All parts arrived and were installed on the VBA Curve Tracer - back to checkout

REMAINING CHIP - LM555 TIMER - ARRIVED AND PUT ONTO THE BOARD

I soldered that chip onto the board and then swung the two boards together. The various potentiometers and switches had to fit through the openings with the rear nuts setting the front panel at a consistent distance from the rear board. 

ADDITIONAL NUTS ARRIVED TO COMPLETE ASSEMBLY

Some of the controls I received did not come with the mounting hardware. The nuts used included various fine and very fine thread patterns and two sizes. I put on all the nuts and then installed the knobs. 

BEGINNING THE CHECKOUT AND CALIBRATION PROCESS

The first step is to verify the proper voltages are present, which is where I discovered my earlier error with the voltage polarity swap. I did discover a zener diode that was soldered on backwards and corrected it. This time, they all checked out properly. +5, -15, +15, -24, +24, +10 and 7.5 are the supply voltages, plus the device under test voltages from the 35, 70 and 200V ranges were verified. 

Because some of the voltages used in this design are isolated, driven by a transformer with no tie to ground, testing is complicated. DMMs or oscilloscope probes must reference the proper ground, either isolated ground or true ground, before testing any point in the machine. 

As one also uses the oscilloscope to display the X and Y outputs for the curves, it is easy to have the cables establish a true ground connection while another probe is going to be used with isolated ground and related circuits. I must disconnect and think through each testing action to accommodate the dual grounds. 

I got through a number of verifications. The step delay function and operation of the device test voltage worked well. The step voltage however was not correct. When I looked at the stepped voltage pattern from the generator circuit I spotted the anomaly.

The generator has a counter chip whose bottom three output bits are used to mix combinations of resistors to produce a stairstep of voltages. Another chip converts the binary value to decimal and the switch that controls the number of steps routes the chosen decimal value to the reset pin of the counter chip. Thus, this will count from 1 to 7 times based on the setting of the switch. 

The scope showed a fairly random looking set of pulses and the number didn't change properly based on the rotary switch. My first impression is that the middle output bit of the counter chip is not being delivered, thus the patterns I observed. Further, without that bit the switch will produce reset pulses based on what it saw and not matching the value of the switch position. 

When I get back to this project I will investigate the circuit board around the generator logic as I suspect a bad solder joint is to blame. 

Comments

Post a Comment

Popular posts from this blog

Bizarre behavior detected in a Raspberry Pi Pico based design and partially understood

ORIENTATION OF THE PRODUCT CAN CAUSE THE PI TO FREEZE AT STARTUP This particular product with a Raspberry Pi Pico would not boot up when sitting flat on the table in its normal orientation, but if tilted or turned it would come right up. When I hooked up a USB serial connection to the debug header, it worked fine even flat on the table.  INVESTIGATION BY DESIGNER AS WELL AS ME I first assumed that I had introduced this problem in the product, but happened to describe it to the maker who was able to reproduce it reliably. I continued to try variations to determine whether it was pressure on a board or component that caused it. I also searched the web for any reports that might cause Pi Picos to fail to boot based on orientation, but found nothing.  The designer found signals introduced on a serial connection trace when sitting in the normal (failing) orientation but not injected when rotated or otherwise turned. Traces were routed from the UART0 on the Pico to a header, for use...
Read more

Incompetent coders - the frustration of using Vivado to build FPGA designs

IDIOT SOFTWARE MUST BE RESTARTED CONSTANTLY TO SEE INPUT FILE CHANGES When I update a source file, often Vivado is somehow working from a cached copy and never notices the changes. It is highly frustrating to fix an issue, rerun a Linter or Synthesis and get the same old errors. The only solution is to shut Vivado down and restart. I remember hitting this many times in past years as I worked with the toolchain, but it has changed from sporadic to nearly constant failure to see the changed timestamp. I can't be the only person running the toolchain under Windows, yet this kind of bumbling misperformance has been a feature for years. 
Read more

Found correct mod for the Arduino Mega 2560 to avoid powering it from USB

Image
USB HANDLING CIRCUITS IN THE MICROPROCESSOR REQUIRE USB VCC TO RUN The reason my first attempt failed, simply removing the poly (resettable) fuse, is that the fuse is between the USB connector and the microchip which needs the USB VCC connected. I had to reinstall the fuse.  BETTER CHANGE IS TO REMOVE A MOSFET THAT ROUTES POWER The Arduino uses a power mosfet and a comparator to decide when the external power is adequate to run the board, disconnecting the USB VCC from the board. It does leave that wire connected to the USB circuitry in the chip, however. With the transistor removed, the USB power never gets to the board so it is always powered by the external (1130) power. This means the board powers up and down with the main system, whether or not a USB cable is connected to anything else. When the board is powered up, the cable is active as a serial connection, exactly what I wanted to happen.  In the picture below, the green circle shows the reinstalled polyfuse. The red c...
Read more