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Old 3rd Dec 2013, 7:49 pm   #1
Skywave
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Arrow Marconi TF1041C - an overhaul.

This thread is a follow-on to my post entitled "Manual for Marconi TF1041C VTVM".
Since I intend to repair & renovate this item, it seems appropriate to start a new thread - appropriately named. My last post in that related thread, in which I started to discuss such a renovation, has been copied here as below, since it is in this thread where that aforementioned post clearly belongs.

------------------

I'm running into big problems with this item. It's time for a general appraisal of my options to get this item working properly.

It has been necessary to carry out a number of further repairs, including replacing the 12AT7 and various passives, all of which being routine repair work, was not a problem. But the problems really began when I tried to perform the calibration: the various settings would not give reliable and consistent results. Apart from the fact that the various pre-set calibration resistors are of the cheap and unreliable open-frame skeleton type, all of which need replacing, (there are other faulty components somewhere, too), a more serious concern is when a comparison is made between the model B and the model C, it is evident that Marconi realised that the model B had some serious design errors related to the calibration, which they have attempted to correct in the model C - which, in itself, is fair enough: such is the nature of the design & evolution of many products.

Now in order to attain a reasonable degree of accuracy and reliability with my unit, it seems logical to rework the entire front panel section so that it becomes a C model - thus making this entire item a true TF1041C, rather than the 'half-and-half' unit that it is now. However, "logical" as that may be, the amount of work and expense involved in doing that rework & re-design is far from trivial, mainly on account of the method and style of the construction of this unit and with a very real possibility that wiring errors could easily arise, leading to more work and trouble.

All of which leaves me in a bit of a quandary. One option I have is to go shopping for a 'real' TF1041C, either to use as a spares source (if not functional), or it that item is fully functional, retain my existing 'TF1041B/C' as a spare parts source. So on that basis, a 'Wanted' request at this Forum seems now to be worthwhile.

► 'Wanted' thread now produced.

-----------------

Al.
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Old 3rd Dec 2013, 8:07 pm   #2
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Arrow Re: Marconi TF1041C - an overhaul.

Whilst trawling though past threads here, I came upon the following:

https://www.vintage-radio.net/forum/...ad.php?t=79571

Quite a lot of useful & relevant information in that thread: could well be helpful to anyone reading this particular thread and contemplating undertaking a repair of one of these items - the thread was useful to me!

Al. / Dec. 3, '13.
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Old 3rd Dec 2013, 8:16 pm   #3
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Default Re: Marconi TF1041C - an overhaul.

Re: my post #1. After much contemplation, etc., I've decided to re-work this item so that the entire unit is to the TF1041C 'later models' spec. This will not be a trivial undertaking , but it appears to be the only option to establish an acceptable degree of functionality and reliability. The 'later models' seem to be few and far between.

Besides, sometimes I like a challenge!

Al. / Dec. 3, '13 //
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Old 17th Dec 2013, 6:15 pm   #4
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Arrow Re: Marconi TF1041C - an overhaul.

Just an interim report. Work is almost complete: numerous interruptions substantially delayed progress. However I hope to be able to send in a full report with photos soon.

Al. / Dec. 17, '13 //
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Old 17th Dec 2013, 10:22 pm   #5
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Default Re: Marconi TF1041C - an overhaul.

I have been interested in getting one of these for a while.

I would be grateful if, when you do your write up, you could elaborate on the calibration/stability issues on the '1041B a bit more. With the C version being quite scarce it is most likely I will end up with a B.

Al
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Old 18th Dec 2013, 7:29 pm   #6
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Default Re: Marconi TF1041C - an overhaul.

Alistair: very soon I will be submitting my write-up. In the meantime, a few remarks from me to try to answer your Q. about calibration / stability issues on the B model - which does indeed seem to be much more common than the 'fully-featured' C version.

