Marconi TF1331 Oscilloscope

[pwdrive]

Its looking promising Alan, That property of IBM label on your 'scope does that mean Including Beckett Mr.

Lawrence.

[AlanBeckett]

Afraid not. It was there when I bought it.
My Tek 547 says Yorkshire Television on it

Alan

   

[AlanBeckett]

I've had another look at the 'ticking' in the dark. It seems to be coming from a loop of wire which goes through the Transformer and feeds the final EHT Rectifier. The arc is over to the Core I think. Actually there are two loops. Earthing the Core cures it although the Core should be earthed via the Clamp. The main problem is that I've only got a couple of minutes to see it and fiddle about. Although the loops appear to have plenty of insulation they are dirty, and of course old. Perhaps I should at least clean them and check the Core Earthing. If that doesn't work I'll change them 'cause I daren't risk damaging the Transformer.

Alan

[AlanBeckett]

Onward and upward, ish.

Having persuaded the Timebase to work, at least most of the time, there was still one thing that was driving me bonkers - the Trace kept dashing off North or South. The Shift Pot would get it back but it was entirely unhelpful. So, what's the problem? As expected the Y-Amps are pretty complex as it's 2 Channel and does A, B, Chop and Alternate. There's also AC/DC Input switching, Normal/Invert for each Channel, the Input Attenuators, shifts and finally the Calibrated Shift. There's also a job lot of Presets. The obvious starting point seemed to be to try all the Controls, and sure enough the B-Channel Normal/Invert Switch, the one I was using, was solid. Well, it's got to be fixed at some stage so it might as well be now. There are two of these switches, one for each channel, side-by-side with the engraving in the middle, so they're a mirror image of each other. The circuit is quite simple in that the switch selects the output from a pair of (balanced) Cathode Followers. The Switches themselves are quite a long way back in the chassis, and are operated via an extension shaft and a Coupler. Oddly, while all the other Switches are normal open frame ones these aren't. The Coupler sits in a slot in the Chassis. Accessibilty is not it's strongpoint as the B Channel Switch is the 'Bottom' one when looking from the side. Step one was to remove the Knob, Shaft and Coupler to see what's what. It's all held together with 6BA Socket Head Grub Screws. Slacking the Grub Screws wasn't too difficult but the Knob was stuck on the Shaft and the Shaft was stuck in the Coupler, at least as far as pulling it out was concerned. After a strong dose of PlusGas, and using tools not normally associated with Electronic Equipment, I got them out. The Gods must have been with me 'cause I dropped one of the Grub Screws and found it again. However, that didn't help a lot as the Coupler couldn't be removed due to the slot in the Chassis, and there wasn't room to get a grip on the Switch Shaft with anything sensible. The first two Attached pictures show it with the Extension Shaft removed but with the Switch in situ - it's the bottom one.

That was yesterday, and today my mate Pete turned up. He's got smaller hands than me and he's Left Handed, which has its advantages. However, despite slackening the Pot that's above it and swinging it out of the way we still couldn't get a grip on the Shaft, so out it had to come. Fortunately, the wiring is quite simple and symetrical, and it's all on one side of the Chassis. Having disconnected the wiring the nut was removed by the old expedient of tapping it round with a long Screwdriver and a small Hammer, and it was out - see Picture 3. However, it was still solid. Another dose of Plusgas and oil, removal of the 'Wafers' and some gentle easing in the Vice got it going. After that it was just a matter of putting it all back together, including all the Valves we'd taken out, and switching on. There was one small panic as some smoke appeared, but it turned out to be some PlusGas, Servisol or 3-in-1 burning off.

So, was it a success? In a nutshell no. OK, the Switch now operates, in fact it feels better than the A-Channel, but it's still dodgy, at least as far as getting a Trace is concerned, although once it's there it doesn't rush off as before.

Further investigations showed that the Input Attenuator Switches don't seem to have all 7 postions so the Knobs are currently off and the shafts are soaking.

Tomorrow's another day.

