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Playmaster Twin 25 stereo amplifier

I am looking for the schematic diagram and or construction details for either the 25W or 40W per channel stereo amplifier brought out by "Electronics Australia" back in the late 70s. The problem is that the output transistors have shorted out. I know that one pair are 2N3055s but as for the other two, I assume they could be MJE2955 but I can't be sure. Can you help?

(D. H., Launceston, Tas).

The Playmaster Twin 25 and 40/40 amplifiers used one pair of 2N3055s in each channel. It was not a complementary symmetry design. If the output transistors have shorted out, look for other faults as well.

We can supply as photocopy of the relevant articles for $8.80 including postage.

6-LED torch has flashing fault

I have just built the 6-LED Torch kit featured in the May 2001 edition. The kit was easy to assemble and the torch worked first go, however once it has been on for a short period it starts flashing. It's sort of like a strobe light, except that it can flash either off or brighter than the usual brightness setting.

The flashing varies from short, quick pulses to slow pulses where it almost goes off completely for a fraction of a second. It will start flashing anywhere from a few seconds after being turned on to about a minute.

(B. G., via email).

This is a strange one. Some of the reasons for this problem could be: (1) The D cell connections could be tarnished, causing a high resistance to the inverter PC board or to the torch negative spring connection. (2) The 47μF tantalum capacitor could be inserted the wrong way or may be faulty. (3) There may be a bad solder joint between the PC board tracks and the MAX1676 IC. (4) The D cell may be discharged to below 0.8V or it is a general purpose type rather than an alkaline type which should perform better. (5) The output voltage is set higher than the recommended 3.9V to 4V max.

As you can see, we are quite puzzled about this fault but we have given you a list of points to check.

Lost speedo mode in Speed Alarm I have built the PIC Speed Alarm from the November and December 1999 issues. The project was well designed and went together easily. It was perfect for about eight months then one day I turned the key and the display said "dash, dash, dash". Pressing the mode button now gives two choices: dash, dash, dash and speed alarm setting. The unit still works as a speed alarm but there is no speedometer readout. Could the PIC microprocessor somehow have reprogrammed itself? (D. K., via email). The speedometer option in the PIC memory has probably been lost due to high temperatures within the car. This can be regained simply by holding the Mode switch closed when power is applied to the Speed Alarm (ie, when the ignition is switched on). This will select the speedo option again and the display will show an S until the switch is released. This option will again be stored in memory and you will be able to select the speedometer display as before.

Volume control for multi-channel amplifier

I need your help with a problem I am not sure how to resolve. I have just purchased a DVD player with a built-in multi-channel DTS Decoder with analog outputs for all 5.1 channels. I plan on plugging my DVD directly into a 6-channel power amplifier I am building. I am using the LM3876T 50W audio chip for each channel.

Would a 500VA toroidal transformer (25-0-25) with a 35A 400V bridge rectifier and two 20,000μF 50V capacitors as the power supply be sufficient to drive all channels to full power into 8 ohms? Would it be better to have dual power supplies to even the load?

My DVD has volume (gain setup) for each channel, so all I need is a way of turning up and down the volume. Any ideas? Can you buy a 5 or 6-gang pot for this purpose or is the volume control on most surround sound amplifiers all electronic? The idea of adjusting all six channels at once individually is a bit daunting.

Second problem: I am repairing an old Technics SU-7200 amplifier. It has blown output transistors. They are marked D526 & B596 and are TO-220 devices. I need two of each one. Would you know where I can get replacements? Is there a replacement type (equivalent) I can use instead of the originals?

I also found a cordless handheld mike at a garage sale. It is a Chiayo Q-309 with an Audio Technica capsule fitted. Its frequency is 203.300MHz. I believe this type of mike used a dual diversity receiver system. I wonder if any of your readers could help me with a receiver to match? Is it possible to change the mike frequency to say suit the diversity receiver that appeared in your magazine that used the FM band?

K. S., Morphett, SA).

Your ideas about home theatre parallel our own. In fact, we have just developed a 6-channel infrared remote volume control - planned for publication in the coming March issue. Your power supply concept should be adequate.

We suggest you try WES Components (Phone 02 9797 9866) for your Technics amplifier transistors.

Query on Theremin alignment

I have just completed assembling the Theremin kit described in the August 2000 issue of SILICON CHIP. I am having a little trouble getting it set up properly. Could this be due to your instructions not explaining how to adjust transformer coil T1?

(E. K., via email).

Coil T1 is not adjusted since all other adjustments are made against this reference frequency. Adjustment of its trimmer slug will result in detuning of the Theremin.

Check the voltages on all oscillator sections of the circuit, at the drains of the JFETS and at the collector of Q4.

Dr Video negative supply query

I recently built the Dr Video Stabiliser from the April 2001 issue but I have not had any success with it. The recorded video is in black and white and is not stable. Both LEDs light up as required but the negative 5V rail measures only 3.5V. Positive 5V is spot on. The circuit board and soldering look OK. The input to the 555 is 11.5V (pins 4 & 8) and I have checked both D2 and D3.

