In the Shop
Harry Leeming G3LLL starts with headphones, which takes him to hearing aids, on to starting a shop and ending with diodes!
Harry Leeming G3LLL starts with headphones, which takes him to hearing aids, on to starting a shop and ending with diodes!
Ialways found that trying to decipher signals buried in noise was much easier if I used headphones, and for many years this was my preferred mode of operation. There is a danger when wearing headphones, however, in that you tend to be less conscious as to how loud the signals are, as you are isolated from room noise, and hence have no reference level. In my case I am paying the penalty of all that listening on noisy channels, as I have become deaf and have to wear hearing aids. Learn from my mistake and when wearing phones use the lowest possible volume setting.
And Talking About Hearing Aids
The age-old problem, with any kind of portable electronic device, has always been the difficulties in storing enough electricity, and of reducing size. I can still remember as a child an elderly gentleman (well he was probably younger than I am now!) who had an earpiece plus a heavy box hanging around his neck at church. This often emitted whistling noises during services, much to my amusement. Later, as technology progressed, slightly smaller aids were developed, still full of valves, and some of these early devices, ended up being sold off on the surplus market. Radio magazines such as PW then published DIY articles showing how to convert them into portable radios.
Move on to this century and due perhaps to the large number of young people who have spent a great deal of time listening to loud music, increased numbers are become deaf at an earlier age, and hearing aids are becoming even more important.
Having for many years experimented with making speech clearer on amateur radio equipment, I was pleasantly surprised at the care that was taken to match the response of the NHS hearing aids I was supplied with some years ago by the Lancaster and Morecambe Audiology department to the response of my hearing. Modern hearing aids are a marvel of miniature solid-state electronics and Audiology has become a very worthwhile career to anyone interested in sound and electronics, and who likes helping people.
Storing electricity is still a problem, however, and recently I was having trouble with my aid’s batteries, Fig. 1, having only a very short life. I started trying to test one that had apparently failed, with my digital meter. It should have read about 1.4V but only registered around 1V. I was just going to throw it in the bin when I noticed an odd effect. The voltage was gradually rising, and after about five minutes was showing a full 1.4V. I refitted it to my aid and it then functioned perfectly for a few more days. What on earth was going on?
I had a search on Google and ended up on Rayovac Hearing Aid Batteries’ website, which had a most interesting article: https://tinyurl.com/y8kx5lg2
The latest batteries for hearing aids use ‘Zinc/Air’ technology. They arrive in a packet and on the top of each battery is a red label. It is not just for decoration, as removing it exposes some very fine holes in the battery’s case. Normal batteries use the reaction between metal, carbon and other chemicals to generate their output, but Zinc/Air batteries use only zinc, which reacts with the oxygen in the air once the red label has been removed.
In my case it would seem that the hot weather had caused me to sweat (or perhaps I had forgotten to wash behind my ears!) and that this had either blocked up the very fine ventilation holes in the batteries or those in the aid’s case, Fig. 2. I can’t help wondering as to how many perfectly good batteries are thrown away just because the holes have got blocked. I have now noticed that if my batteries fail when I am away from home, and have not got a spare, if I wipe the them with a hanky and blow into the holes, I get a few hours extra use from them − worth knowing!
Zinc/Air batteries and rechargeable power cells seem to be popping up in all sorts of places, such as being used to store the output of wind farms, and are even proposed for use in electric cars, so it is worth keeping your eye on developments.
Thinking of Opening a Radio Shop?
If your hobby is radio, computers, or even gardening, it can be quite tempting to make it into a business, but first of all you have to ask yourself if you have the ability to organise a business, and as to what the competition is like? Can you offer repairs and anything special that the nearest competitors can’t match, and have you plenty of contacts in local clubs who are likely to patronise you?
Also, are you or your partner good at record keeping? The tax and VAT inspectors require a complete trail showing all your takings and expenses, where they originated, and where they go to. They are inherently suspicious of cash, and I can remember being grilled and having to show proof that the
cash I had taken had ended up in the bank, and not my pocket. Fortunately, my wife Brenda is gifted in keeping clear tidy paperwork, and took charge of keeping the records, or I should have been lost.
