Many older radios have voltage tables in their manuals or on their schematics. These, together with standard signal tracing and resistance checking techniques can be of great use in quickly finding a problem stage and then isolating a component(s) that have caused a fault. The problem is that many meters in use several decades ago used a lower sensitivity movement than even fairly inexpensive analogue meters today, with a resulting lower ohms/volt value. Therefore in many cases using, say, a modern 20Kohm/volt meter (or worse, a DVM or VTVM with an input impedance of several Mohms/volt), will often result in a significantly higher voltage reading than shown in the voltage table values. The variation in reading between a 1Kohm/volt and a 20Kohm/volt can be very significant (even more so for a DVM or VTVM), depending on the circuit under test. For example, if measuring the HT line in a receiver, there will be hardly any measurable difference, as the differing current drain through the two meters (say 1mA for a 1Kohms/volt meter versus 50mA for a 20Kohm/volt meter) will easily be supplied by the receiver (low impedance) psu circuitry, however, when measuring the anode or screen voltage of an operating tube (high impedance), the extra current drain through the lower resistance meter can make a considerable difference due to increasing the voltage drop across the anode or screen resistor (ohms law) - typically lowering the measured voltage by say 20% or more. Thus if the voltage table indicates that the voltages were measured using a 1Kohm/volt meter and you are using a 20Kohms/volt meter, the measured higher voltages would certainly be enough to question whether the stage under test is functioning correctly (even though the more sensitive 20Kohms/volt meter or, DVM or VTVM is actually reading a much more accurate ‘real’ voltage!). The simple way to resolve this is to shunt your 20Kohm/volt (or higher) meter with a suitable resistance on each range to give approximately the required sensitivity – eg, for a 250v scale, use a (2%) 250Kohm (or 247Kohm) resistor. I made up a unit constructed in a small project box using a rotary switch and covering all the dc voltage ranges of my ‘go-to’ bench multimeter (see photos).
- Article and Photos by Gerry O’Hara -