Homebrew VTVM (Under Construction)
I regret never having built one of Heathkits VTVM kits. I've used DVM's for many years and love their precision. However when you are adjusting a circuit you need to see the rate of change. A DVM can't do that well, you need an analog meter. I've used solid state versions of the VTVM, and they have their advantages, such as being isolated from the power mains. The VTVM is still a classic piece of test equipment, and I wanted one on my test bench. Now that Heathkit is gone (but not forgotten), I decided to create my own. I found two old EICO VTVM's for a buck a piece at a ham flea market. About the only thing useful on them were the meter movements, someone had modified and destroyed the electronics. One of the meter movements was in good shape except for the scale which was badly faded. I was able to create a new scale from the old one with the aid of a photocopy machine, liquid paper, and a sharp pencil. The RCA receiving tube manual has had the circuit for a VTVM since the RC-23 release (or earlier). I decided to build a VTVM based on this circuit. I have a large number of salavged and surplus rotary switches in my junkbox, so I pulled a 'Frankenstein' and created the two required switches from the dead bodies of others. I found some nice 10 turn 20k pots at a hamfest flea market for the 'zero' and 'ohms adjust' controls on the front panel. The multiplier resistors are series-parallel combinations of resistors found in my junkbox. Most are 1%, some are .1%, 2% or 3%. The function and range switch knobs came from one of the junked EICO vtvm's. The box for the unit is a repainted TenTec cabinet. I havn't finished this project yet, all the metal work and part scrounging is finished and all that's left is the wiring. The pictures below will tell it all.
Front view
Inside view
Sub-chassis assembly, wired.