7834 Readout repairs, continued


After my previous antics with the 7834 readout system, the outcome was:

  • U3401 Zeros logic & memory – tek part 155-0018 – the original culprit.  Replaced with one from ebay.
  • U3418 Column decoder –  tek part 155-0014, which I think I killed during repairs. Replaced with one from ebay.
  • U3477 7402 quad 2 input NOR gate – smoked during repairs.  Replaced with a 74ALS02 from stock.
  • It turned out U3232, the row data switch that cycles through the 8 plugin display channels was bad as well,  botching the readout in horizontal A bottom, and horizontal B top & bottom.   Replacement sourced from ebay.

So far, I’ve spent $50 on the scope, and another $50 or so on replacement chips.  Still not bad at all.

Here’s a few shots using the 7B85’s delta time feature, which lets you do fun things like measure frequency, rise-time, and pulse-width directly using the on-screen delay time read-out.  Thanks to Peter on the TekWiki who’s entry on the 7B85 pointed out these features.

These photos are of a rudimentary clock circuit I made using parts from the DF01

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Tektronix 7834 Mainframe readout repairs

DSC_1001(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.  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.

7834 readout

There’s a few things wrong with this:
1) The characters are incorrect
2) 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.

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

Oh, hello:

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

So I started poking around with my 7D20 and found something strange coming out of the U3433, the custom timer chip:7834 readout board

Note the often overlooked screen annotation feature on the earlier 7k scopes 😉

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

09/16 UPDATE:
Heard back form a gentleman on the forum:
1) 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)
2)  He recommended swapping U3429 row decoder & U3418 column decoder – did that, no change.
3) He recommended swapping U3232 row data switch and U3263 column data switch.  oh fuck, those are BURIED.  Standby for some fun.

9/16 UPDATE:
The power supply slides right out, making U3232 & U3263 accessible.  Well played Tektronix.   Swapping them made no change.

Here’s the bottom of the scope – note the plastic rails on the bottom of the PS on the left.  One of those was loose and keeping the PS from sliding out.  Had to pop the bottom cover off to free it.


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

9/17 UPDATE:
what’s supposed to be +4.4v on pin 9 into column decoder U3418 is only +3.84v
what’s supposed to be +14.5v on pin 10 is 14.8v

9/19 UPDATE:
Well, this has suddenly turned into a larger problem.  Yesterday I’d gotten some of the characters to change by paralleling in a decade substitution box.  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.  I shut it down, and when I started it back up, U3447 started to release it’s magic smoke.
I 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.  I’m at a complete loss as to what to do next…

OK – moving on from despair…
With the readout board fully disconnected, the scope powers up and behaves normally (sans readout).  IMPORTANT NOTE:  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.

9/20 UPDATE:
It was U3401 – zeros logic & memory.
I decided to swap out one chip at a time from the 7834 to the known good 7603, in order to asses the damage.  When I got to U3401, the same problem showed up on the 7603.  Total facepalm for not doing that earlier.  All other chips were OK, with the exception of U3418, the column decoder.  So now I’m down:
U3418 Column decoder –  tek part 155-0014
U3401 Zeros logic & memory – tek part 155-0018
U3477 7402 quad 2 input NOR gate – I only have an 74ALS02, not sure if that’ll cut it.

Replacements for U3418 & U3401 en route

9/20 UPDATE:
74ALS02 is fine for U3477, tried it in the 7603 (thanks David)

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’.

9/25 UPDATE:
After 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.  All good.  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.