Circuit Notebook 100 - A Video/Audio Switch-box

RosetteI am absolutely delighted to have been made an Honorary Member of the Club in recognition of producing 'Circuit Notebook'. I would like to express my sincere thanks to the Committee and to the many Editors of CQ-TV for their friendly help and encouragement over the years.

And now, having at last reached No. 100, Circuit Notebook looks back over the half century of Amateur TV circuits choosing a few which have interesting and/or useful features.

But first, a bit of background. My interest in Television began 1950 when I built a TV receiver for my parents, ready for the opening of the Northern BBC Winter Hill TV station in 1951. I used a home-built test pattern generator and oscilloscope to set it up. The receiver had a 12" diameter round Mazda CRT and of course, used valves throughout.

I then began experimenting with TV transmission and built a flying-spot scanner. In 1957 I started transmitting 405 line pictures using a valve transmitter on 436 MHz. These activities with GW3FDZ/T from 50 years ago are described in 'Turning back the Pages' in CQ-TV 224, page 37.

At that time, all my circuits used valves, but in the 1960s I, and many others, gradually moved over to transistors. CQ-TV 48 contained details of a Transistorised Pulse & Waveform Generator by Mike Cox. Inspired by this design I began rebuilding my station using transistors and eventually ICs. I kept a Hard-backed Notebook in the shack and recorded details of everything I built. I constructed a wide range of electronic equipment and a fair proportion of these designs were related to ATV.

From my Shack Notebooks came the Circuit Notebook series published in CQ-TV. I also included (with acknowledgement) circuits from other sources including those from fellow Television Amateurs, in fact anything that seemed interesting and appropriate. I will pick out just a few, starting with No.1 in CQ-TV 68 Nov. 1969

Circuit Notebook No.1. A Pulse Narrower Circuit (Fig.1.)

This simple circuit will produce a short positive-going output pulse when driven by a longer negative-going pulse. The circuit is a practical experimenters dream. With a base resistor of 150kohms the duration of the positive output pulse is approximately equal to1 microsecond per 10pF or for 15kohms the pulse is 1microsecond per 100pF. The circuit shown accepts a line blanking pulse and generates a front porch and line sync pulse signal. The component values shown are for 405 lines (it was 1969!).

The output can drive a further stage to provide a delay. A negative-going pulse can be generated by inverting the circuit and using a PNP transistor. This circuit was used extensively (10 times in all) in 'A Simple Sync Pulse Generator' described in CQ-TV 68 Page 5. An associated 'Simple Video Processing Unit' is also shown.

Circuit Notebook No.3. Vidicon Blanking Generator (Fig.2.)

This simple circuit generates a 20V p-p pulse from a 10V supply. This type of voltage-doubling circuit appears elsewhere, for example, in Circuit Notebook No.78 'Using Coax Relays with High Voltage Coils' where a 24V relay is pulsed from a 12V supply.

Circuit Notebook No.11. D-I-Y Quadrant Fader Mixer (Figs. 3, 4 &5)

This was a mechanical design for a dual quadrant fader using standard 100 ohm rotary pots and Meccano gears. Normal slider pots were not readily available at this time. Two parallel aluminium plates each carried a gear-pot system and the fader levers came out through the front panel, side by side. A buffer video amplifier fed each pot and the output from each slider was fed to a summing/ inverting amplifier. The incoming video signals were from sychronised sources but without syncs, which were added later.

Circuit Notebook No.53. Video Switching Unit (Fig.6.)

Where a mechanical contact is used e.g. keyboard, press-switch, relay etc. the contacts are apt to bounce on closure causing a number of contact connections before eventually remaining closed. The contact usually settles closed after about 1-3 ms but during that time several 'makes' occur which can affect the following circuits. In this circuit the 'Latch Pulse Gen' (lower left) uses a low-cost 4011 CMOS IC, to provide a delayed 1ms latch pulse. This is used in the 4 Input Video Switching Unit described in Circuit Notebook No. 53 and also appears in other similar circuits.

Circuit Notebook No.60. Stepped Frequency Burst Generator for Video Testing (Figs. 7 & 8)

This uses the Maxim MAX038 High frequency Waveform Generator I.C.(a free sample from Maxim) This device will generate square, triangle and sine-waves from low frequencies up to 20MHz. The circuit shown generates sine-wave bursts of 1, 2, 3, 4, 5 and 6MHz each having a duration of 4us with 4us spacing, which is repeated on each line. The p-p amplitude of each sine-wave burst is equal and extends from black level to peak white. The space between each burst is mid-grey. The line sync pulse is 4us, a little shorter than the specified value of 4.7us. The circuit does not provide field blanking and sync signals.

