If you go down the Banggood DIY CRO path. Check these pics, there is a fair bit of noise in the amp. But for the price I built 2 of them & reasonably happy to use them. Just alow for the noise.

 

As for building the scope can't you just add some decoupling caps to tone down the noise????

 

Tried all that & suspect its noise in the amp ic. This is with the input switch in Gnd position.

 

Tried all that & suspect its noise in the amp ic. This is with the input switch in Gnd position.View attachment 89846

It does not look good at all.

Is the IC custom made?

 

The IC is a TL084C (U2) 4x JFET opamp.

 

For starters the 074 has lower noise figure (but same GBW).

 

Try powering from a battery if you're not already doing so. Maybe there's noise coming in from a switching power supply.

 

All testing was battery power. Heres the circuit of the power supply part. Pics of wave forms on Q1 collector & TP25. Both Horz 10us & Vert 1mv/div. Bottom pic is TP25. Have also tried disconnecting the neg 5v power feed, an feeding that with a battery. Still the noise was there.