Gould OS255 Oscilloscope trace question

Thread Starter

nsipkes

Joined Nov 30, 2020
17
Greetings to all. I am a very new user of this forum, so please excuse any missteps. I have an old Gould OS255 scope which, with the exception of some very noisy switches and pots, seems to work quite well. There is a relatively minor issue with the trace. If the trace is positioned near the top of the screen it is quite level. Positioned mid-screen there is a downward dip at the left. Positioned at the bottom of the screen the trace dips at the right. This behaviour is present on both channels and is seen regardless of input (even when set to ground). Am I liable to be looking at a CRT power supply issue, or something a bit more insidious? I am not particularly familiar with scope innards. I'm hoping to use it as a backup to my main scope, a Tek 442, which works perfectly when in a good mood (maybe a later post...) Appreciate any suggestions. Thanks, Neil.
 
Last edited:

MrChips

Joined Oct 2, 2009
23,584
Welcome to AAC!

This is normal with older scopes and not much to worry about.

A number of things can cause this to happen.

1) Stray magnetic fields from surrounding ferro-magnetic objects such as a metal work bench.
Try moving the scope to different locations and see if it makes a difference.

2) Earth's magnetic field. Again try rotating the scope for a different orientation.

Look for a small hole with a trimpot adjustment on the front or back panel which allows you to adjust trace rotation.
 

Thread Starter

nsipkes

Joined Nov 30, 2020
17
Welcome to AAC!

This is normal with older scopes and not much to worry about.

A number of things can cause this to happen.

1) Stray magnetic fields from surrounding ferro-magnetic objects such as a metal work bench.
Try moving the scope to different locations and see if it makes a difference.

2) Earth's magnetic field. Again try rotating the scope for a different orientation.

Look for a small hole with a trimpot adjustment on the front or back panel which allows you to adjust trace rotation.
I tried trace rotate, but that just turns the overall shape around the midpoint.
 

AlbertHall

Joined Jun 4, 2014
11,319
The trace rotation pot won't fix it if it dips in different directions at different verical positions. My favourite would be stray magnetism within, or external to, the scope.
Does it change with different timebase speeds?
Does it change with the scope upside down?
 

Thread Starter

nsipkes

Joined Nov 30, 2020
17
The trace rotation pot won't fix it if it dips in different directions at different verical positions. My favourite would be stray magnetism within, or external to, the scope.
Does it change with different timebase speeds?
Does it change with the scope upside down?
Time base has no effect. Will try flipping the scope, amusing as that sounds. Thanks, Neil.
 

Alec_t

Joined Sep 17, 2013
12,095
Sounds to me as though there is some interaction between the vertical and horizontal deflections. Perhaps a drooping power rail could cause that? Are all reservoir caps in good order?
 

tautech

Joined Oct 8, 2019
205
Without diving deep into the scope armed with the schematics and some test equipment I suspect some component values have drifted from marked values and they are affecting the linearity of the trace other than when it's in the uppermost biased vertical position.
V-Pos part of the circuitry is where I'd go looking.......after the compulsory PSU checks once you have the scope open.

Be careful in there....old CRO scopes and their HV bites !
 

Thread Starter

nsipkes

Joined Nov 30, 2020
17
Without diving deep into the scope armed with the schematics and some test equipment I suspect some component values have drifted from marked values and they are affecting the linearity of the trace other than when it's in the uppermost biased vertical position.
V-Pos part of the circuitry is where I'd go looking.......after the compulsory PSU checks once you have the scope open.

Be careful in there....old CRO scopes and their HV bites !
I do have access to the schematics and will have a look, though at this point the scratchy controls are more annoying. Thanks for the HV reminder. I've been zapped a couple of times in the past on transmitters, and I think now my pacemaker would take exception to that!
 

tautech

Joined Oct 8, 2019
205
I do have access to the schematics and will have a look, though at this point the scratchy controls are more annoying. Thanks for the HV reminder. I've been zapped a couple of times in the past on transmitters, and I think now my pacemaker would take exception to that!
Just work through the basics slowly and deliberately ensuring the PSU meets spec first and hopefully the schematics have DC values listed which make for easy checks with a DMM. Note, most scopes that have DC values scattered through the schematics require the scope to be set in a certain way for those values to be valid. RTFM.

Pots bathed in IPA can sometimes fix scratchiness or the preferred product to properly sort them out is Deoxit. Generally some good amount of exercise helps too.
Good luck.

For those of us that wanna play along you should post up a link to the manual you'll work from.
 

Thread Starter

nsipkes

Joined Nov 30, 2020
17
The trace rotation pot won't fix it if it dips in different directions at different verical positions. My favourite would be stray magnetism within, or external to, the scope.
Does it change with different timebase speeds?
Does it change with the scope upside down?
Alas, flipping the scope had no effect. Shame. It would have been fun watching me get used to using it upside down! Seriously, though, there are no nearby sources of strong-ish fields (all other gear unplugged) and the bench is non-conductive.
 

AlbertHall

Joined Jun 4, 2014
11,319
Alas, flipping the scope had no effect. Shame. It would have been fun watching me get used to using it upside down! Seriously, though, there are no nearby sources of strong-ish fields (all other gear unplugged) and the bench is non-conductive.
So it may be within the scope metalwork which has become magnetised over time or by being, at some time, near a magnet, speaker, etc.
I don't think there is much you can do about that.
I can't see it being anything to do with voltages/signals in the scope.
 

tautech

Joined Oct 8, 2019
205
Oops! Just looked at my original post. I can apparently type faster than my brain. Yes, its an OS255.
As a newbie you need know you can edit posts and it's always a good idea to get any errors corrected for those that might find this thread in the future. Just scroll up and use the edit feature.
 

Thread Starter

nsipkes

Joined Nov 30, 2020
17
As a newbie you need know you can edit posts and it's always a good idea to get any errors corrected for those that might find this thread in the future. Just scroll up and use the edit feature.
Not seeing the edit feature anywhere (?). Only see quote and reply.
 
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