Electronics > Repair

Tektronix 475 vertical position knob problem

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israel:
Hi,

It has come to my hands an scope with so many years in st-by. It is a Tektronix 475 is in very good condition but has several problems.

Necessary repairs in the power supply have been performed successfully, but there is a problem with one of the channels (2).

The problem is that the vertical knob does not work. As time goes up the trace to get lost in the top of the screen.

It's like there DC voltage at the amplifier channel 2.

I have removed the first amplifier IC, and trace can view and move perfectly. Thinking that the problem could be the amplifier IC, I've exchanged. But the result is the same, I have

also disconnected the resistance goes (solder aerially) to one of the pins of the IC (I guess that is the entrance). But still unsuccessful.

In a quick check the voltages, I have seen are all correct and identical to the channel that works well.

I rule out a problem in the power section of the vertical deflection, when removing the IC amplifier channel 2, the vertical knob responds perfectly.


Any ideas or suggestions?

They will be very well received

See video...

http://youtu.be/DMu_bnzYmv0



Sorry for bad my english.


Best regards,

David Hess:

--- Quote from: israel on September 05, 2014, 01:54:42 pm ---The problem is that the vertical knob does not work. As time goes up the trace to get lost in the top of the screen.
--- End quote ---

Does the vertical position control do anything?

The only place a gradual drift can originate is one of the FET inputs which have low input bias current.

Does it still drift when the input coupling is set to GND?


--- Quote ---I have removed the first amplifier IC, and trace can view and move perfectly. Thinking that the problem could be the amplifier IC, I've exchanged. But the result is the same, I have

also disconnected the resistance goes (solder aerially) to one of the pins of the IC (I guess that is the entrance). But still unsuccessful.

In a quick check the voltages, I have seen are all correct and identical to the channel that works well.

I rule out a problem in the power section of the vertical deflection, when removing the IC amplifier channel 2, the vertical knob responds perfectly.
--- End quote ---

My guess is that R39 on the attenuator board is open.  This resistor sinks the FET input bias current to ground.  Otherwise it would gradually charge the input capacitance.


--- Quote ---Any ideas or suggestions?
--- End quote ---

Ask over at the TekScopes@yahoogroups.com email list.

oldway:
Measure the input voltage of the first amplifier of channel 2 and compare with channel 1. (pin 13 of U220 or R62 / R204)
Seems to be a problem of faulty diode(s) CR203/204/205.
NB: back side of the board.
See also R219 (30R).

David Hess:
I agree about the input protection diodes.  I did not see any examples of "drift" in the video.  If swapping U120 does not change anything, then the input protection diodes are the next likely cause of the problem.

If the diodes are bad, then I would grade some small signal transistors for high base-emitter reverse breakdown voltage and use them.  If the diodes are changed, then the input compensation will need to be adjusted.

israel:
Thank you very much all for your invaluable help. Eventually I was able to find the fault, it was a broken one end resistance.   :) :) It was almost imperceptible to the eye, so I pass unnoticed.

The problem is that I could see that one of the amplifiers IC is faulty.  :-BROKE

Now the adventure is to find one of these Is (1St cascode amplifier U220).
 :palm: :palm:


Best regards for all.

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