Marconi Valve Voltmeter TF1041C - repair thread

[Cathode Ray]

Hello forum,
I've started work on a Marconi TF1041C Valve Voltmeter which I aquired from a fellow member here.
Condition is excellent, internals look like factory.
It hinges open from the front to give good access to the PSU and meter
Probes look intact and in ok condition.
It also has the original round pin 5 amp fuse-less plug which I want to keep, it fits nicely into the top lid (nice one Marconi).

Taking my time with this, dont want to ruin it.

First order of business... the large 25v 3000uF cap is reading virtually open, about 10Mohms resistance (dropping as I measure it) and virtually no capacitance reading.

Any chance this will reform?
And if yes, what current is recommended to start with?


Thanks
Ray

[MotorBikeLes]

Don't waste your time. A very useful and accurate instrument, don't burden it with a worn out junk component that was good 50 years ago.
Les.

[Cathode Ray]

Right you be Les.
I ordered one.

Thanks
Ray

[Cathode Ray]

Hello again,
I have discovered the NTC Thermistor in series with the meter has decayed over time and broken into two parts.
It's in parallel with a 100 Ohm resistor (R42).

The thermistor is a Mullard Varite Type VA1040.
Specs found here:
http://www.electrojumble.org/DATA/Mu...o%20VA1040.pdf

Resistance at 25 degrees C is 130 Ohms +/- 26 Ohms (which is 20% tolerance)
Max Dissipation 1W

130 Ohm is hard to get these days, one company in Oz sells them but mad money (50 dollars shipped).

So an an alternative I found this close match, its 120 Ohm +/- 20% inrush protection Thermistor, but it should compensate .. right?
These normally heat up with current flow, but they should also react to ambient changes too.
I reckon it should do the job if I add a 10 Ohm in series with it ?

https://www.ebay.co.uk/itm/Joyin-JNR...55c3:rk:1:pf:0

I suspect the calibration will be off on all ranges due to this change, but if I can fine tune the additional 10 Ohm resistor I expect I can return to the overall meter to the point where it was last adjusted.

What you fellers think?


Thanks, Ray.

[Cathode Ray]

The original has a B- Factor of 4400 whereas the Inrush substitute has a B-Factor of 3400

Will that be close enough I wonder?

Ray

[karesz*]

Ray,
you can order a more close specified NTC from Fabian, 130 Ohm, B=4600, £2,50_OK, from Malta...
Rgds, Karl

[Cathode Ray]

Very good many thanks Karl.

Ray

[Alistair D]

Ray, have a look at the attached picture, I extracted this from a '1041B write off. Currently the parallel combination reads 61 ohms and the temperature is currently around 20C so at 156 ohms for the thermistor it would seem to be in working order.

Yours for the cost of postage. I will confirm how much it is tomorrow.

Al

[Cathode Ray]

Hi Alistair,
Excellent ! How lucky am I eh.

I will PM you my address.

Thanks
Ray

[Cathode Ray]

Progress update...

Alistair kindly donated a thermistor which is in place now.

After resolving another leaky capacitor in the AC circuit and cleaning the wafer switches, the instrument still has some issues which appear be caused by a DC offset.

The DC offset appears to be about +2v and is only evident in the DC+ and DC- ranges, its quite consistent across all the DC ranges and results in Full Scale Deflection in the 1V and 300 mV ranges.
The AC and Ohms ranges seem to work just fine.

If I ground the DC probe tip, the offset dispensers. And I've made sure the DC probe is switched to V with the thumb switch.
Measuring the voltage present at the probe tip with my DMM meter reduces the offset by about 75%, so the source of the DC offset appears to be very high impedance.
There is about 100mv DC present on the tip in all ranges, except the 10v, 300v and 1000v ranges. which have virtually no tip voltage.

There are a couple of disc capacitors 0.01uf which measure ok, but I'm temped to just swat them out.

