[Attachment(s) from Jerry DeHaven
included below]
This post describes a procedure that I used to remove and
clean the Ch1 and Ch 2 input assemblies for a Tek 475A scope.?
My scope was showing intermittent and erratic traces even after
switching ranges and tapping on the range switch.? What I
discovered, and repaired, made the vertical display like new again.
I followed the procedure shown in the 475A Service
Manual, pages 4-24, 4-25.? This manual is available on
line.? The dis-assembly involves removing the Vertical
Preamp Board and the Vertical Mode ¨C Gain Switch Board and the associated Ch
1 and Ch 2 attenuator assemblies.? It sounds formidable
but it can be done.? Take pictures with a digital camera
or draw pictures in your own style to help you retrace your steps for
re-assembly.
The input attenuators are mechanically operated by the
range knobs (Volts/Div) which have cams that operate very small ¡°switches¡±
on the Gain Switch circuit board.? The attenuators are the
little rectangular plastic assemblies which plug into the Vertical Mode ¨C
Gain Switch Boards (Ch 1 and Ch 2).? At first I tried
using contact cleaner on the Attenuator module pins, sockets and
board-mounted switch contacts (see photos) but that had almost no effect on
the quality of the vertical displays.
Looking at the bottom of one of the Attenuator modules I
noticed that some of the solder joints looked quite bad and had signs of
corrosion.? The Service Manual calls for replacement of
the Attenuator Modules.? They are available on line, but I decided to
look further.? This is where you will want to have a
somewhat steady hand, a good work light and an eye loupe or a head band
magnifier.? I use one made by Donegan Optical, the
OptiVisor model with 5 diopter optics.? I bought one at a
local model / hobby immortal.?store.? They are also
available on line, new or used.
Before removing any of the Attenuator modules I marked
them by Channel, and by location.? Then I removed the
modules one at a time, and did a visual inspection of all of the solder
joints on the bottom of the attenuator modules. I even opened up some of the
modules being careful not to bump any of the variable elements (resistors or
capacitors).? Where a solder joint looked bad I applied a
small dot of liquid flux and carefully applied the soldering iron tip and a
very slight amount of new solder if needed.? I did not use
any flux remover (or alcohol).
When I reassembled everything I was very happy with the
results.? In summary, I believe that the main cause of the
erratic displays was the vibration sensitivity of the attenuator module cold
solder joints.? You could pay for new / used attenuator
modules but I found that it is possible to correct the problem first
hand.
Photo 1: Vertical Preamp Board and Vertical Mode¨CGain
Switch Board.? Channel 1 attenuator cover removed to show
attenuators.
Photo 2: Vertical Mode ¨C Gain Switch Board
with all attenuator modules removed to show switch contacts. (Mark the
modules before removing them!)
Photo 3: Vertical Mode ¨C Gain Switch Board close up
showing board-mounted switch contacts. Carefully clean the little ¡°forks¡±
with good contact cleaner or alcohol and a small strip of
paper.
Photo 4: Attaching Vertical Mode ¨C Gain Switch Board to
front panel.? Each input attenuator module attaches to the
front panel with two nuts.? I used an Allen wrench to hold
the nut and a cut up wooden stick to turn the nut onto the screw.
Photo 5: Use a nut driver to tighten the nuts to the
front panel screws.
