Tektronix 2205 oscilloscope - Dot Only, no line - Page 1
Forum section: electronics / testgear

author: Brian.Stover
author_rank: Newbie
timestamp: January 11, 2020, 02:52:25 am
content: I purchased my first oscilloscope on eBay the other day... A bit of a gamble. A Tektronix 2205 for $90 without confirmation of functionality. Other than power light indicators coming on.The scope powers up but only displays a single dot in the middle of the screen. Both channel exhibit this. It displays this way with AC, DC and GND. If I use the calibration signal, it appears as two dots connected by a faint line. The deflection of the two dots appears accurate to the calibration signal. It can be positioned vertically with the positioning knob. The vertical deflection seems to respond appropriately to a DC input as well (used a AA battery for this).I've viewed hours of YouTube videos on this model and I believe I'm using it correctly (it is my first scope though).I assume I'll need to open it up to troubleshoot further. Just looking for some advice on where to look for the issue. Note that I don't have another scope available for troubleshooting efforts... Just a pair of DMMs.Thanks in advance.

author: andy3055
author_rank: Super Contributor
timestamp: January 11, 2020, 05:03:56 am
content: You can download the opertor's manual and the service manual from th links n the right hand side of this page by TEK:http://w140.com/tekwiki/wiki/2205You should check the voltages at various places as per the service manual to begin with.

author: Bud
author_rank: Super Contributor
timestamp: January 11, 2020, 05:53:39 am
content: Adjust brightness of the dot to minimum while you working on repairing the scope, to minimize risk of damage for the display tube.

author: Brian.Stover
author_rank: Newbie
timestamp: January 11, 2020, 06:10:48 am
content: Quote from: andy3055 on January 11, 2020, 05:03:56 amSorry - should have mentioned that I have copies of the operators and service manuals. Checking the supply voltages per the service manual was going to be my first task. In hindsight, I guess I should have held off on posting until that is done. Maybe this weekend if I have time.Quote from: Bud on January 11, 2020, 05:53:39 amThanks for the tip on the brightness. Makes sense.

author: george.b
author_rank: Frequent Contributor
timestamp: January 11, 2020, 06:14:49 am
content: Does X-Y mode work?

author: Brian.Stover
author_rank: Newbie
timestamp: January 11, 2020, 04:37:45 pm
content: Same behavior in XY mode:With channel 1 on the calibration signal, it appears as a dot in the middle of the screen that does not move. Changing the voltage range settings for channel 1 has no effect.With channel 2 on the calibration signal, it appears as 2 dots in the middle of the screen with a faint line connecting them. Changing the voltage range settings for channel 2 changes the spacing between the dots.

author: george.b
author_rank: Frequent Contributor
timestamp: January 11, 2020, 05:52:02 pm
content: Then we know your sweep generator is probably not the issue. X-Y mode bypasses the sweep generator.I would make sure this assessment is correct and check for the presence of the sweep signal. I would then follow the sweep signal path on this circuitry (with the scope not set to X-Y mode) - from the service manual:Somewhere along that path, the sweep signal gets lost. If you find where is it that it gets lost, then you'll have found the problem. The problem is, then, how are you going to follow it. Normally, one'd use an oscilloscope, but you said you only have this one. Well, I can think of some options.First - you could press a sound card into service in lieu of an oscilloscope. Admittedly not the best thing in the world, and there's a chance you could damage your sound card/computer if you do something wrong (i.e. stick it into a voltage higher than 5V without attenuation), but it could be done.Second - in the olden days, when oscilloscopes were a luxury out of the reach of most repair technicians, they'd use signal tracers/injectors. For this, you'd set your scope to X-Y mode again, inject an audio range signal in the X channel, and then use the tracer to trace the signal from the channel input to the deflection output using your hearing. It's basically an audio oscillator and an audio amplifier. You could easily make one yourself.Third - follow the sweep signal using a frequency meter, or a DMM that has one. You'll have absolutely no clue whether the signal looks like it should, you'll have no idea of the amplitude, but it's better than nothing at all, at least to check whether the signal is where it's supposed to be.Keep in mind that the deflection output is in the order of hundreds of volts. Take precautions and attenuate the input of whichever method you choose (if any) accordingly. Read the detailed schematics in the service manual to estimate what amplitude levels to expect in each stage.

author: Brian.Stover
author_rank: Newbie
timestamp: January 11, 2020, 06:03:28 pm
content: Thank you for the detailed response. I had a suspicion that the horizontal preamp or amp block was the right place to start, but needed someone with more experience to validate this.

author: tautech
author_rank: Super Contributor
timestamp: January 11, 2020, 07:01:38 pm
content: At slow timebase settings the sweep generation (triangular ramp waveform) can easily be visualised using a moving coil meter.Understanding CRO principles of operation helps immensely with fault finding.The sweep generator is a singled ended ramp source finishing as as double ended for each horizontal plate drive which are generally in the order of ~60V in X-Y mode with zero signal input and the voltages on the H plates must be equal for the dot to be centered when in XY mode.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2020, 01:38:46 am
content: The horizontal deflection lead might have fallen off of the CRT pin. Start with the simple things like a detailed visual inspection.

author: Brian.Stover
author_rank: Newbie
timestamp: January 12, 2020, 01:58:17 am
content: That's my hope that it's something simple. My garage workbench is being "renovated". So I have limited time to bring it in the house to troubleshoot. Especially with curious 4 and 6 years olds around that like to "help"

author: Mr Lurpak
author_rank: Contributor
timestamp: October 04, 2024, 07:07:12 am
content: Did you ever find out what was wrong?