The front panel meter is secured by two screws. If these are removed, the meter promptly falls out! So a cautionary note there for a start. Behind that panel, a piece of tag-strip is situate and on that strip are mounted several skeleton pre-set pots - part of the calibration resistors assembly (there are a few located elsewhere). In mine, they were in a state of advanced disintegration - perhaps not surprising, given the age of this item. Now that type of pre-set resistor, since it is 'open to the air' is really quite unsuitable for its chosen task in this instrument. So much so, that in the C model, they have been replaced by the circular all-enclosed type and a few additional pots are added as well, removing some of the compromises inherent in the design of the B model. As for the fixed-value resistors associated with each range, many of them were found to have changed value to the extent that the calibration was adversely affected. I solved both resistor problems by replacement hand-selected, fixed-value resistors: my write-up, q.v., explains that in more detail.

Early B models employ a couple of barretters in the PSU: this will be difficult to source if replacements are required. The later B models (usually labelled TF1041C: the serial number is the 'give-away') use the series-regulated PSU that features in the 'fully-featured' C model. That later PSU was - and is - the type in mine.

Hopefully, my write-up will address any other matters of interest you may have.

Al.
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Old 18th Dec 2013, 7:58 pm   #7
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Arrow Re: Marconi TF1041C - an overhaul.

The renovation / re-build of this item is now complete: I am happy with the end result. Whenever I do work of this nature, I always do a write-up of what I have done for future reference, add same to the manual and keep the master document on a computer here. Here is a copy of that write-up.


Marconi VTVM, model TF1041B, S/N JA903/244: a renovation.

Upon initial inspection, this item was found to be in a need of a substantial renovation and repair. It was discovered that this item was a peculiar hybrid design: TF1041B and TF1041C; model C design for the PSU, but a model B design for the measuring part – i.e. all of those components which are mounted on the front panel. This hybrid design - and the general state of this item - was regarded as not being in, and not of, a satisfactory condition for use. The opportunity was taken to upgrade this item to the later model C build, but without the pre-set pots. - which feature in the later C model: these hide behind the front panel where the meter is located and are of a different type to that fitted in the B model. To meet that requirement, individually-selected resistors were chosen for all the d.c. and all the a.c. voltage ranges. (Two drawings show the values of all those selected resistors; photographs were also taken). The resistor selection was made using a variable d.c. source, a variable a.c. source (50 Hz), a Fluke 73 DMM and a Fluke 75 DMM.

Upon calibration test, the voltage calibrations were found to be quite adequate for this item’s intended use.
The Ohms ranges did not require any repair work, including the need for adjustments for calibration.
Repair work of a routine nature was also carried out at this time: some of that work involved repairs to the casework.

Two group boards have been added to mount various components - drawings and photos show these. The meter protection diodes have been relocated from the function switch to the rear of the front panel meter.
Note: these diodes are selenium rectifiers – and are obsolete. An attempt was made to replace them with germanium OA47 diodes – but that failed, since the meter calibration was seriously adversely affected.

Note also that this item requires at least 30 minutes ‘warm-up time’, prior to any measurements. Even then, it is wise to occasionally check the electrical zero setting occasionally – and to adjust it, as necessary.

Al. / Dec. 18, 2013 //
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Old 18th Dec 2013, 8:06 pm   #8
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Default Re: Marconi TF1041C - an overhaul.

Addition to the above post.

Here I have included the drawings which detail the values of the resistors that were selected for the voltage ranges. I used separate resistors for each range. This is a departure from the arrangement that I originally found.
I have also included the component layout for the two additional group boards.

Should anyone who is reading this thread wish to do what I have done, hopefully these additional drawings will be of use.

Al.
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Old 18th Dec 2013, 9:42 pm   #9
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Default Re: Marconi TF1041C - an overhaul.

Marconi TF1041C. Many thanks to Skywave for reporting his work. I have one waiting for restoration, and he has encouraged me to get it onto the bench.
However, it is missing the AC probe. Cable is cut. The rest of the machine needs service but otherwise seems ok. I have a spare EA52 valve, but can anyone offer the probe hardware please. I'll post this in the Wanted section.
Or, I suppose if it would be helpful to Alistair D, then he might be the better man to restore it. wme_bill
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Old 18th Dec 2013, 11:03 pm   #10
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Default Re: Marconi TF1041C - an overhaul.

Al, thanks for that. You seem to have answered all of my question.

The calibration compromises you mentioned were probably within acceptable limits when new but as the components aged a bit it became difficult to calibrate it. Still a bad design if ageing errors cannot cal'd out.

