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Old 26th Jan 2023, 10:19 am   #61
G6fylneil
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Default Re: Standard Beam Approach - Rebuild update

James,
Quote:
"The workers that made this item must have had a really devious assembly plan as there are many solder joints that are hidden...
In the early '80's I was testing some '70's military equipment, which had to fit in a confined space, the units were built and tested on dummy chassis /jig, fitted with lots of hinged panels to give access, and then "dropped" into the final (fully welded) chassis and retested.
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Old 26th Jan 2023, 5:24 pm   #62
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Default Re: Standard Beam Approach - Rebuild update

James,

I continue to watch this project with great admiration for how you tackle each "problem area" head on. This latest one with 3D printing of Breeze connector parts is typical of the approach you are demonstrating - and teaching us all that many things are possible that others too readily dismiss. The resistors are a case in point!

I'd like to hear what is involved in making a "careful measurement" of the Breeze connector parts? I would have thought measuring a fine thread needed some powerful magnifiying glasses, plus some very steady hands and a micrometer at the very least. Do give us some detail on how you did it, and what measurements you had to take to build a successful 3D printing file?


Richard



Quote:
Originally Posted by jamesinnewcastl View Post
Hi All

As I can't get the types of 'Breeze' connector (socket) that I need I've produced a 3D print! The connector series all use the same outer shell and pins in various sizes so I can get sockets with the right shell size but the wrong insert. I've also modelled the two spacer rings. So making careful measurement I've made a 3D file and had the insert that I need printed!

>>Snip>>


Cheers
James
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Old 26th Jan 2023, 7:20 pm   #63
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Default Re: Standard Beam Approach - Rebuild update

Hi Richard,

For the insert I took the pin connector that I do have and put it on my home photocopier. I found that I got less shadowing if left the lid open and turned all the lights off. Next I take a fairly high resolution scan. It's entirely possible that the scan isn't symmetrical but it is not important at this stage.

I have a 3D drawing package - they are abundant these days and I suppose even a 3D CAD package would work. The advantage of my program is that it is mainly for c.g.i and so easily handles imported graphics, it also has 32-bit resolution. I get the program to make a cylinder (I know the diameter so that is precise). Next I project the image from the scanner onto the top of the cylinder. I can adjust the XY scale and do so until it fits - that is easy as I can cause the scanned image to go semi-transparent which lets me see through the image to where I am moving the cylinders. Now all the necessary holes are captured.

I did wonder for a split second if I might have done anything new, but it's not rocket science. Then I remembered when I was an apprentice 50 years ago the company I worked for had a machine that could project the shadow of a production item onto a screen which had go/no go markings on selectable transparent slides - no ideas are new.

Making more cylinders (of a size I have measured on one of the other inserts) I move these so that they are co-axial with each of the scanned pins as I see them. Once I am happy with one pin I can copy/paste all the pins in that row - these can be 'slid' along the axis so that each of those pins will be precisely level with the others (or precisely wrong of course) Any other pins that might need to be in line on other rows can be snapped on the same axis. This process much easier to see than to describe.

Now I have a big cylinder (the insert body) with a number of hopefully precisely placed smaller cylinders (the holes for the pins). Next I make the internal 'step' that stops the pins falling out. All the pin cylinders are 'cut' - but not split. Next the lower part of the cylinder is 'selected' and its X and Y dimensions are scaled - but not the Y. This causes the cylinder to flare. The dimension required is what I measure with a micrometer like tool - can't think what it is called.

So far this is all quick and easy. Next I make the program 'subtract' all the small cylinders from the big one - a Boolean operation. This is a pain as the dopey computer makes a real mess. Doing it by hand would take days so I have to 'pre-cut' the large cylinder to give the algorithm something to work with. Even after that I will spend 2 hours cleaning up the resultant mesh!

Once done - I check that there are no 'holes' in the mesh then export as an STL file and upload to a 3D printers web site! 35 min order with the one I chose, so I made two sets of parts.

The spacers and the cable guide are easy to do - just put in dimensions and the program does them for you!

The thread is actually simple! If I knew what the thread was I could get the dimensions off a drawing, instead I just measured the distance over a large number of threads and divided by the actual number of threads. Next some simple use of the micrometer thingy. Thickness of the shell and threads minus the shell alone = hieght of teeth.

