 60£ (+22£ shipping) analog Tektronix, which is the little brother of the 2225 that Dave actually did a teardown on some time ago. Starting it up, there's a beam on the CRT, knobs work, the horizontal position is very jittery so that needs cleaning or replacing, other than that nothing obviously wrong.

Also got a cheap 60MHz (10:1) probe from a local supplier and tried it for the first time. I don't have a signal generator so first thing I did was try to use the probe adjust to see if anything will show on the screen.
After adjusting the compensation on the probe, the square wave looks great in my eyes, the time base looks a small amount off, but what I don't understand is why, when using DC coupling does the signal go to negative 500mV. Here is a clip to demonstrate what I'm talking about: https://gfycat.com/LividBelatedDoe. Shouldn't it be above the 0 line?

Next thing I did was build a 555 PWM generator that GreatScott provided on his youtube channel (

).
This results in this showing up on the screen:
https://gfycat.com/DarlingMarriedIrishsetter
https://gfycat.com/FrigidMelodicFeline
Now I don't know if this is a product of the PWM generator or the scope itself, but there are two weird things I notice here:
1. that rising edge/begining of the upper line that keeps moving up and down
2. the horizontal jittering of the whole signal that is happening intermittently
Later on I tried generating a square wave signal using an android app and amplifier connected to the phone. That proved to be a relative success, exept after 800Hz the horizontal jitter shows up again. Any ideeas what these problems could be?
Edit:  forgot to mention another troubling issue here - past 1KHz the visible signal on the scope appears to show as one fully visible graph and another dimmer graph superimposed but with a 180 degree offset. I'll get a picture attached later.

I also attached a normal transformer to the scope to look at a sine wave. This was the result: https://gfycat.com/DarkSlipperyHairstreak. The shape looks a bit off. Tried it with a different transformer, same thing. When I tried it with the phone signal generator, it looked better, so again, I don't know what is causing that.

I now finally realize not having a function generator is a major hurdle in diagnosing issues and calibrating this oscilloscope.


Next thing I did was open it up. Internally it looks quite similar to the 2225.
It's clean and I saw no obvious signs of trouble. I've attached some pictures for you to see.

So, if you know how or what I could do to further diagnose this thing without a signal generator I would greatly apreciate any help. Some questions I am facing may simply be from my lack of knowledge so I'm sure there's something more to learn from this.

[Andy Watson]

but what I don't understand is why, when using DC coupling does the signal go to negative 500mV.

Looks good to me. Here is what the service manual says:

The Probe Adjust circuitry, is a square-wave generator and diode switching network that produces a negative-going square-wave ...

Don't rely on the Probe calibration as a means of calibrating the scope - the probe calibration is there for adjustment of the probe compensation - the amplitude and frequency of the signal is not well defined.

The random changes on the rising edges looks like an intermittent connection.

Nice scope.

[---dan]

Hey thanks!

It seems I missed that part when I looked in the manual. Very happy to know that's working fine.
I was not planning to calibrate the scope using the probe calibration. Just used it to look at a basic signal and adjust the probe. Initialy it was overcompensated and the signal in the video is after adjustment.

The random changes in the rising edges and the horizontal jitter do not seem to be affected by moving knobs or the scope around as far as I could tell. Also things seem consistent after warming up, so at least there's that.

[Andy Watson]

It seems I missed that part when I looked in the manual.

May be it is not in the operating manual (?), I was looking in the service manual:
http://w140.com/tekwiki/wiki/2205
(Under the picture on the right side.)

I would give it a few hours use and "get to know it" before worrying about details. It must be nearly 25 years old - there are bound to be a few niggles