Marconi 1041C

[Sinewave]

Following on from my Marconi TF144H/4S thread, there are a few measurements where RF may have affected my readings, it was suggested the use of a valve volt meter. I've got one of those, I thought, but it needs some fault finding/repair.

I really did not want to start another job without finishing the other, but I sort of need a working RF/valve meter for this, I'm hoping I can turn this around and then finish off the TF144H/4S.

So, brief description to begin with. When I bought this 1041C, the movement appeared to stick along the scale. So I investigated it and for some reason it did look as though the needle was at the wrong angle, slightly towards the scale plate. Some careful manipulation and it now moves along the scale nicely.

It's a 200uA movement, it is ever so slightly reading about 1mm to 1.5mm beyond FSD at 200uA, with my calibrator connected directly to the movement.

It responds to measurements, but not accurately. So since acquiring the manual for it I began to make the checks inside.

So the first checks of the maintenance section are the working voltages.

Power supply voltages

Recorded measurement / value from manual

• LT 2 AC, Pins 1-2 = 7.87VAC / 7.5v
• LT 1, DC (for V1) Pins 9-8 = -6.4VDC / -6.3v
• HT +ve, Pins 3-8 = 224VDC / 230v
• HT -ve, Pins 4-8 = 114VDC / 120v

Valve electrode voltages DC

Recorded measurement / value from manual

• V2 Anode = 81.7V / 80v
• V2 Cathode = 4.6V / 3.7v
• V3a Anode = 224V / 230v
• V3a Cathode = 3.96V / 3.5v
• V3b Anode = 224V / 230v
• V3b Cathode = 3.9V / 3.7

The next stage is adjustment of internal preset components. I've not necessarily wanted to dive into moving presets, however I did notice that DC zero not being what it should be, considering that the common zero should be centred with the ability to zero any range being possible.

I did notice that RV7 is not particularly reliable, even just looking at it sends the meter into a frenzy, slight adjustment makes wild swings of the movement, with the inability to make the same reading twice.

At this stage I removed RV7 and gave it a good clean with some Servisol contact cleaner and lubricant. It made little difference, so perhaps I may replace that for something a little more reliable if necessary, so long as it's not another component onwards which is reacting to this adjustment.

I've not gone futher than that so far, but hope to today and we'll see if I can bring this into service.

There are some old carbon resistors which as we know do go wildly out of tolerance with age, so that's next to try.

In the meanwhile any tips, hints and suggestions from those who've undertaken restorative work one of these would be appreciated.

[Sinewave]

Some observations so far when checking the DC full scale deflection from page 26 of the manual.

When the meter is set to 30v and zeroed accordingly, when connected to a 30v source, the 1041C reads 29v but doesn't always go back to absolute zero. Sometimes a little fluctuation.

[Alistair D]

I seem to remember seeing the same problem. Try shorting the probe tip to ground after disconnecting it from the measurement point.

Al

[Sinewave]

From the manual, the seven preset pots are behind the meter surround. They are numbered by reading from left to right as RV1, RV7, RV3, RV2, RV6, RV5 and RV4.

Can I confirm that includes the round pot in this numbering as well as the liner ones?

I'm looking at what I believe to be RV5 for the DC range full scale deflection preset, but what I'm seeing as RV5 appears to be 2k, whereas the drawings say RV5 should be 3.3k.

Also, I can't even see a 2k pot on the drawings. This pot I've removed I can't measure as it appears to have a tear in the wire so is open circuit.

[Skywave]

Once, I owned a TF1041: upon a detailed exam, it appeared to be halfway between the B and the C model. I rebuilt it to the model C spec. It was necessary to replace all the seven pre-set pots and RV5 and other various components. I also re-arranged the entire internal ass'y. to enable future maintenance to be a lot easier. Overall, quite a 'labour of love.' For quite a while, I was happy with it. Then one day, the primary of the mains xfmr. went s/c. That - and the fact that the meter had developed an annoying tendency to stick - rendered the entire item B.E.R.; little salvage only was accrued.

I now own a TF2603: much better in almost every regard. And I suspect (but cannot prove) that the reason for that is that the TF1041 was primarily aimed at the commercial market-place; the TF2603 at the military one.

Al.

[Sinewave]

I'm not sure if mine is a between model B and C version. By the looks of things I'll probably spend a bit of money on new preset pots as these open type rectangle ones don't appear to like being adjusted.

Out of spec carbon resistors will be first I guess, valves are probably ok but we'll see, so far the measurements of the anodes and cathodes were good.

[Sinewave]

Skywave, what did you use to replace the sliding potentiometers?

[Skywave]

I've really got to stretch my memory to answer that one! Much to my surprise, I don't appear to have retained details of the re-build that I did.

However, knowing myself, I probably re-designed that arrangement by fitting new, different types of pots. inside the case. (The originals are flimsy affairs: quite unsuitable, IMHO). From memory, once my replacements had been 'adjusted as required', they were then left alone.
The TF2603 has a similar arrangement, but in that instrument, the pots. are the usual, conventional ones - and are only adjustable when the instrument's covers are removed. (And, upon test & inspection when I acquired it, my TF2603 required a couple of replacement pots, too!)