Circuit Notebook No.61. Not another Fade to Black circuit (Fig.9.)

Then there is the ubiquitous sync separator, the National LM1881. This IC accepts a conventional video signal and produces, composite sync, vertical sync, burst output and an odd/even field signal. It has been used in many Video Clamp and Fade-to-Black circuits. Its popularity resulted in the above title.

Circuit Notebook No.90. Veroboard Video Distribution Amplifier (Fig.10.)

This is a 'bog-standard' video amplifier circuit which could find its way into almost any piece of ATV equipment. The circuit has a gain of x2 and provides overall unity voltage gain when the output(s) are terminated in 75 ohms. The circuit uses the Analog Devices AD817AN. The older Elantic EL2020 would also be suitable.

The above circuit descriptions are necessarily brief, but a Circuit Notebook Index is attached which gives the CQ-TV No. and date and page number of each Circuit Notebook from No.1 to No.100, so that further details can be obtained on line from the BATC Archives.

Circuit Notebook No. 100. An unusual Earth-lift Box (Figs.11 & 12)

This is a circuit that I have used for many years in spite of it being likely to breach Health & Safety regulations. Its purpose is to temporarily overcome a mains hum problem where this is caused by a low impedance inductively-coupled earth loop.

For example, all my video/TV equipment is supplied from the same mains socket and all equipment is bonded together through coax cables and other earthing connections. But, when I connect an oscilloscope (which is fed from a different mains socket) to measure a signal, a low impedance earth loop is formed which causes a 50Hz circulating current to flow. This produces a small signal voltage across any poor coax connectors. These problems were discussed in Circuit Notebook No. 97.

Even after taking all precautions hum loop problems may persist, particularly when making tests on an experimental setup. One way of overcoming this is to temporarily disconnect the earth wire in the plug-top of one of the pieces of equipment. This is to be discouraged as its reconnection may be forgotten and the equipment left in a potentially dangerous condition.

My solution is to use an 'Earth-lift Box'. Here, the mains live and neutral wires pass directly through the box but the earth wire passes through a switch. This allows the earth connection to be broken without having to disconnect the earth wire in the plug-top. Lifting the earth connection stops the circulating current and overcomes the problem.

However, in the event of an earth fault in the equipment it could possibly become 'live'. To prevent this, the switch has several crossed diodes, in the form of a bridge rectifier, connected across it. This allows a +/- 0.6 volt window in the earth wire, to overcome the hum loop problem, but still maintains a safety earth path capable of passing a fault current of 20 amps or so. The Earth-lift Box may be less effective if the equipment has large mains filter capacitors between live, neutral and earth.

Earth loop problems can also occur at TV line frequency. Fig.13 shows a disturbance (with the Earth-lift closed) due to inductive coupling from the scanning coils of an old CRT monitor. Fig.14 shows the effect of opening the Earth-lift.


Hazard warning signWhilst this unit may provide a temporary solution for experimental test purposes, it should not under any circumstances be made part of a permanent installation. Users should be fully aware of the safety hazards of operating un-earthed equipment.

Also see

A Circuit Notebook Index has been prepared so that a copy of any original full article can be obtained by searching the BATC DVD or the CQ-TV Archives, both of which cover all issues of CQ-TV.


Fig.1 Pulse Narrower Circuit

Fig.2 Vidicon Blanking Generator

Fig.3 D-I-Y Quadrant Fader/Mixer, Mechanical

Fig.4 D-I-Y Quadrant Fader/Mixer, Mechanical

Fig.5 D-I-Y Quadrant Fader/Mixer, Circuit

Fig.6 Video Switching Unit

Fig.7 Stepped Frequency Burst Generator, circuit

Fig.8 Stepped Frequency Burst Generator, Waveforms

Fig.9 Not another Fade to Black circuit

Fig.10 Veroboard Video Distribution Amplifier

Fig.11 An unusual Earth Lift Box, circuit

Fig.12 An unusual Earth Lift Box

Fig.13 Line waveform, Earth lift 'closed'

Fig.14 Line waveform, Earth lift 'open'