GOS653G Oscilloscope problem

[Roy57]

Hi,

I am a diy enthusiast who enjoys repairing old audio equipment and saving the environment by recycling.

A friend has given me an old GOS Instek653g after his employer upgraded to digital test and measurement equipment.

The scope seems to work ok but when testing CD player laser eyepatterns the leading edge of the display is cut off. So instead of displaying the characteristic 'triggering' slopes, a rectangular view of the eyepattern is displayed. I have tried all the adjustment on the front panel under advice of experienced techinicians via forums but the problem persists.

Is there anyone who has experience with these scopes or able to assist with troubleshooting and repair?

I do not have access to the GOS Instek 653g service manual but I do have a copy of the user manual and 40year old 20mhz scope.

Any advice and guidance to repair this scope would be appreciated.

Thank you.

 

[jiripolivka]

With any good scope, adjusting the eye pattern requires a careful synchronization. Many scopes offer trigger delay by which you can shift the trigger time to a suitable delay after the observed signal. By this shift you can get the desired eye diagram.
I would not advise to "repair" your scope but rather to test first some simpler signals, to learn how your scope operates. I think it is fine but each type of scope and signal needs some skill to observe correctly.

 

[Roy57]

Sorry about the long delay, I have not had much time to pursue my diy audio hobby but hope to spend some time getting the scope to work like it should. Are you able suggest any simpler signals. I have already tested various power and output sections of a cd player and the scope works perfectly. The only poblem is the eyepattern when testing the laser output.

 

[chuckey]

A timebase on a CRO needs a setup time before it triggers, so the signal comes in the trigger fires and say 10 NS later the time base runs. So if the bit of the signal you are interested in occurs before this time, you will not see it. To overcome this problem, the Y signal is delayed by a delay line in good CROs that delays the actual deflection, so if its delay was 20 NS, you can see a signal 10 NS before the trigger point. The other thing is the trigger level, the higher it is the more the display moves to to left, as the signal needs more time to get to the trigger level.
Look at the "cal" output, you should be able to see its rising edge.
Frank

 

[Roy57]

I will revisit the old GOS653G and again familiarise myself with the control panel. Would you like me to send some pics of the problem if I can?