Repairing a Telefunken OMS 811 oscilloscope


Some time ago one of my truly beloved Telefunken OMS 811 oscilloscopes died (these are used in the Telefunken RA 770 analog computer) - no trace was displayed - nothing...

It turned out quickly that there was not high voltage present at the display tube's connectors and some further investigation showed that the blocking oscillator on the primary side of the high voltage transformer was working. Unfortunately, the high voltage transformer and the high voltage diodes are sealed into a single package so there was no way to dig further into this device (see picture on the right).

Although I always try to repair my machines using the technology of their respective time, I had to divert from this path since I just did not find any suitable high voltage transformer package which would fit into the instrument. So it became clear that I would have to replace the oscillator, the high voltage transformer and the rectifier stages with something else to get the instrument working again.

The picture on the right shows my first attempt: The two transformers were salvaged from an old scanner I found at work in a garbage bin, the two blue heat shrinking tubes contain three 1N4001 diodes connected in series each to feed the two blue high voltage capacitors.

Although this circuit worked quite fine, the resulting voltage of about +/- 1 kV was too low by a factor of two for driving the display tube of the oscilloscope. Back to the drawing board...

Since the basic circuit worked quite fine, I decided to use another high voltage transformer. Fortunately, I had some parts of a defective Tektronix 7000 series oscilloscope at hand.

The picture on the right shows the resulting circuit - the two small transistors have been replaced by a 2N3055 mounted on the back of the metal frame. This circuit was very well capable of delivering +/- 2 kV after rectification which is perfect for the oscilloscope.

The picture on the left shows the completed circuit already mounted inside the Telefunken OMS 811 oscilloscope. It fits quite well into the space previously occupied by the original high voltage power supply and the instrument is working like a charm again. :-)