Help with used Tektronix 2235A - no external signal

[NYEngineer]

Hi all,

I decided to try an analog oscilloscope so I purchased a used Tek 2235A on eBay recently. The seller included a probe and power cord, and included pictures of the scope showing the built-in test pattern on both channels and they looked okay. (That is, the voltage and timing seemed reasonably well calibrated and the shape appeared reasonably square.)

I can reproduce the test signal on both channels with the included probe. The included probe doesn't have a ground strap/lead so I then used a 1x/10x probe from a Rigol 1000-series to test it. Again, the test signal works.

However, I can't get anything except the test signal! I hooked it to a function generator set to 1kHz, 0.5V square wave and nothing. Maybe a tiny little ripple at the scope's maximum voltage sensitivity. I tried various frequencies and amplitudes and still nothing. I tried both channels. I sprayed contact cleaner liberally on the outside and inside of the BNC probe input ports. The FG works fine with the Rigol scope and Rigol probe, but nothing on the Tek.

Does anyone have any idea about what I can try next to try to get this working before I contact the eBay seller about it being DOA?

I'm guessing that the ground just doesn't work somehow, but the probe I was using works fine on the aforementioned cheap Rigol.

Thanks,

Doug

[grumpydoc]

I can't immediately see how you could get the scope's "cal" signal displayed but not the FG output.

Are you certain the FG is working?

[128er]

Did you see any beam without a input signal? Set input coupling to GND, Switch to auto triggermode, center your vertical adjustment knobs. Set an adequate timebase like 1ms for a fast sweep speed. If you see nothing on the screen, try the beam find switch.

Search google for "tektronix troubleshooting your oscilloscope". Its a guide with good basic informations

P.S.: How did you connect your function generator to the scope?

[NYEngineer]

Did you see any beam without a input signal?

Yes. If I configure it as you say, with the "Mode" on A or ALT (but not B), I can get one line at a time with CH1, BOTH or CH2 in the Vertical switch. I can't seem to get both flat line simultaneously no matter what I do. Channel 1 is a nice fine line. Channel 2 is a noisy line. On "Both" the line looked like the Channel 2 line.

As I was playing with this, though, I eventually stopped being able to get a horizontal beam on Channel 2. Beam finder shows it's in the lower left quadrant but no display.

How did you connect your function generator to the scope?

Connected the Rigol probe to the oscilloscope. Connected the function generator's BNC to alligator clip to the Rigol probe, red on the tip and black on the ground alligator clip. I have a straight through BNC cable but didn't try it.

The same configuration worked on the Rigol scope.

I'll try that googling. Maybe I should just open it up and see if anything visually obvious sticks out at me.

Edit: I found the referenced document. Quite a treatise. I will have to work my way through it. I thought maybe opening it up and cleaning it liberally with contact cleaner might be a good first step. The scope came from a smoker's home (which was very evident by the smell).

Are you sure the FG is working?

Yes - it was tested before and after with the low-end Rigol.

Thanks,

Doug

[NYEngineer]

I opened up the scope and didn't see anything wrong with it. I didn't open the power supply area, and didn't take apart any of the shielding - just the outer case.

I sprayed contact cleaner liberally where I could see the slide and rotary switches and worked them back and forth a lot. I tried it on one or two push buttons as well. Put a lot on the inside of the connectors and the ground point.

I did a little air can spraying and made sure all jumpers were connected. I used some isopropyl alcohol on the inside of the case to get rid of a ton of grime - probably cigarette smoke because it really reeked.

Finally, I put it all together again and, lo and behold... The external signal was now showing up on both channels. Urgh. Which means either I cleaned something and now it works, or my previous test didn't work. My FG only goes up to 3 MHz for sine and square, and 1 MHz for triangle, but the signals showed up at all sweep rates for both channels. (I usually inverted CH2.) My guess is that there is just years of grime making things not very reliable that needs a good deep cleaning.

I don't know what several of these controls do yet as I haven't read the manual, but one control definitely doesn't work: the Intensity A/B. Intensity A doesn't do anything, and Intensity B changes both beam strengths.

I now have to learn how to further dismantle this thing and pull the front panel off. Once I get that off I can really put on a lot of contact cleaner on the controls and make it look really nice. I will review the manuals I downloaded and see if it describes how to remove the front panel. A cursory look inside didn't reveal how to do it.

Thanks, and sorry for having an unrepeatable issue here.

Cheers,

Doug

[w2aew]

You may want to take a peek at my YouTube channel (link in signature below).  I have many videos that go through the function of the various controls on scopes like these (vertical controls, horizontal controls, trigger controls, etc.).  Also, if you have the time, you might want to view the "Scopes for Dopes" class that I taught a few years ago.  It is linked in as one of the "sticky" threads in the Beginners forum section.

[w2aew]

Oh, and the fact that the A intensity doesn't do anything, and the B intensity does, tells me that you've got the timebase in the B only timebase mode.  The B timebase is typically only used for delayed timebase operations.  Usually, you'd want to be using the A timebase.  Set the horizontal mode to A, and make sure the main timebase control is pushed in.