Input protection

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orbanp
Posts: 2
Joined: Thu Apr 04, 2019 8:56 pm

Input protection

Post by orbanp »

Hello Everyone,

Last fall I got a kit, assembled it, and it worked perfectly.
However, my joy was short lived, soon the scope developed a DC offset with grounded input and noise on the displayed signal.
When I got the kit I also got an acrylic case, and my guess is that by handling that plastic case I got the first op-amp zapped.
I did replace the op-amp with a TL074 (these days the specs are exactly the same as for the TL084) and now the scope works fine again.

To prevent that happening again I added protection diodes to the input of the first op-amp (U2B), as it did not have any.
I am enclosing the updated schematics and a few pictures.
The protection diode is a BAT54S dual diode.
My kit is a through-hole kit, I soldered the dual diode onto the back of the PC-board.
The diodes are soldered to the C10 and C11 filter cap solder points (+/-5V), the mid-point to the pin2 of the SW2A switch.

I also updated the C2 value from 220pF to 10nF.
The TL074 has an input capacitance of about 6pF and that forms a voltage divider with C2, introducing a 0.5dB dip in the 10mV setting from 10kHz and up when C2 is 220pF. The 10nF cap eliminates that effect. All other parameters of the input remain the same.

The other mod I did, having read the warnings about the LM78L05 power dissipation, was to install a TO220 cased LM7805 instead of the L version. It fits there perfectly.

As mentioned, this is a terrific little scope, kudos to the designers!

Regards, Peter
Attachments
Input_prot1.png
IMG_0282.JPG
IMG_0281.JPG
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