IEC power cords can have poor contacts

Recent remarks about power cords (Mailbag, July 2004) have prompted me to offer some comments about their serviceability. I am involved in testing and tagging a large number of secondhand cords for use in computers, monitors, printers, etc.

In a recent batch of 55 which I tested, nine were rejected due to poor connections between male and female plug and socket. Australian standard 3-pin plugs have a 1.6mm thick male pin but IEC pins are 2mm thick. When I first started checking the resistance of these cords, any slight movement of the plug would produce an open-circuit with some cords and I realised that a "go-no go" gauge was needed.

This was made from the earth pin of an IEC plug and I now make it a standard procedure to test for sufficient contact tension with all IEC sockets, prior to the other tests required.

G. F. Nott, via email

Whatever happened to 230V? A few years back, one of the hot topics in SILICON CHIP editorials and correspondence was the proposal to change Australia’s domestic single-phase mains supply to 230V. I was reminded of this when I purchased a new element for my water heater. Despite being exactly the same part number as the original 240V stamped part, it is now stamped "230V". For a few years I’ve noticed a lot of domestic appliances have been rated at 230-240V. I assumed this was because the same appliances were also for the Kiwi market but now I wonder if it is really in fact to satisfy the pro-230V lobby in preparation for the lowering of voltage. In the UK, they went through the 230V conversion not so long ago, for the same politically correct reasons Australia was faced with. Interestingly however, it turns out that it was "only on paper". The mains voltage was not actually changed from 240V. It might all seem academic triviality but having once lived at the end of the line with the mains regularly dropping to 200V and under, 10V difference was enough to stop an already poorly performing appliance from working altogether. My question is therefore, did we actually go through with it in Australia? If so, the UK approach must have been taken because I am glad to see the mains in my area is still closer to 250VAC when I measure it. John Hunter, via email. Comment: it seems as though the 230V "conversion" has been largely in name only. Good thing too.

Even bigger power supply wanted

The HT power supply using a modified AT power supply in the July 2004 issue is excellent. The only criticism is that as an RF-oriented person, 125W is just not enough; I need about 1kV at 1A for a decent HF linear and yes, I realise the transformer is just not big enough to do this – a redesign using a bigger transformer, perhaps?

The other observation in the photograph of the dismantled supply: note the complete absence of EMI suppression on the mains power input. There is space on the board but the manufacturer just didn’t bother to put it in. Ah well, that’s the price we pay for being run by third-world politicians.

By the way, high voltage transformers are still readily available (see your own advertisers). You just have to pay a lot of money for them. Still, if you are mad enough to want to build a valve audio amplifier, then you will spend it. For RF power, valves still rule supreme on a "bang for the buck" basis.

Andrew Blight, VK3BFA,
via email.

Wide-screen digital TV is a mess

In December 2003, I wrote regarding my thoughts on the current state of Digital TV broadcasting in Australia.

Recently, Aldi started selling their own "Medion" brand digital set-top boxes for the princely sum of $169. I couldn’t believe Aldi’s Cheapo Chinese-made offering: you just plug it in and it works! All the local channels were already tuned in; all I had to do was tell it where I lived and it automatically sorted out the program guide, local time and so on.

You even get an on-screen signal strength meter to help you set up the antenna! The digital channels came booming in! So now it seems I’m getting Digital TV as advertised. If they can just do something about the fact that most of the time all I’m getting is five copies of the existing analog channels, I’ll be ecstatic!

Interestingly, unlike the Thomson units, the Aldi receiver actually does something with the HD transmissions instead of the screen just going blank. You get a series of "stills" which seem to have more resolution than the "live" SD transmissions, although I know that doesn’t really make sense on an SD TV.

The whole HD/widescreen thing seems to be going off the rails somewhat, particularly in the US. Contrary to what was predicted, sales of widescreen sets seem to have gone off the boil. Most of the newer large-screen TV sets I’ve looked at recently have been standard 4:3, including one "HD-ready" Philips 68cm 100Hz job for $999.

They seem to have somewhat lost the plot with large screens. The Philips set offers a choice of 625-line 50Hz progressive scan, 625-line 100Hz interlace, or 1080/1250 line 50Hz interlace. On 1080/1250 you can hardly see the horizontal scanning lines but the tube’s vertical stripe pattern is considerably coarser than even the plain old 625-line horizontal line structure! "HD ready"? Ready for what? Trouble is, the finer the pitch, the more electrons that wind up simply heating up the shadow mask, and the more power consumed, the heftier the scanning circuitry has to be and so on. Most manufacturers have taken the easy way out.

Very few plasma screens offer true HD 1920 x 1080 resolution; in fact a lot of models have only "VGA" 640 x 480 resolution (derived from the visible picture area of ordinary NTSC transmissions).

Meanwhile large-screen LCD displays are racing up on the inside, offering lower cost, lighter weight, less fragility and lower power consumption. The images aren’t yet quite up to current plasma or CRT standard but I doubt many people would notice the difference. What’s really interesting is that many of them are old-fashioned 4:3, not widescreen 16:9!

