Tektronix 7623 Issue with Readout - Page 1
Forum section: electronics / repair

author: Kidon
author_rank: Regular Contributor
timestamp: June 22, 2020, 12:08:17 am
content: Hi,I have this 7623 3-Slot mainframe. It was working fine this morning.I moved it to another room, and when I powered it up, I noticed unusual characters where the readout normally is.Has anyone ever seen something like that before? Can anyone help diagnose and fix the problem?Here are pics of this strange phenomenon...Thanks for your help.

author: wn1fju
author_rank: Frequent Contributor
timestamp: June 22, 2020, 12:44:25 am
content: I have seen the similar thing many times with my Tek 7600-series mainframes. It usually can be fixed by powering the scope off, removing all the plug-ins, taking a Q-tip with alcohol and rubbing the edge connectors of the plug-ins, then insert and remove the plug-in a few times to clean the connectors. Let the alcohol dry for a few minutes and see if that helps.

author: Kidon
author_rank: Regular Contributor
timestamp: June 22, 2020, 07:13:27 am
content: Quote from: wn1fju on June 22, 2020, 12:44:25 amThanks, I’ll try that. Although it does the same thing with any plugin I use. I have several, and whatever combination, it’s the same.

author: Kidon
author_rank: Regular Contributor
timestamp: June 22, 2020, 07:29:19 pm
content: Quote from: wn1fju on June 22, 2020, 12:44:25 amWell, I did and there is no change at all.Can anyone be of some help?? I didn’t do anything before or when it happened, But maybe something got disconnected inside when I moved it from one room to the next???

author: MadTux
author_rank: Frequent Contributor
timestamp: June 24, 2020, 11:26:10 am
content: Since the < and I symbols are close to correct 1 and 2 readout and since the problem happens across different plugins, there has probably something drifted away inside the mainframe.Character skip is also close in range, so fits the symptoms as well.There probably is a potentiometer on readout board or so, to adjust for correct character display.Tekwiki about Tek7000 readout system:http://w140.com/tekwiki/wiki/7000_series_readout_systemTable with current to character map:http://w140.com/tekwiki/wiki/7000_series_readout_system#/media/File:Tek7000-readout-charset.jpg

author: Kidon
author_rank: Regular Contributor
timestamp: June 24, 2020, 12:54:57 pm
content: Quote from: MadTux on June 24, 2020, 11:26:10 am

author: MadTux
author_rank: Frequent Contributor
timestamp: June 24, 2020, 01:28:36 pm
content: Have you checked the row/column match pots in the schematic below? That's where I would start fiddling.Otherwise, do you have a second 7xxx mainframe?If nothing else helps, I usually start swap boards with a known good instrument, to narrow the problem down to a board, to keep my sanity, like if the problem is caused by leakage/noise/bad contacts....There likely is something misadjusted with the row decoder, decoder thinks it's in row2/0.1mA mode while it should be in row1/0mA.

author: Kidon
author_rank: Regular Contributor
timestamp: June 24, 2020, 01:48:49 pm
content: Quote from: MadTux on June 24, 2020, 01:28:36 pmNo I don’t have another 7xxx mainframe.Row/Column match pots? I haven’t seen that yet. In the manual?

author: MadTux
author_rank: Frequent Contributor
timestamp: June 24, 2020, 03:18:18 pm
content: Don't know if it's written somewhere in the manual, but in the schematic, it's R2184 and R2243, in the bottom left, C/D-3/4 coordinates on schematic.I guess these pots are there to adjust the current offset which the character decoder gets. And since the characters are decoded at an offset, I would just fiddle a bit on those pots and see if it improves anything, those trim pots on the readout board don't affect calibration after all and are rather easy to adjust back again.

author: Kidon
author_rank: Regular Contributor
timestamp: June 24, 2020, 03:22:52 pm
content: Quote from: MadTux on June 24, 2020, 01:28:36 pmOk, I see what you’re talking about.I’ve fiddled with these two pots. The column one has some visible effect, but not much, and not in the right area. It changes the “F” into a “J” and the “T” into a “R”. The Row pot has absolutely no visible effect whatsoever.

author: Kidon
author_rank: Regular Contributor
timestamp: June 24, 2020, 03:34:18 pm
content: Quote from: MadTux on June 24, 2020, 03:18:18 pmQuote from: MadTux on June 24, 2020, 03:18:18 pmI don’t have these references on the readout board I have.This is what I have:R2214 Column MatchR2273 Character HeightR2183 Row MatchR2291 Vertical SeparationR2128 Character ScanI’ve played with all of them, but still nothing.

