Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we'll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send us an email.

Substituting a 7406 for a 74LS06

I'm getting started with more complicated PC boards and get both SILICON CHIP and Elektor but since the latter is British I have problems with how to get the ICs they suggest.

So can you suggest a good IC comparison book which would tell me if I can replace the 74LS06 (Low Power) with the 7406 since I can't it at Jaycar or DSE?

(S. V., via email).

In most circuits the 7406 may work instead of the 74LS06. However, if the circuit is running at very high frequencies it may not. You can purchase the 74LS06 from Altronics in Perth and their dealers. See www.altronics.com.au

600W DC-DC converter upgrade I would like to build the 600W DC-DC Inverter featured in the October 1996 issue of SILICON CHIP to run an amplifier I have built. However, the amplifier requires supplies of ±85V. Could I just change the number of windings on the transformer or do I have to change other things like current sensor etc? (A. S., via email). You only need to change the output windings to obtain ±85V. 15 turns on the secondaries should give the required voltage. The current limit is for the 12V input side of the inverter and limits the current if it exceeds 79A. This should not be altered. It will allow the inverter to deliver the 600W of power to the amplifier.

Speed control for Super-8 projector

I am attempting to transfer my Super-8 movies to video using a digital camera. Unfortunately, I am getting a strobing effect which I think I can correct by altering the speed of my projector motor. The motor is 12V DC and rated at 25A. Is it possible to adapt the 12/24V 20A speed control published in the June 1997 issue, to cater for the additional current.

(T. C., Pearl Beach, NSW).

The projector rating of 25A is probably mainly due to the projector lamp. The motor probably draws less than 1A. If you decide to use a speed control you do not want to vary the voltage to the projector lamp.

Valve radio repairs

I have an old valve radio that I would like to resurrect and don't know who does this kind of repair. I was wondering if you know where I might be able to get it fixed.

(G. H., Kensington, NSW).

We suggest you contact the Historical Radio Society of Australia: http://hrsa.asn.au

Feedback problem in Neon Tube Display I am experiencing a problem using your Neon Tube Sound Display (SILICON CHIP November 2001), as sold by Jaycar Electronics. When operating at higher volume levels, I am noticing quite a lot of feedback coming from the Neon Tube Sound Display board into my Subwoofers. I have added a 100kΩ potentiometer in series with the input of the kit to stop the feedback. My system is set up as a mono system delivering 200W. The kit initially worked correctly for the first week of operation, however this feedback problem started. Several friends who installed the same kit to their car audio systems have also experienced the same problem. Is there a fix? (M. B., via email). We are inclined to think that the interference is due to the pulse currents from the neon tube drivers getting back into the signal line to your amplifier. The usual way to solve this problem is to have separate supply and ground lines going back to the battery rather than relying on common supply lines to your amplifier etc.

Negative ion generator circuit wanted

I am chasing instructions to build a 240V Negative Ion Generator. I was unable to find any mention of one in your kit list on the web site.

I realize they are a string of diodes and capacitors in a succession of voltage doubling circuits but I don't understand the Cockroft-Walton multiplier principle enough to design one myself. I know it would need some final resistors to make the emittor pins safe but the problem is working out where to cutoff the voltage doubling to minimize ozone production.

Can you advise me if you have had a design in an early edition or could you do an article on negative ion generators and the Cockroft-Walton multiplier circuit principle? It has so many other uses it must be of interest to readers.

(M. B., via email).

SILICON CHIP has not described a Negative Ion Generator but one was described in the April 1981 issue of ETI. It did use a Cockroft-Walton multiplier. That issue also had a good information article on Negative Ion Generators. We can supply photostat copies of these articles for $8.80 including postage.

13.8V for car amplifier

I have a few questions concerning the 25A amateur transceiver power supply described in your May & June 1991 issues. Was that a switchmode design? Can it be upgraded to 35A continuous? I'm trying to make use of this car amplifier I've got. The only other recourse I can think of is to bypass the existing power supply, replacing it with a conventional 240VAC supply but so far I have been unable to find a circuit diagram for the amplifier.

Finally, have you ever described a power supply above 25A @ 13.6V?

