Query on loudspeaker resonance

Some time ago, after reading your article on the JV80 speaker system (SILICON CHIP, October 2003), I purchased the speaker kit with the intention of constructing my own boxes. I have finally begun, and for interest I thought I’d check the resonant frequency (Fs) of the drivers. I was most surprised to find that they all measured between 34.5Hz and 35.4Hz. Your article gave it as 31Hz and a search of the internet came up with 29.5Hz.

I am just wondering how sensitive the box design is to such a variation in Fs and would like to know how it was measured for the October 2003 article. (K. W., via email).

There are a number of significant points about the measurement of speaker resonance. First, the speaker should be mounted on a baffle, typically one-metre square. Second, the speaker needs to be driven from a low impedance source, less than 1Ω. Driving it from a high impedance source will typically raise the measured resonance by a few Hertz. Third, the quoted resonance figure is not an absolute and has a tolerance which could be plus or minus 5 or 10% (what ever the manufacturer specified).

Finally, we would not expect the speaker box performance to be affected by a small shift in the resonance.

50W DC load circuit question

This is a question about the 50W DC Load described in the September 2002 issue. Why has a 47μF non-polarised capacitor been specified across the output, as opposed to say, a 47μF polarised electrolytic? Mosfet Q1 has an intrinsic anti-parallel diode, so it can’t be just in case the load is accidentally connected in reverse – a regular electrolytic would be protected. (R. Z., Victoria, Canada).

Good question. We realised that if a low impedance, high current source was connected in reverse across the input terminals, the Mosfet would likely be destroyed, as the fuse wouldn’t be fast enough to protect it. Clearly, the lack of robust polarity protection is one of the downsides to such a simple design.

Having said that, you can use a polarised capacitor if you prefer.

FM stereo Micromitter drift problem

I have constructed the FM Stereo Micromitter kit (SILICON CHIP, December 2002). It seems to work fine except that the frequency drifts high after about 10 minutes. All construction appears to be OK. Any suggestions? When it is on frequency, it is quite impressive for such a small unit. (J. T., Winmalee, NSW).

The phase locked loop must not be operating. Check the lock voltage for around 2.5V, as detailed in the setup procedure. Maybe the coil is not wound exactly as shown, with the same direction and position close to the PC board.

Video connection for a hotel TV

Can I make or buy a device that will allow me to view my digital camera output on a hotel TV? That is, a TV with no video input and a secure coax connector for the RF. I thought of an RF modulator but how do I get it to the TV?

Would a dipole antenna cut to length for channel 4 (NTSC) overpower the coax feed? (D. F., Toronto, Canada).

That’s a pretty difficult task. Yes, you can use a modulator but you then need to plug its output into the antenna socket of the TV and then you have to tune the TV set to the modulator’s output frequency. The modulator also has to match the system used by the TV (PAL, NTSC, etc). Most countries of the world do not use NTSC.

You cannot hope to get your modulator’s signal into the shielded coax – you have to make a direct connection to the TV. If the coax cable is secured, you can forget it.

Power boost for Studio 350

I would like to modify the Studio 350 amplifier circuit (January/February 2004) by adding four more output transistors in parallel to increase the wattage. Would there be any problems doing these modifications? (G. M., via email).

It could be done; two extra transistors per side with ±80V rails would give around 500W into a 4-ohm load. However, there would clearly be some difficulties getting it all connected and working reliably. You might be better off building the 500W amplifier from the August, September and October 1997 issues. It uses a similar circuit, with the same output transistors. The PC boards can be obtained from RCS Radio at www.rcsradio.com.au

Comprehensive video source

Having recently successfully constructed both a component video-to-RGB converter and a TV pattern generator (SILICON CHIP designs) has got me thinking. A really useful gadget would be a simple video source, say just colour bars/greyscale but with multiple outputs; eg, composite, S-video, component, RGB, plus perhaps an RF output.

One way to do this is to burn a CD with appropriate jpeg image files and access these through a DVD player with multiple outputs (which I have already done).

However, a simple hardware realisation could be cheaper, more portable and more reliable. How about a future project along these lines? (H. T., Lower Hutt, NZ).

Given that DVD players are really cheap these days, your suggested solution is by far the cheapest and easiest approach.

Using a train detector with Command Control

I am attempting to use the Train Detector For Model Railways (SILICON CHIP, June 1995) with a Command Control System, without success. Can the Train Detector system be made to work with these Command Control systems. (R. S., Bundaberg, Qld).

Unless you have your layout divided up into blocks, which is probably unlikely if you are using a Command Control system, then the Train Detector is not practical. Having said that, you should still be able to get it to work although note that the 4mH isolation choke L1 (shown on Fig.4, page 29) should be inserted in series with the output from your Command Controller to the rails.

Windows-based EPROM programmer

I have built the EPROM Programmer and not being a programmer myself, I need to be able to program (for a pinball machine) 2716 EPROMs (24-pin, 2048 byte), which is priority 1. Also 2316 masked ROMs (or not) and 2732 EPROMs are priority 2.

