Marconi TF1331 Oscilloscope

[AlanBeckett]

Don't tempt fate!

Alan

[AlanBeckett]

Progress, but not entirely down to my efforts.

A couple of days ago I escaped to the Garage and changed the other three EHT Rectifiers. It was a pig of a job, but I prevailed. One end of C509 came out as I was pushing things about which meant I had to tip the PCB up once again and resolder it. Just in passing all the component leads are very short - barely through the PCB - which makes me nervous.

Anyway, all back together and checked, connect up the Meters and 'scope, take a deap breath and fire up. Wait 90 seconds as usual for the click from the Delay Relay and ............ it was slightly worse than before with the PDA at 6kV and the Oscilator anode swinging less than 400v p-p. Oh, dear, or something similar. Everything seamed OK so I decided that I must have generated another dry joint - see comment above about Component Leads - and the PCB would have to come out and have all its joints properly soldered.

Today Al (skywave) called in and I let him loose on it. Having plied him with coffee I gave him my notes, explained where I'd got to and left him to it as I had to do a bit of taxying. An hour or so later I returned to find he'd discovered a short - bent pin, no doubt generated by me - on the tube base and everything in that area was working splendidly. Even the Controls do pretty well what they're supposed to do. On the Screen there's a nice bright little Lissajous pattern which can be focused etc. I suspect it's hum.

So, now I need the Timebase running I think, and another visit from Al

Alan

[Skywave]

Hi Alan,
Yes, an addenda from me - just for the record. You never know when a report on a piece of kit (such as this) might be of value to someone else one day.

The CRT base socket is a 12-pin affair (might be a B12H) and the solder lugs on the socket are fairly large. The trouble is that those solder tags are bent at about 45° to the plane of the socket - so there isn't much clearance between adjacent tags. Now add in the many (relatively large) components (and thick wiring) which are soldered to those tags and not only is it difficult to see the individual tags, but also difficult to see - and thus check - that various tags are not in contact with their immediate neighbours - which was the cause of the problem. The CRT cathode gets about -1200 v., fed to it from that EHT pcb. But the CRT socket pin adjacent to that has a wire with about +250 v. fed to it - and from a low impedance source, too. Since those two tags were touching, the -1200 v. point was being pulled up to + 250v. - and the EHT pcb, not liking that, (understandably), naturally got all upset & confused.

The route to the successful diagnosis was one of 'going back to basics' - since not all the voltages (normal operating conditions) are stated on the cct. diagram. I knew that the final anode should be at about 8.5 Kv. Now this 'scope has a 5 inch tube - so I would expect a final EHT of about 10 Kv. So where do I find the 'missing' 1.5 Kv? That must be at the cathode, producing the required 10 Kv. across the CRT. But on measurement, the cathode wire on the EHT board was at +250 v. Huh? That can't be right! When that cathode feed wire was disconnected from the EHT pcb, the now unconnected pin was at -1200 v. Ah-Ha!
And you know the rest.

Final comments.
It may be possible to obtain a replacement socket whereby the tags are perpendicular to the plane of the socket. Alternatively, adding heat-shrink sleeving to some tags would help.
It's times and events like this that establish the value of 'networking' - and that is only possible through the Internet and forums such as ours - the G.V.R.

All said.

Al.

[pwdrive]

Proper job lads.

Lawrence.

[AlanBeckett]

Flushed with success, and having mowed and strimmed, I decided to have a look at the Timebase etc. There's a Sync/Trigger Amp, Timebase and X-Amp.

It seemed to me that the easy approach would be 'Left to Right', ie start with the Sync/Trigger Amp and work through to the CRT. Switching to Mains Trigger would give a simple signal to look at.

The first problem I found was that the Valve Types are marked on the top of the Chassis but the Valve numbers are below. Ah, well, it can't be too difficult can it? Well yes it can - see attached. The two valves in the Amp are buried under the switches. Lurking in there are at least 4 black Hunts. Access to anything ranges from 'Oh, dear' to 'How the very devil do I get at that?'. You can fill in your own expletives.

Finally, the output signal on R215 is a slightly distorted Sin wave not the nice clean square wave shown on the Diagram.

Trying to look on the bright side this looks like the worst part, and I can only spot one other Hunts in the rest of the Chassis. I could always start at the other end but that sounds like an admission of defeat before I even start.

Alan

               

[pwdrive]

Left to right seems logical Alan as there doesn't appear to be any closed loops between the three diagram chunks you've posted that I can see.
A fully working trigger stage would be a good basis for a logical progression.

Lawrence.

[Skywave]

Finally, the output signal on R215 is a slightly distorted Sin wave not the nice clean square wave shown on the Diagram.

