Tek 577 non linear behavior in both vertical and horizontal deflection amplifier - Page 1
Forum section: electronics / repair

author: rodd
author_rank: Regular Contributor
timestamp: January 12, 2022, 01:08:59 pm
content: Dear all,I have been struggling to fix a Tek 577 curve tracer for some time and I have posted other question related to this project but I decided to create a new topic with this question since it seems to be very specific.After calibrating the 177, I tried to continue with the 577 calibration procedure, but it is not possible to adjust the trimpots.So I decided to try the performance check to try to identify which section(s) of the circuit are defective.It turned out the I found that there were problems in both the vertical AND horizontal gain sections.The "workable" area on the display is a square of approx. +-4div horiz and vertical.So I moved to diagram 7 (Vertical & Horiz Output Ampl ) for serial numbers below B082440, which is my case.First I checked the +200V, which seems to be a little high 220V but since there is no adjustment I guess it is not the cause of the problemMy next step was to measure the voltage between +VERT and -VERT as well as +HORIZ and - HORIZ.The manual says the gain of both vertical and horizontal sections should be 0.5V/div.However what I am observing is that the gain is correct from +1.5V to -1.5 V for the vertical and highly off for the horizonTALHere are my measurementsVertical sectionvoltage(V) deflection (div)2.0 to 5.0 3.11.5 3.01.0 2.00.5 1.00 center-0.5 -1.0-1.0 -2.0-1.5 -3.0-2.0 -3.5-2.0 to -4.0 -3.8Horizontal sectionvoltage(V) deflection (div)2.5 to 4.5 3.92.0 3.41.5 2.51.0 1.90.5 1.10 + 0.2-0.5 -0.5-1.0 -1.3-1.5 -2.1-2.0 to -5.6 -2.8Is it possible that all 4 output transistors (Q1104, Q1114, Q1124 and Q1134) have failed?Any ideas to explain this non linear behavior of BOTH sections?I have attached the schematics of this section for to make things easier for anyone who would like to helpThank you very much in advance,RogerVertical & Horizontal Amplifier.pdf(228.77 kB - downloaded 82 times.)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2022, 02:26:28 pm
content: Are the 5 V still OK ? If the resistors are carbon, one (espcially R1101) may have gone high. It may be worth to check them, as a limited current from the 5 V could explain the ouput stages saturating at a similar level.

author: mazurov
author_rank: Frequent Contributor
timestamp: January 12, 2022, 06:17:44 pm
content: Which variant do you have? I have D2 (no storage) in good working order. I seem to remember similar issue with mine when I just got it. It went away after I cleaned up the selectors.PS Let me know if I can be of any help.

author: rodd
author_rank: Regular Contributor
timestamp: January 13, 2022, 04:20:34 pm
content: Thank you for all the replies.My model has storage (557 D1).I have made a first cleaning with deoxit when I started the fixing job, but I might do this again.I am suspicious of S1125 (beam finder), which should be NC, connecting the -30V to the circuit.If it is open, R1125 will be inserted in series with the -30V supply, limiting the current to both sections.It seemed that I could obtain a linear behavior of the circuit yesterday, but it was very late at night and I have to try again.I will post my findings after more testing.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 13, 2022, 04:40:08 pm
content: S1125 is indeed a possible single point of failure.

author: rodd
author_rank: Regular Contributor
timestamp: January 17, 2022, 07:09:24 pm
content: I fumbled around S1125 and the problem seems to be gone.But since there are other issues with the instrument I can't tell for sure.