Saving the output transformer

On reading Rodney Champness’ informative Vintage Radio section in the June 2004 issue, the text on pages 87 and 88 refers, amongst other things, to the problem of capacitor C11 becoming shorted. As the circuit of Fig.1 on page 85 shows, C11 connects from the plate of the output valve to ground. This is a quite common circuit design strategy to assist with (RF) stability and in tailoring the final audio-frequency response of the receiver.

Should C11 become shorted, it immediately pulls the plate end of the output transformer’s primary winding to ground. One of the unfortunate side effects of this is that the full high-voltage DC supply (HT2) is then connected across the low-resistance primary winding. Not unexpectedly, this can result in a burned out (open-circuited) primary due to the excessive current flowing through it, after which both the transformer and C11 will need to be replaced.

There is a simple expedient which will forever prevent this destruction of the output transformer in radios of any type which share this circuit configuration. The solution is to connect C11 directly in parallel with the transformer’s primary winding – an electrically equivalent arrangement. This is easily done by unsoldering the earthed end of C11 and connecting that end to the high tension line (HT2).

Now, not only will a future short in C11 not destroy the output transformer in the manner described above but the possibility of C11 shorting again is greatly reduced, due to the much lower static (DC) voltage across it.

Graeme Dennes, MIE Aust,
Pakenham, Vic.

Comment: that’s an excellent suggestion. Wonder why those clever designers in the days of yore did not think of that!

Valves have definitely had their day If Grahame Macpherson ("Valve Electronics Never Reached Its Peak", June 2004 Mailbag) dug a bit deeper he would find that in truth, valve technology and indeed circuit design were both highly developed before the silicon age. Valves were imaging, photo-multiplying, counting, displaying, microwave amplifying and even computing, before the first transistor. The only obstacle with using a phase-locked loop (aka lock-in amplifier) was that first you had to design and build one using about a dozen valves, rather than just grabbing a CMOS PLL chip. Think of a modern cell-phone with its computer, display and microwave transceiver, based on thermionic emission. The simple power density required for thermionic electron emission compared to solid state alone make it a non-starter. Research into thermionic devices hasn’t stopped, particularly in the area of display pixels. I’m crazy about valve guitar amplifiers but that’s where it stops. When I’m playing keys, the last thing I want is a bunch of jingles and hiss from a 12AX7A. Gimme an LM833 any time - that’s "blameless" enough. Roly Roper. via email.

DVD dubbing stymied by Macrovision

It was very interesting reading your June 2004 editorial regarding Macrovision on DVDs.

About five months ago, I purchased a Digitrex DVD recorder. The reason for the purchase was to copy all my old home movie camera films and later analog video camera recordings to DVD disks. The movie camera films and analog tapes had previously been transferred to VHS tapes (22 VHS tapes in total). Those tapes are now showing age and the quality of the images is bordering on "wishy washy". It was imperative to transfer them to DVD as soon as possible.

The many problems I encountered were frustrating to say the least. Some of the tapes had developed "sound bars" and whenever the DVD recorder encountered these, the recorder would cease recording and give a message stating stopped due to copyright pro-tection.

These tapes are my own and not commercial movies or but the recorder does not see it that way. Every time I resumed recording, the recorder starts a new title. One disk ended up being 26 separate titles, even though the film was only 20 minutes in length.

Another lesser problem occurs while editing via the TV. After 10 minutes of continuous recording, the output to TV goes into black and white mode.

I contacted Digitrex in Sydney. The service contractors verified the problem and stated there was nothing they could do, due to Macrovision being built into the recorder. I then contacted Digitrex’s sales manager and asked if there was any way to overcome the problem. He said to buy a video enhancer.

My response was "it is laughable that a sales manager would suggest that a customer buy another product to make his product perform as advertised. I want my DVD recorder to do the job it is supposed to do". Plus, I was not going to spend any more money.

The topic of Macrovision now makes me see "red". Your Dr Video (SILICON CHIP, June 2004) will probably solve my problems and I will await feedback but why should I have to purchase another product? I suggest that readers be wary when considering purchases of DVD recorders.

Rex Shepherd,

via email.

Expensive DIY loudspeakers not so attractive

The speaker design information from www.linkwitzlab.com looks tempting unless you’re an old cynic like me. Open baffle speakers as high performance items were around 40 years ago but were never really successful, although they had their strong adherents. For really good sound, the back wave from the driver must be suppressed unless it can be put to use as in a vented enclosure. At $US3800 a pair plus 60 hours labour, the Linkwitz design is unlikely to appeal to local "do-it-yourselfers", as you rightly replied to Paul Rohde (Mailbag, June 2004).

As you are well aware, there are many critical factors in quality speaker design and not much agreement among users. My own speakers are a 4-way active crossover design, mainly because separate amplifiers were needed to control the resonances which some of the drivers exhibited outside their specified operating range.

