Second repair of Wellbrook ALA1530 Mag Loop Aerial

[Radio Fixer]

Back in 2016 I had the Amp for this replaced under guarantee and it has lasted until a few weeks ago.  Then it went low gain with gritty noise.

So it was build a new Amp rather than pay out for a replacement. BVWS members will know I wrote two articles on building Mag Loops and so circuits were at hand.

Went for the simplest with least components that I could string together using Manhattan technique.  It just goes in place of the failed one using the same power and signal feed (12V at 100mA).  The power is spread over 4 transistors (Hfe:  Tr1/2 >130, Tr3/4 >80 and matched) rather than two in the WellB which are also potted. One stage, which is where I started, (https://www.pa1m.nl/simple-active-receive-loop/) would suffice for short coax lengths but two are needed for 50m of coax as in my case.

Left out Ferrite beads on bases and the inter stage screen and it still seems perfectly stable. Took the ‘scope down the garden and no sign of oscillations.  Experimented with a different power feed and changed that. Eliminating DC feed in the Ferrite ‘pig nose’ core offered no improvement.

The circuit board was given a spray with Zinser Spray Shellac which over the years and several Amp builds has been effective at countering any moisture.  Being left powered 24/7 no doubt helps as well.  Put a dribble hole in the bottom of the box just in case.

Won’t bother with any near lightening protection, its flat here and I can’t ever remember a local strike. So see how it goes and it’s an easy repair.

Performance into a HMV 650 is back to stunning again.  Crystal clear R4 and RTE better than before.  Some noise but not bad considering I’m in the SE.  MW is excellent on many stations and SW better than I can remember it, not that there is anything of much interest on the bands.

  LATEST CCT 4 TRANSISTOR AMP.pdf (Size: 24.52 KB / Downloads: 53)
   
   
   

Gary

[Murphyv310]

Hi Gary.
You may remember I did a similar repair to my Wellbrook many years ago with just two transistors. Two of 2N2222A in my circuit. No lightening protection or diodes across the loop. It's 100 times better than the Wellbrook designed amp, which corrodes and actually has more output than the original. It gives useful gain even in the air band. I've even done TV DX on band 1 with it a few years ago.
Glad you fixed the antenna for just a few quid, have fun.

[Radio Fixer]

I remember that Trevor. In my case both Wellbrook Amps were good until they slowly went down hill.
The design was asking a lot putting 100 mA into a pair of plastic transistors and then encapsulating them.

The design was reversed engineered if you are interested (Chems and X Ray).
https://www.george-smart.co.uk/projects/wellgood_loop/
Gary

[Murphyv310]

Yes I have a copy of the reversed engineered amp.
It's surprising that a much simpler amp preforms as well if not better than the original. For me the way the loop was attached to the amp was its weakness, the loop has ferrite that is "moulded" into the loop and amp, it cracked on mine and performance tailed off badly.
I made the amp in a waterproof box and the ends of the loop were inside the new box, once all made up with the ends properly secured into the box the whole thing was potted. If it was to fail the box would need to be smashed to gain access.
What they do need is to support the top of the loop so it isn't effected by strong winds and flex at the entrance to the amplifier.

[Radio Fixer]

No ferrite in mine and no need for it. I and others have made plenty just from bare tube and they work fine. My new AMP solder tags just screw into the WellB loop holes with their wires cut off. Some almalgamating tape is wound around the tags. Perhaps yours was faulty from new and so anything you made was going to be better.

The WellB worked as good as mine on the HMV 650 using the eye tube and the ear as a guide. Both closed the eye completely on R4 as an example. I also built more than one of the "reversed engineered amp", the WellGood, using the Manhatten technique and they worked about the same as any. Some minor differences can be found on SW but not much on MW and LW.

I gave away my last WellGood else you could have a loan and try it but it may still be possible if you are interested. The person it went to still hasnt used it as far as I know.

Gary

[Murphyv310]

I suppose that's possible.
I'm surprised that yours didn't have the ferrite in the loop. Mine was a very early one though. I've also made another using two of those little Chinese RF broadband amps. Unbelievable sensitivity but very prone to feedback and cable routing so not practical really. I eventually cannibalised it for the amps.
There are loads of different ways to make a magnetic loop, lots of info and designs online.

[Radio Fixer]

Been thru most of them Trevor, some are very complex which for broadcast bands isnt needed.
Have you seen this site:  http://lz1aq.signacor.com/
   

Gary

[Murphyv310]

Thanks Gary.
That's overly complex IMHO but if that's what floats their boat then fair enough.
Mine does the job for me until it breaks or goes faulty.

[DXLOVER]

Hi Gary,

I tried to build this amplifier, it overloads, any ideas to reduce the overload? thank you!

BR,
Ray

[Radio Fixer]

Well I assume you have not made too big a loop? One friend made one with 3m sides and that was a bad idea. 1m to 1.5 is certainly enough.

Maybe your component positioning isnt as shown? My first ones had a double sided copper screen between the first and second stages (on the top side). They also had either ferrite beads on all base leads and earlier no beads but low value resistors in series with them on the first pair of transistors. Think value was 10 Ohms.

Are you using a long coax feeder ? 50m plus

Gary