Marconi TF1331 Oscilloscope

[pwdrive]

To keep your feet warm in cold weather Alan I would recomend an original pair of Sorel Arctic Pac Boots, warm as toast.

Lawrence.

[AlanBeckett]

Yes Lawrence, I've got warm boots, I just don't like them.

Alan

[AlanBeckett]

At long last I received all the Resistors. I must have chosen the two most incompetent Suppliers on the planet.

Anyway, I fitted a full set as there seemed little point ignoring the couple that were in tolerence.

It looked very pretty, so I fitted it back in the 'scope along with its new Valves. I also joined A4 to -150v as the Schematic says that's where it should be during the scan, and there's no guarantee that the Timebase will be running.

So, fire it up, wait 90 Seconds, and ........... nothing, no oscillation

Well, there must be something else wrong, so I measured the voltages - see attached.

A2 - +300v - is a bit low but not enough to stop it I wouldn't think.

The RH Half of the 12AU7 looks to be hard on. I expect the -143 on the Grid is Grid Current. The other voltages around there - A6 etc - I supect come from that -150, although I haven't disconnected anything yet to find out.

The above cuts the LH Half off pretty solidly.

The voltages on the 5763 look about right bearing in mind the other voltages, and it is conducting.

There is HT on both sides of R501 so the Transformer isn't O/C.

So, that all looks pretty normal to me for a 'not oscillating' start-up condition.

The 1n across the Transformer is new, but I haven't changed C501. It measures Ok and it is Ceramic, but ......

All suggestions gratefully recieved.

Alan

       

[pwdrive]

If you have - 150v then something must be oscillating?

Lawrence.

[pwdrive]

Ignore my last post Alan.

Lawrence.

[AlanBeckett]

Lawrence,

I thought I'd put a Post on saying that the -150v comes from the Main PSU.
I don't know where it's gone.
Anyway, I'll have a poke about with a 'scope just to be sure.

Alan

[pwdrive]

The 5763 is about at it's limit with the voltages as shown, the PSU feeds seem to be ok, I wonder if it's anything on the external lines that feed the CRT?

Lawrence.

[Skywave]

The 5763: Ia = Ik = 50 mA. So it is conducting hard: it is 'bottomed'.

LH 12AU7: grid @ -143 v.; anode @ + 140 v. Ia = 2.5 mA. But with grid @ -143 v., valve should be hard 'off': something not right there. Suspect possible defective valve.

RH 12AU7: grid and cathode at same potential. So no -ve bias on grid. Therefore valve is hard on. Hence anode @ - 143 v.
But where is the - 150 v. @ grid coming from? It isn't coming from V508, etc., since as we go from V508 to grid of valve in question, voltages become increasingly -ve. Suspect this valve has a grid / cathode internal short.

Conclusion. Try a replacement 12AU7 - and then let me know what happens.

HTH,
Al.

[pwdrive]

The LH triode must be hard off with - 140v on the grid? The 140v is set by the potential divider when LH triode is hard off ie: 280v/2 ?

Lawrence.

[AlanBeckett]

Composite answer:-

I think Lawrence is right - the 12AU7(a) must be off.
The 5763 is conducting about 53mA which puts it just outside its maximum Anode dissipation.
It's not showing any signs of distress but I don't leave it on for very long.
I have tried the old Valves just in case there's a problem with the 'new' ones - no difference.
I'm thinking of disconnecting the wire to A7. That should cut off the 12AU7(b).

Alan