-
Notifications
You must be signed in to change notification settings - Fork 0
/
la30-2.html
226 lines (184 loc) · 9.65 KB
/
la30-2.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>fritzm.github.io - PDP-11/45: LA30 repair II</title>
<meta name="description" content="">
<meta name="author" content="Fritz Mueller">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<!-- Le HTML5 shim, for IE6-8 support of HTML elements -->
<!--[if lt IE 9]>
<script src="https://fritzm.github.io/theme/html5.js"></script>
<![endif]-->
<!-- Le styles -->
<link href="https://fritzm.github.io/theme/bootstrap.min.css" rel="stylesheet">
<link href="https://fritzm.github.io/theme/bootstrap.min.responsive.css" rel="stylesheet">
<link href="https://fritzm.github.io/theme/local.css" rel="stylesheet">
<link href="https://fritzm.github.io/theme/pygments.css" rel="stylesheet">
<!-- Photoswipe -->
<link rel="stylesheet" href="https://fritzm.github.io/theme/photoswipe.css">
<link rel="stylesheet" href="https://fritzm.github.io/theme/default-skin/default-skin.css">
<script src="https://fritzm.github.io/theme/photoswipe.min.js"></script>
<script src="https://fritzm.github.io/theme/photoswipe-ui-default.min.js"></script>
<script src="https://fritzm.github.io/galleries.js"></script>
<script type="text/javascript">
var pswipe = function(gname, index) {
var pswpElement = document.querySelectorAll('.pswp')[0];
var items = galleries[gname];
var options = { index: index };
var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, options);
gallery.init();
};
</script>
<!-- So Firefox can bookmark->"abo this site" -->
<link href="https://fritzm.github.io/feeds/all.rss.xml" rel="alternate" title="fritzm.github.io" type="application/rss+xml">
</head>
<body>
<div class="navbar">
<div class="navbar-inner">
<div class="container">
<a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</a>
<a class="brand" href="https://fritzm.github.io">fritzm.github.io</a>
<div class="nav-collapse">
<ul class="nav">
</ul>
</div>
</div>
</div>
</div>
<div class="container">
<div class="content">
<div class="row">
<div class="span9">
<div class='article'>
<div class="content-title">
<h1>PDP-11/45: LA30 repair II</h1>
Sun 26 November 2017
by <a class="url fn" href="https://fritzm.github.io/author/fritz-mueller.html">Fritz Mueller</a>
</div>
<div><p>Okay, first thing to debug today is the ready light. This is lit by RDY LITE L, pin A16D2 (lower right of
sheet M7721-0-1 in the LA30 engineering drawings). Logic probe showed this was correctly asserted low. Pulled
the lamp and checked it with bench supply, and it checked fine. Verified +10 and ground at the lamp socket as
well, so why isn't it lit? Turns out it is polarized, and the socket is soldered in backwards (?!). Corrected
the socket, and ready light is working.</p>
<p>Noticed that ribbon is stalling occasionally. Ribbon tension seems to be overcoming the clutch on the take-up
side. It seems tensioning drag on the inactive side is too high. Not sure what to do about this one yet;
there is not much adjustable within the clutches, and the service manual only recommends replacement if they
are out of spec (yeah, good luck finding one!)</p>
<p>Repeated the experiment with loopback jumper (A15R2 to A15C2). Turns out I had miscounted backplane pins the
first time. With jumper placed correctly, I am now get a printing response to the keyboard. Not quite right,
but definite progress:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-first-print.jpg"
title="LA30 first print attempt"/></p>
<p>Here I had typed <code>:;L</code>, and then some other letters. You can see evidence of correct pins firing for the
first three characters, though either head movement or pin timing is off. Letter spacing appears more or less
correct for 80 characters per line.</p>
<p>Okay, hooked up the logic analyzer, and started to take a look at the character generator clocking to see if
all the the columns are being clocked out correctly. The logic analyzer shows a malfunction consistent with
