Tek 577 non linear behavior in both vertical and horizontal deflection amplifier

[rodd]

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 problem

My 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 horizonTAL
Here are my measurements
Vertical section
voltage(V)     deflection (div) 
 2.0 to 5.0           3.1
 1.5                    3.0
 1.0                    2.0
 0.5                    1.0
  0                      center
-0.5                  -1.0
-1.0                  -2.0
-1.5                  -3.0
-2.0                  -3.5
-2.0 to -4.0       -3.8

Horizontal section
voltage(V)     deflection (div) 
 2.5 to 4.5           3.9
 2.0                     3.4
 1.5                     2.5
 1.0                     1.9
 0.5                     1.1
  0                     + 0.2
-0.5                    -0.5
-1.0                    -1.3
-1.5                   -2.1
-2.0 to -5.6       -2.8


Is 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 help

Thank you very much in advance,
Roger Vertical & Horizontal Amplifier.pdf (228.77 kB - downloaded 67 times.)

[Kleinstein]

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.

[mazurov]

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.

[rodd]

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.

[Kleinstein]

S1125 is indeed a possible single point of failure.

[rodd]

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.