After eliminating 8 tubes and capacitors as a possible cause, I decided to get smart (like Don Adams) and fire up the old VTVM
I took VTVM readings of DC voltages on the 6GH8, 6GM6 and 12BH7. I swear by a VTVM versus my Fluke DMM for working on tube gear anyway
The first two tubes seemed to be biased OK, the 12BH7 seemed to be in a state of extreme cutoff on both triodes, output stages driving the deflection plates. I pondered the only reference I had, the RCA WO-91 schematic.
Enjoying the warmth of the attic bench, the snowfall and time off work...then
suffering an attack of Technician's A.D.D., I grabbed another recent acquisition, Triumph model 830 Oscillograph Wobbulator
. A very interesting unit from 1943, I figured it was an easy fix
. I reflexively started re-capping it until it responded with basic functions in a similar manner to the one Phil Nelson donated to a Navy Museum. Richard Sears also wrote a nice piece on it in Wordpress. I was suddenly able to understand the signal path of a scope's vertical input with this classic unit, so I then grabbed my faithful vintage scope, the RCA WO-91B to do the actual tracing - measuring.
Tracing the test signal through the stages of the PS120 and got as far as the grids of the 12BH7 with an amplified sinewave. Then nothing at the plates. The tube was definetly in cutoff, but why
Bias network resistors on grids were OK. On to the cathodes, grounded through two 6600 ohm "power" resisitors, so what is the chance both are bad???
They were odd wirewound "sand-sticks" and appeared to be corroded where leads attached.
They both measured OPEN, mystery solved! I located two 'dog-bone' 4 watt 6.6 K resistors in my parts drawer.
The power supply and horizontal stage, I got the last two wax caps hiding on the sweep selector
PS120 horz.jpg
The vertical stages
PS120 vert.JPG
A simple unit showing a simple sinewave
PS120 working 12-31-12.JPG
The Triumph and PS120 display composite video from a VCR
830 and PS120 display video.JPG