Tektronix 7603, readout not working - Page 1
Forum section: electronics / repair

author: DwPetr
author_rank: Newbie
timestamp: January 11, 2025, 06:55:08 pm
content: Hi, I'm new here, so I'd like to introduce myself. My name is Petr and I'm from the Czech Republic. I came to this forum by looking for a solution to a problem with a Tek 7603 oscilloscope where the readout doesn't work. I found threads on this forum where users here were able to fix it.I bought the oscilloscope more than 15 years ago, and even then the readout didn't work, but I didn't fix it because the oscilloscope otherwise works very well. I have 7A26, 7A26, 7B53A plugins in it. The plugins date from 1992, I couldn't find the date on the oscilloscope, but by the looks of it I would believe that it also dates from that time. I haven't been into electronics for a while until I recently got back into it and decided to finally try to fix it.I have downloaded the service manual for the 7603 (year 1974 I think), and I got the manual for the 7633 for the oscilloscope at the time, it is of a newer date. This manual is more comprehensive in terms of diagrams, and in the identical ones, small details are different, e.g. testpoints etc.. but overall both are good.Unlike the 7603 oscilloscope, the schematics from the 7633 oscilloscope also show voltage values ​​at the test points and selected circuits. I read both manuals to try to understand how the readout works, what the connections are.I would like to say that I am new here and have not studied the customs of the community here, so please excuse me if I started a topic or posted photos differently than is customary here. ThanksIn the photo below, you can see that the oscilloscope works in perfect order except for the readout, both amplifiers and the time base show no defects. When the IDENTIFY button is pressed, the beam responds, but the readout fields do not show anything at all. The main beam INTENSITY control responds.The READOUT control does nothing at all when it is turned on, there is no response from left to right on the screen, even after pressing IDENTIFY.I decided to open the device and according to the diagrams and the repairs made here on the forum, I made measurements. First, I checked the functionality of the READOUT rotary switch, it is functional and the resistance changes as it is turned, it has a specific course towards the center, but since I do not have comparative values, I cannot judge whether they are correct, but I will assume that they are. The wiring is also fine, except for the READOUT board is fine. The board looks like thisTo measure the voltage at the test points, I used a Uni-T UT60G multimeter, the multimeter does not have true RMS, it was grounded on the chassis of the device. The procedure was followed according to the manual, i.e. the plugins were pulled out and the values ​​were set according to the manual. The following table shows the values ​​I measured. Values ​​that are green match the scheme. I noticed the problem mainly with:TP2112, where according to the schematic the voltage should be 5.2V, I measured 7V and after inserting the time base plugin, the voltage decreased to 1V. Other plugins had no effect.TP2129 was measured at 1.6V, in the schematic it is 1.8VTP2180 was measured at 8.1V, in the schematic it is 13.8VAnd this is how I continue, see the table, where I will point out the circuit U2284 (155 0020 00) where 4.8V was measured instead of 2V.Getting some IO wasn't a problem, so I bought U2284 (155 0020 00 format generator), 7402(U2120) and 7493 (U2250 Chanel counter), which I tried to replace. Yes, it is not a good procedure, it is very amateurish and dysfunctional, to change parts without the necessary diagnostics, and the effort to save work did not pay off, because there was no change. So at least they were not possibly destroyed.I couldn't do the measurements with the oscilloscope according to the manual exactly, because I don't have a second oscilloscope, and to be honest, if I were to buy a second one, I probably wouldn't need to repair the first one. And I wanted to at least back up my question with some effort, so I put all the plugins back in the chassis and did the measurements according to the instructions. I am attaching the results in the photos, where there are curves from the instructions, and my curves that I measured matched, they are green. The measured values ​​are described from the table according to the multimeter, I did not measure them with an oscilloscope, they would be different. So far, I was only concerned with the progress here. Where the measurements did not match, they are colored red, where there is also a drawing of what the beam looked like. The measurement on pin 12 completely disappeared, as well as in the second photo, and where the readout was turned off with a rotary switch, where the beam on pin 13 also disappeared.I am also attaching the shape of the signal from pin 16a positive voltage should have appeared on pin 14. I have to say that it was here all the time, but very low, about 0.4V. I would also add that the whole time I had the switch on the S2120 board in the upper position (Gate trigd)The voltage on the readout board seems fine, except for the mentioned green wire in socket P2171, where it was only 1V with the 7B53A plugged in.Now I have to re-analyze the instructions and have a deeper understanding of the meaning of the measurements I made, albeit in the wrong way, because I had to have plugins plugged into the mainboard.But maybe someone will have a similar experience, or patience, and since there are a lot of users among the members who understand this issue more than I do, which is probably all of them :-) he could push me in a better direction. I try to be very careful not to cause any damage to the device. It's a nice honest oscilloscope, and while I could afford to buy a new digital one, I'd still rather have this one.Thanks to everyone for the space and any help. And once again I ask for patience if I don't manage to do the topic well the first time.Petr.