Tuomas Vaherkoski

Programmer and electronics hobbyist

Tektronix TAS 250 Oscilloscope

May 31, 2015

I bought three Tektronix TAS 250 oscilloscope with my friends from local seller for only 80 euros, what a bargain! One of them is working perfectly, but two others have some problems with one channel and cursors. So it was time for some troubleshooting. Here is some pictures I’ve taken. Sorry about blurriness in some pictures, they are taken with my phone camera.

TAS 250

Components (Top)

General Control and IO board Mains input Transformer Main board CH2 Input Power and High Voltage board CH1&2 under shield Trigger under shield CH1 under shield CH2 under shield CH2 under shield CH2 under shield All under shield

Traces (Bottom)

Boards Main board Main board (back) Main board (middle) Main board (front) CH2 & EXT Trig Power and High Voltage board Power and High Voltage board (back) Power and High Voltage board (middle) Power and High Voltage board (front) Front panel Control and IO board

Outside

Rear panel General Front panel Test signal

Repairing - Cold joint in CH1 BNC connector

In one of three units we got, the CH1 didn’t show the test signal when probed. It should be 2Vpp 2kHz square wave.

CH1 probe comp signal CH1 no input noise

The problem was in the resistor connecting BNC connector to main board (yellow wire in CH2 & EXT Trig picture). I measured resistance from center of BNC connector to the circuit board with my DMM and it showed infinite resistance. I desoldered the resistor, measured it and it showed 22ohm, just like CH2. I soldered it back and everything is now working. It must have been cold joint or something.

Repairing - CH1 voltage range selection

The second unit had problems with voltage range selection knob. It really took some effort to find this problem, because display showed selected range as it should. However it wasn’t actually switching voltage range selection relays on main board. We did some basic tests for relays and transistors driving them and they seemd to be ok. Testing also showed that there was signal on the base of transistors too. Actually that same voltage was in all transistors when it should be in only one of them at the time.

Logic for buttons and knobs Not so pretty repair

So now we had to locate the chip driving those transistors. It was found from the PCB mountent directly behind the front panel which holds all the buttons and knobs. In picture you can see three 74HC574 D-flipflops, two 74HC245 buffers and PIC microcontroller plus some passives. After some probing we found that 74HC574 (UA23) was driving those CH1 relays through connector labeled as JA23. We measured its output pins voltages and they were all logic high no matter in which position voltage range selection knob was.

We had some problems at removing that 20 pin dip from the PCB because we didn’t have side cutters small enough to cut chip legs. What we came up with was to cut VDD pin of old chip and then bend the legs of new chip and solder it on the other side of the PCB. Not so pretty but hey, the unit works now!


Throttle Quadrant

May 26, 2015

The Idea

I had had an idea of building this thing for a long time. It originally came from very annoying bug/feature of my wireless joystick which loses connection when left idle for some time. What happens then is that the throttle randomly jumps to full power making aircraft to do unwanted maneuvers… I also wanted to be able to control power settings of the plane more easily and realistically.

Throttle Quadrant

Building

This device is actually quite simple on hardware side. It takes power from USB (+5V) so you don’t need any battery. It’s build around AVR Attiny84 microcontroller.

Schematic

The microcontroller uses its internal analog-to-digital converter to measure voltage from center taps of slide potentiometers. There is also AVR-SPI connector for programming the chip and usb connection to speak with computer. Slide pots are color coded as (from left) throttle - prop pitch - mixture, but of course they can control anything you want.

There is still a few free pins (used only by AVR-SPI), so maybe I’ll add some switches and leds some day. It also definitely needs a better case with nice front panel.

Coding

I used VUSB library which implements slow-speed USB stack for AVR microctontrollers. Device is acting as USB HID joystick so the drivers are built in on every modern operating system (Windows XP or higher). Microcontroller runs in a loop which takes measures from ADC channels and then saves them so that computer can query the values.

It would be easy to convert this to any HID device, for example volume controller, you just need to change the usb descriptor.

Conclusion

The device is working great and it was cheap compared to ready-to-go kits so it definitely worth building. I’ve used this with Microsoft Flight Simulator X and IL-2.

If you are interested you can download schematics and source code, it’s open source. Schematic can be opened with KiCAD.