Weird Tektronix 2215 problem: single dot on screen, nothing else works

[Gentlegiant]

Hey all, title is pretty descriptive 

Got this one on ebay last week, it seemed to be able to produce a trace on the listing pictures, but when I got it all it does show a single dot on screen. After negotiating a partial refund with the seller, here I am with a broken scope.

Vertical/horizontal offset does nothing. Tried sending a signal in, dot won't move. I tried every controls, it won't react to anything I tell it to do, except  the focus knob, which has the super useful hability to make the dot bigger or smaller


Anyone know what might be causing this? This is not my first electronics repair, but it'll be the first time I work on a scope, so any advices/insight would be appreciated!

[trevatxtal]

1 Looks like no x or y deflection.
Look in to the x and y supply voltage to the amps.
2  Or the find button is stuck on , this causes the same effect.

I don't have the circuit to hand so unable at present to point in any one direction.
Good luck
Trevor

[wn1fju]

The first step in any test equipment repair (other than a thorough visual examination) should be to check all of the power supply voltages.

I just fixed a similar analog oscilloscope exhibiting the same problem.  The 12V supply read 15V and the -12V supply read -17.5V.  It was a simple shorted pass transistor on the -12V line (the +12V is referenced to the -12V supply).  Once that was corrected, the scope worked perfectly.  I'm not saying that is the problem with yours, but checking the power supply is mandatory.

[Gentlegiant]

Voltage testing today it is then, I'll let you know how it went, thanks guys

[Gentlegiant]

So I tested my voltages, and I think I found the culprit: I'm supposed to get -8.56V on TP500 but I'm getting 2.6V. Service manual tells me to adjust a pot, but when I do I only get 0.2V more or 0.2 less. Definitely not supposed to do that, need to investigate further.
There are other test points that aren't giving me the right values, but they all are pretty close (33V when I need 31.5V, 110V when I need 100V), so maybe once I fix the first one the others will follow. Or not, then I'll have some more work to do lol.

Will update once I make some progress, but in the meantime any suggestion is appreciated

[andy3055]

Check C 972/976 and lift one end of the jumper W 976 to make sure if the voltage goes up (or down since it is the -ve).

By lifting the jumper you will know if something is loading it or if one of those caps are leaking.

[Gentlegiant]

Someone tell me if updating too often is a thing by the way, I'm trying to document the repairs as much as possible to help future generations

I have narrowed to problem down to this part of the board, which is the diode bridge that produces the +8.56 and -8.56 voltage. Top part is working perfect, I get 8.58V no problem.

Bottom part is where the problem is at. I am getting 2.6V on the right side of L972 but -10V on the other. But wait it gets even weirder! I still get -10V when the scope is turned OFF.  My guess is this is because C972 isn't discharging after being turned off, for some reasons I don't yet know. I am able to drop the voltage to 0 when I short it to ground (with a proper discharge probe of course)

I am a bit stumped right now. Are the diodes the problem? Or could it be one of the two filter caps? Could also be the inductor, seeing how the voltage changes by 12V from one pin to the other.


Oh and Andy, W976 don't look like a jumper at all, more like a small resistor or something, like a white diode shaped thingy. By "lift one end" do you mean I have to de-solder it?

Anyway, I'm gonna take a break as I've been on this for the better part of my day. If anyone has an idea as to whats happening, let me know!

[bdunham7]

L972 is open.  Just jump it for test purposes.  If your scope works and you're fussy about things, just replace the inductor. 

[andy3055]

Yes, that is what I meant. From what you have observed, it looks like that C 976 may be leaking. But it is also possible that it is good but something beyond that is the culprit. If you lift one end of the W 976, it will be much easier to determine because, since the cap is still in circuit and any drop like that has to be because of the cap. If not, it is beyond that point more to the right which means it is some other part.

[andy3055]

L972 is open.  Just jump it for test purposes.  If your scope works and you're fussy about things, just replace the inductor.

Quite possible too.

[fzabkar]

I would try reflowing the solder at the inductor pins.

[Gentlegiant]

IT'S ALIVE!!

Well, almost.