If anyone has experience in these meters I would be grateful for a steer on how to debug the DC offset.


Thanks

[Cathode Ray]

Ok progress made, offset was reduced by about 50% by replacing a 0.03uf disc cap, ESR was 85 Ohms.
So I will just replace the remaining two disc caps to be safe.

[Cathode Ray]

Replaced remaining caps and but no further improvements made.

It now reads with about +1v DC offset on the 30v to 300mV ranges
But I don't detect any offset on the 1000v to 100V ranges.
The source of the offset must be quite high impedance, my fingers across the probe tip and ground can almost kill it.

The probe tip has about +100mV DC on it in the 30v to 300mV ranges.
But virtually nothing on the tip in the 1000v to 100V ranges.
so I reckon this tip voltage is part of the problem.

Note, when connected to any voltage source it reads the voltage very accurately in all ranges.

If anyone has one of these, could you possibly test the tip voltage in DC and let me know what it reads?

Thanks

[Alistair D]

Just dug out my 1041C. The reading on the probe tip varies between 0.1 and 3 mV. On every DC range the meter always returns to zero whether the probe is grounded or not. Hope that helps

Al

[Cathode Ray]

Hi Al,
Yes that helps a lot. I know what I'm chasing now.

Cheers

[Electronpusher0]

Hi Ray, glad to see you are making progress. It seems there are several things that need repairing and I am glad it went to someone who has the time to restore it properly, I am not sure that I would ever have found the time myself, far to many projects....

Peter

[Cathode Ray]

Ah Peter, thanks for dropping by!

Yes it's going well, got almost every range to work except for DC , which still works but has a 1v offset from a high impedance source.

The list of faults/replacements wasn't too bad, mainly caps and an intermittent connection to the ground clip.

I'm working on it now, but this last issue has me stumped!

But anyway, that's half the fun.

Cheers and thanks for such a fine meter.

[Cathode Ray]

Ok so I have arrived at a possible PSU problem.

The HT+ line is stated as +230v in the manual, but the regulator can only muster +220v despite having +276v on its anode.
Likewise the HT- is stated as -120, but it's only producing -112V

Adjusting the slider RV12 from 16v to 3v has virtually no effect on the HT voltage, whereas the manual says this should be set to produce +230v HT+.

The voltage between R59 and C7 is also 220v (anode of V6b)

See diagram below.
The resistors are within tolerance and C7 was replaced.

Does this look like a soft 12BH7 valve to you guys?


I'm not sure of this is causing the dc offset problem, but it's a all I have at the moment.

Thanks

[buggies]

The circuit portion you have shown looks like part of page 2 of the pdf that wme-bill provided. His circuit had the ac voltage on the transformer secondary and the total dc on the diodes and caps. I could not quite make out the values but think the dc is 300V - so your 276V is a bit low to start with.
Other than that - and just of interest - what is the variation on the anode-grid junction as you change the preset end to end?

[Cathode Ray]

Anyone good with valve voltage regulators?

As an experiment I decided to remove a valve (12AT7) from the main meter to reduce current drawn and see if the regulator remained at 220v output.

Well it did'nt!
The output went up to 360v, with a 500V input voltage.

The main meter was still drawing some current as I had only removed the main bridge valve but left the other valve in place.

I found an articles on Series Double Triode Voltage Regulator which appear similar to the diagram attached above.
https://electronicspost.com/series-d...age-regulator/
This shows how the circuit works, and it should regulate the voltage.

The 12HB7 is an expensive valve and I don't want to replace it unless absolutely necessary.

Appreciate any input you guys may have.

[Cathode Ray]

George,
I was typing my message above when you sent your reply.
Yes the diagram from wme-bill is similar , it actually says 500v! which is similar to what i read on no load.

When I adjust the slider, the grid of V6a remains fairly constant, matching the output voltage. And the anode remains constant too.


What you reckon, perhaps I'm looking in the wrong place, could the meter be drawing too much current?