UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 28th May 2020, 8:23 pm   #1
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Marconi TF1041B problem

Good evening gentlemen, i'm a new user. I am trying to repair a glorious Marconi Instruments TF1041B voltmeter, after several tests I discovered that the instrument that should have a full scale of 200 microAmpere instead has a full scale of 240 microAmpere, in fact the readings in DC voltage. they are lower than the voltage actually measured. I ask if it is possible to repair it or recalibrate the circuits for the 240 microAmpere full scale. If it is not possible to repair the instrument, can they be replaced?
Redcat2 is offline  
Old 28th May 2020, 10:48 pm   #2
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Hi!

https://elektrotanya.com/marconi_tf1...wnload.html#dl

All the setting–up details, including calibration of the meter f.s.d., is in the book I've linked above you can download free of charge!

The difference between 200uA and 240uA is within the normal manufacturing tolerances of the valve voltmeter circuit and the calibration adjustments detailed allow for this.

Do be aware though, that there are two fairly fragile glass–based B7G based p.t.c. baretters used to stabilize the heater current of the valves, and for this reason, you must not subject the instrument to heavy shocks or bumps, and in particular, move it as little as possible whilst it's switched on.

If either or both of them have failed, this will be given away by the heaters of the appropriate valves fed from them being out.

Spare Hivac XB1 Baretters can still be obtained from this link:–

https://www.ebay.co.uk/itm/280234883233

I recommend you order one or two spare XB1s if you particularly wish to use the instrument regularly for any period of time, then you'll have replacements to hand!

The EA52 Disc–seal UHF diode in the probe is vulnerable, does go low–emission and is also hard to get and expensive, so take great care not to overload the meter on a.c.!

Calibration and performance checking needs a lot of somewhat esoteric equipment and jigs to do it to factory standards, but for repair purposes, only the d.c. zero, d.c. calibration, the a.c. calibration and resistance calibration adjustments are really needed!

Study the bookwork, it's a good one but a bit heavy reading in places!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

Last edited by Chris55000; 28th May 2020 at 11:10 pm.
Chris55000 is offline  
Old 28th May 2020, 11:10 pm   #3
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Hi, thanks Chris,
then the lower than normal voltage indications are not due to the defective instrument?
You say the defect is due to the baretters?
I have the manual, I read it even if it is not always easy to understand.
I tried them all to have a correct reading, but for example with 300 mV dc input
the instrument indicates a maximum of 270 mV dc.
What can I do to correct the reading?
I checked the resistors involved but they are in tolerance. I tried to adjust the variable resistors but without being able to solve.

Last edited by Redcat2; 28th May 2020 at 11:27 pm.
Redcat2 is offline  
Old 28th May 2020, 11:30 pm   #4
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Hi!

I only mentioned the XB1 Baretters because these are physically and electrically somewhat fragile, but they would not affect the calibration unduly, providing the valve heater voltages are correct.

If you can't get enough meter deflection by following the calibration adjustments given and RV7 is already at the minimum available resistance setting, possibly increasing the value of R25 from 750 ohms to 1k0 or 1k2 may help, but please check V3A/V3B by substitution first to make sure V3 hasn't gone low–emission!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 28th May 2020, 11:40 pm   #5
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Hi, thanks Chris, I understand what you wrote to me about the baretters.
The V3A/V3B valve: I tried to replace this valve, but without success.
I will try to replace R25 with higher values ​​in the coming days. I'll keep you up-to-date. Thanks so much.
Redcat2 is offline  
Old 29th May 2020, 8:45 pm   #6
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Good evening gentlemen,
I tried to change the value of R25 but without success: increasing the value of R25 the indication increases but the zero is no longer correct and if I adjust it, the reading returns to the wrong value lower than the real value of the inserted DC voltage (for example the real voltage is 6 Vdc while the one indicated is 4.2 Vdc). What can I do to fix it?

Thanks for the help you can give me.
Hi, Attilio
Redcat2 is offline  
Old 1st Jun 2020, 11:44 am   #7
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Hi!

This looks like the mutual conductance of V3A has dropped to a lower figure than the V3B half of the circuit, so you'll need to replace V3A/V3B with a brand new one and try to get the meter set–up again according to the book before you can go any further!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 1st Jun 2020, 12:09 pm   #8
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Hi Chris, thank you, I will try to replace the V3A / B with a new one and update you on what happens.
Thank you again.
Redcat2 is offline  
Old 1st Jun 2020, 1:05 pm   #9
Alistair D
Nonode
 
Alistair D's Avatar
 
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 2,008
Default Re: Marconi TF1041B problem

I doubt that changing valves will solve your problem. You have already found the fault. The fact that you need 240uA for Full Scale Deflection means that the meter movement is way out of specification.

Low sensitivity and open circuit meters are are known problems on the '1041. I believe what happens is the the hair springs become stiffer and brittle with age. Search this forum for TF1041, there are various threads covering this issue.

