Circuit Notebook 102 - Modifying the Maplin A/V Source Selector L62AB

The Source Selector L62AB from Maplin is a basic device with mechanical push-buttons to do the selecting, shown in Fig.1. All inputs and outputs (video and stereo audio) are on phono sockets at the rear, shown in Fig.2. Any of the 4 sets of inputs can be switched to either or both of the two outputs. It has a built in mains supply and provision for an alternative +18V dc input through a 2.1mm power connector. It was priced at a very competitive £14.99 until very recently (July 2009) when the price was increased overnight to £24.99. It would be difficult and time consuming to build a comparable switching unit, and so at £14.99 I bought one.

The L62AB was of particular interest because one output could, for example, be connected to a video monitor and the other to an ATV transmitter, thus allowing any of the 4 inputs to be previewed before switching it to the transmitter. Also, the output from a receiver could be connected to one of the inputs and this could be viewed by selecting this input to the monitor. All this seemed ideal until I investigated the specification more closely.

The specification shows that there is a video insertion loss of 0.6dB. This represents a 7% loss of video voltage through the unit. In addition, measurements showed that although the video input impedance was approximately 75 ohms, the output impedance was 45 ohms instead of 75 ohms. This may not matter in a domestic situation, but I was intrigued to know why.

The other concern was that the DC supply requirement was 18V, rather than the usual +12V DC. I tried running it from +12V dc but the video output waveform showed sign of distortion. This precluded it from ATV portable use.

I contacted Maplin Technical Support for a circuit diagram for the L62AB but this was not available. So, I decided to find out for myself, even if it meant making the warranty void. A project not to be undertaken by the faint hearted.

Dismantling

I removed the 4 screws (two either side) and opened the case. I removed the four screws two each side of the front panel and removed the front panel, then the two M3 screws holding the PC board and the 6 black screws, at the rear, holding the blocks of phono sockets in place. I then removed the PC board with the push button units and phono sockets attached and started tracing out the wiring.

The PC board contained two transistors, each was connected as an emitter follower, one for each output. The circuit components had been cleverly chosen to minimise the loss of video signal whilst providing a compromise match to 75 ohms at both the input and the output. The basic emitter follower stage is shown in Fig.3.

I decided to remove the two emitter followers altogether and to replace these with two conventional video op-amps (Circuit Notebook No. 90, CQ-TV No.215) mounted on a piggy-back Veroboard fitted above the main PCB. Each amplifier would have a high input impedance so as not to load the input resistors, a gain of X2 and a sending impedance of 75 ohms. The op-amp circuit is shown in Fig.4 and the Veroboard layout in Fig. 5.

Video amplifier board

The video amplifiers and associated components are mounted on a piece of Veroboard, 12 strips wide by 41 holes long, as shown in Fig.5. An 8V regulator, 7808 or similar is mounted on the top of the Veroboard by an M3 screw. This screws into the right hand pillar which also supports that end of the Veroboard. There is no need for a heat sink as the power dissipation is very small. The position of the two mounting pillars is shown in Fig.7. Their position is critical so as to avoid damaging any of the PCB tracks on the underside. The position of the left hand pillar is given in mm, but the position of the right hand pillar is referenced to it in decimal inches to match the 0.1 inch matrix of the Veroboard. The mounting pillar height should be about 18-20mm.

The input and output connections of the original emitter followers on the main PC board are wired to and from the amplifiers, including the DC supply, as listed below. Fig.8. shows the amplifier board installed above the main PCB.

DC Supply

Because the primary requirement was for portable use I decided to re-arrange the internal circuits to work from +12V upwards and to abandon the internal the mains power supply. The revised DC supply is shown in Fig.9. I also removed the mains lead to make it more portable but left the mains transformer in place. If I should need to run it from the mains in the future, then I would use a plug-top power supply.

The changes made to the PC board are given below:

1. Remove R2, R3, R4, R5 (100R) and replace each with 75R 2. Remove R10, R14 (10R) and replace each with a wire link 3. Remove R9, R17 (220R) 4. Remove R7, R15 (10k) 5. Remove R1 and replace with a wire link 6. Remove D2, V1, V2

Fit a connecting pin in the PCB hole where the component has been removed and solder on the track side, as follows:

7. Fit connecting pin to 'base connection of V1' - connects to Input of Amplifier 1 8. Fit connecting pin to 'left hand end of R9' - connects to Output of Amplifier 1 9. Fit connecting pin to 'base connection of V2' - connects to Input of Amplifier 2 10. Fit connecting pin to 'lower end of R17' - connects to Output of Amplifier 2 11. Fit connecting pin to 'cathode of D2' - connects +V supply to input of 8V regulator 12. Fit connecting pin to 'anode of D2' - connects 0V to COM on 8V regulator

The positions of the connecting pins are shown in Fig.6.

Audio Switching

No changes were made to the basic audio switching, which although not providing any proper input-output isolation, performs adequately. The basic audio switching is shown in Fig.9.

The modified assembly is refitted in its case by reversing the dismantling sequence.

Conclusion

The video performance is as expected, unity gain into 75 ohms with a frequency response from 10Hz to beyond 6MHz. The 12V input current drain is 20mA.

The modified Maplin L62AB A/V Source Selector has proved to be a useful piece of kit when operating portable ATV.

My thanks go to David GW8PBX for input and output impedance measurements and for helpful comments whilst modifying his own unit using the above instructions.

Figures

Fig.1. L62AB Source Selector Front

Fig.2. L62AB Source Selector Rear

Fig.3. Basic emitter follower stage

Fig.4. Dual Op-amp Circuit

Fig.5. Veroboard Layout

Fig.6. PC Board modifications

Fig.7. Position of Mounting Pillars

Fig.8. Op-amps fitted to main PCB

Fig.9. Basic Audio switching and Power Circuits