Standard Beam Approach - Rebuild update

[jamesinnewcastl]

Hi Andy

There are some places where I have used a different value, but not many. For example instead of 50 ohms in the meter shunt I used 47, which should protect the meter better. The Fine Tune pot is 500R (marked 470) and not a 400 ohm WW. This would increase increase the bias voltage a little but I suspect that the overall current flow through the back bias resistors is the main contributor to the -HT to chassis potential.

If I could see how the circuit avoids overloading the meter I could check to see what was giving the problem.

Cheers
James

[jamesinnewcastl]

Hi Richard

Sorry, I missed one of your posts, so in reply...

Mis-wiring always a possibility and a major review to 'make sure' has been in the back of my mind for a while. Having the actual wiring layouts of all the parts in the AP is a major help and I suspect that any mis-wiring would have been noted when a 'correct' wire met and 'incorrect one'. So far all I have had is a 'missing' one, which I replaced. I have found a discrepency between the circuit and the wiring of the control box however - a simple resistor name swap where a switch chooses between a 1k or a 20k resistor, that is always on my mind when checking the circuit but so far swapping the wires over hasn't made any real difference. As I have tested I have checked many of the wires in the process of finding where to probe. I have had a number of Eureka moments which (sadly) turned out to be false hope.

I've played with inputting RF for quite a while. I eventually got up the courage to tune the system and pretty much all the trimmers failed but I do have a good channel which tunes up nicely. Or does it? I realised that I have no idea what level of RF I should be putting in. A spec sheet gives the 'sensitivity' as 40uV. Not sure what that means and my RF gen has uV pd and uV emf. One is half the other so I am no better off in picking the right one! I have however watched the agc work by slowly increasing the signal from nothing to as much as the generator gives.

But, what is the right signal level? As I mentioned there is an original test set on the planet but I can't get to it. So, in the face of uncertainty I have switched to a 'known'. That is, no signal. In this DC quiescent state, which the system must have been in 98% of the time, I would not expect the meters to suffer damage in any setting of the controls. I'm assuming that the receiver is designed to be 'ready' to take up its task as it makes a distant approach in this condition.

As the circuits as drawn in the AP 'do my head in', and because the circuits are only drawn in a seperate, individual manner, I have redrawn them to show the complete system. I'll attach a PDF if I can. Also I attach the voltages I found noted onto a simplifed diagram of what I hope is the bias circuitry. I've removed the capacitors as they should not be doing anything once the circuit has settled.

I've tried to interpret what is happening by considering how the system works, or how I think it should perhaps? so...

1. The Glide Meter is pointless on approach (Course) as the system has the agc enabled, so the meter should point somewhere on the scale coming up from some low value when far out and moving to a fixed agc determined point when close to the runway. The pilot can't control anything except the volume and channel in Course. Question is - how is that fixed point fixed by the circuit? In the later receivers a divider across the heater supply was used for bias - nicely stable.

2. The Course meter needs to work all the time in which ever setting - the amplitude of the 'kick' only needs to be enough to see/hear.

3. On switching to Glide, the pilot should be able to adjust the glide pointer to a position of his choosing using the Fine Tune knob - I'd have to suppose that the meter was at zero with a small signal and max at the point the aircraft is very close to the transmitter - to avoid overloading. I suppose that there could be a procedure in the pilots training to set the Fine Tune to somewhere safe, but that seems a little dodgy.

Cheers
James

[trh01uk]

James,

one assumption you are making is that the various excessive currents that the meter is seeing represent a "fault". We don't know that. It may be the designers just specified a meter which could withstand overload conditions. I'm not aware that we have any specification for the meter?

Overloading of meters would definitely offend modern designers - but I doubt whether designers in the 1930s were worried about such things!



Richard

[jamesinnewcastl]

Hi Richard

Given that the Course meter has specially shaped poles it may be possible that the Glide meter also had similar design considerations, but that has never been mentioned in the texts,

Looking at the situation simplistically they put a meter/shunt combination with an fsd of 3.17 mA in the cathode line of a valve that could deliver 10 or so milliamps. A disaster waiting to happen!

While designers of the time may have been blase it has to be remembered that this was a system to guide military and passenger planes down safely in fog. Losing the Glide meter at that point would have been really annoying!

So a meter that could take the punishment would be a simple way out of a situation that calls for more unwanted circuitry.

The bottom line then is to just 'do it' (after looking at the pole design first). While it goes against all my inclinations I think I do have a butchered instrument that I can test the theory on. I'll call it your theory .

If it works that will be brilliant of course!

Cheers
James

[jamesinnewcastl]

Hi Richard

Well the meter hasn't blown up! Not given it a long term test yet but it looks promising - thanks!

I note that I can't zero the meter when in Glide - but - this might be the function of the mysterious resistors that remove some of the 'space charge'. I'm hoping that the 20k resistor is switched in when it should be the 1k!

Now I feel happier about going forward!


Cheers
James

[trh01uk]

James,

sounds like you have a way forward now! As an epilogue, it strikes me that offering a "Fine Tune" pot to adjust the arbitrary position of the meter pointer on the scale does suggest that the pointer can be off-scale under certain conditions.


