AVO VCM Mk1 - UK Vintage Radio Repair and Restoration Discussion Forum

EF80TVVALVE · 10th May 2022, 9:26 pm

Hi all,

I recently picked up a Mk1 AVO VCM (not the 2 panel type but the metal cased type). I'm only just getting around to taking a look inside. Before I go too far ahead with restoration I'd like to make sure the meter is okay. I wondered if anybody knows the impedance of the Mk1 meters? I believe the Mk3/4 uses a different meter and is little over 2kOhm but I can't find any data about the meters used on the Mk1&2 although I've read they are more robust than the later types so I presume thicker gauge wire so maybe lesser impedance? If this checks out okay I will open it up to clean out the dust and re-glue the glass, being careful not to disturb the movement and start on the main unit.

I did apply some power via a Variac, the meter seems to work as it begins to move upwards freely but the safety cut out operates so there must be an internal short causing this to trip.

Dekatron · 10th May 2022, 9:42 pm

Here's a link to a thread with the magazine Radio Bygones where my friend Euan wrote an article on refurbishing/restoring the Mk I/II: https://www.vintage-radio.net/forum/...05&postcount=5 The meter is described in the component list and text.

EF80TVVALVE · 10th May 2022, 10:22 pm

Hi Martin

Thanks for the link, checking this info against my own example suggests that my meter is fine which is promising. I'll do some investigation tomorrow just to make sure there are no obvious shorts or wiring problems. These are much more crowded and full of wires compared to the later models which obviously became much more simplified over time. If all of the wiring checks out then I will begin to check the caps, the tester is completely original and it would be no surprise if a number of these are leaky.

EF80TVVALVE · 25th May 2022, 7:39 pm

Hi all,

Having had some time to take a look at the AVO tester more closely I thought I'd make a follow up post. One of the 1/6A rectifiers had basically become a resistor so I replaced both of these with some BYW96E diodes as recommended in the RB article that Martin kindly attached.

Currently I still have the same issue of the cut-out operating. When I power up the tester the meter gives a reading up to 30mA on the 100mA scale, when pressing the reset button then the meter drops to 0. I'm going to study the schematic further and investigate more, having virtually no experience of servicing these other than previously dismantling the rotary switch of one for cleaning I'd be interested to hear if anybody has encountered a similar fault or can suggest where I should focus my attention.

10th May 2022, 9:26 pm	#1
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	AVO VCM Mk1 Hi all, I recently picked up a Mk1 AVO VCM (not the 2 panel type but the metal cased type). I'm only just getting around to taking a look inside. Before I go too far ahead with restoration I'd like to make sure the meter is okay. I wondered if anybody knows the impedance of the Mk1 meters? I believe the Mk3/4 uses a different meter and is little over 2kOhm but I can't find any data about the meters used on the Mk1&2 although I've read they are more robust than the later types so I presume thicker gauge wire so maybe lesser impedance? If this checks out okay I will open it up to clean out the dust and re-glue the glass, being careful not to disturb the movement and start on the main unit. I did apply some power via a Variac, the meter seems to work as it begins to move upwards freely but the safety cut out operates so there must be an internal short causing this to trip.

10th May 2022, 9:42 pm	#2
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: AVO VCM Mk1 Here's a link to a thread with the magazine Radio Bygones where my friend Euan wrote an article on refurbishing/restoring the Mk I/II: https://www.vintage-radio.net/forum/...05&postcount=5 The meter is described in the component list and text. __________________ Martin, Sweden

10th May 2022, 10:22 pm	#3
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: AVO VCM Mk1 Hi Martin Thanks for the link, checking this info against my own example suggests that my meter is fine which is promising. I'll do some investigation tomorrow just to make sure there are no obvious shorts or wiring problems. These are much more crowded and full of wires compared to the later models which obviously became much more simplified over time. If all of the wiring checks out then I will begin to check the caps, the tester is completely original and it would be no surprise if a number of these are leaky.

25th May 2022, 7:39 pm	#4
EF80TVVALVE Octode Join Date: Feb 2010 Location: Leicester, UK. Posts: 1,433	Re: AVO VCM Mk1 Hi all, Having had some time to take a look at the AVO tester more closely I thought I'd make a follow up post. One of the 1/6A rectifiers had basically become a resistor so I replaced both of these with some BYW96E diodes as recommended in the RB article that Martin kindly attached. Currently I still have the same issue of the cut-out operating. When I power up the tester the meter gives a reading up to 30mA on the 100mA scale, when pressing the reset button then the meter drops to 0. I'm going to study the schematic further and investigate more, having virtually no experience of servicing these other than previously dismantling the rotary switch of one for cleaning I'd be interested to hear if anybody has encountered a similar fault or can suggest where I should focus my attention.