Marconi TF1331 Oscilloscope

[pwdrive]

Yes, possible, it would certainly add to the secondary voltage of that winding, the value is quite low but I guess it depends on the osc frequency.

Lawrence.

[Skywave]

The osc. freq. is 20 kc/s. (approx.): higher than conventional T.V. line timebase, (15,625 kc/s) but comparable.

When V501 turns off relatively quickly by its grid going substantially -ve, a narrow but a large +ve pulse appears at the anode of V501 on account of the back e.m.f. from the L/C cct. in the anode cct. of V501. A substantial proportion of pulse is fed through that 22 pF to boost the -ve rails shown in the top part of the cct. diag. Doing that reduces the size of the winding that would otherwise be necessary to produce the same amount of that -ve. voltage. Again, all very similar to a valve T.V. line O/P stage, except here we don't have the additional complications of the line scan coils, linearity correction cct., etc.
The actual final EHT generation will be the usual 8.5 kV. input to a 2 or 3 diode / capacitor Walton-Cockroft voltage multiplier, commonly known as a 'doubler' or 'tripler', depending on the particular rectification scheme that has been chosen. (I suspect a doubler is used, producing a final EHT of about 15 kV.; 25 kV from a tripler seems a bit excessive for this 'scope).

Of course, there is a certain amount of guesswork involved in some of that, since we can't see the rest of the cctry. that is connected to that EHT generator cct., but I think my overall analysis is fairly close.


Al.

[pwdrive]

Hi Al, that capacitor is a lower value compared to the ones normally found in a TV boost circuit (I think?) Having said that the old TV line output circuit was a bit of a power hungry grunt, so maybe you are on the right track, I must admit the idea had crossed my mind initially before I replied. I seem to remember that the TV boost capacitor provided part of the scans energy, mind you it's been a long time, I presume we are talking electrostatic scan with the 'scope, interesting.

Lawrence.

[Skywave]

Lawrence - I edited my last post whilst you were composing and then sent in yours, i.e., the one immediately above this one of mine.

In a T.V., the line scan coils are current-operated, hence the energy requirements are substantially higher and, of course, the CRT is much larger, too. Plus, the operating freq. here is higher too (by about 28%), reducing the size of the capacitor necessary, compared to a T.V.

Al.

[pwdrive]

"reducing the size of the capacitor necessary, compared to a T.V."

Yes, that is what I was meaning insofar that the scope is less demanding than the TV.

Lawrence.

[AlanBeckett]

Thank you for the comments Gentlemen. Unfortunately I fear that the circuit that I have is a lot different.

So, another sunny day and another couple of hours in the Garage. It can't last, and indeed the Forecast says so

Step one was to snip out the broken Hunts and test it. It really is 0.01uF 400v so at least that matches the Diagram. It appears to be completely O/C and measures about 45pF on my Cap. Meter. So what's new?

Next I tacked in a replacement, and ........ no change. Ah, well.

So a bit of measurement and reverse-Engineering is called for. The PCB is mounted on 6 pillars. Fortunately I have a Nut Spinner just the right size so it wasn't too bad. I removed the EHT Lead and the Bright-up line and managed to turn it over. The Bright-up line arrives on a pillar below the PCB and has -150v on it, so there's plenty of scope (sorry) for getting the back of my hand on it. All is nice and clean and nothing looked singed. While I was at it I put the replacement C502 in properly. I decided a bit of voltage measurement would be a good start - see attached.

V501 would appear to be either cut off or duff. The latter seems more likely as there's only -4.6v on the Grid assuming my measurements are OK so it should be conducting.

I'll have a look for dry joints, measure the Resistors and get a couple of Valves on order. While I'm waiting I'll see if I can sort out the circuit.

Alan

               

[AlanBeckett]

By the way Al the PCB says V1 should be a 5763. Also the presence of the two extra Diodes may imply a Voltage Multplier. I'm not going to worry about the RHS of the Diagram until I get it going. As the old saying goes 'Any fool can make an Amplifier Oscillate, but making an Oscillator Oscillate is more difficult'.

Alan

[Skywave]

Thank you for the comments Gentlemen. Unfortunately I fear that the circuit that I have is a lot different.
Alan

Really? I based my most recent comments on your most recent drawing, as per your post #16: your manually-amended version of the original cct. that you sent in.

Al.

[AlanBeckett]

Sorry Al, but the 'Modifications' were really just the Valve types and Pin Numbers. I've no reason to believe that the rest is the same.
I'll do some reverse Engineering fairly soon, but I fear tomorrow is probably a write-off.
By the way, I see that ECC82/12AU7 has entered the realms of idiocy. Ah, well, needs must.

Alan

[Skywave]

Hi Alan,
That 68R resistor looks like a later design tweak: it puts some -ve bias on the valve so that in the absence of drive from the 12AU7, the 5763 does not draw a heavy current and do nasty things to it and the pri. wndg. of the xfmr. primary. I looked at your table of voltages: what is the voltage at the cathode of the 5763? That's the really critical measurement which we need here. Whatever, it's clear that the cct. isn't oscillating: I think you'll find that R516 is open-cct.

Al.