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it will need a snubber RC circuit connected across it. Without knowing the characteristics of the transformers (or transformers, if more than one unit is involved) we cannot come up with a suitable snubber circuit. Recording video signals on an audio machine Although I have been involved in electronics and computers for a number of years now, I have never yet examined or read about the encoding used for video signals. When I say “video signals”, I am referring to the signal emerging from the “video out” socket on most VCRs. My question is: can this video signal be recorded onto and retrieved from normal audio tape? Obviously, if this was possible it would be a novel and revolutionary way of storing video images. My first thought is that maybe the video signal is too high a frequency to be recorded on audio tape and that the distortion and background noise on the resulting signal would be too great to recreate the original signal. I hope you can prove me wrong, as this would be a great experiment, even if extra circuitry would have to be added to the tape recorder/player involved. If it were possible, video could be recorded on one track and audio on the other. On another matter, maybe you could clear up a problem I am having with a colour monitor I recently acquired. The screen is of the Apple brand and is marked “colour composite monitor”. The video input socket (there is only one socket apart from the mains cord on the whole monitor) is the RCA type, corresponding with the aforementioned “video out” of my VCR. Logically, I thought that just maybe the two signals were compatible, so I connected the screen to the “video out” of my VCR. I then activated the VCR, set the internal tuner to a strong television channel and up came a remarkably clear picture on the monitor in question, except for one minor blemish: no colour! The VCR is not at fault and all adjustments on the exterior of the monitor have been tried. Could you shed some light on the subject? The monitor is not known to be good, so it could well be an internal fault. Unfortunately, I do not have a colour computer video card with the corresponding RCA connections (al- NTSC playback on a PAL TV I am writing to ask for your assistance on a matter which has annoyed me for quite a while. I have a VCR which can do “NTSC playback on PAL TV”. The only catch is that my television is not a multisystem set and, even though I can see a colour picture, the vertical hold is off. That’s OK as long as there is an adjustment for the vertical hold. Unfortunately, my television doesn’t have one. I have heard from somewhere (can’t recall where) that it’s because the tape is outputting 60Hz and our PAL sets only seem to enjoy 50Hz. Is this true? If so, how can I build a device that can convert it to 50Hz – could you send me schematics? If this is not true, then why is it that colour is present and the vertical hold goes off line? I know that when using a “true” NTSC machine, no colour is repro­duced though I have a monochrome card that works just fine with the monitor). Any response would be gratefully received. (A. M., Northbridge, NSW). • Trying to record video signals with an audio recorder is doomed to failure since the bandwidth of typical video signals is at least 4MHz, with the colour intercarrier at 4.49MHz, while most audio recorders are flat out trying to get to 20kHz. We doubt whether even the best audio recorders would be good enough and stable enough to accurately record the line sync signals at 15.625kHz. Even if they were, the resulting recording would only store the lowest of video signals (ie, below 20kHz) and the result would be an extremely blurred grey picture. It has never occurred to us to try it but that’s what we assume the result would be. As far as your computer monitor is concerned, it is likely to be using an American video standard; ie, NTSC. If you had a VCR with NTSC outputs, no doubt it would produce a fine colour picture. However, the Australian video standard is PAL, based on an original German standard and this is incompatible with NTSC composite video signals, as far as colour is concerned. by the TV. It seems that the VCRs that can play back “NTSC ON PAL TV” only decode the colour portion, not whatever is put­ting the vertical hold off. I would greatly appreciate your help on this matter. (P. T., Indooroopilly, Qld). • As you suspect, the reason you cannot obtain a locked pic­ture on your PAL set is that it requires a TV signal with a 50Hz frame rate, not 60Hz as is produced by your VCR when playing NTSC tapes. There is no way that this problem can be solved other than by adjusting the vertical hold control on your TV set. If your set does not have such a control, it may be possible for a local TV serviceman to add this facility to your set and if so, this would be the cheapest solution to your problem. A TV standards converter will not help in this regard since the output signal from your VCR is a mixture of PAL and NTSC; ie, PAL with a 60Hz frame rate. Notes & Errata Mains Music Transmitter & Receiver, May 1995: a number of errors have appeared on the circuit and wiring diagrams for the receiver. C4 is shown as 330pF on the circuit and .0033µF on the wiring diagram; 330pF is correct. C17 is shown as .0047µF on the circuit and .015µF on the wiring diagram; .015µF is cor­rect. C25 is shown as .0047µF on the circuit but not marked on the wiring diagram; the correct value is .0033µF. C28 is shown incorrectly polarised on the wiring diagram but is correct on the circuit. The cathode of diode D2 is shown connected to the junction of C11 and R9 on the circuit but incorrectly shown on the wiring diagram as connected to the wiper of trimpot VR2. To correct this, the track section connecting D2 to the wiper of VR2 should be cut and then linked to the junction of C11 and R9. The circuit board will work as presented but will not mute fully when no carrier signal is present. CTOAN Electronics has advised that all PC boards supplied in the future for this design will be correct. SC July 1995  93