Lobsters retrocomputing - 13 Sep 2026

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Internally the VZ200 is very simple and very cheaply made. There are three main divisions, the logic board itself, a smaller board to the left/west soldered to the main board with connecting jumper wires, and the keyboard, which is attached by a stiff wired plastic ribbon. There are no internal connectors, because that would have added cost and complexity, and for further cost reduction the circuit boards are all low-grade phenolic resin PCBs. Other low points include cardboard washers to prevent the mounting screws from shorting anything (we also saw this on another VTech unit, the unrelated Laser 50) and a plastic cover sheet with slits for the top ports' card edges to reduce dirt getting inside. Also visible on the top right/northeast side is a sprawling heat sink screwed to the fin of a 7805 voltage regulator peeping out from the lower right/southeast corner. This heatsink sits under the ventilation slits in the top case.

Much of the logic board is covered by a large sheet metal Faraday cage serving as an RF shield, festooned with soldered metal braids connecting everything to the ground plane, and then the whole assembly placed on an irregularly shaped metal plate on the bottom with the piezo. As mentioned, in those days the FCC was very strict about radio interference from computers and video games, particularly home units where a Class B device (as this is) had a 10dB lower maximum than a commercial Class A one. Some systems like the Atari 400 solved this problem by effectively encasing the entire system in a molded metal endoskeleton and placing as few holes in the case as possible from which radio signals could emanate. That made for a very sturdy computer but one more expensive to manufacture, so VTech went for this cheaper, hackier approach which was no doubt iterated upon until it just cleared the bar.

One major component is not under the cage, however, and that is the Motorola 6847 VDG video chip, in a plastic carrier (MC6847P) with a date code of 24th week 1983. It's not precisely clear why that is, but it can be seen that the left board is connected to some of its lines.

The chip on the left board is a Fairchild TBA520 manufactured by Telefunken (the date code is probably 25th week 1983). The TBA520 is a PAL synchronous demodulator, a surprising choice, since the MC6847 is usually paired with the MC1372 NTSC colour modulator. The 6847 emits YPbPr (as Y, B-Y and R-Y) video, which in the black and white Lasers only the Y (luma) signal is used. In other systems like the Tandy CoCo the MC1372 takes the YPbPr lines, encodes the colour, and emits a signal suitable for a television set and/or composite video depending on the specific components.

The TBA520 can be made to do the same task, even generating NTSC colour with the right crystal, albeit with more supporting electronics. (I presume it was less expensive than an MC1372, plus there would be the advantage of not having a different chip for Euro and Aussie systems, since the TBA520 can obviously do PAL video too.) Reference B-Y and R-Y signals are generated to the TBA520 using the 3.58MHz (315/88) NTSC colourburst oscillator on the left board, while the B-Y and R-Y signals from the MC6847 are passed on different lines, with the MC6847 running on the same 3.58MHz clock. We then pulse the TBA520's line inputs at the necessary horizontal rate, approximately 15.7343kHz, causing the TBA520 to emit a single NTSC chroma signal (on its G-Y pin) from the 6847's PbPr signals. (This theoretically makes it possible to get proper S-video out of the VZ200, though we're not going to try that this time.) Discrete components then combine the luma and chroma into composite video for both the monitor connector and for feeding into the separate RF modulator.

