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Repaired a dead VT320 serial terminal from the 80s (lemmy.zip)
submitted 1 week ago* (last edited 1 week ago) by litchralee@sh.itjust.works to c/imadethis@lemmy.zip
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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.

logic board of VT320 from above, unobstructed by the picture tube

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.

oscilloscope traces, one showing a square wave pulse for 35 ms with amplitude of 4 volts, and another trace showing a single distorted sine pulse lasting 10 ms with peak amplitude of 200 volts

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.

a tall inductor supported by wood splints, marked as H-LIN

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.

monitor displaying: VT320 OK, Firmware and Set-Up Screens Copyright C 1997, Digital Equipment Corporation

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.

a VT320 terminal with vim open

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[–] litchralee@sh.itjust.works 25 points 1 week ago* (last edited 1 week ago) (1 children)

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 documentThe 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:

VT320-B2 horizontal deflection circuit

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:

simplified circuit from the VT320

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.

  1. At this point, the horizontal yoke current is positive and increasing, being fed by the constant ~1 volt that Q202 is able to maintain. 1 volt is the approximate forward voltage of that transistor, the STM BU406. Because Q202 can maintain a very sturdy 1 volt, this means T101 is also seeing 18.5-1 volts, so it too is a positive current and increasing.

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.

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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)

[–] litchralee@sh.itjust.works 20 points 1 week ago* (last edited 1 week ago)
  1. But not for too long. Now, when C203's voltage has reached zero, the horizontal yoke reaches it peak current. So too does the current through C203 reach its peak.

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.

  1. Now, the long-dormant D201 springs into action, because by being in parallel with C203, will start conducting when it sees a negative voltage. This conduction is from ground, going up to the -0.7 volts that the inductors were inducing. But unlike a capacitor that would be trading voltage and current, a diode does no such thing. This is the start of the scan line, where the electron gun is pointing at the left edge of the screen.

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.

  1. As the rightward scanning electron gun approaches the middle of the screen, the current in the horizontal yoke and T101 is just about to run out and reach zero. If they do, there will be no stored charge in a capacitor to push the inductors through the impending zero-crossing: they will crash out at zero if nothing is done, and the credits will start rolling.

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.

  1. At the precise moment that the electron beam crosses the screen's mid-point, the current in the horizontal yoke is near zero, but Q202's 1 volt influence will excel the current to push through and start its positive ramp towards infinity-and-beyond. The same happens for T101. D201 will have been relieved of duty, because its current must also near zero, thus it cannot have any notable forward voltage at all.

The cycle is complete, and will start again at step 1 in a virtuous cycle, repeating every 15 kHz or so.

the ten-step simplified circuit diagram explanation

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.

[–] Twongo@lemmy.ml 20 points 1 week ago

really cool project! congrats on fixing it. i would be too scared about a capacitor killing me x)

[–] Arghblarg@lemmy.ca 10 points 1 week ago

Nice work! I have an amber VT unit in my storage room.. 220 or 320? It's late, and I'm too lazy to go check. If it ever dies hopefully this will give a hint for diagnosis. I'm almost afraid to go power it on to see...

[–] sixtoe@lemmy.blahaj.zone 10 points 1 week ago (1 children)

That supports sixel! Sweet!

[–] litchralee@sh.itjust.works 10 points 1 week ago (1 children)

Do you have ideas that I can try out with sixels? I do want to see how far of a "modern" workflow can be done on an 80s terminal lol

[–] sixtoe@lemmy.blahaj.zone 5 points 1 week ago (1 children)

I use a CLI Pandora player that I extended to have sixel album art display. Folks play video but you'll be bw limited. It's very easy to render images so web browsing too! Play with imagemagick because it has sixel output and you can direct it to stdout! If an image is needed it can be displayed.

[–] sixtoe@lemmy.blahaj.zone 2 points 1 week ago (1 children)

I wonder how speedy it would be to use byobu or tmux or screen on your terminal and page between panes and sessions. I could probably emulate that using a rate limited SSL connection hmmmmm. BTW I am envious. I used different DEC VT thru the years. I wish I could remember which one was at the weber state college computer lab in 79. I wrote my first lisp program on a PDP 10? Vax thru a DEC terminal, I think. Nostalgia is a strong vibe.

[–] litchralee@sh.itjust.works 2 points 1 week ago* (last edited 1 week ago) (1 children)

I can say that tmux is unusually chatty over 9600 baud, when compared to screen. There seems to be something about how tmux redraws the screen, since it will stutter when loading long output, whereas with screen or no multiplexer at all, the output of "ps" rolls gracefully down the screen, almost as if it were on a paper roll.

I switched from screen to tmux years ago for the ability to subdivide a window into panes, but modern screen has caught up. Not sure about multi-session support, but it would be awesome if my same desktop tmux workflow could be equally efficient over 9600 baud on a VT320.

I did try enabling 19200 baud, but my bargain USB to RS232 adapter becomes unstable at those speeds, so I need to source a better one. 9600 is functional but a tad restrictive when quickly moving around in vim.

So I think 19200 will be a pretty good time. Also, I'm surprised that this terminal can use two different baud rates for TX and RX. I don't think modern hardware even supports asymmetrical rates anymore.

[–] sixtoe@lemmy.blahaj.zone 1 points 1 week ago (1 children)

Are you an oldhead too? 🤣

[–] litchralee@sh.itjust.works 0 points 1 week ago (1 children)

What's an oldhead? I certainly feel old lol

[–] sixtoe@lemmy.blahaj.zone 2 points 1 week ago

I'm 55. Urban Dictionary: Oldhead

  1. Someone who has been in the {scene} before you or your group.
  2. Someone who is still in the {scene} beyond the 'acceptable' or expected age.
[–] AFKBRBChocolate@lemmy.ca 9 points 1 week ago

I sure spent a lot of hours on one of those, connected to a VAX. More 220s though.

[–] myotheraccount@lemmy.world 6 points 1 week ago

Amazing! Thank you for the writeup