The video information signal is runs through a common emitter amplifier on its way to the cathode. We see the signal at the base, but not the emitter or the collector. This meant the transmitter is suspect. A diode test on the BE junction of the transistor reads 530 mV (although that alone doesn't guarantee the whole transistor is working). We thought that the 680 ohm collector resistor had gone bad, since we were measuring infinite resistance on the back of the PCB, but I fixed that by just reflowing those joints. (Maybe other places on the board need reflowed?)

The power supply for the amplifier is 35 V, but we see a 300 mV gunk signal at the horizontal sync frequency (we think) on top of that we don't know if that's acceptable or not (if not, maybe a filter capacitor is becoming dodgy?)

This is the first CRT I've tried to debug, so any advice is appreciated! Normally I'd just get a new monitor, but since this CRT is integrated as part of the Vector Graphic 3 I'd really like to get it to work.

Note that when I say the anode is at 7 kV, I just took what we read on the meter and multiplied it my 100. I couldn't find any information on the Simpson high voltage probe we were using to know for sure what the scaling factor is (I assumed it was 100), and more to the point what input impedance it expected the meter to have. I measured the ancient Simpson meter we were using (it was the only one we found that had connectors that matched the probe) as having 5 Mohm input impedance. If the resistor in the probe was was chosen according to expecting a higher input impedance, then the actual voltage is higher. The voltage marked on the schematic was 11 kV. Since we are seeing traces on the screen we didn't pursue this issue any further.

Support this channel via a special purpose donation to the Georgia Tech Foundation (GTF210000920), earmarked for my work:    • Support This Channel via a Special Pu...  

0:00 Getting to the PCB
1:36 Grid voltages
3:03 Intensity adjustment
3:57 Rolling flicker
4:55 Input signals
6:15 Signal amplifier
8:30 Resistor is suss
9:19 Resistor is exonerated
9:33 Disappointment
9:59 Cathode & collector signals
12:06 Base & emitter signals
13:05 Is this ripple a problem?
15:25 BE junction test
15:50 Reflow all the joints?