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Hi Gang:
Whenever I have a 485, or any other scope for that matter, in my shop for
refurbishing I always tap on every potentiometer with a screwdriver handle
end while watching the CRT. If I see any flickering I replace that pot.
As to the shortened trace I would strongly suspect the X6 voltage
multiplier, U1600. The failure pattern for U1600 is a short trace in the
center of the CRT. Sometimes the trace is not visible for several seconds
after applying power To determine if U1600 is putting out 21KV proceed as
follows:
1) Disconnect all inputs to the neck of the CRT. This opens a return path
for a high voltage arc which will happen next if U1600 is good.2) Apply
power and wait about a minute.3) Remove power and immediately pull out the
high voltage connector at the left rear of the scope. Move the removed end
near the CRT metal shield and if you get a big spark you know U1600 is OK.
If you leave the 6 neck connections on the CRT when doing this test you
will have an excellent chance of killing U660, the vertical amplifier
output. This fact was verified by me the herd way, I lost two U660's
If no spark occurs you know the X6 multiplier is bad and must be
replaced. They are available as a new item from Voltage Multiplier Inc.,
Visalia, CA as item VM176. Price a year ago was $150 each. If you replace
U1600 carefully check R1602, a 15K, 1/2W resistor. It is frequently
toasted if U1600 is bad.
The 485, in my opinion, is the best analog scope Tektronx ever made. It
is versatile, robust, small, has excellent bandpass, reliabile, fairly
easy to repair and has many features that are outstanding.
Good luck but, whatever you do, do not trash it.
Reed Dickinsonreed714@...
On Friday, November 8, 2019, 09:42:50 AM PST, Raymond Domp Frank <
hewpatek@...> wrote:

On Fri, Nov 8, 2019 at 06:32 PM, Abc Xyz wrote:

Shailendra,

How did you go about Finding that Faulty Potentiometer?

I found them because they turn very easily or have bad/scratchy behaviour.

Raymond