Repair of Tenma 72-3060 Oscilloscope

[Jubullu]

I recently bought this scope for $15 to tinker with trying to fix it as a first attempt at electronics repair. I was able to find a service manual for it here: http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Goldstar/OS-9020P.pdf under an LG rebadge.

The scope powers on and the CRT display lights up and the horizontal control seems to work fine as the x direction moves as it should and the t/div knob slows down the trace just fine but the vertical control seems to do nothing at all and no signal shows up when I introduce a signal. So this is where I started. The scope has a channel one output on the rear so I connected the calibration signal of my working scope to the Channel 1 input and connected the output to the Channel 1 input of my working scope and the signal came through fine, if slightly out of calibration. Also when changing the V/div on the problem scope the signal responded accordingly on the working scope.  So I am assuming that the signal is getting lost somewhere between the input, which seems to be working fine for now, and the CRT display.

So my question is where the most likely fault would be and what I should be looking at first. Would this likely be in the CRT control or in the vertical mainamp or some other part of the circuit that I'm not aware of?  Unfortunately, I just don't know enough about how CRT scopes work to know what would cause the signal to not make it to the CRT. My gut says it's probably the vertical mainamp but I wanted to appeal to someone with more experience in CRT scoopes.

Thanks for the help.

[P90]

is the input coupling switch selection set to ground?

[Jubullu]

Yes changing from AC to DC to Ground had no effect

[oldway]

Wellcome on the forum.

Security advise : the CRT of an analog oscilloscope works with high voltages , this can be lethal, you try to repair it at your own risk.
If you are aware that you have not enough knowledge and experience for working with high voltage, ask for help to somebody who knows.

The first thing to do is always a visual inspection: look for burned or broken components, bad solders, bulged or with leakage capacitors, loose connectors or wires, cracked boards,

After this, check all the power supplies .