Quad II test run.

[PJL]

Separating the individual windings seems a high risk strategy as we don't have a full specification for the original and this is a reproduction anyway. Everything that can be done with the transformer apart can be done with it intact using phase, amplitude and resistance checks.

[sparkymike]

OPT is out now. The windings UV and WV cannot be reversed as the centre tap is buried in the other windings. The centre tap for XY and YZ have both ends of the windings going to the turret Y, so they can be reversed if needed.

The transformer needed to come out as it was not potted and also one of the four fixing bolts would not locate with the tapped block in the can, so only choice there was to elongate the hole in the chassis slightly with the trusty Dremel drill and burr. I need to take photos of the core now from various angles before any wires are moved.

Mike

[Herald1360]

Fingers crossed then that U-V-W is at least wired Ustart-finishVstart-finishW 'cos if the CT isn't a start_finish point, the TX is screwed!

[sparkymike]

Hi Chris, my thoughts exactly. I have more patience now than I had in 2006 when this saga first arose, so will press on regardless. It should hopefully be possible to check out the OPT out of the chassis first, and once happy in that respect, replace it in the amp and see what happens. All of the above tests seem to be working from the speaker sockets, backwards. Is it possible to work from the X-Y and Y-Z points instead.
Mike.

[PJL]

Leaving the transformer in circuit does introduce some issues with the various existing connections to earth. Connecting the scope probe earthing clips to different turrets as in GJ's tests introduces further risks with connections being made through the scope earth.

However, unless GJ is willing to disconnect his known good transformer and repeat the exact same tests, you have nothing to compare.

[sparkymike]

Photos of the OPT and the top plate which I made. I am fortunate in having a pantograph engraving machine, so script was no problem..

[sparkymike]

Now in process of writing down which colour wire (or wires) go to which turret.
All the wires were tagged when I fitted top turret plate
A possible breakthrough !! Wire tagged "R" looks to be connected to Q.?? Now need to find Q and see what is what.
Mike.

[sparkymike]

False alarm (pity).The ident tag/label was on correct wire, but had also partially wrapped round wire R.
All wires are on correct turrets according to tags.
Mike.

[Radio Wrangler]

I was going to say that P,Q,R,S,T might screw up your speaker matching, but then I realised that Q is also the overall feedback pick-off tapping. Now raising that tap to more turns will risk overall loop instability. But i thought in an earlier post you said you'g opened the feedback loop. Or was it that the amp was OK provided the loop wasn't closed? I'm finding this thread a bit confusing to follow.

David

[sparkymike]

When feedback loop was removed, the scope trace cleaned up a lot and the rising hum level did not occur. Hum was still present but at a much lower level.
Mike.

[PJL]

Now you have the transformer out, it would be a good time to check the cable runs to the anodes and cathodes

[Radio Wrangler]

Crossed posts

Oh well.

With transformer out. Connect P, Y and V together. These are your grounds. Drive Q with some 50Hz AC 6.3v from a heater supply.

Looking at the schematic

Signal feeds V1 grid. So V1 anode has inverted signal, V3 grid gets inverted signal so V3 anode gets non-inverted signal - Turret Z

V2 grid gets a sample of the inverted signal courtesy of V1 So V4 grid gets a non-inverted signal, so V4 anode has an inverted signal on it - Turret X

V3 as it gets turned on by a rising grid voltage needs its cathode to also rise to apply local negative feedback. But the V3 grid gets an inverted signal so V3 cathode should also get an inverted signal - Turret U

V4 as it gets turned on by a rising grid voltage needs its cathode to also rise for local feedback. V4 grid gets a non-inverted signal, so its cathode should also have a non inverted signal - Turret W

So far, so good. Now I have to wrangle-out the sense of the feedback winding and this is where it gets complicated. Feedback gets applied to the common cathodes of V1 and V2, now if they had been working as a long-tail pair, it would not work, feedback would be nulled. There has to be an imbalance for it to work, and there is. THe feedback winds up in both V3 and V4 grids where it won't do much at all, but it alsoheads for V2 grid, where things are a lot more sensitive.

V2 grid has an inverted signal, so for negative feedback, we need a non-inverted signal. - Turret Q Hooray!

So, Put the earth clips of both your scope channels onto the turrets you linked together P, V and Y.

Put one scope channel onto the turret fed with 50Hz and keep it there.

Use the other channel to probe turrets Q, U, W, X and Z

Q is a confidence check, you should see two waveforms in phase

U should show an inverted waveform

W should show a non-inverted waveform

X should show an inverted waveform

Z should show a non-inverted waveform


I think I've got all this right. I've just reverse engineered most of the design work of the Quad II !

Don't act on anything until G-J has checked my working-out

David

[sparkymike]

Hi David,
I will need to print that out first !! Guess I will need a beer by the end of the day..
I was wondering if I could have put a sig. gen on X-Y and another sig gen on Y-Z and then checked phases through to the output points, both on very low volts ? I do have a couple of similar Advance signal generators if needed.
Mike.

[Radio Wrangler]

No, the scope's been doing fine so far stick with it. It just needs a methodical and comprehensive run through all the windings just to make sure we've found all the surprises.

