[Lead Fingers]

This is the circa 1935 model 027 VTVM:

LF

[radiorich]

Hello Ledfingers very cool !!
Sincerely Richard

[Lead Fingers]

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

[Lead Fingers]

....and here is the four pages from the Philco Test Equipment Manual 1944.

LF

[Lead Fingers]

Back in the chain gang I've taken the important voltage readings in the subject Philco 91 Code 221, the two speaker model with the vanishing 136 ohm resistor.  To start I'm attaching a marked up section of the schematic to aid voltage reading point locations.  I measured the voltages with respect to chassis ground using a Fluke 77 DMM, which has a 10 megohm input resistance, so the circuit loading is minute and inconsequential.  Attached is a marked up partial schematic of the power supply and audio output section.  I'll provide the data in a subsequent post