The programs themselves are available but for the 2716 I need to know how to adapt them or is the 24-pin adapter suitable? (A. M., via email).

The answers to your questions are as follows:

(3). 2316 PROMs are factory (mask) programmed and cannot be erased or reprogrammed. However, some pinball machines will accept 2716 or even 2732 EPROMS instead with a jumper/link change.

Component-to-RGB Converter Modification

I built your Component Video to RGB Converter (May 2004) and the TV displays pure black. I believe this was mentioned in a later issue of your magazine and it has to do with an "RGB flag" signal to pin 16 of the SCART socket on the TV.

Could you please advise a likely fix? I am using a Jaycar RGB to SCART cable (just the three leads).(M. P., via email).

The modification to the Component Video to RGB Converter was published in the August 2004 issue, in the Circuit Notebook column on page 77.

A length of flexible hookup wire is then added to your RGB cable, connected to pin 16 of the SCART plug and fitted with a pin jack plug at the Converter end. When the plug is fitted to the pin jack, this connects pin 16 of the TV’s SCART input to +5V when ever the Converter is powered up. This seems to solve the problem of a "black screen", with sets such as yours needing an "RGB Flag" signal on pin 16.

Notes & Errata Inductance & Q-Factor Meter, February & March 2005: the specifications incorrectly stated the measurement range of the meter and its power requirements. The correct measurement range is 200nH - 999mH and the maximum current demand is about 300mA. Also, the orientation of the ISP header on the overlay diagram (Fig.9) and various photos is opposite to that used on the ISP programmer (SILICON CHIP, October 2002). To use a pin-to-pin cable between the ISP programmer and the meter, install the ISP header the opposite way around to that shown on the overlay. In addition, the 1N4148 diode above IC6 on the overlay diagram (Fig.9) should be labelled D8 instead of D9 and there are several discrepancies in the parts list, as follows: change 9 x 100Ω to 8 x 100Ω resistors; change 1 x 8.2nF to 2 x 8.2nF MKT capacitors; change 6 x 1N4148 to 7 x 1N4148 diodes; and add 1 x 130Ω resistor. The lack of over-range indication in the original release of the microcontroller code is being looked at by the author and we hope that an update will be available on our website shortly. Bass Extender, April 2005: under the "Circuit Details" section on page 62, the text in the fifth paragraph states that the circuit shows a sealed enclosure when it actually shows a vented enclosure. The paragraph should be changed to read: "Accordingly, the values of resistors R1, R2 & R3 on the circuit are for vented enclosures. If you have sealed enclosures, R1 should be changed to 27kΩ, R2 to 47kΩ and R3 to 39kΩ". Clifford the Cricket, December 1994: the 2.2μF electrolytic capacitor is shown reversed on the circuit diagram (Fig.1) but is shown oriented correctly on the parts overlay (Fig.2). Also, the 68kΩ resistor on the parts overlay should be a 10kΩ value, as shown on the circuit diagram and parts list. Bidirectional Motor Speed Controller, December 2004: the text states that the circuit can operate from a 24V battery. However, due to the gate-source voltage limit of the Mosfets, it is only suitable for use at up to 16V DC (ie, from a 12V battery). Fig.1: connect 16V zener diodes to the Bidirectional Motor Speed Controller as shown here for 24V operation. However, it should be possible to modify the circuit for 24V operation by fitting 16V zener diodes between the gate and source terminals of each of the Mosfets (Q3-Q6). The accompanying excerpt from the circuit diagram (see Fig.1) shows how to connect the zeners. Portable PIC Programmer, September 2003: a few constructors have been unable to adjust VR1 to get the required 5.0V output from REG1, as described in the article. If you encounter this problem, replace the 22kΩ resistor in the collector circuit of Q1 with a 10kΩ value and redo the calibration. Although we don’t recommend the use of the kit to program PIC12C508/9 devices, we’ve found that it’s more likely to program these devices successfully if the 1μF capacitor connected to the cathode of D4 is replaced with a 10μF 35V tantalum unit.

Heat-Sensitive Computer Power Supply

Recently, my computer has developed a fault which appears to be in the input power supply switching circuit. When I first switch it on (after it has been off for several hours and is "cold"), it shuts down within the first minute or so. There appears to be no power getting to the computer and everything (including the power supply fan) is off.

I then switch it off at the mains, wait for about a minute and switch it back on again. It then switches on as normal.

I am not really familiar with the circuitry for the power supply, but presume that it contains some "protection" circuit which shuts off the supply if there is a fault. If this is the case, I’m not sure if there really is a fault or if the protection circuit is malfunctioning due to a faulty component.

It seems logical that the fault is in the mains switching circuit. Even if I can’t fix this fault myself, it would be interesting to know how the "switch on" circuitry operates and what protection arrangements are built in to such power supplies.(B. T., via email).

We also believe that the problem could be in the power switching circuitry or at least in the power supply itself. Unfortunately, it could also be almost anywhere else in your system, even the motherboard, which controls the power supply via the PS_ON signal. Here’s one possible scenario that may help explain what we mean: