Simpson 260-1 VOM/Multimeter: History, Repair, Overview, Demonstration, Theory 

Timothyhall, glad you like the video, but I don't understand the purpose of the rest of your comment. Are you responding to something in my video that you disagree with? You mention a schematic; WHICH schematic? The Simpson original, my version, or what?

herosipet, what's to explain? Three DC Volt scales, use whichever corresponds to the selected range, two AC Volt scales according to range, and one Ohms scale. This is very conventional.

The link to the schematic and other 260-1 resources is given in the description to this video. There are only two diodes in the meter, and they are indeed needed for the AC Volts function to work. But they are located in the circuit after the resistor attenuation, and as such as not particularly high voltage. But given this meter's age, I think that they are Selenium diodes, not Germanium or Silicon. Selenium diodes are typically about 30V per layer, and are typically comprised of several layers stacked to achieve the desired rectifier voltage. They can be replaced using Silicon diodes, with one Silicon diode replacing the entire stack of a Selenium rectifier. HOWEVER, each Selenium layer has a forward voltage drop of several volts, whereas a Silicon diode only drops 0.6 or 0.7 Volts. In the 260-1, the post-attentuator voltage applied to the two Selenium diodes is quite low, so I think only one layer is used on each diode, so they can't be seeing more than a small number of Volts. But this has the disadvantage of skewing all the precision resistors in the meter circuit, because they were selected by Simpson to result in the correct current through the meter movement when quite a few Volts are being dropped by the rectifier. If you put in Silicon diodes to replace blown Seleniums, the meter will probably read very high in the AC Volts function, because too much voltage is present after rectification, since not enough voltage is being lost in the rectifier diodes. You might find that you need to replace at least a few resistors with ones of different values in order to get the meter working properly for AC Volt measurements. Study the circuit in DC Volts, AC Volts, etc; to see if there are any resistors unique to the AC Volts circuit, and that resistor(s) would be a good place to start tweaking once you have replaced the diodes.

SDsailor, "for these VOM's" seems to mean this particular VOM. In which case, the manufacturer is long out of business, the resistors are high power precision types with custom values Why would anyone expect them to NOT be expensive?

@@youtuuba So to repair one of these is going to be expensive then. I have a 260 series 5 that is not accurate in some of the ranges. Ohms is good but AC and DC are of on a couple of the ranges.

@@SDsailor7 , while the "Simpson" that made the very early model I show no longer exists, another version of the company still exists in northern Wisconsin. They still survive some versions of the 260 series. You might find that they will still work on yours. You can Google them.......

@@youtuuba Yes I have read that they still make the and they still service them. Thank you

@@youtuuba I also have a bench model vtvm by Simpson it is the model 312, but I think it also needs some work. I downloaded the operator manual to see if I can make it work.