Monday, May 25, 2015

Working on 1053 console printer, attempt to rescue fpga board, and checkout of a Tek 7854 scope


When I reattached the clevis ends to the operational mechanisms, the space was extremely limited. I have attached two photos where you can see the black ends, unconnected, and the limited space to reach them. I was able to get them all attached yesterday. but didn't upload the photos until today. Although I call these clevis ends, there may be a more official name as they are slightly different from the canonical clevis fasteners or pins.

Unconnected clevis joints visible in rectangle in center of picture

Actuator arms with holes where the clevis joint will attach

The flaw I still have in carrier return operation is that when the carrier reaches the left margin and the mechanism releases the clutch latch by pulling the release lever, the clutch latch should be moving back up to idle position promptly and completely; instead, it is oozing up partway.

Latch and release on left, but both rods and all circled parts are moving too little, too slow
I haven't yet found the spring that should be helping the clutch arm snap back into rest position. I have examined the parts catalog diagrams, the theory of operations pictures, the maintenance manual diagrams, and visually inspected the actual machine, all to no avail.

I checked on the escapement and tab operations, which led me to rewind the main spring to improve the quality of the escapement (spacing) and tabbing across the entire span of the carriage. I still have the inadequate unlatching of the carrier return latch to debug, plus sporadically the machine goes into repeated index operations until I fiddle with the interposer lever to get it to re-latch.

This warrants checking for a missing spring on the latch and restore functions of the index and carrier return mechanisms. Another anomaly is in erratic (or actually partial) tripping of the tab function. Sometimes, I push the tab button and it doesn't move or actuate, but if I push the space button it completes the tab operation.

All of these seem to be issues in the interposer trip and reset (or clutch trip and reset). It may be residual sludge from forty years of decaying lubricants, or something else, but that will take a bit more diagnosis.

Watching the tab interposer motion from several angles, it is clear that the backwards pressure on the interposer is not strong enough to activate the space/tab/backspace cam on the operational shaft. The spring is not as stretched as the others and is slanting in a different angle than the others. Could be attached to the wrong place. The spot where it attaches is behind the mainspring and blocked by many other levers and plates, but I will find a way to get to it.

Spring that should pull interposer back but isn't strong enough
My biggest concern remains the restoration of the carrier return latch once it is released. There is either way too much drag somewhere or a spring missing or broken. It should be popping up to its stop position instead of moving half-heartedly up to about 40% of its travel, never fully restoring. If I can get this sorted out, along with the tab issue above, the printer should be ready to cover up and put into place. It will take a bit more time.


I made up a JTAG interface cable for the ztex fpga board, since that is a third way I can load the bitstream on the board. While I might whip up a board to automate the loading at powerup using JTAG, my intention is simply to continue testing the SAC box while I wait for the replacement fpga board to arrive. I have been manually loading the fpga on each powerup anyway, after the failure of the onboard flash chip a couple of months ago, so this won't be much different.

I have an Altera USB to JTAG cord (USB Blaster) which I used to attempt the bitstream load via JTAG, but I couldn't find software to use the cord and recognize the .bit format file built by the Xilinx toolchain. The USB Blaster was found but it asserts that the JTAG chain is not working. I think I have the lines correctly assigned but it isn't worth a huge amount of debugging and testing time today.


I recently acquired a Tek 7854, which is a hybrid machine. It is a four compartment 7000 series 400Mhz analog scope, plus a digitizing processor that can capture waveforms and do quite a bit of processing and measurement on them. It is programmable through its attached keyboard. It has a few problems, which is why I got it free, but they seem to be due to a sync failure between the analog and digital driving of the CRT.

Tek 7854 (not mine but same type, different plug-ins)
Normally, when you have 'readout' on to show plug-in settings on top of an analog signal, the scope alternates painting the CRT with a trace and digitally rendering the pixels of the readout characters. Stored waveforms are displayed in the pure digital pixel mode. A master control either authorizes the analog circuits to drive the CRT plates or the digital circuits to drive them (but alternating so fast the eye doesn't notice).

The symptoms are dashes or interruptions in the traces of analog signals, plus bending/distortion of the characters from the readout function. However, when I click the 'readout' intensity to its 'off' position, the dashed interruptions go away and I have clean traces from the analog scope function. If I set up the analog scope to be silent (e.g. no trace triggered), the readout characters look fine. When I display a stored waveform including readout characters, everything looks fine.

The only problems occur when both are attempting to use the CRT - thus I believe one of the sync signals is bad or the alternation logic is malfunctioning. It shouldn't be hard to debug and fix this. The calibration circuit was put through the four plug-ins installed in the machine. The '2' amplifier of the left plug-in doesn't work, but the '1' amp does and both amps of the right plug-in are fine. Both timebases work well. The vertical mode buttons should light when activated, but the "Left" button does not. It selects properly and lets me view the left plugin but the lamp itself is out.

Keyboard for use with 7854 scope
The attached keyboard is slightly stiff from time but works well. I tried differentiating and integrated a stored waveform, plus conducted various measurements successfully. I feel quite good about the condition of the scope, which definitely replaces my lower bandwidth 7000 series scopes. The machine was $17,000 when new in 1990 (add more for the the keyboard and plug-ins). 

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