Bearing in mind the age of this item, the Hunts capacitors and that report, sounds to me as though this item is a prime candidate for a wholesale replacement of every capacitor whose value is 0.01 µF and above and every resistor whose value is 10 kΩ and above. And from what I recall from seeing of it, a thorough clean out of all the accumulated dust and muckite will help too.
And yes, I do appreciate that doing all that will involve a great deal of work & considerable expense - and will be difficult to do, on account of access to some of those components. I wish you good fortune in your endeavors with it: certainly won't be a 'five minute' job!

Al.

[AlanBeckett]

Back on the job at long last.

I hate leaving things because no matter how many Pictures I take and Notes I write it still takes time to catch up.

Anyway, everything looked OK so switch on, wait 90 seconds, and ...... nothing. What now? A quick poke round with the AVO showed no ht, and lo, two of the HT fuses were blown. How's that happened? Don't know, but replacing them brought it back to life as before with a bright little lissajou figure on the screen. So far so good.

Having pondered on the problem during the gap I decided to change my mind and work backwards. After all, that's what I do with Radios. So, I applied a nice big Sine Wave from my trusty Advance 81A to the X Amplifier and was rewarded with a stretched out figure on the screen. Why was it like that? It looked supsiciously like hum, and my Tek 475 confirmed it. So where was it coming from? I'd changed all the main Electrolytics and most of the Supplies are regulated. A quick poke around showed that the +150v was down at about 80v and there was 5v of hum on it. Now I'd been here before, and it turned out to be dirty valve pins. Sure enough, cleaning the pins of V607 fixed it. I then decided, rashly perhaps, that I should have cleaned all the valve pins to start with, so I did. Switch on again and the +150v says 80v. This was getting a bit wearing. So, what's on the 150v line? Easy answer, an ancient Plessey double Electrolytic - 8 + 8. Must be them, mustn't it? Tacking a spare across them made not one iota of difference. Now what? While poking about with an AVO I managed to get a bang off the aforementioned Electrolytic and no amount of valve wiggling and cleaning would get the 150v back. Next candidate was the 2M2 Anode load, which indeed measured about 2M7, as did the one in the +300v circuit, but changing them made no difference. All the other Resistors measured OK. In desperation I also changed a couple of Hunts lurking in there, but to no avail.

After a lot of pondering and measuring I swapped the ECC83s - V606 and V607. That got the +150v back but killed the +300v. Not good. Later on I had a look on ebay for ECC83s. There's strange phrases like 'Blackburn Code', 'Square Getter', 'Yellow Print' and 'Matched Pair', all of which seem to multply the prospective price by about 50. I was sitting there in a state of gloom and despondency when it dawned on me that there was one in a Black Box Amplifier that I have. Lo and behold I could even locate it, and when fitted everything, at least as far as the PSU was concerned, worked fine.

So, I'm back to square one, ish. There's nothing on the screen now, but the Trace Locate button puts a nice bright spot in the middle. I hope to get back to some proper testing and diagnostics shortly.

One last sting in the tail: when I switched on there was a faint ticking from the EHT Generator. This, I think, was a bit of damp 'cause a gentle waft from my Hot Air Gun at long range fixed it.

Alan

P.S.
That took three goes over 3 days

   

[Yorkie]

Later on I had a look on ebay for ECC83s. There's strange phrases like 'Blackburn Code', 'Square Getter', 'Yellow Print' and 'Matched Pair', all of which seem to multply the prospective price by about 50.

I think the ECC83 is the best example in existence of 'audiophoolery'. A myth seems to have developed - no doubt fed by internet and e-bay, that certain versions of ECC83 have been endowed with magical qualities that set them apart from other versions. These myths, and the prices that the valves command, can only take root if gullible individuals buy into the myth and dig deep into their pockets.

No lab tests to underpin or justify this tomfoolery - just supposition.

(Rubber rings to prevent 'microphony' on EL84s? If they say so).

This current example of a used ECC83 on offer is very typical, and by no means the most expensive of many on offer:

Quote:

Used ECC83/12AT7, Halo "O" Getter with double (Goalpost) supports.
Long ribbed Anode Plates. Has a manufacture code of f91 B8b
Blackburn 1958. BUY NOW £79.00

End quote.

Oh well, I suppose it ticks all the right boxes for those with a low gullibility threshold. Might be just what you need Al to get your scope back on top form

I've just bought a Pinnacle NOS ECC83 for £3.00+£1.99 on e-bay.

Works a treat.

I don't have any equipment that warrants the purchase of exotically priced valves, and I guess I'm not alone in that.

I'm waiting delivery of four boxed NOS EABC80s, 'BUY Now' price £17.65 inc post, with a money back guarantee. I guess this description means something to someone, but it hasn't talked up the price.

1."RWN NEUHAUS" with inclined getter;
2."WF" made in the former Telefunken plant;
3."Tungsram" or "RFT" with solid foil getter and silver, nickel, chrome plate;
4."Tesla" with gray plates;

[AlanBeckett]

I agree David.
I'll keep an eye for a sensibly priced one to replace the one from my Black Box.

Alan