AVO VCM 163 open circuit meter coil. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

bud allen · 28th Apr 2020, 10:53 pm

I am a fanatic about keep the original look of something, even if it is a chore and challenges everything I know. I gave up ICs and transistors about 30 year ago, so going down that avenue is not what I want to do, and I am out of date on that.

I am going to buy a meter and make transparencies of each card to see how well they match up for the arc to the jewel, and distance from zero to FS at 50ua. If it works, I will post the result, but that may be weeks. I will worry about the internal fit to the old case after I solve this front end issue.
thanks everyone.

bud allen · 14th May 2020, 10:26 pm

An update:

In the middle of the night I woke to the idea the meter was actually good. Nothing to indicate the coil was burnt or anything like that. Still no continuity from the bolts to the coil. Curious, I took stranded 30g wire and spread it open so each wire was bare. Hooked the other end to the meter, and using this baby hair wire probed parts of the coil. After some time I found continuity , I found the solder joints were dirty on the tension springs. They needed flux, the flux in the solder was not enough to break thru any patina. After they were re-soldered, the ohms read 2,000 but still not constant. There had to be other solder joints that were bad. Indeed the tiny arm which the coil wire was soldered to was also bad. After re-soldering them, the coil reads 1603. That was the best it could measure with refreshed solder joints from top to bottom, but perhaps it never measured 1500+- when made. Once back together, it hit CAL spot on, on my working AVO. It may be that some other meters were never bad, just the joints were dirty when made and failed.

Final remarks, it took over two hours to set up a jig that would hold the meter, hold the needle steady, and hold my hand steady with a EDS iron to do this soldering. It took an hour just to fix one spring and arm. It is like doing eye surgery while holding your breath, one slip and you are back to looking for a new meter. All this time you must be aware that anything in your hand can hit and bend the needle. Total time was close to 5 hours to fit the meter. And now...I need drink !

Dekatron · 14th May 2020, 10:49 pm

Nice!

I'm glad you got it working again!

I know of one more area where yours might have poor contact, at least two that I have has poor contact in that area - from the solder point on the coil the current flows to the most inner turn of the hair-spring and on my meters there is a solder joint there too, directly opposite to the side where the coil is soldered.

I can't take any good photos of this as I can't focus on this part as it is very small but if you have a loupe you should be able to see it - however trying to solder there will be problematic as the hair-spring will uncoil, when the solder melts, if you can't hold it in place with something so if you go that way be very careful and make sure that you keep it in place.

1600 Ohm is no problem for the gm (mA/V) meter, it will also work for the other meter as long as you adjust the temperature compensating components to allow for this resistance.

Dekatron · 15th May 2020, 2:00 pm

It would be interesting to see a photo of your jig if you haven't torn it down already. I might build one myself and it might help others with similar problems - it might even be possible to access that thir solder joint if the hair spring could be kept in place.

Radio Wrangler · 15th May 2020, 6:09 pm

Keep your eyes open for a good stereo microscope. It's amazing what you can do when you can really see what you're doing.

David

bud allen · 26th May 2020, 7:53 pm

Unfortunately I was so excited about being able to do this, I tore the jig set up down before taking any pix. Not sure if the movement would photo very well with out a really good camera.

The trick was making sure nothing moved, including the needle and YOU. Holding the needle still does much to stop the spring from moving too. However, when the first hot tip hits the bad solder joint the spring will likely let go. You then may need to readjust the zero balance adjust arm so the end of the spring is , once again, over top of the adjustment arm solder point. I had to do this, it sprang back about 1mm and the arm needed to be back under the spring. Moving the arm put the spring end right over the spot and then re-soldering was easy. Remember the solder needed is not much more than the size of a poppy seed and will need a touch of flux.

You must have an arm and hand rest to keep your iron tip from shaking. I wished that there had been a way to hold the actual iron heater tip instead of the tip being 5 inches from the spring. LOTS of light is a must. I even used some eye moisturizer to be sure my eye didn't dry staring at the work.

Hold you breath when you try, just breathing will shake the tip. Be very careful, if the tip catches the spring, you might just warp it or ruin it trying to get lose.

Make sure you don't accidently hit the needle with anything or it will bend. The needle is not like others I have seem where the needle is flat from the movement to the tip. This one is a cylinder until almost the end when it is flattened. I thought about this for a while before I began, and realized if it bends it might break in half or crack.

Hope this written review helps everyone.

bud allen · 26th May 2020, 7:59 pm

Quote:

Originally Posted by Dekatron

Nice!

I'm glad you got it working again!

I know of one more area where yours might have poor contact, at least two that I have has poor contact in that area - from the solder point on the coil the current flows to the most inner turn of the hair-spring and on my meters there is a solder joint there too, directly opposite to the side where the coil is soldered.

I can't take any good photos of this as I can't focus on this part as it is very small but if you have a loupe you should be able to see it - however trying to solder there will be problematic as the hair-spring will uncoil, when the solder melts, if you can't hold it in place with something so if you go that way be very careful and make sure that you keep it in place.

1600 Ohm is no problem for the gm (mA/V) meter, it will also work for the other meter as long as you adjust the temperature compensating components to allow for this resistance.

