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Wellbrook traced-out loop amp schematics uploaded to Photos
开云体育I made a folder in Photos just now and uploaded the schematics
for ALA1530LN, ALA100LN, FLG100LN that I've traced out in the
past. There should be complete info there for anyone to be able to
make their own. The original leaded 2SK715 jfets are now
unobtainable, but there are two SMD equivalents that work just as
well, with the 2SK932 preferred since its typical yfs is the same
as the 2SK715. One should be able to make one of these for around
$50--maybe less. On eBay I see a Chinese seller claims to be
selling an ALA1530LN or an ALA100LN for a very high price of
around $250, for the loop amp alone! I have no idea if it's a
direct clone of the Wellbrook. /g/loopantennas/album?id=296460.??? The name of the folder is "Wellbrook official traced-out schematics ALA1530LN, ALA100LN, FLG100LN". 73, Steve AA7U On 7/17/2024 2:15 PM, Paul V Birke via
groups.io wrote:
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Hi Steve,
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There has been a bit of variation in the build of the Wellbrooks.
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The one I took apart had two 82pF capacitors, each one connected across half of the transformer primary. I suspect they were originally intended to provide better rejection of strong FM broadcast signals, but it was a "happy accident" that they also improved the performance on the upper HF bands.
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The core material was closer to N30 (u' 4,300) (Epcos ) rather than 73 mix (u' 2,500) , which would tie in with the FG100LN additional transformer, that used something like the higher permeability x38 material (u' 10,000). My thinking is that the Epcos N30 cores would also have possibly been easier to obtain in the UK, when the Wellbrook was initially designed, as RS were pretty much the only big electronic parts supplier at the time, and was used by nearly everyone.
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Using back to back diodes across the loop terminals is fine, however that is not true for other types of antenna, with higher feed point impedances.
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I have tried CPH3910 fets and they work, but a lot depends upon your requirements for frequency coverage.
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Regards,
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Martin |
开云体育Hi Mike, One would need a full schematic with the T1 transformer labeled per your drawing to be able to say if you have it correct or not. My schematic in the files doesn't have transformer numbering on it, just phasing dots. Steve On 7/21/2024 7:07 AM, Mike M via
groups.io wrote:
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Mine added too, derived separately, useful for comparison purposes.
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On Sun, Jul 21, 2024 at 04:26 PM, Mike M wrote:
Schematic added: |
开云体育Hi Mike, Yes, it appears your numbering matches with the proper phasing of the one-turn feedbacks. But RV1 is 100 ohms ("100R"), not 100 k ohms. That pot is set so both Source sides are the same voltage. All the 2SK715s that Andrew Ikin used were of the highest Idss version, "W", from the Sanyo datasheet, in the 3 different ALA1530LNs that my group opened up and traced out. Steve On 7/21/2024 9:26 AM, Mike M via
groups.io wrote:
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开云体育Thank you. I should note that my group opened and traced out 3 different ALA1530LNs and all had a single 82 pF cap across the two loop inputs. Apparently you had a variation if yours had two 82 pF? caps in series across the two loop inputs as your schematic shows (and the midpoint of them not going to common/ground either). Steve On 7/21/2024 9:58 AM, Martin -
Southwest UK via groups.io wrote:
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开云体育Martin, on your schematic you've got the resistor feeding the 2 x 1N4148s as 500K, whereas Steve has it at 1K.Should it have been 500R? Mike - M0MLM From:
Martin - Southwest UK via groups.io
[mailto:martin_ehrenfried@...]
Date:
Sunday, 21 July 2024 at 5:58 pm
Subject:
[loopantennas] Wellbrook traced-out loop amp schematics
uploaded to Photos
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Hi Martin,
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On your schematic, at the junction of C1 (82pF) and C2 (82pF) they need to be grounded, or it will oscillate.
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Everett N4CY
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In a message dated 7/21/2024 11:58:32 AM Central Daylight Time, martin_ehrenfried@... writes: ?
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I asked Andrew Ikin one time if he could share what core he used in his preamps for the transformers. He told me he uses the Fair-Rite 2873000302, same as I've shown on my schematic. (Amidon "BN73-302"). That's what Everett and I always use too. Note this is a conductive core, and with the small gauge magnet wire needed for the transformers, it's easy to abrade the coating when winding, so the wire touches the core. You do not want any wire to touch the core--the circuit's IMD will severely degrade, particularly for the T2 output transformer. Use a multimeter set to ohms with one probe to the core and the other probe to any of the (stripped) wire ends--there should be infinite ohms, otherwise you need to rewind it and try again! We always use a thin Teflon tubing spacer when we wind the transformers so the wire doesn't abrade when winding. We buy it on eBay, from China. The proper size is "9L 3.0 x 3.4 mm". Here's one seller of it:
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(I have no experience with the cores that Martin mentioned.) Steve On 7/21/2024 9:26 AM, Mike M via groups.io wrote:
Steve, |
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Martin,
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Comparing your schematic to Steve's I see a few differences:
I assume R1 is to bleed of static charge. What is R6 for?
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Are all of the differences from tracing a different unit or are some things your own changes?
--
=================================================================== Mike M |
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Hi Mike,
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On Sun, Jul 21, 2024 at 08:19 PM, Mike M wrote:
That's how it was
2.7K ? I don't know why it was there, but I double-checked when I saw it, and it was definitely present.
The input transformer was actually 3 + 3 turns centre tapped, but the tap wasn't connected to anything.
That is a mistake, the part was damaged during the reverse engineering process of boiling the assembly to remove the epoxy. I couldn't check with a multimeter as it had snapped in half, and the coloured bands were unclear. I assume it was actually a much lower value, and 1K or 500R is probably more likely.
Static bleed, as the loop is "floating" above ground. The input transformer centre tap was not grounded or connected to anything else.
That's how it was
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I have no idea why it was there. As the unit was hand made on stripboard, it may have been meant to be connected somewhere else in the circuit. Or maybe it's serving some other purpose during assembly and test ?
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I produced the circuit diagram directly from what I saw and measured in a faulty unit I had been given. I carefully picked the resin away from the circuit board after boiling it for an hour to soften the material.
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The transformers were the most difficult part to understand, as the binocular cores were full of resin. I had to cut the windings and count the number of wire ends to determine the number of turns. I measured the resistance and reactance curves of the ferrite material using a VNA, and compared it with test windings on same sized cores using different materials. It was similar to 73 mix, but a closer fit to N30. The wire numbering in my diagram is incorrect, as there are two number 4's.
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It could be that my unit was an earlier version, in comparison to the ones that others have taken apart, due to the "kitchen table" construction, slight differences and errors have probably crept into each build, and the design may also have "evolved" slightly over time.
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I will revise the diagram and repost it to correct the mistakes.
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Regards,
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Martin
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Hi Steve,
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OK, if that's what Andrew said he used, I'll happily accept that.
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The cores in mine were different to 73 mix, but he may have changed makes or suppliers over time, or perhaps more likely, Fair-Rite may have changed the formulation, as they have done with other mixes.
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In the UK, it used to be quite difficult to get hold of different Ferrite materials, unless you bought directly and in quantity. Fair-Rite only became easily available a few decades ago, when the main distributors started stocking their product range.
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On Sun, Jul 21, 2024 at 07:42 PM, Steve Ratzlaff wrote:
He told me he uses the Fair-Rite 2873000302 |
Everett N4CY
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