Weird timebase Hameg 203-6

[mikeyB]

I got a strange thing happening with my scope timebase/sweep. If you look at the waveform, it's as if the sweep is starting off slow then speeding up ie not linear sweep.
The initial cycle LHS is wider/slower tan the last cycle on the RHS.
Any clues/pointers as to what is happening/causing this fault??

As usual, thanks in advance

 

[Philbo King]

Assuming you are in standard scope mode (i.e., not Lissajou) and it does it with all waveforms, it appears the horizontal sweep waveform is nonlinear. It should be a sawtooth with a linear rise. Check the service manual for a horizontal linearity trim pot. If there isn't one, you'll likely need schematics to chase the problem

 

[Audiosohn]

Actually, It is the other way around. Your Sweep starts out fast (on the left) and then gets slower, allowing mor periods of your sine waveform to be written to the screen.

Usually, a sawtooth wave is created by charging a capacitor with constant current.

Being that your sawtooth sweep sort of flattens to the end(top), maybe the Voltage supplying the current source is too low, saturating the pass element?

This is just speculation though. You will need a schematic and another scope to measure!
If you can't get a schematic, try to locate the sweep generator on the PCB by looking after test points or any type of hints on the silkscreen. Try to poke around with another scope.
Take care! CRT voltages are life threatening.
If you come across a Waveform that looks like a Sawtooth that looks like described earlier, you are on the right way.

Then start measuring components or try to retrace the circuit.

 

[mikeyB]

Thanks for the replies.
I've had to fix the scope a few times now and every time has involved 100nF multilayer ceramic caps used as decouplers on the supply rails that have gone down. If I locate the problem and it's a cap again, I'll be replacing all the 100n ceramics (about 40 of them in total).
I now have a schematic which helps a lot!!