Fred- I love this stuff! One observation I made was the first time you added 200 ųA the needle only traveled to 20 on the top Ohms scale- Once you added more juice or 306 ųA the needle now rested at 35 Ohms at the same 200 ųA! I would try sending 500 ųA through the meter and see where the needle rests. 😃👍
The meter is a 200uA movement to read full scale. I don't know why it isn't reading full scale at 200 uA. Could I have damaged it during the balancing? I have no idea.
Well that's my feeling as well. If so in my opinion the meter is no longer any good. Thanks for jumping in, I appreciate your comment as that's also my opinion. Stay tuned, I will have more to come.
Some meter movements make electrical connections to one or both sides of the coil through the meter bearings and the hardware that mounts the bearing. I've seen problems like this in old meters if the bearings or mounts were corroded or coated with some kind of film. This adds a series resistance. It's usually a little more erratic than you are showing it to be. It's interesting that you said you had to loosen the bearing to get it working at all. I wonder if carefully trying to measure the resistance (by indirect means like measuring the voltage drop accross a series resistor) of the movement would tell you anything? Hope this is a new lead to follow...
Hi Mario, thanks for the info. The movement electrical connection is as you stated. I may try to get the movement out to take a better look at it. At this point it's all about a learning experience since I have a subscriber sending me a known good meter. Thanks for your thoughts and expertise, hope you subscribe and comment often.
First guess is that the magnet has weakened. But then as someone pointed out, when you went back to 200uA, the meter was reading higher than when you approached from below. Was that just due to stiction (taping the face or blowing again would show if that was the case). Or is there something else going on? I assume that the meter was disconnected from all circuitry when you were doing this. There must be a similar clock spring on the back side of the movement. These usually can be adjusted as well to center the front adjustment. The combined spring rate, number of coil turns and magnetic field are what determine the meter scale factor.
Thank you for your comment and yes the meter was disconnected. There is an adjustment on the back unfortunately there is no access unless and I would need to melt a hole in the clear plastic case to get access. I am considering doing just that. In additional testing I've found the vtvm when set on any of the voltage scales could not repeat the reading when the power was turned off then back on again, sometimes as much as 5 volts. My next move will be to attempt access to the rear pivot but at this point I feel the meter winding is shorted/damaged and unrepairable. Stay tuned for the next video
I would like to see a schematic. The value of vtvm’s is their very high input impedance. The tubes may be good but it is possible that other components such as resistors and capacitors have changed values over the years, particularly in bias circuitry.. . have seen this happen before on older gear. Good luck!!❤
Thank you, I will look into the components on the board. Not sure how to get you a copy of the schematic but they are available online. In the video here the meter is disconnected from the vtvm circuit while checking the full scale reading.
@@ko2fjb I don’t know much about these meters. Used them many years ago before high input impedance digital meters. Will the indicator do full scale deflection with more than 200 UA?? The indicator will do full scale deflection at or above a certain current level and I would think that the circuitry before it would require scaling to operate and be accurate. Wish I was in the shop with you to help chase this problem down.
@iaov yes, very interesting problem. I think the movement is damaged internally as further testing showed the meter would not repeat a reading if the voltage was turned off then on again, on the same scale.
it's a dellicate mechanism for sure but I doubt you have damaged it Fred. If the permanent magnet has become weaker wouldn't that make the pointer move further across the scale for the same 200 ua of current. A guass meter to measure the strength of the magnet compared to a known good one maybe, I don't know.
I just subscribed to your channel today. I watched your earlier videos on this unit. In one of the videos you mentioned the pointer wasn’t moving freely so you tweaked the pivot screw and freed up the pointer. My understanding is that some movements have jewels on each side like a watch. Some don’t. Regardless if you do or don’t I would say your pivots need a tiny bit of lubrication. I have ruined plenty of movements by simply touching the adjustment screw you touched. That kind of work is microscope work and I’m not well suited for it.
Hi Rob, thanks for subscribing and commenting. You could be right about damaging it when I tweaked the pivot screw. That being said it's about 70 years old and from the looks of it when I got it had a hard life. I don't like damaging anything but fortunately it was free so if I have to put a little into it to get if fixed, I'm ok. I'll be rummaging around at the next hamfest for a 200 uA meter movement.
Oh I’m not blaming you. Please understand that. I have been in the same situation and I have messed a few up. Let me check my shelf in the garage shop. At one point I had a dozen of the V7A VTVMs. I’m a bit of a hoarder as I have probably 12 Simpson 260s and half dozen Triplett 630s. Let me have a quick look see.
So I have 6 on the shelf with meters and a few without. You’re welcome to one? If you give me a few days I can test a movement on the bench here at 200uA.
Very plausible. Bent pivot and/or the jewel no longer in line with the pivot. That also explains the "imbalance" of the needle and the fact that loosening the jewel gives better needle travel. Think of it as if the needle has to go "uphill". It probably took a knock on the head.
@@ivojanssens221 Thanks, that kind of makes sense. I will be taking the meter completely apart in a future video to get a closer look at the movement itself. A viewer is sending me a replacement meter so I can complete the restoration
Fred- I love this stuff! One observation I made was the first time you added 200 ųA the needle only traveled to 20 on the top Ohms scale- Once you added more juice or 306 ųA the needle now rested at 35 Ohms at the same 200 ųA! I would try sending 500 ųA through the meter and see where the needle rests. 😃👍
The meter is a 200uA movement to read full scale. I don't know why it isn't reading full scale at 200 uA. Could I have damaged it during the balancing? I have no idea.
