07-07-2026, 02:51 PM
Up coming is the voltage readings for my "ambiguous" Philco 91 code 221. The values listed in Philco's voltage table for the model 91 receiver is part of the "ambiguity" narrative.
For those who have little or no experience accurately taking voltage readings in an operating receiver (Beware of deadly voltage levels!) the following is predatory:
How did Philco ascertain the voltage values for it service literature voltage tables for a correctly functioning receiver? Was the receiver tuned to a signal generator signal of a certain level, or were the antenna terminals open, connected to a load resistor, or shorted? What type of voltmeter was used to make measurements to establish the values for their Model 91 receivers? Without any extenuating knowledge I'm presuming they used a typical technician grade volt meter such as their Model 020 volt meter (schematic attached).
Study the schematic to ascertain the meter's sensitivity and loading effect on circuits depending on the voltage range selected. Knowing the loading effect when rationalizing the error the the voltmeter causes by its loading effect on high impedance circuits such as audio amplifier grid and plate circuits is the bane of fast and accurate circuit operating assessments for those with a weak understanding of electrical laws - present company excluded, of course.
For example, on the 10 Volt range the loading effect is 50,000 Ohms, which translates to 5000 Ohms per Volt. When measuring a few volts of grid bias the reading will be vastly incorrect and will require correction by Ohm's Law attention if perfection if preferred.
My next post will be the readings taken with my Knight Kit KG-620 VTVM referenced (calibrated) to my Fluke 77 DMM, or directly with my 77. The loading effect of the Knight is 11 megohms per volt, the 77 is 10 megohms per volt.
OH! By the way, a SAFETY BRIEFING is n order: The common lead on a VTVM of the Heath and Knight variety is connected to the case. Connecting that lead to anything other than chassis ground is begging for injury or worse. To measure in circuit voltage differences take two readings referenced to chassis and subtract to get the difference, ....and stay alive.
Stand by for further...
LF
For those who have little or no experience accurately taking voltage readings in an operating receiver (Beware of deadly voltage levels!) the following is predatory:
How did Philco ascertain the voltage values for it service literature voltage tables for a correctly functioning receiver? Was the receiver tuned to a signal generator signal of a certain level, or were the antenna terminals open, connected to a load resistor, or shorted? What type of voltmeter was used to make measurements to establish the values for their Model 91 receivers? Without any extenuating knowledge I'm presuming they used a typical technician grade volt meter such as their Model 020 volt meter (schematic attached).
Study the schematic to ascertain the meter's sensitivity and loading effect on circuits depending on the voltage range selected. Knowing the loading effect when rationalizing the error the the voltmeter causes by its loading effect on high impedance circuits such as audio amplifier grid and plate circuits is the bane of fast and accurate circuit operating assessments for those with a weak understanding of electrical laws - present company excluded, of course.
For example, on the 10 Volt range the loading effect is 50,000 Ohms, which translates to 5000 Ohms per Volt. When measuring a few volts of grid bias the reading will be vastly incorrect and will require correction by Ohm's Law attention if perfection if preferred.
My next post will be the readings taken with my Knight Kit KG-620 VTVM referenced (calibrated) to my Fluke 77 DMM, or directly with my 77. The loading effect of the Knight is 11 megohms per volt, the 77 is 10 megohms per volt.
OH! By the way, a SAFETY BRIEFING is n order: The common lead on a VTVM of the Heath and Knight variety is connected to the case. Connecting that lead to anything other than chassis ground is begging for injury or worse. To measure in circuit voltage differences take two readings referenced to chassis and subtract to get the difference, ....and stay alive.
Stand by for further...
LF
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