Next I do a 2D drawing (spline) of the zig-zag that is the profile of the teeth - to the right size. Next I get the program to 'Lathe' that spline. This basically spins the spline around an axis while generating polygons in space as it goes and so generating a 3D 'concertina' shape. Now, happily, the program can also push the shape forward as it rotates it. If I get it to push forward at the same pitch as the thread it makes a screw! Now all I do is call up a cylinder and merge the two to make the bulk of the backshell.

So, not much skill in any of that thank goodness! And the bottom line is that if I get it wrong I can just tweak the drawings and repeat.


Cheers
James
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Old 11th Feb 2023, 8:42 pm   #64
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Default Re: Standard Beam Approach - Rebuild update

Hi All

Another snag stopping the already slow rebuild of the control unit! The threaded shaft of a rotary switch on the unit was a fair amount smaller than the hole provided. How this could come about is a mystery as the switch was certainly original as it was fully wired in to the unit!. So I have a loose switch which is guaranteed to start slipping about later if i do no more than tighten the nut.

One answer is to fit a collar to the switch that will fill the hole. The right size collar is unlikely to be available and I don't have a lathe and I don't fancy hours of filing!

So my solution was to take some lovely thermosetting plastic, get a blob of it all soft. Then laying the unit on its front press the plastic into the hole in the panel pushing it out to fill the hole. Let it cool and I have a disc of plastic that perfectly fits the hole. All that is required is to cut a hole in that disc the size of the switch thread. Job done!

The hole and shaft don't need to be circular, this might be a useful technique elsewhere.

The process in pictues attached.
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Old 13th Feb 2023, 1:49 pm   #65
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Default Re: Standard Beam Approach - Rebuild update

You can still get shadowgraph machines, these days they have DROs on them, so you can easily measure dimensions, angles, radii, PCDs etc.
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Old 10th Mar 2023, 9:09 am   #66
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Default Re: Standard Beam Approach - Rebuild update

Hi All,

Finally, testing on the main receiver has started. I'm not a radio person so a lot of what is happening is a first for me! Attached is a picture of the rig I'm using.

The AGC circuit means I have a sort of floating earth and I realise that the 3-pack 2uF capacitor in the set that is used in the AGC should have a common positive rather than the common negative which I assumed when I restuffed it! Still not sure I had to break into the nice job I made of it and change to non-electrolytics (imported from America), just to be on the safe side. Repaint, new label...

After more reading I realise that the Local Oscillator needs to be operating all the time so putting just the VR107 in the set I turned it on and hoped. No burning, no smoke, no oscillation.

Lesson 1. Make sure you have added that last wire...

Lesson 2. You won't find any oscillation if you keep putting the scope probe on points that have a decoupling capacitor. Apparently too, the main anode doesn't oscillate even if the lower grids are oscillating. Eventually, after grindingly checking every single component and all the connections, I find a point that is oscillating! Hurrah! Learned a little about Hartley Oscillators though.

Lesson 3. The LO is going at 22.2MHz, but the IF is supposed to be 7MHz! Another couple of hours working out the resonant frequency and it looks like I am missing lots of pFs. I don't know about you but I find that thinking away from the kit, usually in the bathroom, sharpens the mind. Bing! The receiver works on 28MHz (ish) and the LO was at 22.2MHz. Subtracting one from the other and volia, about 7MHz! So I didn't have the problem I thought I had. I have a feeling that the kit will work perfectly and I will be the one being constantly corrected!

The next step is to add in and test the RF front end and put in a signal. Really looking forward to that!

Cheers
James
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Old 11th Mar 2023, 6:00 pm   #67
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Default Re: Standard Beam Approach - Rebuild update

Looking at the R1124 schematic, I am not surprised you had difficulty finding the oscillator signal. Grid 2 of V2 effectively operates as an anode for the oscillator (its fed HT+ via R7). And that's probably the point where you finally find some sign of oscillation taking place.

Unfortunately merely attaching a scope (or other) probe to random bits of circuit is enough to stop an oscillator operating at all due to the loading the probe introduces.


Richard
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Old 11th Mar 2023, 6:11 pm   #68
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Default Re: Standard Beam Approach - Rebuild update

As regards local oscillator frequency, I note from the AP that the R1124 operates down to 30.5MHz. So the lowest oscillator frequency should be 30.5 - 7 = 23.5MHz. Its hardly surprising that your local oscillator is a bit low in frequency after the level of rebuild you have applied! There will only be a problem if you run out of adjustment range on the relevant preset capacitor.


Richard
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Old 12th Mar 2023, 12:59 pm   #69
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Default Re: Standard Beam Approach - Rebuild update

Hi Richard,

That was the point where I found the signal and I had avoided it since I too assumed it might kill the oscillation!