the print behavior: character column clocking resets after two columns rather than proceeding through all
five:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-chargen-clock-
trace.jpg" title="LA30 character generator clocking trace. Resets incorrectly after only two of five
character columns"/></p>
<p>This signaling is mediated by a ripple counter on the M7724 character generator card:</p>
<p><img style="display:block; margin-left:auto; margin-right:auto" src="/images/pdp11/la30-chargen-schem.jpg"
title="LA30 character generator clocking partial schematic."/></p>
<p>So it looks like one or more of these 7474 quad flops has failed. I note on my chargen board that these are
early 70's Nat Semi parts; Mattis had a very similar issue (search "7474" on <a href="http://www.datormuseum.se/computers/digital-equipment-corporation/pdp-11-04">this
page</a>) on his LA30 chargen with
the same parts.</p>
<p>All I have time for this weekend; next time I'll get the chip clip on these for a closer look, then pull and
replace the baddies.</p></div>
<hr>
</div>
</div>
<div class="span3">
<div class="well" style="padding: 8px 0; background-color: #FBFBFB;">
<ul class="nav nav-list">
<li class="nav-header">
Site
</li>
<li><a href="https://fritzm.github.io/archives.html">Archives</a>
<li><a href="https://fritzm.github.io/tags.html">Tags</a>
<li><a href="https://fritzm.github.io/feeds/all.rss.xml" rel="alternate">RSS feed</a></li>
</ul>
</div>
<div class="well" style="padding: 8px 0; background-color: #FBFBFB;">
<ul class="nav nav-list">
<li class="nav-header">
Categories
</li>
<li><a href="https://fritzm.github.io/category/arcade-games.html">Arcade Games</a></li>
<li><a href="https://fritzm.github.io/category/math.html">Math</a></li>
<li><a href="https://fritzm.github.io/category/micros.html">Micros</a></li>
<li><a href="https://fritzm.github.io/category/pdp-11.html">PDP-11</a></li>
<li><a href="https://fritzm.github.io/category/programming.html">Programming</a></li>
<li><a href="https://fritzm.github.io/category/radios.html">Radios</a></li>
</ul>
</div>
<div class="social">
<div class="well" style="padding: 8px 0; background-color: #FBFBFB;">
<ul class="nav nav-list">
<li class="nav-header">
Social
</li>
<li><a href="http://facebook.com/fritzmueller">facebook</a></li>
<li><a href="http://instagram.com/infrafritz">Instagram</a></li>
<li><a href="http://www.linkedin.com/pub/fritz-mueller/a/679/62/">LinkedIn</a></li>
<li><a href="http://jsfiddle.net/user/fritzm/fiddles/">JSFiddle</a></li>
<li><a href="https://github.com/fritzm">GitHub</a></li>
</ul>
</div>
</div>
</div>
</div> </div>
<footer>
<br />
<p><a href="https://fritzm.github.io">fritzm.github.io</a> © Fritz Mueller 2023</p>
</footer>
</div> <!-- /container -->
<!-- Photoswipe -->
<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">
<div class="pswp__bg"></div>
<div class="pswp__scroll-wrap">
<div class="pswp__container">
<div class="pswp__item"></div>
<div class="pswp__item"></div>
<div class="pswp__item"></div>
</div>
<div class="pswp__ui pswp__ui--hidden">
<div class="pswp__top-bar">
<div class="pswp__counter"></div>
<button class="pswp__button pswp__button--close" title="Close (Esc)"></button>
<button class="pswp__button pswp__button--share" title="Share"></button>
<button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>
<button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>
<div class="pswp__preloader">
<div class="pswp__preloader__icn">
<div class="pswp__preloader__cut">
<div class="pswp__preloader__donut"></div>
</div>
</div>
</div>
</div>
<div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
<div class="pswp__share-tooltip"></div>
</div>
<button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
</button>
<button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
</button>
<div class="pswp__caption">
<div class="pswp__caption__center"></div>
</div>
</div>
</div>
</div>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://fritzm.github.io/theme/bootstrap-collapse.js"></script>
</body>
</html>