AVO valve tester MK.2 Mains setup problems - UK Vintage Radio Repair and Restoration Discussion Forum

high_vacuum_house · 8th Jan 2018, 7:02 pm

Good afternoon,
I have recently bought a MK.2 AVO valve characteristic tester. I am having difficulty in getting the mains setup working.
I have set it to check C and to electrode leakage ~ but on using the set ~ control I cannot get the needle into the red line. It is roughly 3/4" over this. The mains links are in the 240-250V position. I can bring it down to the line using a variac though the mains is now at

I think the meter circuit is to blame as bringing the mains down to the meter red line the heater set at 6V reads just over 4 volts AC. Using the variac to increase the mains voltage until the correct heater voltage is reached the mains is back at 230V though the needle is then way over the line.

What is the correct way of going about sorting out this issue without wrongly adjusting things and making the calibration for another part of the tester worse. I notice there are 2 big waxies inside each side of the meter and another capacitor on top of a transformer. Should I replace these at this stage.

Many thanks,
Christopher Capener

HamishBoxer · 8th Jan 2018, 7:56 pm

Wax caps of this age are certainly suspect.

Ed_Dinning · 8th Jan 2018, 8:51 pm

Hi Chris, there is a huge thread on this in the forum. Basically there are quite a few caps that can go leaky, but there are also high stab tight tolerance resistors that may have drifted. All this in a system that is meant to keep the tester "standardised".
A bit of a test and change job I'm afraid, but they are good testers when sorted.

Ed

G1RAO keith · 8th Jan 2018, 9:59 pm

Hi Chris
Change the wax caps by all means but there is a preset resistor marked (S) on the right hand side of the tester on the transformer assembly that is across the meter and is used as part of the set up procedure this may need adjustment along with the other two one marked GM and the other VG ,there should be some info in the threads mentioned by Ed.regards Keith.

Kevin Hoyland · 9th Jan 2018, 10:41 am

Morning Chris.
After last winter i went in to my work shop and had the same problem as you have with my Avo mk2 Valve Tester. To get it back working right I Had to replace the 16K9 Selenium Rectifer and KG1 with Philips soft -recovery Diodes type (BYW96E).The 16K9 had a split in it at one end. Then I cleaned the pins on the EA50 .At the side of the Diodes is a 125K Resistor this is for adjusting the meter my resistor was way out so I had to replace it. Then I replaced all the resistors on the roller switch. I then replaced all the 8uf and 0.05 Capacitors With new ones. Then cleaned all switches with Deoxit D5 cleaner After this the mk2 was back working as good as new. I then did the Avo Calibration on it.
Kevin

high_vacuum_house · 9th Jan 2018, 5:39 pm

Many thanks for your reply,
I have ordered some soft recovery diodes and have some 0.05uF capacitors here. I will go through the resistors to see if any are out of spec.
Christopher Capener

8th Jan 2018, 7:02 pm	#1
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,936	AVO valve tester MK.2 Mains setup problems Good afternoon, I have recently bought a MK.2 AVO valve characteristic tester. I am having difficulty in getting the mains setup working. I have set it to check C and to electrode leakage ~ but on using the set ~ control I cannot get the needle into the red line. It is roughly 3/4" over this. The mains links are in the 240-250V position. I can bring it down to the line using a variac though the mains is now at I think the meter circuit is to blame as bringing the mains down to the meter red line the heater set at 6V reads just over 4 volts AC. Using the variac to increase the mains voltage until the correct heater voltage is reached the mains is back at 230V though the needle is then way over the line. What is the correct way of going about sorting out this issue without wrongly adjusting things and making the calibration for another part of the tester worse. I notice there are 2 big waxies inside each side of the meter and another capacitor on top of a transformer. Should I replace these at this stage. Many thanks, Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

8th Jan 2018, 7:56 pm	#2
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: AVO valve tester MK.2 Mains setup problems Wax caps of this age are certainly suspect. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

8th Jan 2018, 8:51 pm	#3
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: AVO valve tester MK.2 Mains setup problems Hi Chris, there is a huge thread on this in the forum. Basically there are quite a few caps that can go leaky, but there are also high stab tight tolerance resistors that may have drifted. All this in a system that is meant to keep the tester "standardised". A bit of a test and change job I'm afraid, but they are good testers when sorted. Ed

8th Jan 2018, 9:59 pm	#4
G1RAO keith Pentode Join Date: Mar 2008 Location: Bury, Lancashire, UK. Posts: 138	Re: AVO valve tester MK.2 Mains setup problems Hi Chris Change the wax caps by all means but there is a preset resistor marked (S) on the right hand side of the tester on the transformer assembly that is across the meter and is used as part of the set up procedure this may need adjustment along with the other two one marked GM and the other VG ,there should be some info in the threads mentioned by Ed.regards Keith.

9th Jan 2018, 10:41 am	#5
Kevin Hoyland Heptode Join Date: Feb 2009 Location: Rotherham, South Yorkshire, UK. Posts: 559	Re: AVO valve tester MK.2 Mains setup problems Morning Chris. After last winter i went in to my work shop and had the same problem as you have with my Avo mk2 Valve Tester. To get it back working right I Had to replace the 16K9 Selenium Rectifer and KG1 with Philips soft -recovery Diodes type (BYW96E).The 16K9 had a split in it at one end. Then I cleaned the pins on the EA50 .At the side of the Diodes is a 125K Resistor this is for adjusting the meter my resistor was way out so I had to replace it. Then I replaced all the resistors on the roller switch. I then replaced all the 8uf and 0.05 Capacitors With new ones. Then cleaned all switches with Deoxit D5 cleaner After this the mk2 was back working as good as new. I then did the Avo Calibration on it. Kevin Attached Thumbnails

9th Jan 2018, 5:39 pm	#6
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,936	Re: AVO valve tester MK.2 Mains setup problems Many thanks for your reply, I have ordered some soft recovery diodes and have some 0.05uF capacitors here. I will go through the resistors to see if any are out of spec. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television