AVO VCM MK2 Complete Meter replacement - UK Vintage Radio Repair and Restoration Discussion Forum

Matthew kane · 2nd Dec 2020, 10:34 am

Hi all,

Is there any donor meter for the VCM MK2 tester that can be modified to match the original meter specs such as the Simpson 29t used for the later VCM MK3-160 testers?

I have a second MK2 which has its complete internal meter assembly removed except the bakelite housing and scale.

chriswood1900 · 2nd Dec 2020, 11:31 am

If you still have the Bakelite housing I have successfully transferred the guts of a cheap Chinese meter into the Avo housing to preserve the appearance. If you can't get a usable sensitivity use an op-amp to change it.

retailer · 3rd Dec 2020, 12:24 am

Chriswood's idea does work - I got my MKII working by fitting a replacement meter I had in my spares drawer, as long as you can bring it to Avo spec with a combination of shunt and/or swamp resistors, a new scale was made with Inkscape, black perspex was cut to fill in the size difference between the meters, and a LED was set into the perspex and connected to the backlight sockets to mimic the back light which could no longer be seen due to the different meter.

Later on I decided I wanted to keep the look of the original meter, that at some time in the past had been fitted with replacement movement which was not only out of spec but the hand made bracket supporting the scale was off-centre giving nonlinear readings - a new bracket and an opamp meter driver restored the original look of the tester, this is not such a bad idea as the opamp meter driver can be configured to limit current flow to the movement to provide a measure of protection.

joebog1 · 3rd Dec 2020, 1:43 am

please remove that post mods!! the meter is available though.

Joe

Radio Wrangler · 3rd Dec 2020, 3:58 am

Post deleted, but I presume that the cost is still postage only?

David

Matthew kane · 3rd Dec 2020, 9:11 am

Quote:

Originally Posted by Radio Wrangler

Post deleted, but I presume that the cost is still postage only?

David

Sorry was this a question for me?

I've seen some conflicting information about the MK2 meter but just to confirm it is a 100ohm approx 440uA FSD meter? I have a 1mA 100ohm meter slightly smaller than the AVO which I may consider using that opamp driver to shunt the current down to approx 440-460uA.

Station X · 3rd Dec 2020, 10:09 am

I think perhaps Joe meant to post in another thread.

retailer · 5th Dec 2020, 11:53 pm

I believe the original MKII meter is 440uA 100ohms, that is what I worked on when I made an op amp driver for my MKII.

Matthew kane · 9th Dec 2020, 1:49 pm

Quote:

Originally Posted by retailer

I believe the original MKII meter is 440uA 100ohms, that is what I worked on when I made an op amp driver for my MKII.

What was the base meter you worked on with the opamp driver?

retailer · 10th Dec 2020, 12:31 am

The meter that came with my VCM had been worked on sometime in the past - a sticker placed on the back was from a meter/instrument service company - 440uA 110ohms had been written in pencil on the sticker - the internal movement was not the original Avo movement but a replacement fitted with an internal shunt of 1.2k to try to bring it to spec. While the FSD of the meter was 440uA the resistance was closer to 185Ohms, as the AVO meter shunts/multpliers had been calculated for a meter resistance of 100Ohms so the meter would never give correct results. To compound this, the person that fitted the movement had made a bracket to support the plastic scale, the bracket was poorly made as the scale was off centre - slightly offset to the right making the readings non linear. I did some calculations and eventually worked out the movement could not be bought into spec with shunts/multipliers so I made up the opamp driver using the 440uA movement, I did have other donor movements I could have fitted, but that meant time spent making brackets etc so I just went ahead with the 440uA movement. I theory just about any movement would work as all of the heavy lifting is done with the opamp driver - you just need 100 ohms resistance across the input (this is what the AVO wants to see) and then adjust the opamp gain so the movement reads full scale when the 100ohms has 440uA through it.

2nd Dec 2020, 10:34 am	#1
Matthew kane Tetrode Join Date: May 2019 Location: Melbourne, Victoria, Australia. Posts: 66	AVO VCM MK2 Complete Meter replacement Hi all, Is there any donor meter for the VCM MK2 tester that can be modified to match the original meter specs such as the Simpson 29t used for the later VCM MK3-160 testers? I have a second MK2 which has its complete internal meter assembly removed except the bakelite housing and scale.

2nd Dec 2020, 11:31 am	#2
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,481	Re: AVO VCM MK2 Complete Meter replacement If you still have the Bakelite housing I have successfully transferred the guts of a cheap Chinese meter into the Avo housing to preserve the appearance. If you can't get a usable sensitivity use an op-amp to change it. __________________ Chris Wood BVWS Member

3rd Dec 2020, 12:24 am	#3
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: AVO VCM MK2 Complete Meter replacement Chriswood's idea does work - I got my MKII working by fitting a replacement meter I had in my spares drawer, as long as you can bring it to Avo spec with a combination of shunt and/or swamp resistors, a new scale was made with Inkscape, black perspex was cut to fill in the size difference between the meters, and a LED was set into the perspex and connected to the backlight sockets to mimic the back light which could no longer be seen due to the different meter. Later on I decided I wanted to keep the look of the original meter, that at some time in the past had been fitted with replacement movement which was not only out of spec but the hand made bracket supporting the scale was off-centre giving nonlinear readings - a new bracket and an opamp meter driver restored the original look of the tester, this is not such a bad idea as the opamp meter driver can be configured to limit current flow to the movement to provide a measure of protection. Attached Thumbnails

3rd Dec 2020, 1:43 am	#4
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: AVO VCM MK2 Complete Meter replacement please remove that post mods!! the meter is available though. Joe

3rd Dec 2020, 3:58 am	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,871	Re: AVO VCM MK2 Complete Meter replacement Post deleted, but I presume that the cost is still postage only? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Dec 2020, 10:09 am	#7
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: AVO VCM MK2 Complete Meter replacement I think perhaps Joe meant to post in another thread. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

5th Dec 2020, 11:53 pm	#8
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: AVO VCM MK2 Complete Meter replacement I believe the original MKII meter is 440uA 100ohms, that is what I worked on when I made an op amp driver for my MKII.

10th Dec 2020, 12:31 am	#10
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: AVO VCM MK2 Complete Meter replacement The meter that came with my VCM had been worked on sometime in the past - a sticker placed on the back was from a meter/instrument service company - 440uA 110ohms had been written in pencil on the sticker - the internal movement was not the original Avo movement but a replacement fitted with an internal shunt of 1.2k to try to bring it to spec. While the FSD of the meter was 440uA the resistance was closer to 185Ohms, as the AVO meter shunts/multpliers had been calculated for a meter resistance of 100Ohms so the meter would never give correct results. To compound this, the person that fitted the movement had made a bracket to support the plastic scale, the bracket was poorly made as the scale was off centre - slightly offset to the right making the readings non linear. I did some calculations and eventually worked out the movement could not be bought into spec with shunts/multipliers so I made up the opamp driver using the 440uA movement, I did have other donor movements I could have fitted, but that meant time spent making brackets etc so I just went ahead with the 440uA movement. I theory just about any movement would work as all of the heavy lifting is done with the opamp driver - you just need 100 ohms resistance across the input (this is what the AVO wants to see) and then adjust the opamp gain so the movement reads full scale when the 100ohms has 440uA through it.