Ok, I know about that link.
Years ago sitting exams I realised if I wanted an A+ a few extra things had to be done, aside from answering the questions correctly, you have to make it easy for someone else to help you. So I imagined the examiner up late at night, with an ashtray full of cigarette butts and only half a glass of Bourbon left with about 30 papers still to mark, and his dry eyes really getting to him. So the writing had to be legible and crystal clear with enough spaces in the presentation of an essay to be concise and not ramble, the method always worked for me.
When you find a schematic split in two what you need to do is assemble into one if possible in a photo editor. I have done it for you this time, as its your first go. That makes it easier for someone else to comment on it and make suggestions of what and where to test things without having to spend extra time and do extra work.
Because the 5155 VDU is quite the masterpiece, I have added some additional information to help you, by labelling its working parts.
One thing of interest is that it has two transistors wired as a PUT (programmable Unijunction transistor) to make the vertical scan oscillator. The Horizontal scan oscillator uses a chip with an integrated AFC (automatic frequency control system) to lock the H scan rate to the incoming sync pulse rate. This is much like a TV in that, due to the fact it has separate H & V scan oscillators, it does not have to rely on the sync pulses from the computer to run and it can create a raster scan with no video signal/syncs plugged onto it. This is quite unlike many other computer VDU's, such as those in the PET or the IBM5151, that require an H drive pulse for the H scan and EHT system to run. So if the CRT/raster scan is blanked out with the brightness control turned right up, you cannot blame that on missing signals from the computer. The separate H scan oscillator makes the VDU totally resistant to damage from any abnormal signals (regardless how disordered they are) coming from the computer.
Other features that make this VDU good , as you will notice the video signal is coupled into it via a capacitor. This causes the DC information and black level to be lost, but they clamped the signal later and that re-stabilizes the video black level. The video output stage has a Cascode configuration and that has a very wide bandwidth to over 7MHz. Due to that, and the CRT having a small spot size (smaller than a typical TV CRT) the resolution of this VDU for fine detail is excellent. Though when it is fed with a color signal, it goes against it, in that severe dot patterning is seen, but if you remove the color carrier, by disconnecting a resistor on the computer's video card, it substantially improves the video image.
This unit has a typical Horizontal output stage & flyback transformer. It is a configuration where the 12V supply is introduced via a diode (check that) to the primary, and an auxiliary winding charges a boost capacitor. In normal operation therefore, the boost voltage is in the 16V vicinity, higher than the supply voltage.
The HOT (horizonatal output transistor) collector current scans the right half of the raster, then when it is cut off by the drive voltage, you get half a cycle of resonance from the flyback transformer (FBT) seen as a high voltage pulse on the collector of the HOT (This is transformed up and peak rectified to create the EHT for the CRT), however the collector voltage is prevented swinging negative because the ERD (energy recovery diode aka damper diode) conducts and the magnetic field of the yoke and FBT is controlled to a linear ramp like decay to scan the left side of the raster. One interesting feature here, as is the case in all VDU's the active drive from the driver transistor stage is such that when the driver transistor is conducting, this switches the HOT off. The HOT is only switched on by the collapsing magnetic field from the small driver transformer's core, stored from the previous part of its operating cycle. The output stage also has the typical magnetic H scan linearity coil and a width coil in series with the yoke coils and the special coupling capacitor there, known as the S correction capacitor. Do not replace that one with an ordinary capacitor, it is a special part.
In any case, since it seems currently that the H scan stage is not operational and there is no EHT there are a few simple checks to do first. One is to check that the 12V is arriving at the circuit. The second is with the unit in the powered OFF state, check the fusible resistors there (they label them "failsafe" resistors) especially the one feeding the primary of the driver transformer. With no H drive to the B-E circuit of the HOT, the circuit will be dead. Then after that, there are many more checks including checking the Horizontal drive from the IC, via the driver stage to the HOT's B-E junction and the HOT itself and then the FBT as required, post what you find so far.
PS: I had previously determined that the IC in this unit is the Motorola MC1391P.
It is strange how that part always gets the blame first especially on youtube.
