07-10-2026, 11:40 AM
Hi Rich,
I got that image by way of Google Images. The schematic and some info is in the Philco Test Equipment Manual 1944. RadioMuseum has an instruction manual with about a dozen pages but you are limited to three one page downloads per day so I didn't bother to get it.
Those first VTVM's were more laboratory work creatures than service tech's tools. No isolation resistor at tip of DC probe as far as I can tell from the limited info (what else is new with Philco docs?). The later VTVM's like the Heathkit, Knight and Eico kit copies became the economical tool for hobbyists. With the 1 megohm isolation resistor tipped DC probe you can measure tube element voltages in RF and IF stages without throwing the circuit out of whack and thus getting erroneous readings.
I'm collecting readings from my "ambiguous" model 91 code 221 which I will post soon.
LF
I got that image by way of Google Images. The schematic and some info is in the Philco Test Equipment Manual 1944. RadioMuseum has an instruction manual with about a dozen pages but you are limited to three one page downloads per day so I didn't bother to get it.
Those first VTVM's were more laboratory work creatures than service tech's tools. No isolation resistor at tip of DC probe as far as I can tell from the limited info (what else is new with Philco docs?). The later VTVM's like the Heathkit, Knight and Eico kit copies became the economical tool for hobbyists. With the 1 megohm isolation resistor tipped DC probe you can measure tube element voltages in RF and IF stages without throwing the circuit out of whack and thus getting erroneous readings.
I'm collecting readings from my "ambiguous" model 91 code 221 which I will post soon.
LF
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