Could the low negative rail be the problem? How do I fix it as I have changed the 555 and both 220μF capacitors? Even with pin 3 open circuit the voltage out of the 555 is only 5.4V.

(R. L., via email).

The circuit won't work without -5V. If your 555 is working properly, you should have about 10V peak-to-peak at pin 3 and about 8-9V at the input to REG2. Are you sure that you do not have a CMOS 555 (eg, 7555) instead of an ordinary 555 which will deliver a lot more current?

Transformer for TENS unit

I am enquiring about the TENS Unit project described in August 1997. I am trying to adapt this to a slightly different application but am having trouble sourcing the EF25 ferrite transformer. Are you able to tell me or put me in touch with the original kit supplier or advise where I might source this from.

(R. S., via email).

The EF25 is now difficult to obtain. You could use the EFD25 which will require some extra holes to be drilled in the PC board. These transformers are available from Farnell. The catalog numbers are 200-300 for the cores (two required), 200-311 for the bobbin and 200-323 fro the clips (two required). Phone New Zealand 64 9 357 0646.

Using ECU signal for speed alarm

I have an enquiry regarding a Digital Speed Alarm kit that I purchased through Jaycar recently. It was described in November & December 1999. The instructions call for the use of a coil and magnets for detection of the speed of the car. My query is can I use the speed sensor wire from the ECU of my car and if so, how? My car is a 1986 VL Commodore.

(C. M., via email).

The speed signal from the car computer can be used to drive the Speed Alarm unit. Simply connect the signal to the 1kΩ resistor input connecting to pin 2 of IC2a. This is instead of connecting L1.

Boosting headphone signals

Have you ever had an article on converting the headphone output of a device to a line level signal suitable for feeding into an audio amplifier? I have a laptop which has a composite TV out signal but only a headphone socket for audio. It would be nice to be able to hook the entire thing into the home theatre.

(M. C., via email).

Have a look at the PreChamp featured in July 1994 (also featured on page 36 of the October 2001 issue); you will need two for stereo. You probably only need a gain of 10 or so, so the 100Ω resistor at the emitter of Q1 should be changed to 220Ω or 270Ω.

Beat triggered strobe does not work I recently brought a Beat-Triggered Strobe project, as published in the August 1998 issue of SILICON CHIP. I got it together fine and it worked fine on oscillator for a day or two. Then I mounted it in the box and decided to try the line input and I got nothing. I spent some time checking the circuit, PC board layout etc and I noticed that the circuit specified a 1μF capacitor just after D1 but in the PC board layout diagram it specified a 100μF (which was supplied). I put in a 1μF unit and that fixed it for about 10 minutes, then it stopped altogether. All that happens now is that the neon goes on, permanently. I did some testing and I think the neon is staying on because the 0.1μF 250VAC in series with T2 is only discharging to about 68V. Beyond that, I have no idea what's wrong. Can you help please? (T. D., via email) The capacitor at the cathode of D1 should be 1μF. Trigger transformer T2 is probably at fault. Replace this and the circuit should work. Check the voltages across the 0.1μF capacitor and the 6.5μF capacitors before working on the circuit. A high voltage across them could give a nasty electric shock.

Video colour inverter wanted

I am a student at Swinburne University, Melbourne. I am currently doing a project involving video editing and production. I am looking for a video colour inverter to add in those effects I have sometimes seen on TV. I was wondering if you had a kit or project that takes a standard composite video signal and inverts the colours.

(G. W., via email).

While we have not described a video inverter project, you could adapt the circuit of the Video Fader we published in August 1993. You need to strip the sync off the video signal, invert the video and then add the sync back again. The August 1993 circuit will show you how. We can supply this issue for $7.70 including postage.

Stepper motor control needed

I am after a circuit that controls a 12V motor. It would control the number of turns in either direction the motor can make; ie, I want the motor to complete 50 revolutions in a clockwise or anti-clockwise direction. It would also allow the number of revolutions to be stored for later retrieval (multiple storages would be nice). Basically, I want to be able to open and close curtains using a motor and be able to open them a preset amount.

Do you know of a circuit that would suit my needs and where I could get it?

(B. S., via email).

We published a number of articles on Stepper Motor control from a PC during 1997 but one entitled "Manual Control Circuit For A Stepper Motor" in the June 1997 issue is probably the most appropriate for your application. It will drive a stepper motor in one direction or the other for a fixed time. We can supply the issue for $7.70 including postage.

Extra inputs for AV distribution amplifier

I would like to add additional (switchable) inputs for the AV Distribution Amplifier project (November 2001) to cope with the situation of, say, using a Cable TV box, two VCRs and a DVD feeding a TV and also feeding the output back to the VCR's line-input for recording/dubbing, plus (sound) to a hifi system. This situation must be quite a usual setup in the home and would cope with recording the 'input' device at any time, whether the TV was on or off.

Is it just a case of buying a video switch box to use before the distribution amplifier or is there a more elegant solution? Or indeed has a project been designed for this purpose?

(B. F., Brighton, Vic).

The best way is to use an AV switch box ahead of the distribution amplifier, as you suggest. Trying to build input source switching into the amplifier would be messy. A switch box should be fine.

