Marconi TF1331 Oscilloscope

[pwdrive]

I'm wondering if that circuit should (under normal conditions) self oscillate with the 12AU7 pulled, the phasing of the primary windings would seem to indicate so? It might help to narrow things down? The downside is that if the 12AU7 s pulled and it does oscillate then there will be no control over the EHT by RV500 (if that's what it normally does)

Lawrence.

[pwdrive]

If you can determine that all is ok component wise in the circuit associated with the 12AU7 then I guess it's down to no or insufficient feedback.
This is usually one of two things, either the feedback loop voltage is not enough or non existant or it is being overly damped.
From what you say the continuity of the primary windings are ok, the 1nf across the primary is ok as is R501.(6.8k) C501 is a ceramic, if it was leaking it would be evident, I have known ceramics to go o/c, might be worth eliminating that prospect.
If all that is ok then it would normally point to damping of the transformer either by internal leakage, overcurrent draw on the secondaries or the dreaded shorted turn on any winding, here's hoping it's not the latter.

Lawrence.

[Skywave]

The LH triode must be hard off with - 140v on the grid?
Lawrence.

Yes: it must be. The cathode is at 0 v., so with -140 v. on the grid, the valve cannot be conducting.

But can anyone tell me why C518, 1 nF, is there? What's it supposed to do? At first, I though that is passed a -ve going spike from the half-wave rect., until I saw that C508, 0.01 uF, is connected to 0 v. Hence, C518 is effectively short-circuited to 0v. by C508. And, perhaps more to the point, where is the -150 v. at the grid of the RH 12AU7 coming from?

Al.

[AlanBeckett]

More Tests:-

First off I disconnected the wire to A7. Everything was much the same. In retrospect it's a pretty dumb move as it effectively puts +150v on 12AU7b grid wrt the Kathode. That turns the Grid/Kathode into a Diode, and that's where the -150 on the Grid comes from.

Next I tried it with the 12AU7 removed. Again it doesn't make a great deal of difference to the 5763 voltages, and it still didn't oscillate.

Finally I decided I'd change C501 and C518 - no Al, I don't know what C518 does. Once again it made no difference. The two Capacitors I removed measure OK and are >200M on my BM100 Megger.

I was standing there scratching various peripheral bits and pondering what to do next when I noticed that R501, which lurks virtually under the Transformer, hadn't been changed. Indeed, I hadn't ticked it off on my BoM. Changing it was a pig, and it made not one iota of difference.

So, what next?

I'm nervous about the 1n across the Transformer. Although I'm certain I read the original Hunts OK it seems a desperately small value. I could slip in a 10n just to see what happens.

I don't like the conditions on the 12AU7, although they seem reasonable if the circuit isn't oscillating, and hence should be the normal startup conditions. I could use a Bench PSU suitably arranged to turn the 12AU7b off(ish).

I'm desperately trying not to have to attack the Transformer, but .......

Alan

[Skywave]

Hi Alan,
This is turning out to be a bit a saga, isn't it?

The one thing about that cct. that I'm not altogether sure about - and I think that this is the critical part of this whole saga - is what constitutes the feedback path? I can't see that it is comprised of the HV rect. diode V508 and thus round to the grid of the triode, via the RV500 pot. Seems to me that the feedback path is 5763 anode, primary of T500, C501 / R501 and back to the grid of the 5763, using the fact that the primary of T500 is centre-tapped to HT (a.c. ground) with each end of T500 thus being 180° out-of-phase with the other, the whole winding comprising the inductive part of a tuned cct., the 1 nF being the capacitor of that resonant cct. (Assuming that the osc. freq. is in the region of 15 kHz, that makes the effective inductance about 100 mH).
So, on that basis, since it isn't oscillating, the suspect items within that loop are the 1 nF, C501 & R501 - all of which I believe we have eliminated as the cause of 'no osc'. I also assume that C503 is also above suspicion: its role is quite critical in this cct..
The next likely cause then is excessive loading on the transformer secondary. So, disconnect all the loads on the secondaries at the transformer itself and see if the oscillation now starts. If it doesn't, then the next likely candidate has to be the transformer itself. Now if we do get to that stage, we haven't unequivocally proved that the transformer is indeed faulty, but there are ways of testing a suspect-faulty HV transformer - provided, of course, that it is removed from the item under test for an assessment. And if you do get to that point and are unsure of such a test procedure, give me a shout.

Al. / April 20 //

[Refugee]

Would C518 be something to do with stopping the EHT climbing up during flyback when there is no beam currant?

[pwdrive]

The transformer primaries and C501/R501 remind me of the old style Blocking Oscillator, the discharge rate of C501 being taken care of by the action of the 12AU7 which is adjustable via RV500. If all the components asscotiated with the oscillator and its control circuit are ok and there are no obvious mechanical shorts or open circuit connections then the non functioning would point to excesive secondary loading or shorted turn/leakage within the transformer.
I cannot say with any certainty as to wether or not C518 is crucial to the initial oscillator fire up.

Lawrence.

[Refugee]

If C518 was badly leaky it would turn the 12AU7 off hard at the first hint of oscillation. It provides some AC feedback bypassing the normal DC feedback path.

[AlanBeckett]

Mornin all.

I've checked the position of C518 and it is indeed where the Diagram says it is. I've changed it. I admit I still don't understand its function.

I agree that (excessive) Secondary loading may kill the Oscillator. It shouldn't be too difficult to disconnect things.

I'm still dubious about the 1n across the Primary. The 'old' EHT generator - see attached - which is different, had 10n across the Primary. Could things really have changed by a factor of ten?

I've no reason to believe there's a problem with C503, but I'll check/change it.

Up and at 'em.

Alan

   

[pwdrive]

Hello Alan, the oscillator in the old and the new circuits look like different types of oscillators hence the large change in the capacitor value across their two respective primaries, the frequency determining capacitor in the old circuit would appear to be one across the primary C502 (.01uf)
The frequency determing capacitor in the later circuit is I think C501 (.01uf) this would tie up if it was a blocking oscillator, if that is the case then you will notice that both capacitors are of the same value so far as the frequency determing element is concerned.
The capacitor (1nf) across the primaries in the later diagram could be a "boost" capacitor, it's charge being obtained during the 5763's sharp shut off period and aiding the primary during the 5763's scan time.
I could be wrong of course but the whole set up reminds me of a TV LOPTX stage.

Go to it Al.

Lawrence.