Marconi TF1331 Oscilloscope

[AlanBeckett]

Al,

I agree. The next step is to take the back panel off and see what's on the tube base.
Attached is the combined EHT and Tube Diagram. Bear in mind that this is the Manual version.
The Focus and Astigmatism do change the shape a bit, but the Brilliance and Focus do nothing.
The Shift Pots move it about so that's promising.

Lawrence,

Herewith, 'The Blob'.
It's not as bad as it looks, the brightness just fools my phone. However, I don't like to leave it on too long.

Alan

       

[pwdrive]

Thanks Alan, so to clarify have you established the PDA voltage yet?

Lawrence.

[AlanBeckett]

No. I need to connect up my big Resistors.

Alan

[Skywave]

I reckon that you're almost there, Alan.

So, brilliance and focus controls have no effect on that resultant display. Hence, question: as each control is rotated, does the voltage on the slider of each pot change? And if not, do the voltages change when the wiper of each is disconnected from the tube and any components that are between the tube and the respective wipers? If the voltages do now change, that suggests that there is a fault in those in-between components . . . the next logical component that then comes under suspicion is the tube itself.
However, if the PDA EHT is significantly low, then the electron velocity within the tube will be low - and that will upset all the settings for the rest of the tube electrodes, of course. Which brings us to the point where you are now: is the PDA EHT voltage correct?

I'm sure you'll let us know. But it's a pity you don't live closer - you could then borrow my Brandenburg 30 kV EHT meter. (A 'left-over' from my colour TV servicing days).

Al.

[pwdrive]

Just a thought when you check the PDA voltage, if you get a reading that doesn't make sense make sure there is no significant RF on it.

Lawrence.

[AlanBeckett]

Another quick escape to the garage.

Connected 9 of my big resitors to the AVO on the 1kv range and fired it up. The PDA starts at about 6.5kv and creeps up to 7.5kv over a couple of minutes. So, a bit low but not disasterous. I think another couple of Rectifiers should sort it so I'll get them on order before SWMBO spends all my money

My big bright blob has reverted back to the previous faint Crab Nebula so there's something intermittent in there. I hope it's not the Pots although they all feel quite smooth.

Yes Al, there are several high value resistors associated with the Tube Electrode voltages. One is on the Brilliance Pot and the rest are on the Tube base. Some measurements called for.

I think I've identified the source of the funny stain on the inside of the back panel. There's more of it inside the end of the tube shield so I suspect it's foam plastic or some such from inside. It will be easier to see when I get the back panel off.

Alan

       

[pwdrive]

Keep at it Alan, once you have a focused dot on the display you will be away.

Lawrence.

[AlanBeckett]

I will young Lawrence.
I've ordered more Rectifiers but unfortunately I don't expect them until Monday at the earliest.
Watch this space.

Alan

[AlanBeckett]

A quick update.

If the Rectifiers arrive on Monday it will be a miracle, it being a Bank Holiday

Took off the back panel which was stuck with the leaked goo. Also had to disconnect the External Kathode connection and Fan. Fired it up just to check everything was as before. Got a wisp of smoke from a couple of big WW Resistors which live just below the Fan. They normally live in a howling gale, however it didn't seem to do any harm.

Eased off the Tube Base - Ceramic. All nice and clean but gave the sockets a brief spray of Switch Cleaner just to be sure.

Measured the two Resistors on the Base, R517 and R521 - both OK ish.

Measured the Resistor in series with the Brilliance Pot which is a bit high so is a candidate for change. It isn't on the Manual Diagram and I can't find MR502 and 503. In all honesty it hardly seems worth the trouble bearing in mind the high value Resistors in series.

Discovered the location of R501 of 502 - hidden under the edge of the top chassis. No designations though, unlike all the others.

Fired it up again and checked the PDA - down at 4.5 kv

While poking about discovered the reason for the low PDA and presumably the intermittant bright blob - dry joint on the V507 Anode. It's a wire loop through the PCB and a Ceramic Bead. I must have disturbed it while changing R515. Everything is desperately short but I managed to get the loop and Bead back through the PCB. However, while trying to reconnect the Anode Wire I broke it off, right by the glass of course. Insert Emoticons for swearing, throwing things and jumping about.

[OK: - Joe]

So, it looks as though I'm doomed until Tuesday.

Alan

           

[Joe]

At least you haven't necked the tube.

- Joe