Electronic Workcenter E100 Oscilloscope: Trigger issues

[Mex]

A while ago I acquired an old analogue scope but it's trigger is faulty, changing any of the trigger settings seems to have no effect on the displayed waveform.

I was wondering if anyone has any information on this scope, I can't find anything out about this device, other than a picture of a version of it branded for the Open University. https://www.flickr.com/photos/adream608/14428942319

Any suggestions on where would be a good place to start diagnosing issues with triggers on analogue scopes.

[ez24]

If I were you I would add the word "Oscilloscope" to the title.  There are a lot of scope guys here and that would get their attention.  I thought it was something else.

[floobydust]

I would further test the trigger portion, to see if it ever works. 
So no sweep ever if switch on TRIG (compared to AUTO)?
AC and DC coupling, on source Ch1 and Ch2, also external trigger, on the slowest sweep speed, I would test. Does it have LINE trigger?
Most analog scopes will trigger once if you quickly move the TRIG LEVEL pot back and forth, to force a trigger, post trigger pick-off circuit.

Guessing it's usually an oxidized switch, or the trigger input section got overloaded and killed a transistor.

If you can't find a schematic or service manual, I'd follow or (need another scope) the signals at the trig aux input and switches, to at least take you to that PCB section.

[Mex]

If I were you I would add the word "Oscilloscope" to the title.  There are a lot of scope guys here and that would get their attention.  I thought it was something else.

Thanks ez24, I have updated the title.

[Mex]

 
So no sweep ever if switch on TRIG (compared to AUTO)?

Yup, nothing. 

AC and DC coupling, on source Ch1 and Ch2, also external trigger, on the slowest sweep speed, I would test. Does it have LINE trigger?
Most analog scopes will trigger once if you quickly move the TRIG LEVEL pot back and forth, to force a trigger, post trigger pick-off circuit.

No line trigger.

Guessing it's usually an oxidized switch, or the trigger input section got overloaded and killed a transistor.

If you can't find a schematic or service manual, I'd follow or (need another scope) the signals at the trig aux input and switches, to at least take you to that PCB section.

Ok cool, I will check out all the switches in that area, it's worth a try as won't take long. I do have another scope now, so I can try and trace the signal paths.

Thanks for the advice.

[oldway]

Without diagrams, I think it will probably be necessary to make some reverse engineering.

Clean the contacts.
Check the supply voltages, connectors and cold solders
Check for bad electrolytics condensators.
Inject an external trigger signal and follow the signal.
Check transistors and diodes in circuit with a "component tester" like Huntron or Hameg.
Good luck.

[Armadillo]

The trigger potentiometer, you should first adjust to detent [if available],  or lowest level and try again.
Alternatively, you will have to use contact cleaner to clean this potentiometer.
This set the level for internal comparator between signal level and this voltage level set by the potentiomenter to trigger the circuit.
So there is a DC level source and the signal source to look at.
The signal level must be greater than the DC level to start the wave.
Can you tell us what signal are you using and what level is that?

But do the simple first.

[oldway]

I have obviously assumed that you know how to use an analog oscilloscope and that you are 100% sure that there is a failure in the trigger circuit.

Is that the case ? 

[Chris56000]

Hi!

Do you still have this instrument with the trigger fault on it?

If you do I will be able to assist with circuit diagrams in the not too distant future, as I have won one of these and will begin drawings when I receive it!

Chris Williams