A few months ago, I was gifted a non-operational DEC VT320 serial terminal. This is a 80s/90s text-only CRT monitor, which was only displaying a thin vertical line rather than a proper picture. I've done some electronics repair in the past, but nothing this old or involving a high-voltage tube. But I figure that it would make for an interesting project over the summer, whether or not I can recover it. If nothing else, it is quite a retro talking piece.
Fortunately, DEC -- aka Digital Equipment Corporation-- made a lot of these things, and also published troves of service information, which was the norm back then (#RightToRepair). Among the documents that can be found online included the schematics, which would be of incredible aid. That said, the quality of the scan is pretty poor, and it took quite a bit of cross-referencing of component identifiers with the parts list to confirm what couldn't be read.
Before opening up the monitor, I did some preparatory research, to identify areas which would be worthwhile to investigate, and also to make sure I'm not going to shock myself in the process. My findings showed that because the CRT was able to display something beyond a single dot in the center of the screen, the issue would not be in the high-voltage generation circuitry, but rather the downstream circuits. Specifically, in the horizontal deflector path; this explains why the only image is a vertical line, because horizontal control was lost.
Using the field replacement guide, I got the monitor open and then discharged the anode using a screwdriver attached to the ground strap. There was no spark or sound, which can happen if the bleeder resistor was still intact. A good sign as to what's probably still working in this monitor, but I take no unnecessary risks around potentially high voltage.
The innards were reasonably laid out, basically existing as components that adorn the CRT display itself. The PSU, main board, and "arc protection" board were all easy to identify, although the latter was more like a connector than a board. I quickly ruled out the PSU by checking its output voltages, so the issue must be on the main board.
It took a while to extract the board, since I didn't want to break any of the 40-year old plastic clips. But once out, I began matching the board to the diagrams and examining for any obvious damage. No obviously blown caps, no evidence of thermal events, no components rolling around on the bottom.
At this point, I stopped to do some very thorough circuit analysis of the circuit diagrams, to absolutely understand what I was going to do. This actually took two tries, since each attempt revealed faults in my understanding, and I had to go back to the thinking chair. This part took a few days, until I finally internalized the circuit's behavior. As it turns out, this wasn't necessary, and I'll probably include it as a later comment, just for posterity.
When I returned to the board, I made a plan to solder some trace wires, to verify my expectations when powered on. And indeed, after reinstalling the board, my oscilloscope confirmed that the H sync signal was intact and the power transistor was functioning as expected.
Narrowing the search, I took the board back out and started tracing the lines on the PCB surface -- it was fortunately only a two-sided board -- and then compared my observations to the schematics. This revealed a difference, where the Horizontal Linearity inductor (H-LIN) was not showing connectivity, despite visibly being attached to the trace.
Closer examination revealed that of the inductor's three legs -- two for electrical connections, one extra for support -- one had developed a hairline fault. This physical damage broke continuity, likely from the shock of impacting something. I made a repair by constructing a wooden splint for physical support, and then soldered over the leg for electrical connectivity.
For good measure, I completed the exercise of verifying all other components, which showed that all other resistors, capacitors, and connectors were intact and likely working. And with that, I reassembled and powered on the monitor to see.
And it works! In total, I probably spent a week on-and-off working on this. I will say that this is a strange machine to have, since I've been able to hook it up to a modern Linux machine and use it as a serial TTY. I even wrote most of the text of this post in vim.
This is a description of how the horizontal deflection path in the VT320 operates, pieced together from the schematics, the very helpful Sam's Repair FAQ page on CRT deflection, and the June 1996 Electronics World magazine article (page 472) referenced by the FAQ. I had to relearn much of my inductor and current rules from university, since I mainly focused only on voltage behavior -- because that's mostly all I needed to program small microcontroller projects.
aside on how to navigate the VT320 schematics document
The linked schematics PDF has multiple separate documents parceled together, with some being very similar. The first page (PDF page 1) is the cover sheet, giving the document numbers for each document to follow. The subsequent documents are organized by section (eg "Power Supply" or "Arc Protection Module") and each section's first page is a cover sheet for that section.For example, the "CRT/Bezel/Yoke Assy" section is made of two documents (E-AD-7024240-0-DBU and K-PL-7024240-0-DBP), with the first being the cover sheet and the second being the parts list. These are on PDF pages 18 and 19. This PDF has all sections in the order listed on PDF page 1.