A friend wanted some really good speakers and was unimpressed with the commercial units on offer. We even listened to a pair of monster speakers (about 2.5 metres tall and entirely impractical for that reason alone) which retailed for, from memory, in the region of $30,000 and with a phenomenal power handling capacity. They were very disappointing in the mid-range; quite unclear and "fuzzy" considering the very high cost. I believe this to have been because of the use of a first-order filter to the mid-range driver. All the other speakers we heard had colouration or poor transient response, or any number of other problems.

The drivers are, of course, critical and we settled on Morel for the mid-range and tweeters. He already had a pair of Vifa woofers and while I think the Morel range would have been slightly better, I used the Vifa drivers because they were to be used only to 200Hz anyway. I was also impressed with the specifications for the Dynaudio range of drivers which I believe would have also performed quite well.

The cabinet was designed to minimise diffraction and was consequently too complex for home construction. The crossovers are 4th-order between tweeters and top mid-range, 3rd-order at the bottom mid-range and 2nd-order to the woofers. The crossover points are 200Hz and 2kHz. All inductors are air-cored and were hand wound as there were no suitable components readily available commercially.

The woofers and ports face rearwards and the internal design of the enclosures is such that sound absorbent material is not necessary, there being a minimum of parallel faces, with the use of baffles to minimise standing waves.

A cabinet maker provided very handsome enclosures for about $800 the pair, the drivers cost about $600 and the crossovers about $400. So, with other odds and ends, there was not much change out of $2000. My friend is very pleased with the result. The response is very smooth and the transient response and "sound stage" are excellent. This is about as inexpensive as really good speakers can be made for, in my opinion.

The cost, however, is unlikely to be attractive to many DIYers and the difficulty of making the enclosures is a big disadvantage. The only criticism we have had of the speakers is that they have no colouration and are too neutral. I always thought that was the very thing good speaker design should aim for! I am envious that they sound much better than my old 4-way speakers!

I’m sure you are correct in your view that most readers’ budgets do not stretch to esoteric and expensive speaker designs.

Alan March,
via email.

Airway museums in Melbourne & Adelaide

Those whose interests were stimulated by the article on Instrument Landing Systems in your June 2004 issue may be interested in the Airways Museum near Melbourne, which has examples of airways equipment as described in your article. It also has communications equipment, photos and archival material concerning aircraft and airways activities spanning many years. Details are at www.airwaysmuseum.com

Information on a similar museum near Adelaide can be obtained by phoning Eric Kelly on (08) 8443 7651.

Ron Rye,
via email.