Alan

P.S. No ticking, so perhaps it was just a bit of damp

           

[AlanBeckett]

Another plendid day. Baking hot, which keeps everyone out of my way, and no hope of doing anything in the garden. It was between 90 and 100 in the Garage, even with the fan running. Perhaps I'm a Lizard really

So, what about the rest of the Controls? The only one that I couldn't turn properly was the A Channel Input Attenuator. The B Channel seemed to have loosened up overnight. At first glance it looks impossible to get at anything, but in practice it's quite simple. There's an Input wire - Resistor actually - from the Front Panel Connector and an Output wire - again a Resistor - from a Feedthrough at the back. It's a complete Module which is held in place by two screws into Bushes at the back and the usual Nut and Washer at the front. After removing them it comes out, with a bit of wiggling, as a complete lump. The application of PlusGas and oil, plus a bit of easing in the vice got it all working. So far, so good, but there's a couple of Hunts in there and it seemed a pity to get this far without changing them. One of them is the Input (AC) Capacitor and should be 0.1u (100n). However, I'd only got 0.22u at 630 volts so I used that. The other one is 0.01u (10n) in the Output circuit so I changed that as well for a 630volt one. Getting it back in was pretty straightforward apart from getting the Washers and Screws in being pretty fiddly. While I was at it I did the same with the B Channel as well. The Gods weren't entirely with me as I dropped another Grub Screw and this time I didn't find it.

There was one other job to do while it was upsidedown and that was to change a couple of 2M2s in the PSU circuit. I'd tacked in a couple of 1/4W ones when I was getting it going 'cause I hadn't got any big ones at the time. Fired it up and checked the voltages - all OK.

Now for a real test. Got it right-side-up, give it a Signal and twiddle the Controls. With a bit of fiddling a twiddling, a Sine Wave appeared

Before getting too excited a few comments:-

That's the B Channel. The Shifts work OK but the Gain Control does some very odd things. By the way, it's better than it looks in the picture, although it's only about 2kHz.
Switching to the A Channel produces a line and that's about all.
Just like the old Tek 'scopes - 545 etc - you actually only get the middle section of the Tube.
Alternate and Chop don't work, yet.
It attemps to Trigger but that's about all I can say. On the other hand I knew that the Trigger Amplifier wasn't working - it's riddled with Hunts.
After about 10 Minutes, maybe less, the Trace gradually faded away, and not even the Trace Locate Button would produce anything. Methinks one of the Tube Voltages has gone walkabout. However, switching it off, waiting a bit and switching back on got it back for a short time.

It's cooling down a bit now so I need food, a shower and some beer

Alan

               

[pwdrive]

I'm still following all this Alan, that looks like a sinusoid to me, things are on the up, keep working them switches, sometimes a cause of all sorts of unusuals, been bloody hot here too, had to wear my Sun hat this afternoon...Do you know anything about apple trees?....Maybe something wrong with ours, let us know.

Lawrence.

[AlanBeckett]

So, to the mysterious disappearing trace. I've had a frustrating few sessions with this one.

Initial diagnosis was easy - the EHT Generator was fading out. The Oscillator Anode voltage should be over 400v p-p and it gradually dropped to about 200v p-p. At that stage the Rectifier Heaters weren't lit and the PDA, and presumably the other voltages, died, but why? Switched off to move the Probes about, switched on again and it didn't come back. Ah, well, at least the fault is solid, which always makes it easier to find, doesn't it? Had to go away for a couple of hours and when switched on again it all came back. The PDA was around 7kV and for the next hour it was solid. End of Part 1.

Went down the Garden this morning to do the usual things which includes switching on the Waterfall Pump. I actually switched the 'scope on - I'm using the socket switch for safety - and didn't notice for some 7 hours. When I noticed there was a moment of panic in case something disastrous had gone wrong, bearing in mind that the Fan isn't running, but it was sitting there quietly working with a nice bright trace - odd. Switched off as I had to go out, and anyway I figured I'd let it cool down. Came back this evening, switched on and it's solid as a rock. Well over 400v p-p on the Anode and over 7kV on the PDA. Even the Pot works, although it doesn't change the PDA by very much.

In all honesty I haven't a clue. Perhaps it's simply drying out completely. I'm still not convinced that it's right yet, but I think it will do for the minute so I can get on with the rest. I've bought a couple of wire-ended 680pF 10kV Caps just in case they're the problem.

Any suggestions gratefully received, but I can't guarantee immediate action.

Alan

[Refugee]

I would expect it to be a small electrolytic capacitor getting hot as it reforms. The short periods of switched on time will have reformed it now. It will most likely be all right now on.

[AlanBeckett]

There are indeed a number of small Electrolytics in it although not associated with the EHT Generator. However, you never know what side effects reforming them may have.

Alan

[Refugee]

There may be one hiding that you have not spotted. You did have to do some reverse engineering on this scope.