(T. C., via email).

The 25A power supply used a phase-controlled Triac in the primary of a large transformer and used 2-stage LC filtering in the secondary. So it was switchmode but not in the same way as today's PC power supplies. It is not practical to upgrade the design and we have not produced anything larger.

Any practical 13.8V high current supply will cost far more than your car amplifier is likely to be worth.

Reversing switch not recommended I have built the 240VAC 10A motor speed controller from the November 1997 issue and it works great. I would like to put a reverse switch in the circuit. Have you got any ideas or circuit programs that I could use to make this happen? Also, I would like to restrict voltage output to 180V DC so that I can run a 180V DC motor. (W. V., via email). The normal way to provide reversing is to use a DPDT (double-pole, double-throw) switch wired as changeover switch. We showed how to wire such a switch in the simple train controller featured in the February 1993 issue. But we strongly recommend against incorporating a reversing switch into the 10A speed controller. If you reverse the motor while it is running you will cause catastrophic failure to the major high voltage components in the circuit. However, since you mentioned that you are using a 180V DC motor we strongly suspect that it is a universal motor, ie, a series-wound brush motor. If so, then it cannot be reversed, unless you reverse the connections to the field windings. If you want to limit the output voltage, you will need to limit the range of the speed control pot VR1. As a suggestion, try increasing the 1kΩ resistor in series with VR1 to 10kΩ and the 8.2kΩ to 12kΩ . To confirm these changes, you really need a true-RMS voltmeter. Incidentally, if you had built our half-wave Drill speed controller published in the September 1992 issue, the output would automatically have been limited to about 170V, suitable for a 180V DC motor.

Protection board has reversed diode

I have just purchased the speaker protection kit for the Ultra LD amplifier (SILICON CHIP, August 2000) from Altronics in Perth. I connected it to a 30V supply. It says to use a 35V-0-35V supply but I only have a 30V toroid.

When I applied power to it, the PC board tracks on the 35V inputs between the solder pin and the two diodes lifted and the diodes started to crack and also the transformer started to make a hum. It's like the diodes are reversed or there is a lot of resistance or something. Could you help me with this?

(C. B., via email).

It sounds as though you have wired one of the diodes the wrong way round. That will blow both diodes and do the damage you describe. The cathodes (white stripe end) of both D1 & D2 should connect together onto the same section of the PC board.

Where is V_CC/2 in DI box?

I've just bought and built the DI box (SILICON CHIP, August 2001) and I am up to the testing part, where it says to connect the multimeter to pin 6 of IC2 and V_CC/2, then adjust the VR5 trimpot. So where is V_CC/2? And what should my multimeter be set to?

(A. L., via email).

V_CC/2 is half the supply of 9V so it equals +4.5V. It is derived by the voltage divider consisting of two 100kΩ resistors. You can see it on the circuit immediately below iode D3. On the PC board, you can find it on the end of the 27kΩ resistor (not the end which goes to pin 3 of IC2). You will need to set your DMM to its lowest DC range and then adjust trimpot VR5 to the minimum possible voltage (ie, 0V) when measuring between Vcc/2 and pin 6 of IC2.

How increase light dimmer rating I know that the Touch & Remote control light dimmer (SILICON CHIP, January 2002) is limited to a load of 250W but I was wondering if there is a way of making it capable of handling a 450W load? (R. B, via email). The limit of 250W is set by the Triac's dissipation limit. To increase it requires the addition of heatsink to the Triac. This is not easy to do because of the very confined space on the PC board and the limited amount of space within the wall cavity that the dimmer is installed in.

Switched control for speed circuit

I have built a motor speed control from a circuit published in your June 1997 magazine. I am a novice builder but managed to get the circuit to work as intended and control a 12V motor. I would like to know is if it is possible to replace the 5kΩ speed control pot (VR1) with a bank of resistors of different values that can be switched in and out of the circuit with a rotary switch. This would give a stepped range of speeds for the motor.

I have tried this but can't get the circuit to work properly.

(D. C., via email).

Just get a single-pole 12 position rotary switch and wire eleven 470Ω resistors around it. The wiper of the switch then becomes the wiper of your "switched" pot.

