Marconi TF1331 Oscilloscope

[AlanBeckett]

You may well be right there young Al.
There was zero volts on the 5763 Cathode and the 68R is OK.
I see your point about R516 assuming the circuit is the same. Everything around the ECC82, apart from the first Anode is very Negative.

Alan

[Skywave]

It's starting to make sense now. V501 isn't conducting because it's cut-off: control grid @ -4.5 v.d.c. Hence, no anode current, no volts on cathode, anode voltage @ +360 v. V502b, strapped as a diode, has biassed off V502a, (normally getting a pulse from the osc. to complete the feedback cct.), making its cathode -ve and thus the grid of V501. Until V502b gets going, nothing is going to happen. And it won't get going if its grid return resistor to 0v. is O/C, (which I'm sure it is), since then that grid is heavily -ve too.

Al.

[AlanBeckett]

I wasn't expecting to get back to it today due to a previous Lunchtime engagement, but that was reasonably quick and then my mate Pete turned up so I was able to escape.
So .....
First we had a poke about.
R508 looked as though it had been a bit warm, and sure enough it measured around 1M instead of the 680k it should have been. I changed it for one I had handy - as is usual my 2W kit doesn't include that value - but it made no difference.
All the other Resistors we measured were reasonable.
After some pondering I decided I'd remove the PCB and take it back to the house for a bit of Reverse Engineering. After some photographing and marking we removed the wires and fished out the PCB. At that stage Pete spotted another duff Hunts lurking across a couple of the Transformer tags. Fortunately enough had survived to read its value, 0.001/1n. Guess what I haven't got in stock? I had a momentary panic when I realised that the PCB is bolted to the chassis at 4 points via the track that runs round the LH edge, potentially making lots of my tests usless. However it turns out that there's alo a wire in the 'Back Connections' that is also a Chassis connection.
So, I'll order some bits and work out the circuit.

Alan

           

[Skywave]

Interesting; looking at your last-released cct. diag., the only 1 nF caps. I can spot are C505 and C511. C505 has only one connection to the transformer; other 'leg' of C505 goes to V503, and C511 doesn't connect to the transformer at all. I suppose that it is just possible that one 'leg' of C505 is wired to a dummy tag on the transformer, but considering the new / additional R in the cathode of V501 and the valve-type changes, I suspect that this board is a new design: hence, your last-released drwg. is N/A; hence, your reverse-engineering attempt - which I hope is revealing.

Al.

[pwdrive]

As a matter of interest Alan, what's the 3 db bandwidth for the Y amp on the marconi's specs?

Lawrence.

[AlanBeckett]

Sorry Lawrence, I didn't answer your question.
The Manual claims 15MHz.

I haven't been completely idle, and I'm glad I've brought the PCB back to the house, 'cause the temperature has plunged.

So, see attached. I'm not expecting anyone to do anything with the scans, just be horrified. It must be one of the most turgid PCBs I've ever had the misfortune to come across.

The good news is that everything seems OK and the Transformer has continuity with no signs of distress.

I'm now constructing the Circuit.

Alan

           

[pwdrive]

Thanks Alan, that's a very usefull bandwidth for most radio work, I'm sure you will have it buzzed up before too long.
Been like spring today here, just starting to turn as I write, forecast's a lot colder.

Lawrence.

[AlanBeckett]

It's been damned cold here the last couple of days, so apart from getting all the usual domestics done I've been working on the Circuit of the PCB - see attached.

A few comments:-
It's not the same as the Manual version!
I don't understand the function of C518.
The Blanking signal is connected to A9 - 21 on the Manual Diagram.
I think I'll have to sort out the external connections before firing it up otherwise I can see the 12AU7 going bananas without a big negative voltage on Pin 7.

Any comments welcome.

Alan

   

[AlanBeckett]

Drawing update.
A7 was on there twice, and one of them shouldn't have been there at all

Alan

   

[AlanBeckett]

Still no bits, or at least not all I need, so a bit more investigation.

I needed to find where all the external Connections went so that once fired up the (Voltage) feedback would be there.
These are the pink wires in the braid on a previous picture. I found most of them OK, but there were two which went into the braid at the PCB end but didn't seem to come out of the other. A count showed that there were 9 at the PCB end but only 7 at the other. There was nothing for it but to unclamp the braid and pull it back a bit. The answer was that a wire at the PCB end went a short way into the braid then looped round and came back out again. Ah, well, you live and learn.

I also moved some bits round on the Diagram to make it a bit more logical and more like the Manual Version, and added the external bits I identified - see attached.

With luck the bits should come tomorrow and I can get on.

Alan