This highlights another major stuff-up. As one of your other correspondents has pointed out, if you set your digital receiver to 16:9 letterbox, a lot of the time you simply wind up with a three-quarter sized 4:3 image, since the stations routinely transmit 4:3 sourced material "vertically letterboxed"! Worse still is vertical-letterboxing of already horizontally-letterboxed SD material!

This is because the authorities blithely assumed that everyone would by now own 16:9 TV sets. What should have happened was a mandate that all digital set-top boxes be fitted with a
16:9 "flag" signal output and all new 4:3 TV sets set up to switch automatically from this. Ah well, hindsight – the only perfect science!

Keith Walters,
via email.

"No More Gaps" not safe as high voltage insulation I was reading your May 2004 issue when I came across the reader’s question regarding the use of Hot Glue as an insulating medium. I have used Hot Glue a number of times but was worried when I saw the reader refer to the use of "No More Gaps". A couple of months ago, I was working with another engineer on an audio project. We experienced problems with air escaping out of the input connectors on a bass speaker cabinet. To quickly fault find our problem we covered the socket in "No More Gaps" due to its thick consistency. The next day we couldn’t find our input signal and to our surprise the No More Gaps had shorted the input socket. We discovered that the no more gaps was highly conductive either wet or dry. We left the PC board for a number of days with no change to the results. This meant two things. Firstly, as a mains insulation medium it is very dangerous. However, as a cheap conductive RF coating it works exceptionally well and for only a fraction of the cost of the spray-on coatings; it is not very pretty though. Robert Martin, via email.

CFLs and timebase correctors

I totally agree that CFLs do not live up to their claims. I bought a fairly large quantity of CFLs from K-Mart early in the piece and they all failed way short of their claimed life. Some that failed within weeks or a couple of months were replaced under warranty, but I got sick of taking them back. They were in the vicinity of $24 to $30 each and terrible rubbish! Some were mounted in ordinary light fittings and several were mounted in plain surface-mount sockets in my darkroom (with a low ceiling) where I did most of my CAD work at the time.

The reduced amount of heat compared to incandescent lamps was the main reason for using them there and the availability of "daylight" types was another. Ultimately they all failed prematurely and were a total waste of resources and money.

I noticed that when they failed the lamp-base was often discoloured, indicating extreme temperature inside. I also found out (the hard way) that as the lamps approach their end of life, the fluorescent tube can become extremely hot. I have opened a few failed lamps to see a pretty low-tech circuit with two or three transistors, electrolytic and film capacitors, a ferrite-cored transformer and a few diodes and resistors on a poorly assembled, badly soldered PC board (usually made in China). Philips and GE CFLs are better but still not anywhere as long-lived as an incandescent bulb or conventional fluorescent tube.

I would say that the lamps fail mainly because the heat dries out the main filter cap which then has insufficient capacitance to filter the 340V or so used to power the HF power oscillator. I have not attempted to repair any of these failed lamps because (1) the tube is usually blackened at the ends or it breaks when the base is pried open; (2) the filter capacitor is very tiny and I have not seen the capacitance/voltage/small size combination available here; (3) it’s pretty much impossible to safely reassemble the base after opening and (4) they’re too dangerous to play with.

Unless the manufacturers of CFLs can lift their game, I can’t see that these lamps will ever perform as long as expected. The right solution might be to have the inverter in the roof where it can be big enough to be constructed from more reliable components and dissipate its heat, or perhaps just go back to the good old choke and starter setup with a plug-in tube. Additionally, I imagine that millions of CFLs must draw a rather spiky current from the mains with their simple half-wave or full-wave capacitor input filters which I thought the supply authorities were trying to minimise these days.

Regarding the Video Enhancer & Y/C Separator in the August 2004 issue, I might be wrong but as far as I can see, the device does not gate out any Macrovision pulses. Many people would want to use this gadget to view commercial DVDs or VHS tapes. Black level clamping is definitely not compatible with Macrovision, so they will need another device in line with the input to get rid of it. I think you should have mentioned this in the article or perhaps it might have been an idea to build this facility into the design.

Finally, regarding Rex Shepherd’s letter in "Mailbag" for August 2004, I get quite a few calls from people who are trying to put original VHS (or Betamax) tapes onto DVD, either using DVD recorders or PC-based video capture cards and internal DVD drives – unsuccessfully. Basically, the video signal from 99% of VHS players is too jittery and noisy for the A-to-D and MPEG2 converters in DVD recorders and capture cards to cope with. The other 1% (a handful of high-end JVC, Panasonic and Sony machines, etc) have timebase correctors (TBCs) built-in and their output can be processed quite readily.

These high-end machines quite often have advanced video noise and chroma noise reduction built-in (such as JVC’s very effective 3D noise reduction) and the improvement in quality of the resultant DVD is well worth the investment, but these machines are starting to become rare as the DVD recorder becomes more affordable and starts to displace the VCR.

