Friday, February 20, 2015

Progress on 1442 card reader reassembly and repair of Vectrex console

Back home from my trip by early afternoon and into the garage for a quick bit of work


I was able to finesse the push rod into position, after finding an unusual angle where I could insert forceps holding the rod at precisely the right angle, then maneuver the other end slight into the upper socket.

At this point I am reassembling the card feed clutch, ensuring I get the right lubrication in place. I had to drive the taper pin out of the index marker and remove it, since the pin wasn't inserting all the way, plus it let me open up to address the lubrication.


I did some more troubleshoot of the console, using my digital scope and the documentation. While I have a test cartridge now, I can't activate it until i can read the screen, a real catch-22, so I am proceeding 'blind' at the moment.

The Y signal looked similar to the images in the manual, but the X signal was decidedly odd. That would be consistent with the folding of images on screen, where lines of an object are not in their proper locations, inside seemingly randomly distributed on the screen.

The console uses a single digital to analog converter to produce analog signals for horizontal, vertical, brightness and sound. Since three work, I assume this is correct and moved forward in the circuitry. The X signal is directly routed, while the Y, brightness and sound signals are multiplexed through a mux/demux chip. The muxed signals work properly, so I skipped past this part as well.

LM347 Op Amps are used to integrate the voltages and move the beam on the screen, with the inputs to the op amp controlled by digital switches (a 4066 chip). I looked at the X signal circuits, which are one of the four opamp sections in the LM347 and two of the switches in the 4066. Since Y and other parts work, it is not a complete failure of any of those devices, but it could be a failed section. Passive devices like capacitors or signal connection (wiring/connector) problems could also be at fault.

Each op amp has one input hooked to a common 'zero reference' voltage which is shared among X and the other outputs which work correctly. The other input of the op amp comes from the DAC through a switch section of the 4066, turned on by the 'ramp' line from the microprocessor. This line also turns on the Y and remaining signal integrators, so it is presumed to work correctly, but if the digital switch is not operating or distorting the signal, it could impact the results.

At the end of a scan line, a different section of the 4066 digital switch is activated by the 'zero' signal from the microprocessor, which will short out or discharge the capacitors that are the feedback loop on the op amp. The capacitor feedback causes the op amp to be an integrator, sweeping the beam from one side to the other as the capacitor charges, then it is reset for the next line.

I will put my scope on the digital switch output and the gate signal, to see if it appears to change and do reasonable things. I plan to compare the working signal's switch, such as Y, with the failing X switch. I have to check both the reset ('zero') and the sweep ('ramp') switches, both for the X output and the reference behavior of the Y output. This means four sets of observations, which ultimately will finger a suspect or clear the digital switches.

After that, I can observe the ramp of the op amp circuit for signal X and the reference behavior of signal Y. This will help me determine if the op amp is correctly generating the sweep signal. Depending on the outcome, I can check passive components, e.g. the feedback capacitor, if they might cause the op amp output to be wrong.

All this requires several tiny attachments to hook onto individual legs of a DIP chip package without shorting to adjacent pins. I didn't have them at hand today, so will resume testing tomorrow with the ability to execute the tests above.


I received some final parts needed for the wiring but didn't get out to the shed in time to do anything today. 

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