Hello Everyone,
I have a Tek 2225 scope which has an oddity which im trying to fault find.
Overall the scope works fine, however, when set to 0.05uS/Div there is a clear ringing on the display. It does it with the input on any Coupling AC/DC/GND on any Voltage Division too. It is still visible on 0.1uS and 0.2uS/Div although it does get scaled as you might expect. It moves as you move the waveform around also.
Probing on the Preamp stage (J30 and J80 connectors), there is a switching noise/ringing on a 18kHz frequency. There is a positive spike then approx 27uS later it sags with a negative spike, before the positive spike reoccurs approx 27uS later.
I have done a fair amount of tracing the noise through and have found it is on the +8.6V supply, but at the main bulk decoupling caps the noise is less than 3mV Pk to Pk, which is within limits according to the service manual, but as you trace along the board, it gets worse and there is a 741 opamp, U225, which appears to be the furthest away is almost 12mV Peak to Peak which is outside of the limits.
The noise on the Preamp stage is in phase with the noise on the +8.6V rail, but i cannot quite find the source of this noise. It is independent of the sweep speed, and is still present in XY mode.
I know there are a fair number of 100nF Tantalum caps, but they typically fail short circuit rather than open circuit. There are a fair few 47uF Electrolytic caps which ive mostly replaced (on the S10 board), but the handful on the main board are actually on the -8.6V rail. Incase it is an open Tant, i have tried tacking on parallel 100nF caps on the +8.6V rail and even removed the Link from the PSU and fed in an external 8.6V supply which only seemed to make things worse, i suspect due to the cable length and stray inductance/resistance, the noise i then had was approx 30mV Pk to Pk. So it was removed along with any additional caps, so its back to "stock" configuration.
The issue occured beforehand, but I have already recapped all the main PSU circuit electrolytics on all supply rails and the switching frequency of the PSU does not seem to fit with the 18kHz noise either. So i dont think this has come about because i have changed the caps. Ive inspected my work too etc and cannot see any dry joints etc.
Im a bit stuck with this one and cant quite work it out. Does anyone have any suggestions?
Thank you :)
