Tektronix 2215A square wave issue - Page 1
Forum section: electronics / repair

author: Waterdog
author_rank: Newbie
timestamp: March 06, 2025, 04:07:53 pm
content: First post here. Have been restoring old tube radios for a few years now, using a RCA VoltOhmyst WV-98C VTVM, Fluke 1911A freq. counter and a B+K 2050 Signal Generator. It has served me well for the old stuff but I had purchased a Tektronics 2215A in good working order from a local Electronics repair shop to step up to better accuracy for FM etc. I then stopped for a few years before learning to use it. Recently got back into it and started learning more about it, reading lots here and other sources. I did a start up and a probe check with it. Channel 2 (Y) gave me a nice Squarewave with my 10x probe. When I checked Channel 1 (X) with the same probe, I got this asymmetrical squarewave. (See pics) I tried different probes, played with the probe compensator, and still have the same form. I then tried the Wavetek 180 on Channel 1 and got what looks like a better Squarewave. I'm a total newbie with O-scopes, so I'm reaching out to find if I have problem with the scope. Is this a malfunction of the scope? Is this adjustable internally? Is this just me being a inexperienced idiot? Is there a way to do a Autoreset with this unit? I have studied the Service manual, read thru forums and watched videos on repair, but cant seem to find a solution matching my issue. Any thoughts? Thanks in advance for any help.

author: Grandchuck
author_rank: Super Contributor
timestamp: March 06, 2025, 05:48:55 pm
content: Usually, probe compensation needs adjusted with that waveform distortion. Perhaps the scope itself has an input problem on one channel.I thought it was the probe but then I reread the post and the same probe was used for both photos.

author: BennoG
author_rank: Frequent Contributor
timestamp: March 06, 2025, 06:39:33 pm
content: Different probes were used (see photo's)The generator created a nice square so it is probe compensation on the gray probe.Benno

author: Grandchuck
author_rank: Super Contributor
timestamp: March 06, 2025, 06:46:16 pm
content: I thought so at first, but to quote:"When I checked Channel 1 (X)with the same probe"

author: bdunham7
author_rank: Super Contributor
timestamp: March 06, 2025, 06:51:41 pm
content: The photos don't show the same probe being used for the bad waveform, but the OP said he tried things several different ways and perhaps those photos just show us the resulting waveform. ALso, keep in mind that the signal generator will have a very low 50R impedance so if there is something like excessive input capacitance on one channel, it won't matter nearly as much when your source is 50R instead of the 9M you get with a 10x probe.The first thing I'd suggest is changing the input selectors to DC. If the problem persists, the first test I'd do is to take ONE of the probes, set it to 10X and connect the probe end to the signal generator and then connect its BNC to CH1 and CH2 successivelywith nothing else connected to the scope. If there is a clear difference between the two, then there is some variance in the inputs.Also, if the OP could tell us the exact model numbers of the probes and whether they were calibrated on CH2 or have been left alone.

author: fant
author_rank: Contributor
timestamp: March 06, 2025, 09:11:56 pm
content: If the problem happens only in the AC position, but on DC position is OK, it is the series input capacitor inside the oscilloscope used to reject DC that lost capacity.If it is on both (dc and/or AC) could be a capacitor on the signal path.Open the scope and with the generator on channel 2 and the probe on the channel 1, follow the signal path of channel 2 on the channel 1.Mandi

author: Waterdog
author_rank: Newbie
timestamp: March 07, 2025, 02:36:09 am
content: Thank you all for your input so far. Sorry if I was lacking in some info. I used 2 Tek P6122 probes, 10Mohm 11.0pf, 1.5Meter and a Tek 221 and a Tek HI- Volt probe, swapped them all around and adjusted each of their Compensators, same wavy squarewave. I left all settings as you see them. I also tried every different combination on the panel to see if it would change. Always remained the same. Could adjust all different ways, but unable to adjust the square to a symmetrical shape. I did try switching DC, same result. I plugged a probe into both channels Hi and Lo from the Wavetek and into both X and Y Channels, Had a slight change in one pattern. Would that be that they were not both LO channels.? When I used the Wavetek, I could generate the square wave you see in the 3rd pic down. I'm not sure that the probes are the problem, based on trying all 3.... Is there a fine tune pot or adjuster internally to adjust or compensate to factory? I feel thats it either an internal adjustment or part failing... but I'm just a newbie. (looking for emoji that is wearing a beanie with a propellor on it!) All thoughts are highly appreciated. Great people on this forum!

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2025, 04:26:33 am
content: Tek 221? You have another scope to help with testing?Instead of trying everything every which way and lamenting your inexperience, lets try a structured approach. This way when you are done, not only will your scope be fixed but hopefully you'll know why!First, lets eliminate confusion by picking just one of the probes (I'd recommend one of the 6122 types) and putting the others away out of reach for now. I've posted an excerpt of the schematic for your scope so we can all reason through this together. What appears to be happening is that your square wave is being low-pass filtered. See here for a basic explanation of how that all works:https://www.electronics-tutorials.ws/rc/rc_3.htmlThe most likely explanation is that there is too much capacitance present at the input of the affected channel. It's not apparent to me exactly where this excess capacitance would be. Since using the Wavetek as your source gives you a better square wave, it seems to me that this excess capacitance would need to be in the passive front end before the amplifiers. So let's do a few tests to track this down.Start with one probe and as I previously posted, set it up connected to your Wavetek (however you manage that) and a 1kHz signal at about 2Vp-p. This will be about how it is configured in your photos except you should use the HI output. Connect the probe BNC to CH2 (the good one?) and adjust the compensation for an optimal square wave. Then transfer the BNC to CH1 and confirm you see the distorted wave. Both channels should be set to 1V/div (10X) and DC. Make sure the red CAL knobs are turned fully clockwise until they click. Please report back, preferrably with photos. Note whether the waveforms are distorted and the apparent amplitudes. Do not adjust the settings to get a better picture.Once you've confirmed that the issue is as we expect, then you'll need to test what happens if you feed both channels the exact same signal. To do this, use that BNC tee and two BNC cables (I see one, I hope you have another) and connect each BNC cable to CH1 and CH2 and then to the legs of the tee. Then put the BNC connector of the probe on the center connector of the tee, leaving it connected to the Wavetek. You should get a signal on both channels and while it may not be much of a square wave, it should still be good enough for our purposes. Note whether the waveform on both channels is very similar or if it appears more distorted on CH1. Then switch the CH1 input only to GND and see if the signal on changes and if so, how much. Go back to DC on both channels and if they look very similar, unplug the BNCs one at a time from CH2 and CH1 and observe the respective changes on each signal.This shouldn't take long and after you do it and report back, I'd suggest redoing the test at different amplitudes. The 1V/div @ 10X (0.1V/div actual) has the 10X attenuator engaged, so perhaps increase the amplitude on the Wavetek to the maximum and test the 10V/div @ 10X (1V/div actual) range and then with 500mV on the Wavetek and the 500mV/div @ 10X (50mV/div actual) range. This will test all three attenuator settings.