Side Projects

[andrew Ivimey]

Groan it gets worse - my fault this time.

thoughtful ... but....

[Mike H]

No worries.

Those boards remind me of Quad 303s ( a bit) A bugger to replace components as the pcb tracks would lift at the earliest opportunity.

Yep I had a wee bit of similar bother, top left where those 4 diodes are, there's been altogether too much heat in this area, both from previous repair (the big metal oddities) and burning out of the other two, so the tracks starting peeling - I managed to save them, I put a layer of solder on one to strengthen it. After that I was mindful of not using too much force - i often had to clean out the holes with soldering iron and a needle so needed a delicate touch. And there were no more track problems.

[Dave the bass]

Onto actual practical things...

Does anyone remember these?
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Marconi-015.jpg
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(Clue: DTB freebies one Owston meet.)

Ha! Yes, its true

[Mike H]

Finally got used! Altho actually I've used the diodes a few more times too. Still have a couple of packets left!

[Dave the bass]

Coolio! I'm glad they came in useful Mike, for some reason we had kazzilions (approx.) in a draw in our old workshop.

[pre65]

Coolio! I'm glad they came in useful Mike, for some reason we had kazzilions (approx.) in a draw in our old workshop.

I had some of the rainbow thingies, but don't remember the diodes.

[Mike H]

Changed the capacitors on the front panel, redid input mains wiring betterer, (mains lead was missing [cut off] when I first got it) and checked out some other things...
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Marconi-018.jpg (150.11 KiB) Viewed 8438 times

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Then it was lunchtime! So I made lunch and et it, and watchrd a bit of YouTube. Then it was teatime! Then I came here.


We can't be too far away from the end now, still To Do is mount the small toroid somewhere, then wire them both up.

[Mike H]

Oh and, one of the 220 µF had a dry joint, the solder had completely failed to tin the lead.

60 years old balls-up... but just about impossible to see in situ...

[andrew Ivimey]

Love it.

[Mike H]

Having a bit of an issue with fuses - don't know why I left it this long, or why I've never checked before, but, took the front panel cartridge fuse out ... it's blown, then some wire has been soldered across it, which is subsequently broken.
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Marconi-019.jpg (104.34 KiB) Viewed 6804 times

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Also, it's 15mm, which you can't get anywhere. Well RS have it in 3A, only. Seems to be an obsolete size.

But anyway I think I really want fuses for all the secondaries as well - bit nervous about turning it on TBH... if it did all go catastrophically t!ts up, it would be nice if the transformers could be saved.

These 2 long tracks at this end of the board connect the low power transformer secondary for the control circuits, and at the bottom, 4 rectifier diodes bunched together. If a screw or nut dropped in here and bridged across these 2 tracks, say bye-sy bye to your transformer...
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Marconi-020.jpg (73.78 KiB) Viewed 6804 times

[Mike H]

Figured where to put the small transformer. Made sense to put it in the space vacated by the original, rearmost large 1,000 µF, (both replaced by 1 x 2,200 µF).

Just need to sort out the spagetti of wiring – I'm nornally more used to solder tags on transformers – also want to add inline fuses.
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Marconi-022.jpg (97.59 KiB) Viewed 6588 times

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Marconi-021.jpg (148.18 KiB) Viewed 6588 times

[Mike H]

Every morning I come out and say to myself right need to get on with this. Then I get side-tracked.....

Anyhoo, did occur to me once I get it working, will need to calibrate it. But I am/was a bit confused about how to do the current limit set-up.

Snippet of relevant bit of schematic below - it looks a bit odd, and I've double-checked if I traced it correctly multiple times, and it keeps coming out the same.
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Curent lmiti detail-1.jpg (108.44 KiB) Viewed 6448 times

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The BC161 & 2N457 are a Darlington pair and the output of the linear regulator. The 2N319 is the current limiting transistor, its emitter being connected to the output terminal (wire 'Bk10'). Imagine a CCS where a resistor in the emitter (or source, if a MOSFET) path creates a Voltage drop to turn on another transistor to 'pull down' the base (or gate) of the CCS transistor.
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CCS nTJ9N.jpg (11.92 KiB) Viewed 6448 times

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Well this works the same way, more or less, and the resistor is variable. VR1 is a preset, and VR2 is the front panel control. I was trying to get my head around what it does, so I could set default positions for the pots for setting up.

