Monday, April 17, 2017

Op amp lab, vacuum tube curve tracer and disk tool work, plus vacation and other things

There has been an extended gap in this blog, encompassing a vacation in Maui, work on my home, and quite a few other tasks that temporarily took higher priority. While I still have quite a bit of work we plan to do on the house, I was able to get back to some of the electronic projects.


I constructed the uTracer kit before I left for vacation but two areas were not working properly at that time. The board produces anode voltage, filament power and communicates, but neither the screen voltage nor the grid bias voltage are responding. 

The screen voltage section is a mirror of the anode voltage circuitry. It has a boost converter that will build up the voltage in a large 'reservoir' capacitor, driven by pulses from the microcontroller. A analog to digital converter chip samples the voltage present on the capacitor and delivers it to the processor. 

When the sampled voltage is below the target level, pulses are applied to the boost side to increase the voltage. When the sampled voltage is above the target value, other microcontroller pulses discharge some of the voltage. The voltage on the reservoir is thus maintained at its chosen level, to be delivered  to the tube under test in short pulses through a high voltage switch portion of this circuit. 

The voltage on the reservoir capacitor never climbs above 19 volts, the input supply voltage to the board. The logic to issue the boost and the discharge pulses is part of an interrupt processing routine in the microcontroller, fired off regularly by timer pops. The main logic path in the controller has to wait to see the sampled voltage reach the target level before it does any testing, but if the boost converter doesn't reach the target level, it just freezes in the main loop.

I examined the board and its solder joints very carefully, but everything looks good. I then put the scope on the microcontroller pins that deliver the boost and discharge pulses, and saw the logic trying in vain to grow the voltage. 

Since the controller is doing everything right, I turn my attention to the components that might be malfunctioning. The FET that drives the boost function might not be conducting properly. The transistor that implements the discharge function might be stuck on or shorted. The high voltage switch could be malfunctioning and draining power as fast as it builds up, although this is less likely. Other hidden component failures or short circuits are the other, less likely cause.


I continued to pore over the traces and other evidence to figure out where the read operation goes awry when the Alto reads cartridges that I have written with the tool. I see a time interval occuring between the header and label records of the sector that is different by about 8 word times, depending on whether a cartridge is written by me or by an Alto. The outcome is misreading of the label record and a checksum at the end.

The same flaw causes the data record to be misread, thus injecting the wrong data into memory, dooming the bootstrap operation. I haven't yet figure out the cause but will continue to study everything I can until I figure this out.


I attended a one day class at Texas Instruments on designing with op amps - intended for engineers who knew the basics and were designing with them already, but who needed to understand deeper issues such as stability, noise and distortion that weren't clear from the idealized model of an op amp that is used in most textbooks. My friend George from the 1401 Restoration Team also attended.

Part of the course was a board with several types of circuits installed and plug-in modules to try out different op amp part numbers. One could inject various signals to the op amps and watch the results on a scope, to reinforce the lessons of the class. They provided a discount to buy the board for home use.

I have a Digilent Analog Discovery which provides function generators, oscilloscopes and other bench tools to drive and display the op amp results, similar to the NI Virtual Bench used in the TI class but much less expensive. I set it up to experiment further with the op amps.

Another discount from TI gave us six other op amps for use in evaluation. I naively assumed these were premounted on the daughtercards just like the ones supplied with the test board, but instead I received six small SOT-23 chips and a set of empty PCBs to solder them to.

Thus, I had to haul out the stereo microscope to solder these in place, before I could make use of these new op amps on the board. As well, it takes special clamps and tweezers and the steady hand I no longer reliably have. 

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