How to eliminate plugpacks

I have been looking in to ways to try and eliminate my bulky power packs for my computer peripherals. I read the article on the PC Powerhouse (SILICON CHIP, December 1999) and found it won't quite do what I need but it is a great concept. I have seven power supplies all up: 4 x 12V, 2 x 9V and one 7.5V, all of which are rated at 1A DC. I am not sure what they are actually drawing but I am sure they are not all 1A each. I would like to be able to cater for additional power packs in the future. As I only have two power points in the room and power boards are useless to try and fit these things on to, I am hoping you might have a better suggestion.

(R. A., via email).

Depending on current drains, you may be able to use one or two 12V DC plugpacks to run all your 12V accessories, then one 9V instead of two 9V plugpacks and so on. That way, you don't build anything but you can eliminate most of the plugpacks.

The only point do you have to watch is to make sure that all the peripherals are negative chassis, ie, the 0V line connects to the earthed metalwork of the peripheral device.

Maximum cable length for water level gauge What would be the maximum length for the figure-8 cable between the sensor and the display unit in the water level gauge featured in the April 2002 issue of SILICON CHIP? (J. E., Silverwater, NSW). This is not something we have checked but it should be OK with cables up to at least 10 metres.

Battery charger with UC3906

I am looking for an article describing a "Sealed Rechargeable Battery Charger". In particular, it uses the UC3906N IC. Could you please advise me which article I am looking for and if I can purchase a reprint from you?

(A. B., via email).

We first featured the UC3906 in July 1989 in an article entitled "Intelligent charger for 12V gel batteries as just a PC board. Later, it was featured in a full-blown charger for 6V/12V batteries, and the data sheets were in March 1990.

Subsequently this charger was substantially revised in August 1992. However, if you want a comprehensive charger which includes SLA batteries we would recommend the Multi-purpose fast charger featured in the June & July 2001 issues. We can supply all these issues (except August 1992 - available as a photostat) for $7.70 each, including postage.

Questions about SteamSound Simulator In October 1991, you published a design for a Steam Sound Simulator (Mk.II), based on an earlier design. I have built such a unit and power it via a conventional voltage controller throttle using the modifications specified in the article. Could you answer three questions? Firstly, can the input to IC1a from the white noise generator be altered to reduce the steam volume as it appears to swamp the chuffing output from the VCO? Or conversely, can the output of the VCO be increased? Secondly, can the rate of the sawtooth oscillator (Q2) be altered to increase its spread? Currently, with variable resistors for Ra and Rb, the best I can manage is a chuffing range of 3 - 8V (ie, no chuff at 3V and maximum chuff rate at 8V). Thirdly, how can I remove the motor whine etc from the circuit? A ceramic capacitor of anywhere between 22nF (.022μF) to about 100nF (0.1μF) across the brushes does nothing. The whine swamps the chuff and steam and increases in pitch with motor speed. Interestingly, using the same motor does not produce any problems with the Diesel Sound Generator. By the way, I have built and used two steam whistle/diesel horns, one as a steam whistle and the other as a diesel horn and they sound really great. The diesel sound generator works really well, although I upped the value of the zener diode to about 15V to increase the frequency spread. (P. S., Brisbane, Qld). It is not a question of the steam volume swamping the chuffing output. It is more likely that you have insufficient modulation via D8. The most likely solution is to increase the gain of IC1a by increase the 560kΩ resistor to say, 1MΩ . You then may need to reduce the gain of the following IC1c stage by increasing the 5.6kΩ resistor to 10kΩ . We do not see any easy way of increasing the chuff range, assuming that you have followed the procedure in the article for selecting Ra & Rb. Motor whine is a problem. If the motor itself is close to the SteamSound PC board there may be nothing you can do since the motor field is inducing noise directly into the PC board. If there is a reasonable space between the two, you may get some improvement by increasing the capacitors associated with the 78L05. Also try connecting a 10μF capacitor across the 4.7V zener diode.

You can buy products mentioned in this article here : KC5322 : NEON TUBE SOUND DISPLAY KIT SC11/01

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