You will probably find my thread where I had exactly the same problem with my own 1041C. I was lucky enough to find a movement from a 1041B. Alas the scales are slightly different and the scale plates cannot be swapped. It is now on the to do list until I can redraw the scales.

Al
__________________
I won't tell you how I discovered that.
Alistair D is offline  
Old 1st Jun 2020, 9:11 pm   #10
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Evening Al!

There must, somewhere, be design data appertaining to the valve voltmeter circuit that allows you to calculate the maximum f.s.d. meter current in order to choose a readily available meter movement – Heathkit chose a 200uA f.s.d. movement in their design.

When I worked at Electroservices we had access to a precision variable current calibrator that had 0.01uA min. resolution per step so that even the most delicate movements, such as the AVO VCM 30uA f.s.d. type, could be safely tested to f.s.d.

If any Member can furnish me with design data for the v.v. circuit that covers the f.s.d. current I'd greatly appreciate it – I don't have a TF1041B but I could shunt a spare Heathkit movement I have to 250 uA f.s.d. and then see if the basic TF1041 circuit, made in temporary form for one range, can be made to operate it.

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 1st Jun 2020, 10:28 pm   #11
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Evening All

Alistair D, I agree with what you have written, but I was hoping that a change could be made on the electronic circuit to have more current for the full scale, that is 240 uA instead of 200 uA.

Chris I don't understand what you intend to do!

I tried to replace the galvanometer with an equivalent resistance of 500 ohms with a digital multimeter connected in parallel and inserting the full scale voltages of the ranges 300 mV, 1V, 3 V, 10 V and 30 V reading on the multimeter always about 100 mV which I think is correct (500 ohm X 200 uA = 100 mV).

Attilio
Redcat2 is offline  
Old 2nd Jun 2020, 6:19 pm   #12
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Good evening everyone

I am now convinced that the problem is the galvanometer.

Can a replacement be found?

I read that maybe can buy the TF1041C galvanometer but have to change the dial with the scales.

Anyone know where buy a galvanometer for the TF1041B (or C)?
Redcat2 is offline  
Old 2nd Jun 2020, 10:02 pm   #13
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Hi!

My intention was to build a temporary single–range version of the TF1041B, e.g., 0–15V, to see if the design can be made to operate a 250uA meter – if this works I can then come up with a repair solution for yours that will not need a replacement meter movement!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 2nd Jun 2020, 10:27 pm   #14
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Hi Chris, thanks,
it would be interesting if you can realize your idea. I look forward to hopefully positive news, given the difficulty of finding a 200 uA instrument in working order.
Redcat2 is offline  
Old 2nd Jun 2020, 10:49 pm   #15
Alistair D
Nonode
 
Alistair D's Avatar
 
Join Date: Jan 2010
Location: Edinburgh, UK.
Posts: 2,008
Default Re: Marconi TF1041B problem

The question is, will the galvanometer remain at 240uA? I seem to remember that mine had risen to nearly 300uA.

Al
__________________
I won't tell you how I discovered that.
Alistair D is offline  
Old 2nd Jun 2020, 11:16 pm   #16
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Hi Al
Good question !
It has stood still for several years and was already at 240 uA. I hope it is seasoned and not worse over time.
Redcat2 is offline  
Old 3rd Jun 2020, 3:03 pm   #17
Sinewave
Octode
 
Sinewave's Avatar
 
Join Date: May 2016
Location: Oxfordshire/Bucks borders, UK.
Posts: 1,604
Default Re: Marconi TF1041B problem

For my B version and C version, I need to find some suitable replacement for those sliding pots which are set then glued at the factory during initial set up.
__________________
Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.
Sinewave is offline  
Old 12th Jun 2020, 1:23 pm   #18
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Quote:
Originally Posted by Chris55000 View Post
Hi!

My intention was to build a temporary single–range version of the TF1041B, e.g., 0–15V, to see if the design can be made to operate a 250uA meter – if this works I can then come up with a repair solution for yours that will not need a replacement meter movement!

Chris Williams
Hi Chris,
sorry, do you have a good news for my problem?

Thanks
Attilio
Redcat2 is offline  
Old 12th Jun 2020, 3:38 pm   #19
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Marconi TF1041B problem

Hi!

I tried the Marconi circuit on one range, and it does indeed only provide about 215uA maximum, enough to set up a movement in good condition with the stated f.s.d. of 200uA but not your defective one with a f.s.d. of 240uA!

You could try reducing the anode–to–grid upper coupling resistor between the first pair of triode halves, R22, from 680k to 560k, this change allows the cathode of the first cathode–follower triode to rise a little more positive than the cathode of the second cathode follower, this might be of help!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 12th Jun 2020, 5:01 pm   #20
Redcat2
Triode
 
Join Date: May 2020
Location: Vergiate, Varese, Italy
Posts: 18
Default Re: Marconi TF1041B problem

Thanks Chris, I'll try then I'll update you.

See you soon
Attilio
Redcat2 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:08 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.