Perhaps the meter has a few shorted windings acting as a shunt.
Well that's my feeling as well. If so in my opinion the meter is no longer any good.
Thanks for jumping in, I appreciate your comment as that's also my opinion.
Stay tuned, I will have more to come.
Some meter movements make electrical connections to one or both sides of the coil through the meter bearings and the hardware that mounts the bearing. I've seen problems like this in old meters if the bearings or mounts were corroded or coated with some kind of film. This adds a series resistance. It's usually a little more erratic than you are showing it to be. It's interesting that you said you had to loosen the bearing to get it working at all.
I wonder if carefully trying to measure the resistance (by indirect means like measuring the voltage drop accross a series resistor) of the movement would tell you anything?
Hope this is a new lead to follow...
Hi Mario, thanks for the info. The movement electrical connection is as you stated. I may try to get the movement out to take a better look at it. At this point it's all about a learning experience since I have a subscriber sending me a known good meter.
Thanks for your thoughts and expertise, hope you subscribe and comment often.
All that comes to mind is "weakened" magnet - provided that 200uA is actually real 200uA. That the springs would become "stronger" is quite unreal.
First guess is that the magnet has weakened. But then as someone pointed out, when you went back to 200uA, the meter was reading higher than when you approached from below. Was that just due to stiction (taping the face or blowing again would show if that was the case). Or is there something else going on? I assume that the meter was disconnected from all circuitry when you were doing this. There must be a similar clock spring on the back side of the movement. These usually can be adjusted as well to center the front adjustment. The combined spring rate, number of coil turns and magnetic field are what determine the meter scale factor.
Thank you for your comment and yes the meter was disconnected. There is an adjustment on the back unfortunately there is no access unless and I would need to melt a hole in the clear plastic case to get access. I am considering doing just that.
In additional testing I've found the vtvm when set on any of the voltage scales could not repeat the reading when the power was turned off then back on again, sometimes as much as 5 volts.
My next move will be to attempt access to the rear pivot but at this point I feel the meter winding is shorted/damaged and unrepairable.
Stay tuned for the next video
I would like to see a schematic. The value of vtvm’s is their very high input impedance. The tubes may be good but it is possible that other components such as resistors and capacitors have changed values over the years, particularly in bias circuitry.. . have seen this happen before on older gear. Good luck!!❤
Thank you, I will look into the components on the board. Not sure how to get you a copy of the schematic but they are available online.
In the video here the meter is disconnected from the vtvm circuit while checking the full scale reading.
@@ko2fjb I don’t know much about these meters. Used them many years ago before high input impedance digital meters. Will the indicator do full scale deflection with more than 200 UA?? The indicator will do full scale deflection at or above a certain current level and I would think that the circuitry before it would require scaling to operate and be accurate. Wish I was in the shop with you to help chase this problem down.
@iaov yes, very interesting problem. I think the movement is damaged internally as further testing showed the meter would not repeat a reading if the voltage was turned off then on again, on the same scale.
it's a dellicate mechanism for sure but I doubt you have damaged it Fred. If the permanent magnet has become weaker wouldn't that make the pointer move further across the scale for the same 200 ua of current. A guass meter to measure the strength of the magnet compared to a known good one maybe, I don't know.
I just subscribed to your channel today. I watched your earlier videos on this unit. In one of the videos you mentioned the pointer wasn’t moving freely so you tweaked the pivot screw and freed up the pointer. My understanding is that some movements have jewels on each side like a watch. Some don’t. Regardless if you do or don’t I would say your pivots need a tiny bit of lubrication. I have ruined plenty of movements by simply touching the adjustment screw you touched. That kind of work is microscope work and I’m not well suited for it.
Hi Rob, thanks for subscribing and commenting. You could be right about damaging it when I tweaked the pivot screw. That being said it's about 70 years old and from the looks of it when I got it had a hard life. I don't like damaging anything but fortunately it was free so if I have to put a little into it to get if fixed, I'm ok. I'll be rummaging around at the next hamfest for a 200 uA meter movement.
Oh I’m not blaming you. Please understand that. I have been in the same situation and I have messed a few up. Let me check my shelf in the garage shop. At one point I had a dozen of the V7A VTVMs. I’m a bit of a hoarder as I have probably 12 Simpson 260s and half dozen Triplett 630s. Let me have a quick look see.
@robbo6460 I didn't take it that you were blaming me but you're probably right.
Hey, didn't think you were but you're probably right.
So I have 6 on the shelf with meters and a few without. You’re welcome to one? If you give me a few days I can test a movement on the bench here at 200uA.
Bent spindle ? Can you back off the bearings a little and see if it moves better ?
Thanks, I'll give that a try.
Very plausible.
Bent pivot and/or the jewel no longer in line with the pivot. That also explains the "imbalance" of the needle and the fact that loosening the jewel gives better needle travel.
Think of it as if the needle has to go "uphill".
It probably took a knock on the head.
@@ivojanssens221 Thanks, that kind of makes sense. I will be taking the meter completely apart in a future video to get a closer look at the movement itself.
A viewer is sending me a replacement meter so I can complete the restoration
Did it do that before the meter balance?
I'm not sure as the zero pot was bad and the needle was sticking so i loosened (slightly) the screw to free it up.
Static charge.try gently wiping the meter face with a dryer sheet. You could try a different 12au7 that has worked for me in the past,best of luck.
Thank you, unfortunately I tried all that and it didn't help.