author: MadTux
author_rank: Frequent Contributor
timestamp: June 24, 2020, 09:36:44 pm
content: That distorted IDENTIFY is interesting though. If I understand Tek7000 readout function decription correctly, IDENTIFY is done on timeslot TS1 and there is not supposed to be any more characters after the IDENTIFY command.The "Zeros and Logic IC" then triggers something in the row/column decoder, so that they generate currents so that IDENTIFY appears on screen. As far as I know, there shouldn't be any more characters after the IDENTIFY command, all other timeslots should be 0mA on row/column.Those 2 dots on 2th and 3th character/timeslot TS2/TS3 means that something goes wrong during those 2 timeslots. First thing I would check if all the timeslot lines have valid pulses on them, maybe a timeslot line has double pulse on it or something which causes the row/column decoder to go crazy. Should be easy to measure with a second scope, since they all appear in sequence.Other interesting thing to do would be to set a single readout plungin into 100µV or 100mV mode (with only that readout on screen), probe testpoint 8/10 with another scope and see if those voltages look similar as the example from Tek 7904A manual. (TS1:R3/C2: "add 2 zeros", TS2: R2/C1: "inverted", TS3 with R2/C10 for ">" uncal symbol, TS4: R1/C2: "1", TS8: R4/C2: "µ", TS9: R5/C2: "V")

author: Kidon
author_rank: Regular Contributor
timestamp: June 24, 2020, 10:07:41 pm
content: Quote from: MadTux on June 24, 2020, 09:36:44 pmI do have several other scopes I could use to check that. I just don’t know how to do it (yet).It’s a learning curve at the moment.On a side note, I did remove the readout board to check the grid bias and other voltages (some were a bit off) since I got this scope a few days ago with 7 plugins, along with a 466 at a very attractive price (for Europe, I guess...). They were both quite filthy, and in need of a calibration. So I’m also trying to follow all the procedures, starting with this one.But I agree, it is interesting.

author: jxjbsd
author_rank: Regular Contributor
timestamp: June 25, 2020, 12:43:37 am
content: Interesting topic, my 7623 oscilloscope also has a problem of trajectory interruption. It is necessary to understand the principle of the analog character generator, so that we can really understand where the problem is.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 25, 2020, 09:02:05 am
content: In my case the problem was an IC that stores characters (PROM) on readout module.First of all look at the manual and try to locate which IC generates characters thatshould be displayedbut are wrong.Often this guides to one specific IC.Then locate the IC on PCB and try to swap with IC known to be good.

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2020, 04:04:08 pm
content: The lines between the readout board and plug-ins are multiplexed placing the outputs from the plug-ins all in parallel. So check the readout with only one plug-in installed, and move it between slots, because one malfunctioning plug-in can cause the readout from the other slots to malfunction.

author: Kidon
author_rank: Regular Contributor
timestamp: June 25, 2020, 11:12:58 pm
content: Quote from: David Hess on June 25, 2020, 04:04:08 pmI have tried all the 7 plugins I have individually, as you suggested, and the result is the same.

author: Kidon
author_rank: Regular Contributor
timestamp: June 25, 2020, 11:16:38 pm
content: Can anyone who has a 7623 tell me if something is connected to that location and to what/where?See pict.Although I don’t see any wires up in the air anywhere, maybe I missed something...Maybe it disconnected while I was moving the scope....?

author: MadTux
author_rank: Frequent Contributor
timestamp: June 26, 2020, 12:37:18 pm
content: Not connected, see pics:https://amplifier.cd/Test_Equipment/Tektronix/Tektronix_7000_series_mainframe/7623A/images/7623_rechts.jpghttp://w140.com/tekwiki/images/f/f4/USA_Tektronix_7623A_Right1.jpgIf you have too much trouble repairing that board, you can also send it over to me for a little cash, I'll fix it.

author: Kidon
author_rank: Regular Contributor
timestamp: June 26, 2020, 01:07:11 pm
content: Quote from: MadTux on June 26, 2020, 12:37:18 pmUnless you’re located 1 mile from my place in Paris, you’re probably way too far.

author: Kidon
author_rank: Regular Contributor
timestamp: June 26, 2020, 01:08:09 pm
content: Quote from: MadTux on June 26, 2020, 12:37:18 pmLink returns a 403 error

author: David Hess
author_rank: Super Contributor
timestamp: June 26, 2020, 09:18:12 pm
content: Quote from: Kidon on June 25, 2020, 11:12:58 pmThat is actually good because it narrows the possibilities.Reseat the connectors and ICs on the readout board. Check the connections between the readout board and interface board which is where the plug-ins attach.

author: Kidon
author_rank: Regular Contributor
timestamp: June 26, 2020, 09:52:50 pm
content: Quote from: David Hess on June 26, 2020, 09:18:12 pmThanks. Will do and report back.

author: Kidon
author_rank: Regular Contributor
timestamp: June 26, 2020, 10:59:15 pm
content: Quote from: David Hess on June 26, 2020, 09:18:12 pmWhile checking for possible disconnect wires, I noticed this.Both transistors have very bad and cold solder joints, and the one on The left side is actually moving. Could very well be related. It might have moved during transport from one room to the next and cause this behavior.It’s not gonna be easy to resolder. Not the most accessible area...Could that be the problem...?

author: jxjbsd
author_rank: Regular Contributor
timestamp: June 27, 2020, 06:55:57 am
content: The 7000 and 400 series are the transistors in the Tektronix oscilloscope, which are basically installed in this way, usually without too many problems. You can try to compress it.