I've not changed any of the tuning components. The coils are all made from heavy gauge wire and the variable caps are unobtainable - well you can get them but I have no idea what the values are as the AP only states 'Trimmers'. I am loathe to adjust them either as they are prone to simply breaking! I have some RAF memos that were moaning about the reliability of that particular make when this equipment was new, so tens of years on I'm not risking it, yet.

I've started playing with the input stage - the input signal is applied to a tap on a coil. The amount of (thick) wire between that tap and ground is about 1cm! The mystery of RF is how that does anything except short out any signal!

I've also noticed that the Antenna for the main receiver is equipped with a 'loading coil'. From what I've read this just allows a shorter antenna but it is another thing to get my head around! From stuff on the internet it looks like it might well have been needed.

Cheers
James
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Old 12th Mar 2023, 8:14 pm   #70
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Default Re: Standard Beam Approach - Rebuild update

James,

I can imagine that a standard quarter wavelength whip antenna would be inconveniently long on a plane. At 30MHz, we have a wavelength of 10m, so the whip would be 2.5m long. A quarter wave is about the most basic antenna you can devise which produces a resistive impedance in the region of 50 ohms. That would match conveniently to coaxial feeders of course.

If the antenna is shorter than quarter wavelength then it looks like a capacitive impedance, and to cancel that out a series inductor aka "loading coil", is fitted.


Richard


Quote:
Originally Posted by jamesinnewcastl View Post
Hi Richard,


I've also noticed that the Antenna for the main receiver is equipped with a 'loading coil'. From what I've read this just allows a shorter antenna but it is another thing to get my head around! From stuff on the internet it looks like it might well have been needed.

Cheers
James
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Old 16th Mar 2023, 7:48 pm   #71
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Default Re: Standard Beam Approach - Rebuild update

Hi All

Several days of confirming that I really am no RF engineer have passed. I thought that the RF input stage would be a snip - pop in a signal and watch it amplifed at the anode, or possibly on the input to the next stage.....

So, at times I've connected the scope probe to the same point on the chassis where the earth clip is and found a nice big signal. I've seen the ripple on the power supply everywhere. The IF oscillation all over the input stage. No apparent gain at all in the first two stages but unpowered the first tuned circuit resonates very strongly. Are the valves any good? Is the AGC shutting everything down?Nothing behaving itself....

Finally, after many, many, 'switch it off and go away to think moments' over the last week I put in the third valve - an AGC controlled IF amplifier stage. This was the second time that the 'only thing left to try' was another valve, not that it did much! I put the valve in and........ lovely wobbly signal, all sorts of things going on - a mess. But by sheer chance I changed the input frequency and HOORAH, a huge (8V pk-pk) sine wave appears!! Trembling hands on the calculator and it's at 7MHz, the IF!! Photo attached.

I'm now having a rest. Sadly it has all been luck rather than my knowing what is going on and I have three valves left to fit so I am not confident at all for the future outcome.

During the last week I've been hampered by my SSG1000 randomly shutting down the frequency range I need, so I have been not finding any signal due to there being none going in, took ages to notice that of course. Long wait while it decides to work again. Side track to replace the front control PCB with a spare I bought ages ago, no joy. Then while investigating the issue I found that probing one of the diodes caused the signal to reappear - dry joint! I was about to buy a replacement signal generator - the equivalent would have been 2200!

Also I managed to absent mindedly swap BNC plugs over and spent some time probing around the circuit with the output of the signal generator! A long scream from the 50V overload protection sounder lead to several sweaty moments where a 2200 bill came back into focus. Amazingly the SSG1000 survived!

Cheers
James
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Old 17th Mar 2023, 9:40 pm   #72
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Default Re: Standard Beam Approach - Rebuild update

Hi All

Valve 5 in now. This is the one that extracts the audio from the incoming signal. See the YouTube link below for a video simulating the aircraft swinging from too far left to too far right on approach. Dots - then silence then dashes.

https://www.youtube.com/watch?v=p0YYN8qF7vg

The receiver is really only responding to the RF, converting it to an IF and then detecting the audio, the effect of flying across the 'beam' is all done by the box you can see me operating in the video. In real life of course the SBA kit would be moved across the beam on an aircraft - I've not got that far - yet!

There is a bad joint somewhere as you can hear a mad sqeak towards the end of the video but I'll fix that later. Next I need to get the dials going, that's the job of valve 6, the last one!

Cheers
James
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