Commercial killer triggered by station logo

Regarding Max Maughan’s query about some sort of device that can detect the station logo and hit pause on the VCR, etc (page 5, April 2006), it did sound like a great idea. So good in fact that Elektor published a circuit for it in 2004. See this website: www.elektor-electronics.co.uk

Matt Crump,
via email.

Comment: thanks to those other readers who sent in the same reference.

Concern about passive DI box

I have a concern regarding the passive DI box published in SILICON CHIP, May 2006. In the text on page 64 you state: "The resistor for the ring output also prevents the possibility of the signal from a stereo source being shorted to ground . . ."

This is clearly not the case with the way that the circuit is drawn. The two ring connections are directly connected together and the resistors and transformer primary are in parallel with this link.

There is no isolation between the ring connections of the input and through sockets; therefore a mono connector plugged into the through socket will still short the ring to ground. Additional isolation resistors would need to be provided between the two sockets to prevent shorting of stereo signals. This will reduce the signal level available from the through jack which may or may not be desirable depending on the application.

Rodney Baker,
Walkley Heights, SA.

Comment: the resistors are included to provide mixing of the signal from stereo to mono. If a mono plug is inserted into the second socket then it will short the ring of a stereo plug that is in the other socket. This would also be the case in any DI box that has mono sockets.

Typically, when a stereo plug is used, you would not be using the second socket or if you needed to you would use a stereo connection. If such a stereo socket was plugged into a standard DI box, the ring signal would be shorted to ground. This would not be the case in our DI box.

In virtually all situations, the second socket is used when the signal goes both to the public address system via the DI box’s XLR output and to an amplifier via the 6.35mm jack connection. These signals would be mono and so cause no problem.

Experiences with a Battery Zapper

I have been following with interest your development of the Battery Zapper and would like to share my experiences with a commercial unit.

I live on a farm that has 18 vehicles that need a battery for operation. Many of these vehicles are only used intermittently and battery maintenance has been a problem.

After premature failures and a costly replacement program, all of the vehicles were modified to take standard-sized batteries so that only several batteries are needed between all of the vehicles. The modifications even extended to the ride-on mower. When not in use, the batteries are shelved and connected to a float charger. However, premature failure was still a problem and a Megapulse brand unit was successfully used to extend the battery life by rotating the unit between the batteries while on the shelf.

A visit to a scrapyard revealed a large quantity of batteries that appeared to be in good condition. Several batteries were purchased to determine if the Battery Zapper could rejuvenate them to a usable condition. From my experiences, the answer is yes and I have since been able to provide good batteries for all of the vehicles at a negligible cost.

An interesting fact I have learnt is that high-quality batteries respond the best. Cheap batteries generally are a waste of time. As there is no price difference at the scrapyard, I make a point of carefully selecting prospective batteries by brand and by using a heavy-duty "battery load tester" to check for open cells. Back in the shed, following a week on the Battery Zapper, I have a very usable battery.

I had read with interest of the original SILICON CHIP Battery Zapper (July 2005), however as I was satisfied with my methodology, I had no pressing desire to build one. However, after reading about the improved design of Battery Zapper (May 2006) and the ability to monitor what is happening, I have decided that I need to build at least one unit. Every farm should have one!

Chris Ryan,
via email.

Easter egg helps remote repair

They say that necessity is the mother of invention. A co-worker presented me with a problem: his remote which controls his entire hifi/TV system was working apart from the volume button. His dilemma was that a replacement was going to take almost two months to ship and his hifi had no external volume control, so he couldn’t use it until he got a new remote control.

I tried cleaning the remote’s board and keys but the multimeter still showed very high resistance on the offending key’s rubber pad. The working keys had resistance of about 15kW-30kW, whereas this one had 1MW. I knew which keys were working by viewing the infrared LED via my mobile phone’s inbuilt camera.

Being just after Easter, I had an Easter egg lying on my desk, so I peeled the foil off and glued it to the offending key and hey presto, a working remote! It’s not exactly hi-tech but sufficient to last two months.

Trevor Nuridin,
via email.

Servicing a belt-drive turntable

I recently repaired a Philips GA212 belt-drive turntable and I thought the process might be of interest. I originally bought it in about 1975 and had not used it for 10 years. I decided to digitise some of my LPs and built a small preamp for the magnetic pickup.

However, when I switched on the turntable, it did not rotate. I removed the platter and found that the capstan whirred but the platter did not turn. The drive belt seemed to have had melted into black goo on the base plate of the turntable.

Checking the internet, I found this was a typical problem. However the net also indicated that it is a very good turntable, so I thought it was worth trying to resurrect.

I tried many solvents but in the end only paint thinners would shift the goo. Unfortunately the resultant (now much thinner) black goo then ran down into the suspension system which took a lot of cleaning to remove. It also took off some paint but that was no problem.

Be warned! Take great care when applying thinners that none runs into the works or touches plastic. Use a cotton bud or cotton wool and squeeze out the excess thinners before applying so it does not run or drip. The goo was also wrapped around the capstan and this was very difficult to remove. It also stained the capstan’s brass so it is not a nice composition. I accidentally scratched the capstan while scrapping off the goo so I had to polish it while it was spinning, with fine wet-and-dry emery paper.

I then had to buy a drive belt. I put a piece of wire around the belt path to measure the length and it was 555mm. WES Electronics had a 550 x 0.5 x 5mm belt for about $10. While this worked OK, it has proved to be a little loose and sometimes falls off the platter drive wheel. I shall buy a smaller belt which I hope will be tighter.

The final results were quite good. I reassembled the whole drive system and it works well. Placing a fingernail gently against the side of the platter to simulate the small load of a pickup to the servo system is not a problem. The strobe markings on the platter showed hardly any change in speed.

John Rich,
via email.

Hearing loss is a real disability Top Marks for bringing to our attention, and for your concern, about hearing loss in the Publisher’s Letter for the May 2006 issue. But to my mind you didn’t go quite far enough. To tell someone that "they may go deaf" has very little impact. Instead, they need to be told that: (1)You will soon be guessing what people say, even after they have said it several times. (2)Your children, grandchildren and others will think you are a bit simple because you don’t properly hear what they say. (3)You will have trouble with telephone conversations. (4)Conversations in a crowd will be almost impossible and you will say "yes" and "no" in what you hope are the right places, while you try to look intelligent. (5)Your family will be quite intolerant when you have the radio or television so loud. (6)When you go to the annual meeting of your favourite club or to a business meeting, you will not be able to understand what is being discussed and won’t be able to take part, for fear of making a fool of yourself. (7)You will have the inconvenience and discomfort of having to wear a hearing aid or maybe two, to survive in everyday life. (8)You will have to buy the said hearing aid(s), which could cost you more than your computer. (9)If you ever take an interest in "real" music or the theatre, you will not be able to enjoy it to the full. This is the reality of what hearing loss is all about! Clive Singleton, Wainuiomata, NZ.