VTVM Circuit Description - DETAILED

Be prepared. Bring food, water and an overnight bag:
In this video I walk you through the theory of operation of a typical Heathkit or Kyoritsu VTVM. The circuits are very similar between brands with only minor differences. Sometimes the meter drive circuit is found at the Plates of the 12AX7 or whatever tubes are used, but otherwise the idea is the same throughout.

[@kj4daq]

Thank you. You explained well. I have two Heathkit VTVMs Models IM-18 and V-7A purchased at a Hamfeast. I will work on them as they have issues.

[@DerekHerbst747]

@@kj4daq if you replace the capacitors and recalibrate it will probably be enough. Good luck 🤞

[@electronicsoldandnew]

Lekker! I always get lost when I look at meter circuits due to the switches everywhere. Great job. 👍

[@billcrowell5096]

Yes, Derek's tracing of the paths was very helpful. The schematic was clear which helps a lot.

[@billcrowell5096]

This is a most excellent presentation. I've sent the link to a number of fellow ham radio operators. I have a VTVM that I use to calibrate voltage-controlled oscillators when I repair radios as a digital meter cannot integrate quickly enough at radio frequencies. I prefer the VTVM to an oscilloscope for this purpose.

[@DerekHerbst747]

Thanks for the Kudos. I have taken another look at the video, which is an unedited, "warts and all" video, and have been contemplating making a cleaner version, (maybe a series of videos) to better explain the function and design. I will probably do that using a FET VOM, which is very similar.

[@billcrowell5096]

@@DerekHerbst747 That sounds very good. I need to find the schematic for my VTVM and also change out the rest of the capacitors. I don't own a FET VOM and should probably add one to my collection. Most of the time, I use my Rigol DMM for measurements, but DMMS don't integrate fast enough for RF - this fact is lost on most of the younger players. If you have a favorite FET meter, let me know and I'll try to get one. 73 de N4HPG

[@DerekHerbst747]

@@billcrowell5096 have a few. My go-to is a Kyoritsu K-200, but I also really love my Grundig UV-4 and the UV-5 is also amazing.

[@spondulix99]

Can you comment on the input resistance and/or impedance vs. input frequency of the meter at the various scales of the ohms and voltage, both ac and dc, functions? Thanks.

[@DerekHerbst747]

The meter has a constant 1.4 Megaohm input impedance for AC and 11 Megaohm for DC. The AC voltage accuracy (5% FSD) is not as good as the DC (3% FSD) because the wire wound resistors in the Range selection circuit have an inductive element, which skews the AC measurement with varying frequencies. The stated frequency range for 3% accuracy is 30Hz - 500kHz. Some manufacturers include parallel capacitors in the range select circuit resistors to offset this phenomenon.