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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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2nd Sep 2016, 9:16 pm | #1 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,010
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A RF-robust 9V supply...
For a while I've tried various power supplies to feed my Roberts R606: unregulated 9V "Wall-warts" and a couple of supposedly-quiet SMPS. None were really satisfactory - either giving a whine audible in the 'silent' gaps of music, or seemingly introducing strange burbly hash [intermodulation] - presumably because the diodes were seeing RF and creating the FM-0equivalent of 'modulation hum'?.
So I decided to do the job properly. A 13V 400mA transformer with built-in 125-degree thermal-fuse was in the junk-box along with a diecast box and some DIN-connectors. There were also a matching 1980s-era Belling-Lee fuseholder/indicator-lamp and an old ferrite toroid. Add in a "Velleman" LM317-based variable-voltage regulator board, and an hour or so of assemblage. I added 1000pF capacitors across each of the bridge-rectifier diodes to minimise noise. The DC side is entirely isolated from the rest of the PSU. The "Power On" lamp is a 28V 40mA bulb wired across the transformer secondary - the idea being that it helps to reduce the peak AC voltage fed into the bridge-rectifier - and, after all, there's no point rectifying/smoothing/regulating power that's only going to light a bulb. Though a 28V bulb, it glows quite bright enough when fed with half its design-voltage. The toroid on the output - wound with red/yellow wire - is intended to keep the PSU's end of the DC output RF-decoupled (at Band-II FM frequencies) from the rest of the circuit. On the input side there's a 100mA fuse, and the mains-lead is rather well anchored by way of a 20mm IP67-rated compression-gland and a couple of cable-ties. The result? Burble-free FM reception, and in tests it can supply around 300mA continuous without getting at all hot. If I wanted more, a friend has offered to machine me a brass thermal-transfer block to sit between the LM317 and the case. A R606 doesn't take that sort of continuous current even when turned up to 11. As to the blue paint-job? I was doing some work on the engine of a friend's Ford RS2000 Mk.1 Escort, and had to respray the camshaft/cambelt-covers. Ford RS-Blue was in the spraygun, so why waste it? |
2nd Sep 2016, 9:45 pm | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,970
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Re: A RF-robust 9V supply...
The built in PSU in the 606MB is a fairly straightforward affair, though it is regulated and obviously linear.
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2nd Sep 2016, 9:55 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,010
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Re: A RF-robust 9V supply...
Mine's not a 606MB, alas.... hence the need for an external, linear supply.
I'm a bit of a stickler for silence-when-there-should-be-silence on FM radios; the other supplies I tried didn't achieve the sort of no-signal silence I got when using batteries. With this supply I can use an xtal-controlled oscillator on 100MHz to generate a local unmodulated carrier and I get no burblies at all. On this little one I even arranged for the DC cable between the radio and the PSU to be a 1/2-wavelength long at 102MHz [guessing the velocity-factor of twin PVC-insulated wire] so that the point where it connected to the PSU and to the radio would be high-impedance. |
3rd Sep 2016, 11:14 am | #4 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: A RF-robust 9V supply...
Half a wavelength merely ensures that each end will see the same impedance (assuming no losses), not necessarily a high impedance.
A halfwave antenna looks high impedance at one end because the other end sees infinite impedance. It is high and not infinite because of radiation loss. |
3rd Sep 2016, 7:46 pm | #5 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,010
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Re: A RF-robust 9V supply...
You're right about the impedance at the ends: which is why I put a bifilar toroid on the PSU DC output (so providing what should be an infinite-impedance to common-mode currents at that point).
Reflected 1/2-wave up the power-lead that should in turn be a high impedance at the radio itself - so discouraging the power-lead from also acting as a 'radial' for the radio's own antenna. I dislike having DC supply-leads also contributing to the RF ground.... |