Re: Tektronix 454


Roger Evans
 

Erik,

I am about to go on holiday so here is something of a long list.

Make sure the Ch2 V/div switch is cleaned with IPA and rotated back and fore a few times.

It is just possible that one of the C149A - J trimmer capacitors has developed a short circuit, perhaps with some foreign object.

Read the theory of operation section of the manual, it shows how the V/div switch has three separate functions, input attenuator, and two sets of gain switching. There is a table showing which parts are switched in at each V/div setting so it helps to narrow down the source of the problem.

Make sure the CH1 preamp and the main Y amplifier are close to being correct. With the input set to GND the trace should be centred on the screen with the vertical position control around midway for all V/div setings. You should be able setup the Ch1 step attenuator balance according to the instructions in the manual. Also the V/div setiings should be close to correct. This is to make sure that the Ch2 preamp is the only source of major imbalance.

If you are following the schematics in a scanned version of the manual beware that the original has blue lines showing where components are located on particular circuit boards. These scan in as black lines and even have (oversized) dots where they cross real interconnections.

Set Ch2 input to GND and at least to begin with choose a V/div setting where the beam is on screen. If you can, centre the trace vertically else go as close as possible to centre. Measure the voltages at the grid of V113, cathode of V113 and collector of Q24 and compare with the manual.

Measure the voltage at R137 and see how it varies with the vertical position control.

Measure the following: collector Q124, collector Q134, emitter Q134, collector Q144, emitter Q144 and compare with the manual. How do they change when you rotate the V/div switch? Is there an obvious correlation of the voltages that disagree with the manual with which segments of gain switching are in use?

If you are suspicious of any of the transistors then swap them, one at a time with the corresponding part in Ch1. Maybe clean the legs while they are out. If you are not sure of the significance of any of the measurements then repeat them on Ch1.

If you get this far and all is well then refer to my previous e-mail for probing around Q164, Q174, Q184 and Q194.

Regards,

Roger

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