Instrument landing systems have attitude Congratulations on an excellent article on Instrument Landing Systems in the June 2004 issue. At Charles Darwin University, we are undertaking a research and development project using tablet computers for document deployment and other flight functions in aviation. The graduate research students we involve in this project have an excellent IT background but minimal background in flying, so this article will provide some of this background. Having said that, I would also like to offer some small comment on the article, in that – probably as expected in any description of a highly procedural operation – there are three small errors that I noticed as follows: (1) The diagram on page 12, section (4) indicates "continues until approaching the centre line, then turns to intercept ...". This is far from the whole story. Depending on whether the procedure is a genuine DME arc intercept (a specified instrument procedure) or simply a manoeuvre chosen by air traffic control or the pilot to position from the inbound track to the final track, this is typically flown at just over 10 nautical miles from the runway. At that distance, an aircraft flying at a typical 150 knots covers just over 250 feet (76 metres) per second. On the localiser, the distance from full-scale deflection to centred amounts to only 2.5° offset, which at 10 (nautical) miles is about 2500 feet. Thus, there would be only 10 seconds from seeing the needle move from full offset to having the aircraft lined up on final (a 90° turn). In instrument flying, turns are normally held to so-called "rate one" or 180° per minute. Following through from this, a rate one turn through 90° should take about 30 seconds. Anything faster and the martinis in row three get sloshed (rather than the passengers). Not good for the first class types! What actually happens here is that the pilot uses a so-called lead bearing, from where he/she turns to intercept the final course at an angle of 30° or less, then uses the localiser needles to position on to the final approach course. (2) The same diagram says "... Having followed the ILS as far as the middle marker ... the pilot must either..." see the runway and land, or get out of there (my re-phrasing). This is not correct. On a precision approach, the pilot follows the ILS to the minimum descent altitude, typically 200 feet above the runway. It is this altitude which defines the missed approach point on a precision approach and certainly, this is close to the middle marker. Pilots receiving training to upgrade from visual to instrument flight ratings are taught to "bounce off" the minimum altitude. If they don’t – if they go below this by only a few feet – they fail their instrument flight test. If they go much further below it without visual reference, they get a more serious reprimand! (3) Page 17 calls the top middle instrument an "attitude indicator". It is more than this. It also indicates the bank of the aircraft and is an "artificial horizon" or AH. By the way, I suspect that many of your readers share with me a love of technology. I once read of an American brain surgeon who said he was a simple person: "...never happier than guiding his fishing boat back through a fog-bound bay by radar". I am the same. I have experienced landing an aircraft at Essendon (Melbourne) where we (myself, wife and two kids) broke out of clouds at tree-top height to see the runway lights, then touched down just before midnight. I get the same thrill when I throw a few chips together with a bit of solder and the whole lot works. Wow! Thanks for a great magazine, Professor R.S.V. (Bob) Pascoe, Head of School/Associate Professor (Computer Science), School of Information Technology, Charles Darwin University. Daniel Field replies: (1) The diagram in the article shows the intercept from a 20-mile DME arc. Professor Pascoe’s argument is based on an intercept from 10 miles, where the localiser course width is only half that at 20 miles. So his argument does not really relate to what is in the article. Second, he argues that a plane travelling at 250 feet per second and starting 2500 feet from the line being intercepted would have to make the 90° turn in 10 seconds. Basic geometry tells us that the plane would actually fly along an arc of radius 2500 feet. On a 90°turn, that arc would be about 3900 feet long and so the turn would actually take more than 15 seconds rather than just 10. 90° in 15 seconds is a "rate two" turn (double the "rate one" 180° per minute) which is certainly not unheard of and needn’t spill any of the martinis (being consumed with seat backs upright and tray tables locked away!). In fact, if you double the distance out to 20 nautical miles as shown in the diagram, then the intercept manoeuvre would start twice as far from the line, giving the pilot a very nice, standard, "rate one" turn. Having defended the diagram I must concede that Professor Pascoe’s point about the "lead bearing" is quite right. When intercepting the localiser, it is more usual to intercept it at a shallow angle. A shallow intercept is less prone to errors such as overshoot. In drawing the diagram, it was a balance between simplicity, accuracy, and illustrating the point (viz, how VOR, DME and ILS can be used). Perhaps I should have called it a "Possible ILS Approach" rather than a "Typical ILS Approach". (2) I fully accept Bob Pascoe’s point here. My background is technical not operational, so I can sometimes miss the details that are part of operational practice. (3) Ah yes, the "whatsamacallit". The artificial horizon goes by more aliases than a KGB spy. One textbook lists the correct names for this instrument as bank and pitch instrument, artificial horizon, gyro horizon, attitude indicator and attitude gyro. It gets even more interesting once extra functions are added, with names such as director horizon, command indicator, flight director and attitude director indicator, denoting the addition of Autopilot functions (to name but one example). The name "Artificial Horizon" (or simply "AH") is certainly the most common. "Attitude Indicator" usually refers to any instrument that gives information about the pitch and roll attitudes of the aircraft. Professor Pascoe has interpreted the word "attitude" in its limited sense of "pitch attitude" only (which is certainly the most important attitude to a pilot). The artificial horizon displays both pitch and roll attitude, so it is perfectly reasonable to call it an attitude indicator. But to be completely truthful, the reason I left it as attitude indicator (rather than the more widely used "Artificial Horizon") was to accommodate your layout artist’s (Ross Tester) little joke. Since "Attitude Indicator" is not actually incorrect, I thought it would be nice to leave it in.

Silent PCs need not have fans

I read your article on silencing PCs in the July 2004 issue with interest. Noisy PCs are not a new phenomenon. I recall using one of the early IBM PCs and it reminded me a jet aeroplane taking off.

In your articles, you focused exclusively on how to build a top-of-the-line system that did not require you to wear earmuffs. There is an alternative approach – start with a system that doesn’t need forced-air cooling.

For example, VIA make a range of CPUs that don’t need active cooling. While they are only the 500-800MHz region, this is plenty fast enough for normal multi-media or office work. The power consumption is low enough that the power supply doesn’t need a fan either, so the only moving part is the disk drive. You can build a whole PC that needs less power than a top-end CPU. For details see: www.mini-itx.com

You can buy them locally at (eg):

www.traverse.com.au

Also, you mention cleaning the heatsink compound off when changing heatsinks. Some heatsink compounds may include beryllium oxide. This is highly carcinogenic and you should be careful to avoid contact with it.

Peter Jeremy,
via email.

Cheap humidity sensor

In your June 2004 issue on page 91, M. H. asks about the availability of a cheap humidity sensor.

There may be an answer on page 403 of the 2004 Jaycar catalog. Cat. QM-7204 shows a Thermometer/Hygrometer "gadget" that sells for $27.95. This little beauty must have some sort of humidity sensor in it. Careful dismantling and "reverse engineering" could yield a useful humidity sensor, along with a temperature sensor (unless everything inside is integrated onto one big chip).

Anyway, $28 is a small investment and may solve the need for a Picaxe
datalogger. If not, just put it back together and you still have a useful gadget.

Brad Fuller,
via email.