UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Marconi Valve Voltmeter TF1041C

Cathode Ray · 28th Jan 2019, 7:55 pm

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.

28th Jan 2019, 7:55 pm	#4
Cathode Ray Pentode Join Date: Dec 2018 Location: County Meath, Ireland Posts: 242	Re: Marconi Valve Voltmeter TF1041C - repair thread 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. Attached Thumbnails Last edited by Cathode Ray; 28th Jan 2019 at 8:07 pm.