Electronics > Repair

(Solved. with pics) Got a new scope but it has some issues Tenma 72-6820...

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Hi all, newbie here with a few questions. I got a 50 buck chuck scope off ebay as Dave suggested.  It's a Tenma 72-6820 100MHz scope and after a bit of fumbling around I did get the test Cal to show properly on the CRT so I think it basically works but it seems a bit off of the grid lines though so I think it needs some adjustment.

The bigger problem though is this scope has those infernal rotary encoder pots that just spin around till you hear a beep.  :palm: They are used for nearly every control Ver/Hor Position, Volts/Div, Time/Div, Trigger level etc. Of course they don't work very well and turning the knobs can make the adjustment go either way or just not move at all. Weird thing is all of them seem to have a problem so they are either trashed, need to be cleaned or there is another component causing it.  Can you give me some ideas what to check when I crack it open? Also I haven't found a service manual yet if someone knows where one is.

Here is a pic of the Cal trace I got...

The Cal waveform is NOT a precision waveform.
Your display of it is as one might expect.
Sometimes there is provision to adjust both frequency and amplitude, but for it's intended use it is not necessary.
Encoder problem is concerning, might be worth removing one to check it's functionality.


The switches inside the rotary encoders are probably at bit crusty, it may help to turn the rotary encoder a lot.
If not post a photo of the encoder(s).

Try driving it for a bit longer before you start to take it apart, you may find that when one switch is in a certain position another has no effect and that is how it is meant to be, if possible find the manual as well.

I don't know if it will help you any, but I'm pretty sure that is also a GW Instek GOS-6103.


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