485 mostly working, need advice re switches


Robert Simpson
 

Would appreciate some advice
Background

Originally this 485-1 only worked on DC channel 2. All PS voltages OK, very low if any ripple. Found burned input resisters, now replaced. Cleaned attenuator contacts. Using home made power extender cables and 4V compensation signal from an R7603 as as a test signal, found dual FET Q34 bad in channel 1, replaced. Also suspected 10X and 100X attenuator modules, replaced those. Now get 40mv output on both channels 1 and 2, AC and DC.
However, Scope only displayed 2 volts (1 volt per division setting) using the 4 volt input signal. Tried adjusting vertical gain, but could not get full 4V. Found over value resistors feeding delay line: R407, R408, R409, R427, R428. Replaced.
Readjusted vertical gain and now get 4V display at 1 volt per division setting with vertical gain pot in mid third of its adjustment range.
Let scope run for awhile when signal started to wiggle. Top of Square wave moving up and down about half a graticle division. Turned scope off and later tried again. Stable after several hours. Still stable again this morning.
Maybe a cap reformed?
Need to lube some sticky switches like the 50 ohm to 1meg ohm input selectors.
Would Deoxit F5 work?
Bob

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