VAT can be a real headache. This is levied on all shops with an annual turnover of over £85,000 and if you get your paperwork wrong, it can cost you a lot of money. When VAT started I was told that everything I sold, that was my property, was liable for VAT, which was then levied at 10% and which was added to the final purchase price. When I purchased new equipment from a registered supplier this was no problem. VAT was added to the price I paid, and I was given a VAT invoice with their registered number on it, with the VAT I had already paid clearly shown. When Brenda submitted the ‘VAT Returns’ to the VAT office she was able to deduct the VAT that we had paid to the supplier, and we only paid VAT on our profit.
The real problem started with secondhand equipment obtained from private customers.
Yes, VAT was then only 10% but it still meant that if I part-exchanged or purchased a rig from a private non-VAT registered customer for £300, serviced it and sold it for £330, I had to pay £30 VAT. Hence this wiped out my profit and the payment for my work, even more so when the level of VAT was increased. I stopped selling any second-hand equipment that was my property, and arranged to sell it on a ‘commission sale’ basis, but this involved extra paperwork.
VAT is much higher nowadays so this could have killed the second-hand business, but fortunately there is now a somewhat simpler way of accounting for VAT on secondhand goods. However, there are still traps for the unwary who don’t get their paperwork or computer records correct and you may need to pay an accountant to help you setup a legally watertight system.
I heard of one trader who got himself into dead trouble. Over a length of time a VAT inspector had kept a record of the secondhand equipment that had been in his window, and had disappeared, presumably sold. At the next VAT inspection the trader was challenged to account for the several thousand pounds VAT that would have been chargeable on these goods. “Oh, they were not mine so I only declared VAT on my profit” was his
defence. “Where then is the paperwork, with the names and addresses and signatures of the owners, proving they owned them, and not you, at the time you sold them?” He could not produce these, so it cost him a lot of money.
There are many other things that you must think of before starting a shop, but that is enough for this time
Switching Diodes
The contacts on relays, band-change and other switches in electronic equipment do tend to give trouble, and replacing or cleaning these on amateur radio and hi-fi equipment was ‘a nice little earner’ when we had the shop. Modern rigs have replaced most of these with ‘reliable’ switching diodes, so this source of trouble is disappearing, only to be replaced with more mysterious faults.
Have you started to hear your own voice in the speaker as you are transmitting, had your rig burst into oscillation, found that the receiver has become very noisy, or come across other odd effects? These are quite possibly caused by a leaky switching diode.
The switching on most modern rigs is carried out by placing a positive voltage on either the transmit (TX) or receive (RX) line. These will be marked on the circuit diagram something like ‘TX 8V’ or ‘RX 8V’ (or whatever voltage is used).This voltage will be applied to the various amplifying stages and relevant switching diodes, to turn them on and off in the TX or RX modes as applicable. If you check the voltage on these lines, only one should carry a positive voltage when in the appropriate TX or RX mode, and there should be no positive voltage, and sometimes a slight negative voltage, on the other. If there is a slight positive voltage on the line that should not be active, there is probably a leaky switching diode. The most likely diodes to fail are those nearest to the antenna, and tracing these will involve removing circuit board connector plugs, and a little unsoldering using trial and error to track down where the voltage is coming from.
Schottky barrier diodes are normally recommended as the most efficient type for RF switching, but some of these seem very susceptible to being damaged by voltage spikes. I had quite a bit of trouble with the switching diodes located near to the antenna input of the FT-757, and in the end started replacing them with normal IN4148 ones. These had a much higher voltage rating, and while they were not intended for RF use, I could detect no difference in performance.
Even when using these I did have the odd repair which ‘bounced’ back with the replacement TX/RX switching diode having failed. I could find no reason for the failure, and in desperation I replaced the failed diode with a much larger 1500V 2A mains rectifier diode. Once more I could detect no difference in performance and the rigs did not fail again. I did warn the customers, however, to advise any future repairer as to the reason why the large diode had been fitted, as otherwise he would have been tempted to replace it with the ‘correct’ one.