David

[sparkymike]

All ready for the test.Scope probe poised at the ready. Just waiting GJ's approval.
One good thing out of this task is that I am now much more conversant with the scope. Previously it was a bit of a novelty for me and never had the need for it until now.I have learnt a lot during this exercise. (or should that be exorcise) .Later on I need to sort it out, as the trace is a bit on the wide side. I do have the service manual so that will come in handy.
Mike.

[Radio Wrangler]

That's good. There are people who avoid scopes, saying 'everything you need to find out, you can find out from a multimeter, I don't need a scope'. Well, there is some truth in that, but multimeters don't show everything directly. Spurious oscillation can be deduced as a likely cause when you see sudden jumps in what should be stable DC conditions, but there can be other causes. A scope clears up any uncertainty and the hunt becomes more understandable. Phases of windings could be checked by wiring windings in series to see if voltages add or subtract. Tedious, but it'll get you there. The scope shows you directly.

So life is actually simpler with a scope, and although you could do without one, it takes a lot more experience to deduce things from just a multimeter. For someone climbing the learning curve, scopes create pictures and the human mind is a pattern recognition machine par excellence. Learning is a lot easier with one than without. Universities know this and their electronics labs have a basic scope per bench, and a lot of experiments are to make circuits that produce standard text-book waveforms.

If you watch a design-level electronics engineer at work, he'll go straight in with a scope first. The multimeter rarely goes off its Ohms ranges, or if he needs to measure current. Voltage metering comes in when he needs more precision than the scope offers.

There's a sort of progression. People start out comfortable with multimeters, but are fearful of oscilloscopes - they look fearfully complicated. Then for some reasons they bust their fear of scopes and realise that the things actually help a lot. They're still fearful of spectrThey are S***ing expensive, yet there are plenty around. They must be doing something to justify their putchase! so the next rung on the ladder is when someone comfortable with spectrum analysers has to breach a fear threshold and start using a network analyser. Those things look so esoteric! Nope! they're just a box with a screen and a load of buttons/knobs, but they do something useful when you need it. You may never need one, but they too can work to make things clearer, to help with understanding. It's surprising just how many graduate level engineers are scared silly of them. But they aren't really fearful objects, they are useful tools which help a lot when you need them.

We can't directly see electronics working. Test gear forms windows through which we can see one aspect or another. Have a few different windows, each showing different aspects and what is going on becomes clearer.

David

[sparkymike]

Now a weird thing. Just had trial run to check Q (confidence check) and I am getting two waveforms but only the earth lead is connected on the other scope
This other wave form is a sort of ragged a/c form. Is this set-up acting as a radio receiver ?
Getting more puzzled by the minute,
Mike.

[Radio Wrangler]

Yes, the scope input impedance is very high so a bit of metal or a human connected to the tip of the probe will pick up 50Hz electric field from all the wiring around you, plus fields at SMPS frequencies, radio signals and all sorts. Just touch the tip of the probe and watch to see what your own body is receiving, now touch something earthed at the same time and watch the level drop.

We live in an electrically noisy environment. There is so much muck being spewed out of switch mode power supplies, ADSL lines, VDSL lines and mains-borne LAN extenders that the shortwave bands are becoming unusable near urban areas. I can even see increased noise floors at 145MHz that turn on and off erratically, but correlate with noise appearing at 7MHz.

David

[sparkymike]

I was starting to think about Faraday cages round the OPT !!
Mike.
I will re-arrange the earth wire to common point.

[Radio Wrangler]

You're skirting another popular myth.

In the case of what you see when you touch the scope probe to something without a path to earth, a FARADAY cage would actually work. Because your scope probe tip and what you're touching is ahort compared to a wavelength, it behaves as an electric field sensor and isn't very sensitive at all to magnetic fields.

A Faraday screen is a very special form of screening, it is designed to stop only electric fields and to let magnetic fields pass freely. From a point of view of stopping electromagnetic waves, theyonly do half the job.

For some reason or another people have started putting the Faraday prefix on every time they talk of screens. It's a bit like always adding the adjective 'flat' whenever tyres are mentioned..... "I've just been down to town and had four new flat tyres fitted to my car ready for the MOT" It sounds silly.

Michael Faraday invented the transformer with its two windings and its magnetic core. His first one is at the royal institution, I think, and it's a toroid. Now various people said the output was coupling by electric field, by capacitance. They understood capacitance so they felt more comfortable with it. Faraday thought otherwise and invented the shield named after him to screen the secondary from the electric field of his primary. He could demonstrate one on the bench stopping electric fields, but still passing magnetic ones, then he demonstrates a transformer with an inter-winding 'faraday' screen to prove it worked magnetically.

So if you have any posh transformers with an earthed screen (a gapped turn of copper foil with a single earth wire from it) that's it! a real Faraday screen in the purest historical sense.

So when you see Faraday screen, shield, cage mentioned, take care. It is either someone who knows what he's doing being very carefully specific, or else it's an indicator that someone hasn't found out about the difference yet.

David