HTH,
Al.

[Sinewave]

What I have observed so far, is now that I have gathered several drawing sets for the 1041C, there appears to perhaps be different versions/revisions as there are at least two 1041C drawings showing different values for components.

What I need to try and find out, is how I 'version' them, how I know where mine sits in this and which drawing to use....

[Sinewave]

It gets shoved further to the back of the list. I ordered lots of different resistors which I didn't have, to be sent two lots of the incorrect value, then to be told there's about a month lead time on one of the ones to be re-sent as their entire stock of that value is incorrect

[Sinewave]

Out of interest, do these movements have adjustable shunts?

[Sinewave]

In the image attached, the red line running adjacent the dashed line. Is this supposed to be the wire link? I know there was a wire link in place but it didn't appear to be connected as this drawing shows, unless the dashed line means something else?

[Sinewave]

This is the area, I put the link back in after I changed the resistors.

Perhaps they're individually 'linked' to suit setting up at the factory?

[WME_bill]

Marconi 1041C
Yes. Short out as required on initial test. There is a note on my circuit diagram which says: "Value selected during factory testing. Nominal Value shown".
For setting up the DC calibration. See paragraph 3.6.4 of the manual.
This takes up the 5% tolerance on the 100M resistor at the top of that chain when calibrating the 100v range.
I suggest leave any existing links in place unless you have changed the 100M or 2.9M resistors in the chain.
wme_bill

[Sinewave]

Thanks Bill.

I've changed a lot of resistors, so this will probably need a complete set up. I wouldn't know where to place the link at the moment. In that chain only the 100K, 47K and 220K resistors have been changed. The 2.9M resistor is ok.

[Sinewave]

I managed to get all of my replacement resistors for the out of spec resistors. The meter appears to be a lot more settled, but does need a good set up.

I'm going to try and dissolve the glue from the rest of the slider variable resistors and see if any of those move without breaking the wire. I'll give them a good coating of contact cleaner and lubricant before attempting to move them.

In the meanwhile I've still been trying to locate replacements as one did fracture. I can't really find anything unless I ditch them entirely for the round type as found on the later 2600 series.

[Skywave]

Those pre-set resistors: what I wrote earlier about what I used was wrong: I've had a memory flash-back. What I did use was Bourns 20-turn trim-pots, type 3059Y. These are available on e-bay - and probably elsewhere. I think I used a piece of Veroboard to mount them on, that bit of Vero being shaped to fit. Other, similar types are available, not Bourns, but shorter in length. That type might be easier to fit in the space available.

All the work I did on mine was a long, long time ago: above is written from a fading memory. I no longer own a TF1041C; I no longer have any documentation on one, including the one I had, which I substantially modified mechanically, internally. I do recall that the pre-set Rs. which Marconi fitted were very intermittent & caused problems - so I replaced every one.

HTH,
Al.

[Sinewave]

Originally Posted by Skywave

Those pre-set resistors: what I wrote earlier about what I used was wrong: I've had a memory flash-back. What I did use was Bourns 20-turn trim-pots, type 3059Y. These are available on e-bay - and probably elsewhere. I think I used a piece of Veroboard to mount them on, that bit of Vero being shaped to fit. Other, similar types are available, not Bourns, but shorter in length. That type might be easier to fit in the space available.

All the work I did on mine was a long, long time ago: above is written from a fading memory. I no longer own a TF1041C; I no longer have any documentation on one, including the one I had, which I substantially modified mechanically, internally. I do recall that the pre-set Rs. which Marconi fitted were very intermittent & caused problems - so I replaced every one.

HTH,
Al.

At my usual place, CPC for some reason I couldn't quite find what I were looking for.

Now, as you've just suggested the Bourns 3059Y I had a google and they look very familiar. It just reminded me of a stash of pots I had tucked away. Mine are Bourns but they're not the 20 turn pots, they're a lot courser than that. I don't quite have the right value, but I've popped one in the RV5 position and have proved that I can make adjustments, I'm just still a little out which will be related to the values I have.

And, during all of this, a 2604 has turned up! It's complete, working but needs adjustment/fettling and even has the original AC probe grounding sleeve!

Anyway, 1041C first.....

[Sinewave]

Skywave, were you able to find exact values? I'm wondering that if the exact values aren't used, whether it affects the linearity of the readings shown on the 1041C

[Skywave]

"Exact values". Can't immediately recall, but I think that those pots. are wired as simple variable resistors as opposed to potentiometers. In which case, I probably chose the next highest value of R for any given one that I replaced. The linearity of the scale was not affected.

However, if wired as potentiometers, a study of the cct. diag. should reveal if the value of the track is critical. If it is, I would be temped to wire an appropriate value fixed R in parallel with the track to obtain the required R value and then simply use the slider as required. The idea being that the resultant effective track R value satisfies the design requirement and the slider simply taps off a voltage level as required. A quick analysis of the resultant currents should be indicative.

As regards the Bourns pots., mine were very old NOS - and I'm sure that they were 10-turn, not 20. I've also seen 10-turn pots. (other manufacturers) with a transparent case, which was about one inch long - thus shorter that the Bourns.

Al.