UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - New movement fitted into AVO CT160 meter case

retailer · 12th Apr 2022, 2:14 pm

I used what I think is the Crowthorne tubes circuit - it isn't anything special and is quite commonly seen in opamp application text books - it is much the same as the one posted by Craig, he covers the operation of it nicely - in place of R2 I've used a 1K trim pot - R1 in my case is 3250 ohms so I went through my stock of resistors and found a 220k and 3k3 that my Fluke tells me makes 3249 ohms when in parallel. My movement is 200uA fsd at 750 ohms - out of the dozen or so meters I have it is the only one that fitted inside the Avo case and also has a pointer long enough to cover the Avo scale
Even though I have mine made and working I'm going to make a second one that fits exactly on the rear of the original Avo case, this will do away with a PCB hanging out to the side - it will give me some more experience with Fusion360 and CNC PCB board milling, I'm still on the steep part of the learning curve with these.

12th Apr 2022, 2:14 pm	#5
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: New movement fitted into AVO CT160 meter case I used what I think is the Crowthorne tubes circuit - it isn't anything special and is quite commonly seen in opamp application text books - it is much the same as the one posted by Craig, he covers the operation of it nicely - in place of R2 I've used a 1K trim pot - R1 in my case is 3250 ohms so I went through my stock of resistors and found a 220k and 3k3 that my Fluke tells me makes 3249 ohms when in parallel. My movement is 200uA fsd at 750 ohms - out of the dozen or so meters I have it is the only one that fitted inside the Avo case and also has a pointer long enough to cover the Avo scale Even though I have mine made and working I'm going to make a second one that fits exactly on the rear of the original Avo case, this will do away with a PCB hanging out to the side - it will give me some more experience with Fusion360 and CNC PCB board milling, I'm still on the steep part of the learning curve with these.