External TBCs are also very effective and offer controls for brightness, contrast, saturation and sharpness (such as this one: www.questronix.com.au) at a reasonable cost. In my experience, most tapes need a bit of adjustment, especially if they are getting "wishy-washy"!

I can see Mr Shepherd’s point but it is a technically complex subject and I doubt that many electronics salesmen would have any idea what a TBC is, let alone recommend that one is necessary for a decent transfer to DVD. Given a good quality video signal, Mr Shepherd’s Digitrex should perform as claimed. If it doesn’t, then he has a strong case for a replacement or a refund. A "stabiliser" such as your "Dr Video" will achieve little if anything as it will not affect the stability of the video signal at all and cannot replace missing/degraded sync as caused by tape dropouts or bad tracking. Only a TBC can achieve that.

Joe R,
via email.

New PCs have puny speakers

Saw the article on silencing PCs in the July 2004 issue. I have just gone through a similar scenario with my new P4 and ended up building a PWM NE555 controller for the internal case fans using a thermistor to control the pulse width and therefore the speed of the case fans.

But have you noticed that new PC cases have a different form of "noise maker" in place of the small dynamic loudspeaker of days of yore? This has a very limited bandwidth/output when compared with the small speaker in older cases and shoots the PC in the foot for many users. Why? Because the motherboard over-temperature and processor fan failure warnings are via this little transducer. For those folks with tinnitus or industrial deafness, this little transducer’s output falls outside their audible range.

Changing the transducer back to a conventional PC dynamic speaker solves the problem. Isn’t progress wonderful?

Brad Sheargold,
via email.

Comment: progress is wonderful; it’s just that some parts of progress are more wonderful than others!

Saving the output transformer

I read Graeme Dennes’ letter in Mailbag for August 2004 with interest, concerning my Vintage Radio article in the June issue.

I quite agree with Graeme’s comments about the placement of the capacitor (C11) from the plate of the audio output valve to earth, in that there is a danger of a short circuit occurring. I talk about voltage stress and leakage of capacitors in this month’s article.

Some manufacturers did place the capacitor across the speaker transformer primary and it is my policy to do the same. As Graeme points out, one of the functions of this capacitor is to assist with RF stability. There is a small amount of IF signal amplified by the audio amplifier and most manufacturers weren’t concerned about filtering this out – as long as the set worked.

When wired straight from plate to earth, this capacitor is more effective. If wired from plate to the HT line, the bypassing of the RF (or actually IF signal) is not quite as effective, particularly if only an electrolytic capacitor is used as a filter.

Electrolytic capacitors are not renowned for good long-term RF filtering. With a lack of good filtering, a small amount of this IF signal remains on the supposedly bypassed and filtered HT supply line and is then fed back into the IF and hence instability can occur. I have struck this many times, and with proper bypassing and filtering, a set that is marginally stable often becomes a first class performer. Then the plate bypass capacitor (C11 in this case) can be wired across the speaker transformer as Graeme suggests.

I haven’t advocated the procedure suggested by Graeme in my articles as I have been concerned that should I advocate doing this and it then causes instability in the receiver, I have not helped a newcomer to get his/her set going properly. A new high-quality high-voltage (at least 600V rating) polyester will be more reliable than the older paper capacitor it replaces anyway.

Perhaps restorers can try the capacitor wired in both ways – the earthy end to chassis or to the HT end of the speaker transformer. If the set performance is identical with the capacitor wired either way, then it can be assumed that the wiring of the capacitor across the transformer primary is OK.

However, if the performance of various valves is down or the alignment is a little out at the time of the tests and later on a new valve(s) is put in or the alignment touched up, unexplained instability could occur which may be due to this circuit alteration.

Rodney Champness,
via email.

CFLs and car electrics I agree with your short life observations on CFLs but perhaps a more important point is that they all contain a drop of mercury in the tube and these drops add up. Given their much higher complexity, it would be interesting to see a proper total energy audit, including construction and waste disposal, for both types. I hope your response to the letter on saving the output transformer (Mailbag, August 2004) was tongue-in-cheek since many of the old mantle sets I repaired in my youth had a shorted .001μF waxed-paper capacitor across the o/p transformer primary (incidentally cooking the output bottle). I’m sure Rod Champness will confirm. The same arrangement is used in guitar amplifier reverb line drivers dating from the same era but failures are very rare. Regarding the letter on a safe battery charger for cars (Ask SILICON CHIP, August 2004), I have seen notices in engine bays advising the disconnection of the battery when welding or battery charging. Car service folks these days often have a "keep alive" gizmo consisting of two 6V lantern cells and a cigarette lighter plug to preserve the memory of the various items connected to the 12V bus. One car I was asked to look at after the owner had disconnected the battery for welding had such terrible amnesia it wouldn’t even crank! Roly Roper, via email. Comment: no doubt if CFLs lasted as long as claimed they would have a favourable energy audit. That battery backup scheme is not really safe; it should have an isolating diode to prevent mishaps.