However, it's quite obvious that there are duplicate sections with the same name: there are two "Monitor, Logic Control Module" and "Power Supply" sections. This is because this PDF concerns both the North American and International variants of the VT320. In my case, I have the VT320-B2 (North American) variant, so I start on PDF page 3, look at the column labeled "B2", and the rows with a "1" are the documents that are relevant. In the case of the "Monitor, Logic Control Module", I would be looking for document 70-24577-01.
That document is at PDF page 53 (observe the box labeled "top document number" in the bottom-right box). This sheet has two columns, so we use the column labeled "-01". It shows us that the document for the "Monitor Logic Control Board" is B-DD-5417189-0-0. This document can be found at PDF page 8. So finally, we have reached the correct document applicable to the North American variant.
To save some confusion, there are the PDF page numbers for the sections relevant to the North American VT320-B2 that I have: Monitor Logic Control Board (pg 8), CRT/Bezel/Yoke Assy (pg 8), Power Supply (pg 20), Bottom Assy (pg 25; has no cover sheet), Arc Protection Module (pg 27), Power Supply Assy (pg 42), Cover Assy (pg 50).
The variant number is given on a sticker behind the terminal, but a quick way to identify is that only the North American version has a fixed cord; the International variant has an IEC 60320 socket.
The trouble with understanding the VT320-B2's horizontal deflection circuitry is that it looks like this, taken from PDF page 12 of the schematics:
It's a blurry scan of the hard-copy schematics, but it will have to do. What we can clearly see is that from the left, P6 and P7 carry ground and 18.5 volts into this circuit, with the 18.5 volts going directly into the flyback transformer (T101). At the top left, we see the "Hsync" signal coming in from DC7081 video IC (see PDF page 10). From the "_H" designation, we know that this signal will periodically go high to indicate the start of the horizontal blanking interval. The magazine article goes into more detail as to what this means for the CRT.
Using transformer T201 as a divider, we can regard everything to its left as being in the "low voltage" realm, and everything to its right as the higher voltage section. However, do not mistake this for the really high voltage section (EHT, as it's known in CRT monitors) to the right of the bigger T101.
The low-voltage section is understood as an RTL inverter made from R201 and Q201, with D202 protecting Q201 from negative base voltage and C201 removing the DC bias from Hsync. Moving leftward, we see R221 and C221 forming an RC snubber circuit in parallel with Q201's collector-emitter, which will protect Q201 from voltage spikes when Q201 suddenly switches off.
By action of the above, when Hsync is high, Q201 will sink current from T201 to ground. This current must flow from 18.5v through R203. Thus, R203 acts as a current limit for T201. The adjacent C202 is relatively large compared to R203, so C202 will moderate any changes in current through R203, which also means it moderates current through T201. Overall, this means T201 either has almost no current (when Hsync is low) or a very precise, unvarying high current (when Hsync is high).
Because T101 is a transformer, the currents will be reflected to its secondary coils, which is the start of the higher voltage section. The dots in the transformer icon indicate that when Hsync is high, current will flow out of the transistor Q202's base very quickly. And when Hsync is low, a small current is allowed to flow into Q202's base. Because Q202 is an NPN BJT transistor, it must have inward base current to turn on. But conversely, to quickly turn off an NPN BJT transistor, the charges remaining in the base must be sucked out by a current leaving the base, ideally very fast. The behavior of the low voltage realm has guaranteed this for us: fairly graceful turn-on and very fast turn-off of Q202. Thus, this transistor shouldn't run too hot.
