The Tektronix 432 is a 25 MHz dual-trace portable solid-state oscilloscope.
There is also a rack-mount version, the R432.
Each 432 vertical signal path starts with the coupling switch,
then a rotary cam attenuator,
then a dual-JFET source-follower buffer amp,
then a 155-0050-00 differential amplifier (switched to 155-0050-01 after serial number B239999).
The vertical and horizontal output amplifiers are fully discrete, using silicon bipolar transistors.
Triggering is done by a 4.7 mA tunnel diode that is fed by a discrete trigger amplifier.
The 432 consumes 55 watts maximum and weighs 20 pounds.
P31 phosphor was standard. P7 was also available.
This shows the area around the BNC input connectors for channels 1 and 2. This photo was taken when the owner was cleaning the vertical gain rotary switches which had become flaky after 45 years.
View from the rear. The vertical gain attenuator switches are under the metal covers.
When the metal covers are removed, one can see the shaping/compensation capacitors. These can be unplugged, each one has 6 legs.
Another view of the shaping capacitors. Work on one channel at a time to avoid mixing these up. Carefully clean the legs with isopropal alcohol and paper towels.
In order to access the rotary gain switch fingers, one must first remove these BNC connectors, which means unsoldering the central wire (connects to a resistor/inductor feed-through) plus a debouce capacitor and lead going to the x1 x10 indicator lights.
Close up view of BNCs. These need to be unsoldered for removal, but nothing else must be unsoldered to access/clean the vertical gain rotary switch.
View of the input preamp. This board needs to be unplugged only, but not removed or unsoldered. It has four 2-pin connectors. Then it can be removed, still attached to the compensation capacitor carriers.
Lifting off the assembly consisting of the 2 compensation capacitor carrier plus the vertical preamp board plus the push-button switch board for CH1/CHOP/ALTERNATE/CH2. The switch board has two plug connectors below that must be removed. It helps to have long fingers or a good pair of needle nose pliers.
Second view under the assembly being removed.
The rotary switches exposed. Each one has four contact fingers underneath. They are cleaned using small (5mm) wide strips of paper soaked in isopropanol alcohol (2-propanol).
One can just see the four contact fingers peeking out. You can get paper under them using small tweezers. Patience and calm are required.
A view of the variable-gain potentiometers. Note that the bodies are both cracked in the same place. Both had been noisy, but the noise was largely eliminated by sweeping them back and forth a dozen times. That may have scrubbed off the oxide.
A view of the contact fingers underneath the capacitor shaping blocks. Each channel has 8 NC and 8 NO fingers (the other half are on the bottom). These are easy to reach and clean as described. So each channel has a total of 20 contact fingers to clean.