Old Tek' Scope Only Showing Mains Noise

[ed_reardon]

Hi all,

My venerable old Tektronix 2211 is misbehaving,  it's not showing anything useful really except mains noise.

I can get a lovely clear 50Hz signal on the screen (about 2v P-P) but apart from that I can't get it to display anything.

The only thing that DOES work is 'scoping out the probe adjust pin on the 'scope itself displays fine. I've tried a few different probes.  I can get nothing useful AC or DC.  I've double and triple checked the 'scope settings. 

It started doing this a few weeks ago then I left it and it behaved again, but now it seems firmly stuck doing this,  just as I'm about to test and comission a little project!

I have had issues with the earthing in this house,  could this potentially be related to loss of mains voltage reference?

Cheers,
Ed

[ed_reardon]

A firm whack and its working again.

Time for a tear-down methinks!

[newbrain]

I can get a lovely clear 50Hz signal on the screen (about 2v P-P) but apart from that I can't get it to display anything.

The only thing that DOES work is 'scoping out the probe adjust pin on the 'scope itself displays fine. I've tried a few different probes.  I can get nothing useful AC or DC.  I've double and triple checked the 'scope settings. 

Most probably, a broken ground connection to input bnc.
The probe calibration oscillator is internally grounded and works OK, anything else is missing ground reference.

[ed_reardon]

I agree,  it's working lovely this morning,  but it's been doing this occasionally for a good long time but last night it took a good whack to free it!

This 'scope is very old, I'm trying to persuade the boss to upgrade it (this is an old 'scope from the workshop that I have at home for doing a few bits of home-work) but until then I shall nurse it along.

It is a very good 'scope when it works, but the switchgear is getting very crusty (the poor thing has been used all day-every day since 1988) so it's time for a major overhaul.

These old Tek 'scopes are built tough though,  I'd gladly recommenced one to anybody working on a tight budget.

[David Hess]

Do both vertical channels show the same noise?

What is displayed in XY mode?

What exactly is it showing?  Is it 50 Hz or 100 Hz?  If it is 100 Hz, then the problem is almost certainly the 2200uF power supply input capacitor.  If it is 50 Hz, then one of the diodes in the input bridge rectifier may be open.

I have never seen the schematics for the 2210/2211 so I am going by the 2225 which as far as I know has the same basic design.

[jh15]

Percussive troubleshooting never hurts

[newbrain]

Percussive troubleshooting never hurts

Think of the children 'scopes!
It seems a sort of abusive relationship: there's violence but also some kind of love.
I don't know whether you can bring a 'scope to couple counseling...

[salbayeng]

I'm inclined to agree with an earlier poster about having a bad earth connection from a front panel BNC.
I've had this happen before.
You can usually verify / work-around this by jumpering from a good ground on the "external trig" or similar rarely used BNC, (or the ground on the "Test signal") to the ground on your probe.
Note that scope probes themselves are notorious for going open circuit on the ground lead.