Tektronix TDS 520 repair

Update: Added a link the Debug Interface schematics (pdf)

The Tektronix TDS 520 is a digitizing oscilloscope from the early 90s. It offers 500 MHz analog bandwidth, 500 MS/s sampling rate, 2 full-featured channels and 2 auxiliary channels with a limited vertical sensivity. Although being at age nowadays, this oscilloscope has perfect properties for a prominent place in the shack at home.

My TDS 520 was saved from the scrapheap.

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Setup an older eGalax Touchscreen on Pi3

I got a nice small monitor with touchscreen from a scrapped payment terminal. The touchscreen is manufactured by eGalax and has a normal USB-interface. Very nice for my Raspberry Pi. Except that it needs some work to get it working

sudo apt-get install xserver-xorg-input-evdev 

Add to /usr/share/X11/xorg.conf.d/40-libinput.conf :

Section "InputClass"
        Identifier "evdev tablet catchall"
        MatchIsTablet "on"
        MatchDevicePath "/dev/input/event*"
        Driver "evdev"

PiVPN suddenly stops accepting client connections

PiVPN for Raspberry Pi is a wonderful and easy OpenVPN setup tool. I use it a lot with various devices to access my home network. But it suddenly stopped accepting connection from those various devices. This is how I solved the issue…

This is what /var/log/openvpn.log showed:

Wed Jan 23 20:09:47 2019 VERIFY ERROR: depth=0, error=CRL has expired: CN=tim
Wed Jan 23 20:09:47 2019 OpenSSL: error:14089086:SSL routines:ssl3_get_client_certificate:certificate verify failed
Wed Jan 23 20:09:47 2019 TLS_ERROR: BIO read tls_read_plaintext error
Wed Jan 23 20:09:47 2019 TLS Error: TLS object -> incoming plaintext read error
Wed Jan 23 20:09:47 2019 TLS Error: TLS handshake failed

So… The certificate has expired.

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uController PSU

Every (home)lab needs a good variable powersupply and so does mine. I wanted to design something with a microcontroller to set the voltage and current limit. Using a microcontroller also brings the possibility to use an LCD and fancy rotary encoders for control. The control loops themselves are pure analog.

I made this powersupply mostly from components I already had.

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Interface a rotary encoder with Atmel AVR (interrupt style)

When I wanted to gently interface an incremental rotary encoder with an 8-bit Atmel AVR microcontroller, I couldn’t find a nice example. That’s why I wrote this little text. Many other howtos, manuals and descriptions are fuzzy, incomplete or too complicated to do the (easy) job.

This article describes three things: the hardware (just the minimal hardware) and two pieces of code. The first is just a simple interrupt based program and the other uses timers to make bigger steps when you turn faster.

I use Atmel AVR studio as IDE and the Through hole USBprog to program the microcontroller.

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Tube amp

The idea to build a tube amp arose at a party, where all good ideas start. It stayed just an idea for about two years, due to other important things. This winter (2008/2009) the amp got my attention again. So I bought some books, googled for examples, searched explanations and gathered other info. Arnova and I then developed the first drafts for the amp. Later we visited some electronics fairs to get tubes and sockets. I have to be honest, eventually I bought new transformers and tubes. At the fairs I couldn’t find any proper power or output transformers, suitable chokes and half of my EL84 were bad. Reconsider buying old(er) parts.

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