Jumping the inductor got it working (hell yeah, thanks bdunham and andy ), I get a trace alright, but the scope won't draw on the lower 1/4 of the screen, it looks like it's clipping the wave. I thought it was the -9.3V rating I was getting, but I was able to adjust it to -8.56V using R946. Maybe I need to adjust another pot somewhere else. I'll get on that next, and I'll continue to update as I progress.

Also, setting CH1 to GND and back to DC causes glitches in the display, like it cuts the V/div by half or something. It gets back to normal if I tap the switch a couple times without changing the setting. I'll have to look into that too but it's not my priority right now.

I've included images of the "clipping" I'm speaking of below. Thank you guys again for your help!

[Gentlegiant]

I would try reflowing the solder at the inductor pins.

That did the trick, no need for that ugly jumper cable now, thanks man

[bdunham7]

The ground switch issue may just be a cleaning issue.

Does the clipping happen on both channels? 

What happens if you just leave the input open with the selector to 'GND', trigger to AUTO or LINE and then move the trace up and down with the vertical position knob?  Can you move it off the bottom of the screen or does it hang up in the same spot?

[Gentlegiant]

I think the same way about the switches, nothing some deoxit won't fix.

Yep both channels are clipping at roughly the same point. I did your test, both channels stop moving with 1/4 of the screen left to cover whether they go up or down. Pic included below.

[bdunham7]

Yep both channels are clipping at roughly the same point. I did your test, both channels stop moving with 1/4 of the screen left to cover whether they go up or down. Pic included below.

I'm not completely clear on what you mean.  Are you saying you can't adjust the trace off the upper side of the screen either?

[Gentlegiant]

Yep, I realise now that I am only able to draw on the top of the screen when I intentionally clip the wave by setting the V/div too low. When the V/div is adjusted correctly it clips at the same point bottom is clipping.

I am including a video to better explain what I mean:



Also is that humming sound normal?

[bd139]

Primary side power supply does not sound happy. Same as the 2235 I had which exploded shortly afterwards as described here: https://www.eevblog.com/forum/testgear/tektronix-2235-repair-thread/

Really not a fan of this series of scopes.

[bdunham7]

Also is that humming sound normal?

No! 

You need to address that immediately before you incur sudden catastrophic damage.  When I work on these, I use an AC power supply that allows me to regulate voltage and see current (BK1655).  You should find a way to measure the input current if you can.  Also, check C906 (be careful, it can hold a good zap) by disconnecting or removing it, measuring its capacitance and leakage current.

[Gentlegiant]

Sounds good (no pun intended) I'll get to work on the hum sound right away

I'll try testing C906 and mesuring the scope input current, alltough I only have a multimeter in my shop at the moment, so I don't know if I can without buying more equipement... fingers crossed for C906 being the problem

A user on another thread mentionned a bad diode bridge could also cause this issue so I'll add that to my list of tests to run

[bdunham7]

alltough I only have a multimeter in my shop at the moment, so I don't know if I can without buying more equipement...

This is not acceptable.  EEVBlog has a 3-multimeter minimum.  Also the justification of 'buying more equipment' is the main reason we are all here.  Of course you need more equipment....to fix your existing equipment, among other things.

[Gentlegiant]

I'll have to get on that too then, I haven't bought any equipement since school, but I guess being one of the big boys mean having 3 multimeters and an extra scope lying around hahaha

Also C906 doesn't exist on my 2215? What cap did you mean? I'm guessing the huge one?

[bdunham7]

Also C906 doesn't exist on my 2215? What cap did you mean? I'm guessing the huge one?

OK, I was looking at the wrong diagram.  C937 perhaps? 

[Gentlegiant]

OK, I was looking at the wrong diagram.  C937 perhaps? 

Yes, this seems more likely, I'll have to do the tests tomorrow since I need a set of torx bits to remove the high voltage shield and my local canadian tire just closed for the day
At least it's on sale at 12$ at the moment so I guess I'm in luck 

[MathWizard]

Consider your self lucky, I wish I had that scope here. So I could put it on the pile. I guess I have a few scopes I could work on right now....1 with a lot of physical damage, 1 with a brunt PCB area, 3 that need real tune up's. But I better learn somewhere before I really tear them apart.


This reminds me I need a bigger bench/shelf because I haven't used my CRT scopes in ages, and I'll appreciate them a lot more now.