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 08:44:48 am
content: Quote from: jxjbsd on June 27, 2020, 06:55:57 amThat is scary!!! It goes against ANYTHING I’ve ever learned about soldering!When you pull on it a bit (the left one), it actually comes right off the board. And without it there is nothing on the display. So I guess it must be making some contact that way. It also tells me this IS NOT the source of the issue.I just don’t feel confortable at all having two transistors hanging inside flimsy and highly unreliable solder joints like that... I will try to solder the back properly.Unless someone else can confirm that, for some obscure reason defying the laws of good practice, this is how it should be, I will try to solder them back.

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 08:46:36 am
content: THIS IS SOOOOO FRUSTRATING! IT DRIVES ME NUTS NOT TO BE ABLE TO SOLVE THIS.I WANT TO THROW IT OUT THE WINDOW!! I PROBABLY WILL.

author: hnjmkl
author_rank: Regular Contributor
timestamp: June 27, 2020, 10:41:40 am
content: those transistors are mounted on zero-height sockets on the PCB, very reliable Tek sockets, I have a 7623, I'll see if a shielded cable comes out of that female connector.

author: hnjmkl
author_rank: Regular Contributor
timestamp: June 27, 2020, 10:55:42 am
content: Nothing is connected to the female connector on the PCB

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 12:02:41 pm
content: Quote from: hnjmkl on June 27, 2020, 10:41:40 amThank you.

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 12:34:03 pm
content: Quote from: hnjmkl on June 27, 2020, 10:55:42 amI sent you a PM.

author: jxjbsd
author_rank: Regular Contributor
timestamp: June 27, 2020, 03:29:26 pm
content: Quote from: Kidon on June 27, 2020, 08:44:48 amNever change it. You should trust Tektronix engineers that there will be no problem in doing so. Of the more than 10 Tektronix oscilloscopes I have repaired, none are due to transistor contact problems.

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 04:32:58 pm
content: Quote from: jxjbsd on June 27, 2020, 03:29:26 pmI am trusting them 100%. But 40+ years and heat can also take their toll on equipment.Not changing it though. I put the transistor back in place, and it works just fine.It was just very disturbing to see solder joints looking the way it did.Thanks for your input.

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 04:55:39 pm
content: Ok, so I found this in the manual (see Picts), but there are no mentions anywhere as to where these chips and Test Points are...And the characters I highlighted are exactly what I have on my scope instead of 1,2 and 5.So obviously the “ROW” potentiometer needs adjusting. The problem is that it has no effect whatsoever on anything...

author: Kidon
author_rank: Regular Contributor
timestamp: June 27, 2020, 08:31:55 pm
content: I don’t know what I did beside turning it on it’s side to tighten some bottom screws (in cas I hadn’t mentioned it, I got it a few days ago), and it Is now just working again...Something is apparently very sensitive and finicky in there...

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2020, 12:06:58 am
content: The transistors and some ICs and other parts are mounted using Berg Miniserts; some information is included below. Later the transistors were soldered in place.Reseating parts in the Miniserts will at least temporarily fix intermittent connections. The Texas Instruments IC sockets used by Tektronix during that time have reliability problems which is why I suggested reseating various ICs.If I find or suspect a problem, I tend to replace the Berg Miniserts and TI sockets with collet socket pins also shown below.

author: Kidon
author_rank: Regular Contributor
timestamp: June 28, 2020, 12:20:49 am
content: Quote from: David Hess on June 28, 2020, 12:06:58 amMuch more reliable.Quote from: David Hess on June 28, 2020, 12:06:58 amThis is probably what I did unintentionally, when I flipped it over to the side to tighten the bottom screws.But I still have no clue what had move and got back into place. Because I had already reseated anything I could physically reach.Quote from: David Hess on June 28, 2020, 12:06:58 amAnd I did, but it wasn’t the IC’s apparently. But good to do anyway.Quote from: David Hess on June 28, 2020, 12:06:58 amI would too, but I don’t feel like disassembling the thing to get to those hard-to-reach places.I already took the power supply off to install the “Pushbutton Illumination” (modification 040-0586-01), and that was a drag.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2020, 12:48:25 am
content: Quote from: Kidon on June 28, 2020, 12:20:49 amIC and transistor reliability was lower back when these instruments were designed, or at least they thought they would or could be lower. Eventually they improved enough that the sockets were less reliable than the ICs and transistors.

author: hnjmkl
author_rank: Regular Contributor
timestamp: June 28, 2020, 04:53:20 am
content: For my part I'm glad you solved it, it was a problem of false contacts.