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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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17th Feb 2019, 3:51 pm | #1 |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
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600Ω Radford LDO3?
I have a Radford LDO3 audio signal generator (thanks Chris!) that I would like to use for setting up some equipment that operates in the 600Ω environment.
Radford did offer the LDO3 in both 150Ω and 600Ω versions - unfortunately mine is the 150Ω one! The circuit diagram shows the alternate attenuator resistors that are needed to convert it to 600Ω but the values are 1706.9Ω, 1157.9Ω, 790.5Ω which I doubt I'll be able to find easily. Is there another way that I can reliably drive a 600Ω load with the generator as it is?
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17th Feb 2019, 4:03 pm | #2 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
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Re: 600Ω Radford LDO3?
Put an additional 200 ohm load across the output and the output will be 6dB down on the nominal.
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17th Feb 2019, 4:39 pm | #3 |
Octode
Join Date: Sep 2004
Location: St. Albans, Hertfordshire, UK.
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Re: 600Ω Radford LDO3?
I would have thought at those frequencies and impedances you could parallel up resistors quite successfully.E.G.
1800R and 33K, 1200R and 33K, and 820R and 22K get close to the values you are looking for.
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17th Feb 2019, 5:22 pm | #4 |
Nonode
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Location: Resolven, Wales; and Bristol, England
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Re: 600Ω Radford LDO3?
Yes, Richard, but these are in the attenuator, so there are ten of the 1706.9 and eight of the 1157.9.....
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17th Feb 2019, 6:00 pm | #5 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
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Re: 600Ω Radford LDO3?
How about a 1:2 transformer, accepting the limits to its performance ?
Ken |
17th Feb 2019, 6:04 pm | #6 | |
Octode
Join Date: Sep 2004
Location: St. Albans, Hertfordshire, UK.
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Re: 600Ω Radford LDO3?
Quote:
A 1:2 transformer would work, maybe an AF output transformer with a number of seconday taps perhaps? (used as an autotransformer) A minimum loss resistive pad would work, but you would suffer nearly 12 dB attenuation.
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17th Feb 2019, 6:27 pm | #7 |
Moderator
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Re: 600Ω Radford LDO3?
The minimum loss pad suggested would mean no mods to your LDO3 and do you need the top 12dB of its output capability?
Alternatively, for the funny resistor values, look up the E96 series. There are 96 values per decade and it's the natural series for 1% resistors, so this will get you within 2% worst case which ought to be close enough. You'll find E96 series values at RS and Farnell without any bother and at reasonable prices. expensive resistors start at 0.1% types. E96 values are ratiometrically spaced by the 96th root of 10, then rounded to 3 significant figures. Aren't calculators wonderful! 1150 Farnell have 1% at 7.5p or 0.1% at £1.70 1694 Farnell have 1% 0.5% and 0.1% at similar prices 787 Farnell have 1% but 0.1% is no longer stocked I've just looked Alternatively, contact Holsworthy and have a chat. I know they made custom resistors on the exact values for 50, 75, 300 and 600 Ohm attenuators of round number values. They may have some lurking in stock. We used to buy them from them. David
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17th Feb 2019, 6:32 pm | #8 |
Hexode
Join Date: Aug 2014
Location: Featherstone, West Yorkshire, UK.
Posts: 386
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Re: 600Ω Radford LDO3?
Farnell list the A262A2E transformer; 150 ohms in, 600 ohms out.
Kevin |
17th Feb 2019, 6:42 pm | #9 | |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
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Re: 600Ω Radford LDO3?
Quote:
I did try a 200ohm resistor as barrymagrec suggested, it drops the -20dB output close to 38mV rms which is not far from -26dB. I don’t yet know what will happen when I connect a load…
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25th Jun 2019, 12:24 pm | #10 |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
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Re: 600Ω Radford LDO3?
With apologies for letting this thread fall into a coma. I needed a quicker solution so fired up another sig gen which has a 600Ω output.
The Radford exhibits a strange characteristic on the output. It settles nicely with a clean output signal, but after a few minutes the output level cycles up and down quite slowly - period is about 2 seconds. Investigation of the level control circuit using the finger probe suggested that either an op-amp or the FET driver were heat-sensitive. Freezer spray suggested the same. However, replacing both components leaves me with no improvement. The only thing that stops the slow level variation is to briefly switch to another frequency band and back again. But it's only a temporary sticking-plaster. Unfortunately the published circuit of the Radford LDO3 is a little shy on detail, and I suspect I may be missing something blindingly obvious.
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25th Jun 2019, 1:32 pm | #11 |
Nonode
Join Date: Jan 2009
Location: Bristol, UK.
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Re: 600Ω Radford LDO3?
Does it have a thermistor or a tiny light bulb in there ?
These will be in the feedback loop to control the gain in order to just get the circuit to oscillate without distortion and may have been changed for the wrong type ... dc |
25th Jun 2019, 4:30 pm | #12 |
Heptode
Join Date: Nov 2009
Location: London, Ontario, Canada
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Re: 600Ω Radford LDO3?
Hi Richard,
1706.9 Ohm turns out to be the exact parallel of 1.8k and 33k of the E12 series. Similarly, 1157.9 Ohm is the exact parallel of 1.2k and 33k, E12 series. You can find those resistors cheaply in your desired accuracy. I used the "Resistor CAD" app to find these equivalents: https://www.circuitsonline.net/downl...r-cad-2.0.html Regards, Peter |
25th Jun 2019, 7:01 pm | #13 | |
Nonode
Join Date: Aug 2010
Location: Resolven, Wales; and Bristol, England
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Re: 600Ω Radford LDO3?
Quote:
Unfortunately that software is a Windows application, and I'm one of those stubborn engineers who refused to be drawn into the Seattle camp....
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26th Jun 2019, 6:00 am | #14 |
Dekatron
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Location: Sleaford, Lincs. UK.
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Re: 600Ω Radford LDO3?
There are loads of electronic calculators online to figure out paralleled R's, RC time constants and lord only knows what else. https://www.qsl.net/in3otd/parallr.html
Andy.
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26th Jun 2019, 6:27 am | #15 | |
Nonode
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Re: 600Ω Radford LDO3?
Quote:
Once I've sorted out the output pulsing.
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