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<title>fritzm.github.io - PDP-11/45: LA30 repair III</title>
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<h1>PDP-11/45: LA30 repair III</h1>
Sat 02 December 2017
by <a class="url fn" href="https://fritzm.github.io/author/fritz-mueller.html">Fritz Mueller</a>
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<div><p>Digging in on the flip-flops identified as potentially problematic in the previous post, found that E5 had
failed. Pulled, socketed, and replaced; character generator now correctly clocks all five character columns:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-chargen-clock-
trace-fixed.jpg" title="LA30 character generator clocking trace after repair. Runs correctly to five character columns"/></p>
<p>After this repair, characters were printing full width, but two problems remained: about half of the
characters printed in response to typing on the keyboard were the wrong character, and the top row was not
printing at all on any character:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-second-print.jpg"
title="LA30 second print attempt -- incorrect characters and top row missing"/></p>
<p>Looking at the incorrect characters problem first, it was clear that bit 4 was not being received by the
character generator correctly. I was a bit worried that the SMC KR2376-17 scanner/ROM on the keyboard
assembly might be at fault, since Mattis had had some trouble with his. This is a pretty cool part; a
combined scanner and code translator, with an internal oscillator, rollover logic, debounce delay, and
flexible interfacing:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/kr2376-schematic.jpg"
title="KR2376-17 keyboard scanner/encoder internal schematic"/></p>
<p>...not to mention the very cool vintage ceramic/gold packaging (see below.) Fortunately, inspection with an
oscilloscope showed that the outputs from the scanner were just fine; chasing downstream, the problem was
found to be just a loose pin (SS) on the keyboard cable Berg connector. With that sorted, we now have this:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-third-print.jpg"
title="LA30 third print attempt -- characters correct, but still missing top row"/></p>
<p>For the final issue with the top row not printing, verified that the problem followed a particular G380
solenoid driver card when swapping them around, and that with a functional G380 in the appropriate backplane
slot pin 1 fires and prints correctly. Inspection of the problematic G380 revealed a failed power transistor
and blown associated micro-fuse; replacement parts on order.</p>
<p>For the ribbon advance issue, I pulled the ribbon motors and disassembled their top-side reduction gear cases
in order to gain access to the upper rotor bearings. Cleaning and lubrication of these bearings, plus a few
more taps with a mallet after reassembly, achieved an improved bearing alignment. With the increased output
torque, the ribbon now advances reliably.</p>
<p>Other minor items: Replacement vibration isolators arrived, and were installed. Threaded inserts in the
fiberglass top shell that had pulled out were reattached with epoxy.</p>
<p>Have some more travel coming up for work, so may not be able to get back to this for a bit. Next steps will
be repair of the failed solenoid driver channel, calibrations, then any debug necessary on the M7910 interface
card for the PDP-11.</p>
<p><img src='/images/pdp11/kr2736_thumbnail_tall.jpg' title='The SMC KR2736-17 keyboard scanner/encoder in the LA30' onclick='pswipe("pdp11",74);'/>
<img src='/images/pdp11/G380_thumbnail_tall.jpg' title='G380 solenoid driver card from the LA30, with failed parts pulled' onclick='pswipe("pdp11",75);'/></p></div>
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