Richard

[jamesinnewcastl]

Hi All

Getting near the end now. The attached photo shows what is basically a 'new' main receiver. The only thing remaining is to clean up the connectors for the junction box.


Cheers
James

[trh01uk]

Is that "new" as in "one you previously restored to pristine condition"?


Richard

[jamesinnewcastl]

Hi Richard

A mix of (internally) new and refurbished parts while maintaining exactly the same outward exterior. It's also been taken back to the original production version.

It's what it would have looked like when first built with everything working and in tolerance. (OK, some very small differences.)


Cheers
James

[Sparky67]

Some remarkable research and build work there James… Well done!

Martin
G4NCE

[ScottBouch]

Hi James,

I have just read through the majority of this topic, and wanted to say how fascinating I found your journey!

I'm messing with slightly newer vintage kit, building a T5 Lightning flight simulator using original parts, unmodified where possible, so am familiar with the general feel of the kit you are working with, and the desire for preserving originality. see: https://www.scottbouch.com/mcfs/ligh...5t55/index.htm

Making your own resistors look like the originals was just inspired! and other re-manufactured components looked fantastic too. Well done!

Brilliant stuff, a big pat on the back!

Ref Breeze plugs, I have a contact with some (many) if you wanted to find original parts, but your replicas are just fantastic, it would be a shame to replace them! PS: my Breeze research is here with plenty of documentation: https://www.scottbouch.com/mcfs/ligh...sey-breeze.htm

Cheers, Scott

[ScottBouch]

I meant to say last night that through the sim development work I have recently learnt about ILS, but I didn't know about its predecessor!

thanks for sharing this wonderful journey!

[jamesinnewcastl]

Hi Martin

Thanks for the compliment, it's really satisfying to get something working again!

Hi Scott

Thank you too, I have to say that I wouldn't undertake your project, that simulator would be too much for me! I had just a glimpse of the items you are looking to rejuvinate and noticed your comments about the turn and slip indicator. If the ball were replaced with one with a metallic core, might you be able to move it with a suitable pair of electromagnets?

You might consider magnetic actuation for the artificial horizon too. Possibly using a shaped metallic core in the central ball.

I'm also in the 'thinking' stage of bringing to life an earlier cockpit!

Cheers
James

[ScottBouch]

Oh really, what cockpit is it? One of the good things about a Lighting era cockpit is that most kit is electrically interfaced, a lot of older aircraft used a lot of mechanical instrumentation, so is harder to simulate.

I will be doing a display at CockpitFest in a few weeks at Newark Air Museum, if you've not been to the event before, its well worth a visit for seeking advice and parts.

Cheers, Scott

[jamesinnewcastl]

Hi Scott

Nothing firm on the cockpit yet. I see from another post that you are looking for an AP, have you tried the research room at RAF Hendon? They have a great many APs.


Cheers
James

[ScottBouch]

In the past I have spoken to the archives at Kew, they hold a lot, but the cost of scanning makes it quite prohibitive.

I will try Hendon though, thanks for the tip!

I always believe there should not be a tax on knowledge or education, and fully support sharing freely.

I always freely share APs when I can help others out, as the end result is seeing another restoration project progress! One day I will take the time to publish my AP library online for anyone to freely access.

I have been lucky to have some significant help from others who have scanned a lot and shared with me to help build up the library.

Cheers, Scott

[jamesinnewcastl]

Hi All

Finally after more than 5 years my SBA project is at an end! All the active parts are working though there are a couple of oddities. All the resistors and capacitors were restuffed with the exception of Micas and the tiny ones that I physically could not hollow out. None of the remade resistors melted fortunately, even after hours of operation. The coils were original - no need to change them. Everything was cleaned, resprayed and labels remade. I have many of the mating connectors but as I changed my mind about a grand recabling I don't have a complete set - and I've lost one of the ones I did have annoyingly.

Also I found one of the original voltmeters that were supplied for the calibration of the kit on the aircraft. An original test set exists in Europe though not restored as far as I know. That test set had a power supply in a seperate box but I've never seen any of those. I never found either of the original aerials or the field stength meter used to calibrate the transmitter pattern.

Pics of the rebuilt items are attached along with AP pictures of the missing items. Note that the Stirling had one aerial in the main mast so I don't have a pic of that but the standard one is shown. The small box to the bottom left is not an essential part of the system but it allows the pilot to listen to the SBA or the Aircraft intercom or both. The bowden cable allows the pilots control box to change channels in the main receiver, I've not installed it in place (lazy!).

Loads of documents and associated information is also being archived.

Altogether it was great fun and a learning experience!

Cheers
James

[CambridgeWorks]

James, if there were a UKVRR award for dedication to an historic project, you deserve it! This is a great achievement!
Rob

[trh01uk]

James,

this is a project that rightly has a number of admirers, myself included.

One last question......what did you conclude about the overloading of the meter?


Richard

[M0FYA Andy]

James, congratulations on completing this project!
It's been fascinating to read all your updates.

Andy