Peak hold for tacho circuit

I would like to know whether a peak-hold function could be added to the Auto Tachometer circuit in the April 2000 issue? I would like to use the unit in a Formula 500 Speedway car which is powered by a single cylinder 2-stroke engine.

Different tracks require different gearing and this feature would be ideal for checking for peak RPM, as it becomes very difficult to keep an eye on the tacho when peak revs come at the end of the straight right when you're sliding into corners, trying to avoid cars in front of you while getting mud thrown at your visor etc.

(J. H. Perth, WA).

Unfortunately, the entire program memory contents of the microcontroller in the tachometer circuit has been used to provide for all the features. Without extensive rewriting of the code there is simply no space to include a peak-hold feature.

The accuracy of the peak hold would also be in doubt since many race engines simply change RPM too quickly for a reliable measurement particularly over the 0.6-second update time for your single cylinder 2-stroke engine.

One suggestion would be to set the bargraph to operate over a narrow range of RPM so that peak RPM can be seen as one of the lit LEDs. This will provide a guide as to RPM reached within the limits set for the bargraph.

Questions on the Ultra-LD Amplifier

The Ultra-LD Amplifier in your latest editions looks very interesting but I have a couple of questions regarding the specifications and possible modifications. Is the distortion rating at 1W output? If so, what's the distortion rating at full rated power? Are the distortion harmonics predominantly low or high order?

Are there any improvements that can be made to the amplifier in the area of sound quality? For instance, I'm not that keen on the crimped connectors but can always attack those with a soldering iron once the kit is up and running.

How does the amplifier respond to an increased bias current? I would prefer to do most of my listening in class A. I'm aware that this would make the fan run more often. Ignoring the background noise issue, are there any problems with increasing the bias a bit?

Finally, you don't seem to be too concerned about any lack of Vbe/gain matching; is there anything to be gained by sourcing matched output transistors? A few people I know love to rant about how anyone can achieve extremely low THD with lots of negative feedback; is there anything to be gained by reducing the amount of feedback and taking other measures to ensure that the amplifier's overall gain is still suitable?

(G. W., via email).

As you can see from the graphs on pages 62 & 63 of the December 2001 issue, we measured the distortion at 90W across the full audio spectrum (Fig.15) and at 1kHz from 500mW to full power (Fig.16). Distortion at full power at 1kHz is less than .003%. The distortion is predominantly 2nd & 3rd harmonic. We cannot suggest any improvements for sound quality - if there were any, we would have incorporated them. We would not recommend solder connections for the crimped connectors - there is not likely to be any improvement at all.

If you wanted to listen in class-A you would have to increase the bias current to several hundred milliamps. This would greatly increase the overall dissipation in the amplifier and may require an improvement in the heatsinking for the bridge rectifier. It will also increase the hum radiation from the transformer and therefore prejudice noise levels in the amplifier - we wouldn't do it.

The only likely improvement obtained by matching the output transistors would be improved current sharing - there is unlikely to be any other measurable improvement.

Wiring layout is the most critical aspect of a low distortion amplifier. A relatively small change in the wiring layout of the Ultra-LD can easily double the THD! Against that, any playing around with the feedback parameters is fiddling around the edges.

We would strongly advise against making any modifications to the circuit or layout, unless you have access to an Audio Precision System One or similar distortion measuring system and a 100MHz scope, to validate any changes.

PIC project questions I have a couple of questions regarding two of your PIC based projects: (1) Is it possible to change the software of the "Automotive Thermometer", published in the October 2001 issue, to indicate tenths of a degree, ie; XX.X? If necessary, a longer 'integration' period would be acceptable. Also, could the software be changed to automatically toggle between internal/external temperatures - say with a delay of 30-60 seconds? The existing toggle button could still be retained to immediately change the mode and be used for calibration. (2) I built the "Rain Gauge" (published June 2000) from a Jaycar kit about a year ago. It works well but I would like to know what changes have been made to the software (I notice you have a new version available for download). I would also like to know if the kit design (software/hardware) could be modified to blank the displays during a main power failure; ie, the backup battery would only drive the clock/counter circuitry without wasting power on the LED displays? (C. S., Lemon Tree Passage, NSW). The software for the Thermometer could be altered to alternate the display readings between internal and external temperature. Alternatively you could connect up a 555 timer operating as a free running oscillator to "close" the switch momentarily every few seconds. Measurements of 0.1°C are not possible because this display resolution is not available and also because the sensors do not give 0.1°C accuracy. Most accurate temperature sensors which give 0.1°C accuracy/resolution only operate over a narrow temperature band. These are used for body temperature measurements. Commercial thermometers claiming 0.1°C accuracy over a -55°C-150°C range using a thermistor sensor are rather optimistic. The Rain Gauge does not have any facility to shut down the display during a blackout using software. It would require an additional circuit to detect power loss to interrupt the display supply.

WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.

You can buy products mentioned in this article here : KC5279 : IMPR. SPEED ALARM KIT 11/99 XC0305 : WIRELESS RAIN GAUGE