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Old Oscilloscope repair

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I have an elderly Telequipment S51B oscilloscope, that I'm given to understand was all the rage in 1970's science labs.

The little beauty still fires up, and after a few minutes to warm and some knob twiddling I get a nice green dot march across the screen as you'd expect. However, no-matter what I plug into the front of it, I can't get it to respond to an external signal (the y deflection knob works fine). A work colleague had a brief look at it a while back and suggested one of the valves on the input amp was kaput and gave me a set of schematics and a datasheet shortly before he retired (never to be seen again).

Now the 2 puzzles I face are which valve to replace, and why has he given me a datasheet for an ECC83 valve when my little scope contains only ECC88 and ECC80 valves. Can I use an 83 in place of the 88's?

The following puzzle is where to obtain whichever valve it is I need to get the scope working properly again.

Any thoughts anyone?




New Member

A replacement for ECC88 would be 6DJ8 AND your scope isn't equipped with an ECC80. Look closer and you will see it says ECF80 (Position V6)
Replacement for an ECF80 would be 6BL8.
Your college disappeared and maybe for the best. There are NO ECC83's/12AX7's in this scope. Beware, even if the ECC88/ECC83 tubes are similar the heaters are connected differently and besides in an o'scope you really need the right characteristics of a tube to be able to trust the readings.

You MAY experience a defective tube, BUT I would do some measuring of Anode, Grid & Cathode-voltages on all tubes before discarding any. Grid resistors/cap's, anode resistors, cathode electrolytics are all components to be checked in case you come up with unexpected voltages, i.e. +on a grid, low anode voltage etc.

You have the schematics, now go do something usefull with it :D


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But first check that all tubes are lit.
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