Now we get to the part that required me to really study the magazine article and Sam's FAQ, ultimately leading me to produce this diagram to help make sense of it myself. This is, essentially, a 10-scene theatrical production, where our stars are: Q202 (discussed above), the horizontal yoke (a coil wrapped around the CRT tube, for which our final objective is to create a sawtooth current waveform), C203 (the feedback capacitor), D201 (the bypass diode), and T101 (the flyback transformer). Everything else is somewhat superfluous to an intuitive understanding of the circuit, so we momentarily ignore those others parts. Instead, this is the simplified circuit to understand:
Because the intended waveform is cyclical -- it happens once for every line on the screen -- we need to start the analysis at some part of the cycle. This is the same exercise as from the magazine article, but with a modified circuit. We will pick the point just after the electron beam has swept past the mid-point of the screen, heaving further rightward.
Because T101's center tap (pin 1) is at a sturdy 1 volt, the magnetic field of T101 causes pin 2 to be a voltage less than 1 volt. We will never know for certain what the actual voltage is, since it depends on the turn ratio of T101. But it's probably near zero or it can even be negative. If negative, though, it cannot be lower than -0.6 volts, because otherwise D201 would turn on and force the voltage to remain at -0.6 volts.
We have now reached the right-most edge of the screen. This is when flyback begins, to get the electron gun back to the left side, by a negative current in the horizontal yoke. Q202 turns off, fast. The currents in the horizontal yoke and T101 have nowhere to go, except that C203 has been sitting there this whole time, with almost zero volts (or -0.6 volts, per the above). A capacitor with low voltage has low impedance, so being the path of lowest impedance, C203 takes up all the current, going from 0 to peak current in basically no time.
Because C203 is now taking current, it must be storing it as an increasing voltage. Since a capacitor with higher voltage has higher impedance, this is what's reducing the current through C203, which means both the horizontal yoke and T101 are reducing in current. But importantly, they are still positive currents, just getting smaller. Also, the exact graph of this reducing current is sinusoidal, not linear.
By trading current for voltage, C203 is now at its peak voltage, somewhere around 200+ volts DC. And importantly, there is no current left, so this means the horizontal yoke and T101 also have no current.
As expected for a circuit composed of both capacitors and inductors, they will complementarily trade this enegry back and forth, exchanging current and voltage like hot potatoes. With C203 being at its high voltage, and with both the horizontal yoke and T101 at low/zero current, current will flow through them but in the opposite direction as before, increasing in sinusoidal fashion. This is bog-standard for LC circuits.
Up until this point, the fates of the horizontal yoke and T101 were aligned. But now, as C203 has dropped down to 18.5 volts -- the same as the other end of T101 -- this means T101 has reached its peak current and will soon start to reverse. But interestingly, the horizontal yoke keeps increasing for just a bit longer.
(cont)
If the circuit were just of these two inductors and one capacitor, then the next step would be for the voltage in C203 to invert, thus acting like a brake that reduces the current through the horizontal yoke and T101. And this does actually happen, but it quickly get interrupted.
The diode's non-linear behavior will hold that -0.7 volts very steadily, when carrying this much current. The consequence for the inductors is that, per the equations for inductors, a stable voltage will force a linear current ramp, not the sinusoidal one that we saw earlier when the capacitor was active. This causes T101 and (more importantly) the horizontal yoke to start reducing in current at a linear rate, but at this moment, the current's magnitude is still negative. For now.
However, at a time between step 8 and 9, Hsync finally went low, meaning that Q202 is starting to slowly turn on. Note that D201 is still carrying the (now almost-zero) current, barely maintaining a negative voltage, when measured at T101's pin 2. But as Q202 turns on, it forces the voltage at T101's pin 1 to be 1 volt (Q202's forward voltage, discussed earlier) and holds it there. Per the inductor equation earlier, a fixed voltage induces a linear current ramp in inductors, and both the horizontal yoke and T101 respond accordingly.
The cycle is complete, and will start again at step 1 in a virtuous cycle, repeating every 15 kHz or so.
The last few details of the VT320's circuit are to understand that R204, D202, and C206 (located just above the horizontal yoke in the schematic) are just there as a one-way snubber circuit, to protect the parallel Width and H-LIN inductors if the horizontal yoke is unplugged. All other components are explained in the magazine article, such as C207 being the S-capacitor.