Tektronix 555 Restore / Repair

I am still working on the blog ..please do checkout my video until then for the progress.


This is an active project and I plan to document all by the end of it..

This was one of my dream project, to restore and document a 555. As it is, it is a monster with close to 111 tubes in it. So here is how I decoded it and repaired it.

As always the first step is to clean and bring to a cosmetic and hygienic condition that I can work on it. This is the most boring and time consuming part. Ive spend weeks cleaning this up.

Here is a video explaining the whole process of cleaning




First part. This is a two part oscilloscope. There is a power supply unit and then the “Indicator unit” which is the oscilloscope. Both are connected via an “interconnecting cable”

So its easy to break it apart in to two parts and focus on one at a time.

To start with, they can not work without each other.

So the first attempt is to get the power supply working and tested without the Indicator Unit.

So here is the pin out of the interconnect cable. Basically we need to put a bypass for the Thermal cut-out in the indicator unit to get the power to the heater transformer going.


Thanks to FirielNXD @ youtube for updating this: Important for 240V units:The fan will only spin up with the indicator unit disconnected on scopes configured for 120V, it will not do so with 240V units as the power supply and indicator fans are connected in series in 240V units, rather than the parallel setup seen in 120V units.

Second part is to supply heater voltages to the tubes which get regulated heater supply from indicator unit.

Here is a map of tubes in the power supply unit with source heater power.


So the basic hack to get the power supply unit working by itself is as below.


However, remember the power supply will not regulate as there is no load.So basic power up can be achieved and tested to ensure all rails are alive and regulates/follows the -150V rail.

Also the interesting part of heater regulator for the indicator unit. A 12-AS15 senses the 6.3 out of the heater transformer inside the indicator unit and uses it to control the current through the saturable reactor, which sits in series to the primary for the heater transformer inside the indicator unit.


The whole process, with schematic is explained in the video.

Now to plug-ins. This unit came with a type H and type L vertical Plug-ins. Detailed restoration and explanation of the same down below here.


Next to H-Plug-ins. Both type 21 and 22. Here is the video explaining the Tek 500 series sweep circuit based on type 21 and overview of type 21/22 plug-ins with details of components.


Few details around the Horizontal Section of typical 500 series scopes. This is based on type 21 plug-in. High level map of the stages involved in sweep generation.


Here is the breakdown of the trigger section


And here is the breakdown of the sweep section

Sweep Sections.png

Here is the simplified signal flow through the sweep section.


Next : Restoring Indicator Unit.

Here is the map of the indicator unit top section


Detailed video of the indicator unit coming soon !



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