Tektronix 465 Vertical Deflection Issues

[netwinder]

Hello EEVblog forum,

I have a Tektronix 465 oscilloscope (no options) and its been a very good unit. However, something that i didn't take much notice to when i bought it was the vertical deflection issues.. What i mean is, the grid installed is 10x10, if i adjust the position of the trace manually it will only go up 2.5 - 3 divisions before the origin slowly moves to a halt. Same goes from moving from the origin down, it will go down about 2.5 divisions and stopped. So far I've tested all my voltages and they are all in spec. I have adjusted the gain pot on the vertical output amplifier, and that in theory should solve my problem but it does not; all it does is deflect the beam just a 2-3 millimeters () The deflection coils aren't getting enough power to deflect the beam to the extent that is necessary, according to the Textronix service manual the "vertical deflection factor accuracy" averages out to be 4 - 4.7 divisions.

I have done a performance test for the vertical, which consisted of

• Checking the trace rotation (pass)
• Checking ALT and CHOP modes(pass)
• Checking the balances on both channels (pass)
• Checking the position range and centering (pass)
• Checking the beam finder (pass)
• Checking the deflection factor (failed)

To elaborate on the check of the deflection factor, the test consisted of connecting a 20 millivolt signal to ch1, and then set to the 5 millivolt range. The vertical deflection is supposed to be 4 divisions, and given the 3% tolerance that works out to 3.77 to 4.12... I ended up with 2.5, thus a fail.

• Checking the variable volts/division range (pass)
• Checking the output amplifier low frequency compensation (pass)
• Checking both channels volts/division compensation (pass)
• Check both channels transient response (pass)
• Checking the bandwidth of both channels (frequency generator does not go to 100 MHZ, the bandwidth is probably fine)
• Checking the cascade gain and bandwidth (pass)

Its fairly clear that the oscilloscope is perfectly fine besides this deflection issue..

Note: The vertical amplifier on the Tektronix 465 came in two flavors, one being an "IC" version, one being a discrete version. They have differences in terms of possible adjustments that can be made, just to note that mine is the discrete version, which gives you the adjustment points for gain, HF (high frequency?) compensation, LF (low frequency?) compensation and centering adjustment. There are 3 HF compensation pots and  2 LF compensation pots.

http://bama.edebris.com/download/tek/465(2)/465.pdf  <-- There's a link to the Tektronix service manual.. Just a few helpful pages I've been using:

Page 261 has the vertical adjustment board's adjustment points

Pages 59 - 62 has the performance check

Pages 81 - 92 has all the calibration checks for the vertical


Any ideas as to how to solve this vertical deflection issue?

[Andy Watson]

Measure the voltage on each of the Y plates as you vary the Y position control - should find that both voltages vary equally and in opposite directions. From the symptoms that you describe I would guess that one half of the Y amplifier is not functioning.

[netwinder]

They do vary equally, as i can move the trace equally up and down.. Any ideas how to go about fixing it however?

[tautech]

If you haven't got this already it should be useful:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[netwinder]

Thank you very much. Ill go through troubleshooting the vertical section and post any findings