Hi Martin. Yes that also needed to be re-soldered. And if soldering the spring is not nerve-racking enough, that tiny point and that coil wire is almost invisible to you eye. This is where extreme caution is needed. If you tip hangs on the wire, you will break it tring to get lose.

I posted the rest below.

28th Apr 2020, 10:53 pm	#21
bud allen Triode Join Date: Nov 2015 Location: Key West, Florida, USA. Posts: 36	Re: AVO VCM 163 open circuit meter coil. I am a fanatic about keep the original look of something, even if it is a chore and challenges everything I know. I gave up ICs and transistors about 30 year ago, so going down that avenue is not what I want to do, and I am out of date on that. I am going to buy a meter and make transparencies of each card to see how well they match up for the arc to the jewel, and distance from zero to FS at 50ua. If it works, I will post the result, but that may be weeks. I will worry about the internal fit to the old case after I solve this front end issue. thanks everyone.

14th May 2020, 10:26 pm	#22
bud allen Triode Join Date: Nov 2015 Location: Key West, Florida, USA. Posts: 36	Re: AVO VCM 163 open circuit meter coil. An update: In the middle of the night I woke to the idea the meter was actually good. Nothing to indicate the coil was burnt or anything like that. Still no continuity from the bolts to the coil. Curious, I took stranded 30g wire and spread it open so each wire was bare. Hooked the other end to the meter, and using this baby hair wire probed parts of the coil. After some time I found continuity , I found the solder joints were dirty on the tension springs. They needed flux, the flux in the solder was not enough to break thru any patina. After they were re-soldered, the ohms read 2,000 but still not constant. There had to be other solder joints that were bad. Indeed the tiny arm which the coil wire was soldered to was also bad. After re-soldering them, the coil reads 1603. That was the best it could measure with refreshed solder joints from top to bottom, but perhaps it never measured 1500+- when made. Once back together, it hit CAL spot on, on my working AVO. It may be that some other meters were never bad, just the joints were dirty when made and failed. Final remarks, it took over two hours to set up a jig that would hold the meter, hold the needle steady, and hold my hand steady with a EDS iron to do this soldering. It took an hour just to fix one spring and arm. It is like doing eye surgery while holding your breath, one slip and you are back to looking for a new meter. All this time you must be aware that anything in your hand can hit and bend the needle. Total time was close to 5 hours to fit the meter. And now...I need drink !

14th May 2020, 10:49 pm	#23
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO VCM 163 open circuit meter coil. Nice! I'm glad you got it working again! I know of one more area where yours might have poor contact, at least two that I have has poor contact in that area - from the solder point on the coil the current flows to the most inner turn of the hair-spring and on my meters there is a solder joint there too, directly opposite to the side where the coil is soldered. I can't take any good photos of this as I can't focus on this part as it is very small but if you have a loupe you should be able to see it - however trying to solder there will be problematic as the hair-spring will uncoil, when the solder melts, if you can't hold it in place with something so if you go that way be very careful and make sure that you keep it in place. 1600 Ohm is no problem for the gm (mA/V) meter, it will also work for the other meter as long as you adjust the temperature compensating components to allow for this resistance. __________________ Martin, Sweden

15th May 2020, 2:00 pm	#24
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO VCM 163 open circuit meter coil. It would be interesting to see a photo of your jig if you haven't torn it down already. I might build one myself and it might help others with similar problems - it might even be possible to access that thir solder joint if the hair spring could be kept in place. __________________ Martin, Sweden

15th May 2020, 6:09 pm	#25
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: AVO VCM 163 open circuit meter coil. Keep your eyes open for a good stereo microscope. It's amazing what you can do when you can really see what you're doing. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

26th May 2020, 7:53 pm	#26
bud allen Triode Join Date: Nov 2015 Location: Key West, Florida, USA. Posts: 36	Re: AVO VCM 163 open circuit meter coil. Unfortunately I was so excited about being able to do this, I tore the jig set up down before taking any pix. Not sure if the movement would photo very well with out a really good camera. The trick was making sure nothing moved, including the needle and YOU. Holding the needle still does much to stop the spring from moving too. However, when the first hot tip hits the bad solder joint the spring will likely let go. You then may need to readjust the zero balance adjust arm so the end of the spring is , once again, over top of the adjustment arm solder point. I had to do this, it sprang back about 1mm and the arm needed to be back under the spring. Moving the arm put the spring end right over the spot and then re-soldering was easy. Remember the solder needed is not much more than the size of a poppy seed and will need a touch of flux. You must have an arm and hand rest to keep your iron tip from shaking. I wished that there had been a way to hold the actual iron heater tip instead of the tip being 5 inches from the spring. LOTS of light is a must. I even used some eye moisturizer to be sure my eye didn't dry staring at the work. Hold you breath when you try, just breathing will shake the tip. Be very careful, if the tip catches the spring, you might just warp it or ruin it trying to get lose. Make sure you don't accidently hit the needle with anything or it will bend. The needle is not like others I have seem where the needle is flat from the movement to the tip. This one is a cylinder until almost the end when it is flattened. I thought about this for a while before I began, and realized if it bends it might break in half or crack. Hope this written review helps everyone.