TMK Multimeter model TW-20S - UK Vintage Radio Repair and Restoration Discussion Forum

John Harwood · 12th Oct 2013, 4:03 pm

I've just picked up a TMK model TW-20S. It 'works' but the needle is not swinging full scale. What I need is a circuit diagram.
Here's something odd... The two AA batteries are wired in parallel, so the meter is working on a nominal 1.5 volts. Has some oik played with it in the past, and re-soldered the battery holder the wrong way?
If anyone out there has a schematic that they could share, I'd be eternally grateful. Also, any knowledge or suggestions would improve my understanding.

Phil G4SPZ · 14th Oct 2013, 10:27 pm

Can't help with a circuit diagram I'm afraid, however if what you mean is "the needle is not swinging to full scale on the ohms range" then that does indicate that the two batteries should be in series, giving nominally 3 volts, rather than in parallel as the oik has left it.

Are you able to test the meter on various voltage and current ranges to see whether the needle can actually swing to full scale, and how accurately it reads?

John Harwood · 16th Oct 2013, 7:50 pm

Phil G4SPZ... Thanks for your prompt reply. Since posting my request, I have been tinkering with the TMK meter, and tried the cells in series... The needle swings way too far and still doesn't return to zero.
So I took the thing entirely to pieces and discovered that the meter movement hair spring was not firmly attached. A bit of trial and error, and a 4.5mm spanner, (I knew that spanner would come in useful one day) and the retaining nut was nipped up at exactly the right place to give the needle a chance of zeroing on the adjusting cam.
The battery is now back to 1.5v in parallel, and everything works fine. Maybe I posted for help too early... I should have had more confidence in my ability to get to the root of the matter and perform a fix.
I'm now going to tackle fixing the economy... Watch this space.

Phil G4SPZ · 16th Oct 2013, 8:45 pm

Well done John, and it's a bold man who tackles meter movements and hairsprings, but you seem to have affected a cure and kept yet another vintage item out of landfill

The economy may take you a little longer, but post a question here if you get stuck. In the meantime, we'll keep watching the news...

Phil

glowinganode · 16th Oct 2013, 11:38 pm

Quote:

Originally Posted by John Harwood

Here's something odd... The two AA batteries are wired in parallel,

I'm sure I've come across this before, I think it's a way of providing the current required for a low resistance range, whilst maintaing a slim physical profile.

Rob.

12th Oct 2013, 4:03 pm	#1
John Harwood Retired Dormant Member Join Date: Oct 2011 Location: Chesterfield, Derbyshire, UK. Posts: 12	TMK Multimeter model TW-20S I've just picked up a TMK model TW-20S. It 'works' but the needle is not swinging full scale. What I need is a circuit diagram. Here's something odd... The two AA batteries are wired in parallel, so the meter is working on a nominal 1.5 volts. Has some oik played with it in the past, and re-soldered the battery holder the wrong way? If anyone out there has a schematic that they could share, I'd be eternally grateful. Also, any knowledge or suggestions would improve my understanding.

14th Oct 2013, 10:27 pm	#2
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: TMK Multimeter model TW-20S Can't help with a circuit diagram I'm afraid, however if what you mean is "the needle is not swinging to full scale on the ohms range" then that does indicate that the two batteries should be in series, giving nominally 3 volts, rather than in parallel as the oik has left it. Are you able to test the meter on various voltage and current ranges to see whether the needle can actually swing to full scale, and how accurately it reads? __________________ Phil Optimist [n]: One who is not in possession of the full facts

16th Oct 2013, 7:50 pm	#3
John Harwood Retired Dormant Member Join Date: Oct 2011 Location: Chesterfield, Derbyshire, UK. Posts: 12	Re: TMK Multimeter model TW-20S Phil G4SPZ... Thanks for your prompt reply. Since posting my request, I have been tinkering with the TMK meter, and tried the cells in series... The needle swings way too far and still doesn't return to zero. So I took the thing entirely to pieces and discovered that the meter movement hair spring was not firmly attached. A bit of trial and error, and a 4.5mm spanner, (I knew that spanner would come in useful one day) and the retaining nut was nipped up at exactly the right place to give the needle a chance of zeroing on the adjusting cam. The battery is now back to 1.5v in parallel, and everything works fine. Maybe I posted for help too early... I should have had more confidence in my ability to get to the root of the matter and perform a fix. I'm now going to tackle fixing the economy... Watch this space.

16th Oct 2013, 8:45 pm	#4
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: TMK Multimeter model TW-20S Well done John, and it's a bold man who tackles meter movements and hairsprings, but you seem to have affected a cure and kept yet another vintage item out of landfill The economy may take you a little longer, but post a question here if you get stuck. In the meantime, we'll keep watching the news... Phil __________________ Phil Optimist [n]: One who is not in possession of the full facts