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Tektronix 7603 Oscilloscope - Repair Log

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David Hess:

--- Quote from: sean0118 on August 22, 2021, 05:33:39 am ---I went through the calibration procedure for the horizontal gain of the 7603 (as best I could without a calibration module). First I set the 4V ref to DC and calibrated that to my multimeter, I set it back to 1kHz and swapped the vertical and horizontal modules. This gives vertical traces spaced horizontally by the 4V ref, which allowed me to calibrate the horizontal gain independent of the horizontal module.
--- End quote ---

Standardization is only required if you expect to swap modules between oscilloscopes which was never really intended.

Before I got a standardizer plug-in, I modified my 7A16A amplifier with a couple of test points at the interface so that I can connect a multimeter to measure the DC and AC signal voltage at the interface, which as I recall is 25 millivolts per division but someone should check me on that.  Many multimeters have plenty of resolution and accuracy at that level to calibrate the mainframe better than required.  AC calibration can be done with a low frequency square wave and either AC RMS or AC averaging with a correction factor of 11%.

sean0118:
Okay it died again!  ::)

I left it on for a bit then came back to find the trace was gone. Measuring the power rails I found they were all much lower than usual, which pointed to an issue with the -50V rail as it's the reference for the others.

When turning the scope on I found the -50V rail increased very slowly and never reached -50V. I suspected the 5.6uF electrolytic capacitor C889 in the reference circuit had gone leaky, however I lifted one leg and found it to be okay.

I then found R890 (5.49k) had gone open. It biases the zener diode reference, so without it the zener voltage was much lower than normal and C889 charged very slowly.

I replaced it with a CPF15K4900FEB14 (5.49k, 1%, 25ppm, 1W), it's much smaller than the original resistor, however the service manual specs it as only needing 0.5W. I also re-soldered some of the regulator pins as they looked possibly cracked and some had way too much solder. One or two of the pads were already lifted slightly so it may have been touched up before.

With R890 replaced the scope works again and all the power rails are in spec.   :D








youngwug:
What is the measured value after removing the R890(5.49k)?

sean0118:

--- Quote from: youngwug on October 28, 2021, 09:32:05 am ---What is the measured value after removing the R890(5.49k)?

--- End quote ---

Hey sorry I just saw this, I think I measured that but I can't recall what the value was. I just measured the voltage over it as 40.39V (-49.32V to -8.93V), that's only 7.4mA / 0.3W so I think it's okay, I'm not sure what caused the original resistor to fail.

I think something else is wrong with it now though, the trace keeps fading out, disappearing completely at times. The trace intensity adjustment seems to have no effect. The display intensity for the 1ms label etc does work fine though...

Anyone have ideas what I should look at first?

edit: I forgot to mention, the power rails (-15V, +5V, +15V, +50V, +130V, -50V) look good, both DC and ripple. I'll have a look the the manual to check the CRT bias etc.

edit2: Now that it's had time to warm up the trace isn't disappearing but it's brightness is unstable and the trace intensity knob still has no effect. Seems to be a similar issue to this:
https://www.eevblog.com/forum/repair/tektronix-7313-trace-disapears/

David Hess:
The DC restorer which levels shifts from low to high voltage in the z-axis circuit path could be leaking before failure.  You might inspect it in a dark room and see something.

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