In the diagram, the bottom of each pot is, literally, the fully anticlockwise end for the wiper. VR2 is wired up as simply a variable resistor and It becomes apparent that if VR2 is set fully anticlockwise, it is zero Ohms, more or less. Zero (fully anticlockwise) means minimum current output. In that case, twiddling VR1 does nothing except vary its resistance between +9V supply rail ('+Vs1' in red ink) and 'Gn43' which is the base of the 2N457. The preset, VR1, is not for setting minimum current then.

If instead VR2 is set fully clockwise, or maximum resistance (= 1k), it starts making more sense. In the following the red line is all the points shorted together if VR2 is at max. (fully clockwise), and VR1 is at min. (fully anticlockwise) ...
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Curent lmiti detail-2.jpg (111.82 KiB) Viewed 6443 times

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The wires 'Or20', 'Yl21', 'Vt22', 'Yl45', and 'Gn43' are basically all connected together by, at most, a very few Ohms (pot tracks end resistances).

In LTspice simulation it becomes apparent that VR1 must be advanced a tenth or a bit more of travel off the bottom end of its track, else the current limit transistor (2N319) is permanently on. This makes sense because its base (blue line) is directly connected to 'Gn43' and the 2N457 base if VR1 is at min., so it will be getting the forward Voltage drop of the 2N457 be junction (plus the V-drop across the 3 x 0.5 Ω in parallel). In other words, VR1 sets the Voltage threshold where the 2N319 turns on. Turning it clockwise increases the threshold.

Right then, so it seems we should get VR1 say 20% away from the min. end of its track, or whatever it needs to be to make sure that the 2N319 stays off and the regulator works normally (and we can then set the Voltage output range and calibrate the Voltmeter). With VR2 set fully clockwise (max. resistance), then VR1 is used to set the maximum o/p current. The ammeter is 2.5A fsd, but there is a dark band after 2A, which I take to mean 'don't go here', rather like the red zone on a tachometer.

Laters!

[Mike H]

There was a mistake in the last drawing, you may want to refresh ....

[Mike H]

Sorry it went quiet for a while there - every morning I come out and say right really must get on with this, then get sidetracked into something completely different.....

So, finished wiring up the transformers, and started putting it back together. Front panel on first, then I noticed mains switch (a large double-pole double-throw toggle) is a mite too close to the choke frame, or rather its solder tags are. Plenty of thread on it so it can easily be otched forward.

Not. Nut on front of panel is a knurled nut not a hexagon, and was not wanting to play ... holy crap ... as our American cousins like to say, it was 'on there'. Had to go and get the 'angry pliers' ...
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Marconi-023.jpg (101.06 KiB) Viewed 6292 times

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It finally succumbed to brute force and swearing, and got pretty beaten up in the process. Into the bin with it. I have spare same type switches so I went and 'borrowed' a hex nut. Switch otched outward, new nut on, proper job. Below photo of extra gap thus created ...
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Marconi-024.jpg (71.45 KiB) Viewed 6292 times

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Top view with front panel fitted...
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Marconi-025.jpg (143.66 KiB) Viewed 6292 times

[Mike H]

Marconi-026.jpg (143.29 KiB) Viewed 6287 times

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Alright, back panel on, can't be put off any longer, time to try starting it up...

I figured one of four possible scenarios -

1. It catches fire (again)
2. Nothing happens.
3. Works, but with problems.
4. Works, no problems.

It turned out to be #4. "A triumph of planning and forethought"
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Marconi-027.jpg (105.76 KiB) Viewed 6287 times

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Put the Fluke on, then setting up was a doddle. Tweak the Volts preset for 50V at fsd (50V range). Then set the Voltmeter 50V scale preset. The 10V scale option I had already sorted when I first got it - a 20 Ω resistor in series with the Voltmeter movement (replacing a low resistance but burnt out wirewound preset thingy, but how was it burnt out? We'll never know). The 10V scale is slightly over but will do.

Next current limit - I was right, panel control fully clockwise, then tweak the preset for max. current, I set 2 Amps.

Below, demo of current limit, with front panel knob, setting for half an Amp - as I mentioned before, pushing the calibrate button throws a 2.2 Ω resistor across the output...
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Marconi-028.jpg (80.4 KiB) Viewed 6287 times