{"id":240,"date":"2014-09-14T19:52:27","date_gmt":"2014-09-15T00:52:27","guid":{"rendered":"http:\/\/paulcarbone.com\/blog\/?p=240"},"modified":"2017-09-24T19:36:31","modified_gmt":"2017-09-25T00:36:31","slug":"tektronix-7834-mainframe","status":"publish","type":"post","link":"https:\/\/paulcarbone.com\/blog\/tektronix-7834-mainframe\/","title":{"rendered":"Tektronix 7834 Mainframe readout repairs"},"content":{"rendered":"

\"DSC_1001\"<\/a>(attempted) Repair log continues below<\/p>\n

Picked this up about a year ago, and used it solely with my then slightly misbehaving 7D01<\/a>, in combination with the DF01 Display Formatter.\u00a0 Because the DF01 was gen’ing all the characters, I never noticed that the readout feature was misbehaving until I went to use a pair of conventional plugins.<\/p>\n

\"7834<\/a><\/p>\n

There’s a few things wrong with this:
\n1) The characters are incorrect
\n2) The top and bottom of the vertical plugin readout are supposed to be for channels 1 and 2 respectively, but they both seem to respond to channel 1 only.<\/p>\n

Regarding the incorrect characters, it appears that from this table<\/a>, the characters are shifted one to the left of what they should be. ‘0S’ should read ‘1V’, and ‘4m’ should read ‘5u’\u00a0 Columns and rows are selected by way of a current loop between the plug-in and mainframe, using .1ma steps between 0 & .9ma.<\/p>\n

Oh, hello:
\n\"rowmatch\"<\/a><\/p>\n

“Score!” I thought,\u00a0 but sadly turning it does nothing.\u00a0 Also, my columns are the issue, not the rows.\u00a0 UPDATE – Column Match doesn’t do anything either.<\/p>\n

So I started poking around with my 7D20 and found something strange coming out of the U3433, the custom timer chip:\"7834<\/a><\/p>\n

Note the often overlooked screen annotation feature on the earlier 7k scopes \ud83d\ude09<\/p>\n

So there should be a negative going pulse on pin 16 that corresponds to the waveform on pin 10.\u00a0 Also note that negative portion of the waveform on page 10 are shorter where the pin 10 pulse is missing.\u00a0 I went ahead and ordered a replacement U3433, we’ll see if that does anything.\u00a0 In the mean time, I’m off to the tek message board…<\/p>\n

09\/16 UPDATE:
\nHeard back form a gentleman on the forum:
\n1) What I’m seeing on the output of U3433 is not uncommon and due to display skip (which is described in the manual, I just have to wrap my head around it)
\n2)\u00a0 He recommended swapping U3429 row decoder & U3418 column decoder – did that, no change.
\n3) He recommended swapping U3232 row data switch and U3263 column data switch.\u00a0 oh fuck, those are BURIED.\u00a0 Standby for some fun.<\/p>\n

9\/16 UPDATE:
\nThe power supply slides right out, making U3232 & U3263 accessible.\u00a0 Well played Tektronix.\u00a0\u00a0 Swapping them made no change.<\/p>\n

Here’s the bottom of the scope – note the plastic rails on the bottom of the PS on the left.\u00a0 One of those was loose and keeping the PS from sliding out.\u00a0 Had to pop the bottom cover off to free it.
\n\"DSC_0989\"<\/a><\/p>\n

\"DSC_0992\"<\/a><\/p>\n

resocketing a few of the transistors fixed the missing readouts on the left horizontal bay, and the upper\/lower duplication I was seeing.\u00a0 Now every bay has it’s readout, it’s just wrong.<\/p>\n

9\/17 UPDATE:
\nwhat’s supposed to be +4.4v on pin 9 into column decoder U3418 is only +3.84v
\nwhat’s supposed to be +14.5v on pin 10 is 14.8v<\/p>\n

9\/19 UPDATE:
\nWell, this has suddenly turned into a larger problem.\u00a0 Yesterday I’d gotten some of the characters to change by paralleling in a decade substitution box.\u00a0 Today I tried again but paralleled the wrong resistor (R3418), and I as was trying to pull the lead (live – dumb) I shorted something, heard a snap from the power supply, and the display went crazy, then dark.\u00a0 I shut it down, and when I started it back up, U3447 started to release it’s magic smoke.
\nI can only suspect that one of the legs of the power supply has gone awry, but the power supply wont fire up when not under load.\u00a0 I’m at a complete loss as to what to do next…<\/p>\n

OK – moving on from despair…
\nWith the readout board fully disconnected, the scope powers up and behaves normally (sans readout).\u00a0 IMPORTANT NOTE:\u00a0 When disconnecting the mini coax connectors, note that some of them have shield potentials of 15v, so take care to make sure they don’t touch the chassis.<\/p>\n

9\/20 UPDATE:
\nIt was U3401 – zeros logic & memory.
\nI decided to swap out one chip at a time from the 7834 to the known good 7603, in order to asses the damage.\u00a0 When I got to U3401, the same problem showed up on the 7603.\u00a0 Total facepalm for not doing that earlier.\u00a0 All other chips were OK, with the exception of U3418, the column decoder.\u00a0 So now I’m down:
\nU3418 Column decoder –\u00a0 tek part 155-0014
\nU3401 Zeros logic & memory – tek part 155-0018
\nU3477 7402 quad 2 input NOR gate – I only have an 74ALS02, not sure if that’ll cut it.<\/p>\n

Replacements for U3418 & U3401 en route<\/p>\n

9\/20 UPDATE:
\n74ALS02 is fine for U3477, tried it in the 7603 (thanks David)<\/p>\n

In retrospect, I should have suspected U3401 given that ‘>’ worked but ‘IDENTIFY’ didn’t, since it’s U3401 that signals the column & row data switches to run through the sequence of characters to spell ‘IDENTIFY’.<\/p>\n

9\/25 UPDATE:
\nAfter an in situ check of components around the affected areas, I pulled U3418 & U3401 from the 7603 and installed them in the 7834, held my breath, and power’d it up.\u00a0 All good.\u00a0 The soon-to-arrive replacements will make the 7603 whole again, though for now it doesn’t really matter, since it’s really only host to my 7D20 & 7D01.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

(attempted) Repair log continues below Picked this up about a year ago, and used it solely with my then slightly misbehaving 7D01, in combination with the DF01 Display Formatter.\u00a0 Because the DF01 was gen’ing all the characters, I never noticed … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":277,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[26,1,16],"tags":[63],"table_tags":[],"_links":{"self":[{"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/posts\/240"}],"collection":[{"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/comments?post=240"}],"version-history":[{"count":19,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/posts\/240\/revisions"}],"predecessor-version":[{"id":1106,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/posts\/240\/revisions\/1106"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/media\/277"}],"wp:attachment":[{"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/media?parent=240"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/categories?post=240"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/tags?post=240"},{"taxonomy":"table_tags","embeddable":true,"href":"https:\/\/paulcarbone.com\/blog\/wp-json\/wp\/v2\/table_tags?post=240"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}