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Heathkit ID-22 Beam Switcher

June 20, 2018

This device was intended to permit viewing two signals on a single beam oscilloscope. The input signals would have to be synchronised and though I haven’t tested this, probably of the same frequency. To cut straight to the chase, here are two signals displayed on a Knight KG-630 at 1kHz:Heath ID 22 Knight HG 630 1KHz

At 10kHz:Heath ID22 Knight KG630 10KHz

The chop rate can be selected from 200, 500, 1100 3700 Hz (as measured on my unit and rounded to the nearest 100). Here is the schematic:heathkit_id22_electronic_switch_sch

The circuit is quite straightforward. There are two input cathode followers connected to switch tubes.  An astable MV generates the chop squarewave that is used to alternately bias the switch tubes off via driver cathode followers. The separation between the traces may be controlled by altering the relative “ON” DC potential of the switcher cathodes. A further cathode follower buffers the output to the oscilloscope. Cathode followers are also provided to allow a sync signal to be taken from either input, the scope must allow an external sync signal. This is so that the scope doesn’t “see” the chop waveform and try to sync to both that and the signal, resulting in a useless muddle on the screen. It is necessary to play with the chop rate along with the scope sweep frequency to get a decent display. Both the pictures above were taken with the chop at 500Hz.

As designed, the sync CFs are connected to the input grids. I found that this resulted in the input being loaded down especially as the frequency increases so I relocated the sync CF grids to the input CF cathodes. MUCH better.

Amazingly, the coupling caps in this specimen appear to be of fairly low leakage so I left them. I did add a B+ resistor to get the B+ within the electrolytic voltage specification. This is due to increasing line voltage over the years. However, the capacitor must have been marginal voltage wise, even back in the day. Other than lubing all tube pins, switches and pots, it worked. For audio application where the frequencies presented to each input will be the same, I imagine this device would have been a boon in the days when professional scopes were too expensive for most amateurs. On the other hand, I expect most people played with it, as I have!







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