Tek 465B Preset Potentiometer Problems


Shailendra Krishan
 

Hello Group,

I have this Tek 465B which was stored away for 5-6 years, and brought out again recently for routine use. As I switched this on, I noticed a few problems.

1. No output on calibrator loop
2. Horizontal trace shrunk to about 7 divisions, not filling the full screen
3. Unstable trace when in channel 1 mode, jumping up and down vertically by about one full division at random
4. Difficulty in getting a stable trigger, could be due to the above mentioned problem in channel 1

I started to troubleshoot the simpler problems first, 1 and 2.

Problem 1 located to R4274, Horizontal X1 gain potentiometer (1K Ohm). This pot was found to be open on the slider terminal, hence acting as a 1K fixed resistor. Replaced this pot and done X1 gain adjustment. Problem fixed.

Problem 2 located to R4293, Amplitude calibration potentiometer (50 Ohm). This pot too had EXACTLY the same problem and was found to be open at the slider terminal. Pot replaced, and the problem fixed. Amplitude calibration done.

Both the pots are Bourns 3352 open cermet type. The question is, whether this is a known problem with Bourns 3352 pots when used in Tek 46x scopes?
Is it probable that there are more such pots in the machine, which are causing the channel 1 problem mentioned above?

I am new to fixing scopes, hence thought of asking the group for inputs.

I am yet to start work on problem 3 and 4, and if I can get group inputs about pot problems, it might help to speed up the work.

Thanks

Shailendra

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