Avo 8 & 9 mk iv - UK Vintage Radio Repair and Restoration Discussion Forum

dfriedman · 2nd Oct 2019, 8:02 pm

Hi to all
I'm in Toronto, Canada. I got in an online auction a bunch of valves - which tested good on my AVO VCM III. Also got 3x AVO 8's in Peterborough, Ontario, Canada.
That was few months ago - I finally got to check the 1st AVO. On the back it says the above title.Problems:
DC - Reads 1.33 the actual value.
AC - reads 117 instead of my trusted Amprobe AM91 DMM = 123v(true RMS). I think I've some unused AVO rect bridges.
Ohms: needle goes backward & when pressing Reverse, it shoots over range on all 3 settings.
Any hints aside of what's mentioned in the repair manual? I've to get back to the meter. I've not yet checked the DC or AC currents.
- Solution about the 15v battery. I soldered 5x CR2430 = 16.3v. These lend themselves to solder, but any of 3v from China(off eBay) do not take to the solder!
I've cal the VCM III myself. Currently also working on a VCM I = 2 panel AVO valve tester.
ThanX in advance, Dany, Toronto

pmmunro · 3rd Oct 2019, 9:00 am

The problem you describe sounds like an open circuit in the current shunt chain (universla shunt). This is designed to be permanently connected across the movement and to draw 12.5 microamperes of the 50 needed for full scale deflection. It greatly affects the damping of the movement.

If you don't have the service manual, send me a private message.

PMM.

2nd Oct 2019, 8:02 pm	#1
dfriedman Triode Join Date: Mar 2016 Location: Toronto, Ontario, Canada Posts: 18	Avo 8 & 9 mk iv Hi to all I'm in Toronto, Canada. I got in an online auction a bunch of valves - which tested good on my AVO VCM III. Also got 3x AVO 8's in Peterborough, Ontario, Canada. That was few months ago - I finally got to check the 1st AVO. On the back it says the above title.Problems: DC - Reads 1.33 the actual value. AC - reads 117 instead of my trusted Amprobe AM91 DMM = 123v(true RMS). I think I've some unused AVO rect bridges. Ohms: needle goes backward & when pressing Reverse, it shoots over range on all 3 settings. Any hints aside of what's mentioned in the repair manual? I've to get back to the meter. I've not yet checked the DC or AC currents. - Solution about the 15v battery. I soldered 5x CR2430 = 16.3v. These lend themselves to solder, but any of 3v from China(off eBay) do not take to the solder! I've cal the VCM III myself. Currently also working on a VCM I = 2 panel AVO valve tester. ThanX in advance, Dany, Toronto

3rd Oct 2019, 9:00 am	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avo 8 & 9 mk iv The problem you describe sounds like an open circuit in the current shunt chain (universla shunt). This is designed to be permanently connected across the movement and to draw 12.5 microamperes of the 50 needed for full scale deflection. It greatly affects the damping of the movement. If you don't have the service manual, send me a private message. PMM.