The use of the skeleton pots in a piece of valved equipment where there will greater convection currents due to heat and therefore greater dust ingress is a no-no as well.

I am going to paste this thread into a file on my own PC in case it ever disappears from here.

Al
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Old 18th Dec 2013, 11:15 pm   #11
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Default Re: Marconi TF1041C - an overhaul.

Bill can I snap you hand off. Yes Please. The meter that Skywave was one sold on here I think. I saw it first and offered to buy it. I saw something else I wanted and did not really have the funds for both so I deferred to him. I subsequently regretted my decision. Please PM me your price + postage. Thanks.

Al
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Old 19th Dec 2013, 10:46 pm   #12
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Default Re: Marconi TF1041C - an overhaul.

I have the Marconi TF1041C ,or so it says on the meter dial..
I haven't used it for absolutely ages.
Is the "C" variant the better one ?
And what are the differences ,and why are they rare.
Edit I have some paperwork that came with the meter complete with schematics.
Hardly ever used it,can someone tell me,what is the working freq of the Marconi TF 1041C.
Any info would be very much appreciated.
Much obliged and the very best regards Roger.
Edit to verify that I do indeed have a model C...
What serial no should I have ?

Last edited by Roger-M0TDM; 19th Dec 2013 at 10:53 pm.
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Old 19th Dec 2013, 10:56 pm   #13
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Default Re: Marconi TF1041C - an overhaul.

Sean, I have now studied the circuit diagram(1041B) a bit more closelyand one thing concerns me a little bit.

The rectification is all solid state but there is no mains fuse to protect the transformer in the event of rectifier failure. Looking at the specification of 240V @ 30W a fuse of 250mA would probably fit the job. Certainly a lot less than a 2 or 3A plug fuse.

Just a thought.

Al
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Old 19th Dec 2013, 11:34 pm   #14
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Arrow Re: Marconi TF1041C - an overhaul.

Quote:
Originally Posted by Roger-M0TDM View Post
I have the Marconi TF1041C ,or so it says on the meter dial..
Is the "C" variant the better one ?
And what are the differences, and why are they rare.
. . . can someone tell me: what is the working freq of the Marconi TF 1041C?
To verify that I do indeed have a model C...
what serial no should I have ?
All the TF1041C VTVMs that I have seen have the model number and serial number not on the meter dial but on the front plate at the bottom. But Marconi was never that consistent with matters like that!

The TF1041C seems to come in two 'flavours': the 'hybrid' version (see my posts above) and the 'full' C model. The difference appears to be in the serial number formats. The format, e.g. JAxxx/yyy signifies a 'hybrid' C model; the format ABCDE/FGH signifies the 'full' version.

Both versions claim to measure freqs. up to 500 MHz: the manual covers the specs. in great detail.

The 'full' C models are a design improvement over the 'hybrid' models, mainly on account of ease and reliability of calibration - again, my comments in the posts above refer to this. By the same token, any of the C models are a design improvement over the earlier B models. I have no knowledge of the A model; I do not know if such an animal even exists!

The TF1041x was superseded by a later design: mainly a change of case. Apparently, much of the circuitry remained the same as the model C 'full' version - so I have been led to understand.

HTH,

Al. / Dec. 19, '13. //

Edit.
Roger: Have received your P.M. and have responded to it accordingly.

Al. / Dec. 19, '13. //

Last edited by Skywave; 19th Dec 2013 at 11:45 pm. Reason: As stated.
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Old 19th Dec 2013, 11:41 pm   #15
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Default Re: Marconi TF1041C - an overhaul.

Well mine says no 54274/210 and as you say it says TF1041C on the front case it self my apologies for the confusion.
Regards Roger.
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Old 19th Dec 2013, 11:53 pm   #16
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Thumbs up Re: Marconi TF1041C - an overhaul.

Quote:
Originally Posted by Alistair D View Post
. . . there is no mains fuse to protect the transformer in the event of rectifier failure. Looking at the specification of 240V @ 30W, a fuse of 250mA would probably fit the job. Certainly a lot less than a 2A or 3A plug fuse.
Al
And a valid & worthwhile observation, too. I wonder how I missed that! Never mind, it should be a simple matter to add one: I'll modify mine A.S.A.P.

Cheers,
Al. / Dec. 19, '13 //
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