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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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28th Apr 2017, 7:15 pm | #1 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Home made capacity tester and GDO
Again from the latest local electronic surplus equipment and junk sale, I couldn't resist these at the opening bid of a quid each, if only for the parts!
The capacitance tester is actually a nice little unit, and with a full battery fitted it works well, other than reading a little high on the higher value capacitance ranges. It's quite power hungry taking nearly a quarter of an amp from a fully charged rechargeable PP3 9 volt battery - the one fitted to it has gradually come back to life over several recharges. I'm sure someone out there will recognise the magazine article that these were originally constructed from. If I can get the circuit for the capacitance tester I can give the appropriate pot/s a twiddle to get the calibration a little better as it's obviously drifted over time. It seems a shame to break these units up for parts after someone has devoted their time to make them from the original construction article, although I may scrap the GDO. |
28th Apr 2017, 8:01 pm | #2 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,763
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Re: Home made capacity tester and GDO
The GDO won't be of any use without the coils of course, but maybe it came with them? Easy to check the calibration at various points by looping a test lead round the coil, connecting the lead to the input of a frequency counter and reading off the frequency. Which of course assumes that you have both the coils, and a counter!
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David. BVWS Member. G-QRP Club member 1339. |
28th Apr 2017, 8:58 pm | #3 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Home made capacity tester and GDO
No coils for the GDO unfortunately, hence the reason that I'll probably scrap it. There's so much stuff at these sales that the coils were probably in another box of bits sold earlier. It's quite a laugh at these sales, the GDO was held up and it looked as if no one was going to offer the minimum bid of £1. I hadn't noticed it when I had a quick look round just before the start of the sale, so I stepped forward to take a closer look and that's always fatal as you're then shown all the useful components that could be removed, and as I seem to be known as the one likely to give a quid for items that no one else wants I had to say "oh go on then"! He actually pointed out the nice meter and I tried to get out of it by saying that I'd got loads of meters already, which is true, but I gave in when I noticed the nice ceramic octal valve socket for the coils to plug into on the top end of the chassis. There were a lot of sets of drawers of components and I did make the mistake of buying the only set that was actually empty, and that was the only set I got as I was outbid on all the others, but the set of drawers will be useful.
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28th Apr 2017, 9:09 pm | #4 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Home made capacity tester and GDO
Those sales always confuse me. I ended up with half a Marconi spectrum analyser once and another guy ended up with the other half, which he mysteriously wanted £250 for from me after winning it for £22 which I declined. I offered him my half for £150 which cost me £12. Easiest £150 I ever made. Then he had to go and find the cable to link the two halves together...
I'd probably wind some coils for that myself. I tried this many years ago as a child with a couple of smashed valves out of an old jumble sale radio and cut my fingers to ribbons |
28th Apr 2017, 9:29 pm | #5 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Home made capacity tester and GDO
Yes, I'm sure someone else walked out with the coils in another box or bag of bits. There were actually some large paper sacks full of cables and wires that were sold for a pound, it's just that I couldn't be bothered to look through them all in the short time before the sale and these were under the tables so are easily missed. I actually took some other folk with me so that they would bid for things to save me buying yet more junk - and they did! One of my pals bought a whole box of mixed transformers for a quid. I usually buy the transformers, but I think I've got enough, so didn't bid. I did buy the filthiest and most tatty AVO that I've ever seen for a quid, no one else wanted it, and I can't believe that it actually works perfectly on all its ranges. It's more accurate than any of the AVO meters that I've already got and just reads fractionally high on the 10mA range - it's an old model seven and has actually cleaned up quite well...and you can never have too many AVOs - or can you?
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29th Apr 2017, 12:18 am | #6 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,642
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Re: Home made capacity tester and GDO
Depends who you ask!
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29th Apr 2017, 1:28 am | #7 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Home made capacity tester and GDO
That's very true Bill
I've just counted and I've only got six and not all of them work 100%, particularly due to an open circuit movement in one of them - a common problem, I believe. |
29th Apr 2017, 5:07 pm | #8 |
Heptode
Join Date: Jan 2011
Location: Croydon, London, UK.
Posts: 773
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Re: Home made capacity tester and GDO
That looks like the superb Direct Reading Capacitance meter designed by the late Alan Wilcox, published in the May 1976 issue of Television. The article is on the forum here:
https://www.vintage-radio.net/forum/...ad.php?t=73676 One big advantage of this circuit is that it isn't particularly upset by leakage. Consequently you don't get the "the capacitor has doubled in value" effect often seen with the DMM capacitance ranges. Paula |
30th Apr 2017, 5:40 pm | #9 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: Home made capacity tester and GDO
Many thanks for that, Paula, you've got it in one! That's the one from May 76. I agree that I particularly like this circuit due to, as you say, not being particularly affected by leakage which is nearly always the problem with these testers. I have to admit that I do quite enjoy messing with other peoples 'hombrew' kit from the past. Now I've seen the article, one thing jumps out straight away, and that's the fact that it's only supposed to have a current drain of 3mA, a bit less that what I quoted earlier. I checked it again and it was about 170mA and dropping, which will basically be the poor old battery failing. So there's obviously a fault which is likely to be that little regulator transistor or its controlling zener diode, or both, or possibly a constructional error and it's always been like it from when it was first built. That will account for the slightly high reading on the higher capacitance ranges that drops over a short time as the battery struggles. It'll be getting the full 9 volts (which it shouldn't) which then drops quickly due to the fault draining the battery.
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