For comparison, here is the inside of the Aussie VZ-200. The heatsink-7805 assembly can be seen here as well, but the big difference is that the logic board is longer and the left board, which we now know to be the colour encoder, is sitting on top of it and the 6847. The additional circuitry under the colour encoder is what coerces the 6847, which requires a 3.58MHz NTSC colourburst frequency signal and expects to generate a 60Hz 262-line interlaced NTSC display, to generate a credible 50Hz 312-line interlaced PAL one instead. The 6847 is an autonomous device and draws the screen independently of the CPU. When the 6847 is done, it emits a signal, typically used as an interrupt to the CPU, but here also used along with the horizontal sync line to drive a series of discrete logic counters. These counters intermittently redirect the VDG's clock signal, halting it for a certain number of screen lines each time to pad out the frame by another 50 lines. These added lines also necessarily reduce how often the VDG is free to scan video RAM and draw the next frame (i.e., (262/312)*60 is ~50.385), thus achieving the reduced refresh rate. The colour encoder board here has a 4.43361875MHz PAL colourburst crystal, but uses the same TBA520 part to generate the chroma signal. Because of the added circuitry required (including a long power wire) and the fact the colour encoder board now needs to be stacked on top because of the limited space available, this is good evidence that the VZ200 was designed first and foremost for the United States: the case fits the NTSC colour encoder board and its shorter main board more elegantly, and fewer components were needed overall (cheap!). Adding PAL components for the Euro and Aussie versions made the system more complicated and slightly more expensive to manufacture, which wouldn't seem like a desirable initial design result, and the American market was of course much larger - assuming you could actually sell any. The redesign of the VZ-300 gives up even fewer of its secrets. The RF modulator and heatsink-7805 assembly persist, but everything else is sheathed inside the soldered-down Faraday cage, even the VDG and colour encoder board. Small apertures allow trim adjustments for the video, but that's about it. Still, through the holes we can see the MC6847 ... ... and a real Zilog Z80, with a date code of 18th week 1986. Back to the Yank VZ. To be able to check continuity we're going to need to get the keyboard out, so we'll start by removing the small screws affixing it to the top case. The keyboard appears to have been installed at the factory by slipping it behind this plastic strut, screwing it down, and then soldering on the ribbon and epoxying it for strain relief. Unfortunately the strut is preventing the keyboard's removal and we can't cleanly reverse those steps, so I decided to just break the strut to get it out. The screws will still hold the keyboard in place when we put it back. We can now separate the PCB from the rubber key sheet (no individual keys fortunately) and slide it out, still attached to the motherboard. Turning it over, the keyboard PCB has multiple contact pads that are connected when the conductive nubs on the bottom of the key sheet press against them. The traces carry the lines from the keyboard matrix and intersect at those contact points. To avoid having to make a multi-layered board, traces crossing traces without connecting them are separated from each other with a strip of insulating material, and the crossing trace then bridged using what looks like conductive paint (cheap!). With this in mind, if we map out the traces starting from the top (north) row at the 5th position from the left (west), which is the 5 key, we have our line of bad keys. It starts at 5, then goes southeast to T, G, and B, and then east (right) to the N. That seemed suspiciously like a continuity break to me, so I verified continuity between the 5 key (the closest to the ribbon cable) and the ribbon cable, and got what appeared to be a good connection. Next I looked at where the ribbon cable attaches to the logic board. This is also soldered on. Checking the same contacts, there appeared to be continuity from the 5 key to the logic board as well, which would rule out the connector and the ribbon as causes. Now we need to open the Faraday cage to see if there's a break inside on the main PCB. The Faraday cage is held on by metal tabs passing through the motherboard and soldered beneath it. Eventually I'm going to completely remove the Faraday cage because I don't care about RF leakage and it's just in the way (and I may need to do some work on the board later, though we'll talk about that after we fix this problem), so I decided to just clip the tabs with angle cutters. One of those tabs is here over by the 7805 ... ... and the other by the MC6847. The other two tabs are in the back corners and were pulled down flush with the board which might have made it hard to cut them without scuffing traces, so I just bent the RF shield back at this point. Now we can see the main components. This is a single-sided board (cheap!), so there are no hidden pieces on the underside except for the piezo at the bottom, and you are therefore looking at just about the entireity of what's there. A few months ago we did an exhaustive teardown of a 1985 Canadian video titler, the Scriptovision Super Micro Script, which has a 6802 CPU (a microcontroller version of the 6800) and a 6847 VDG, RAM, ROM and a simple keypad for entry. In that article I mentioned that those were enough to make it almost a home computer (replacing the ROM on the Super Micro Script to make it one) because home computers like the VZ-200 have a similar architecture. Now we'll prove the comparison is valid.

In this view, the MC6847 is the large DIP on the far left/west side. The chips in the back, going from left to right, are a Hitachi HM6116P-2 2K static RAM used for video memory, the CPU, a clone SGS Z80 (the former state-owned SGS Microelettronica "Societa Generale Semiconduttori" of Italy prior to merger into STMicroelectronics in 1987) with a date code of 19th week 1983, a Hitachi 74LS139 and 74LS32 used as part of the address decoding for the memory mapped I/O range, and lurking in the back right (east) corner a 74LS174 6-bit flip-flop serving as a latch register. In the front, also left to right, are a Hitachi 74LS245 octal bus transceiver which bridges the shared 2K video RAM between the CPU and VDG, a Hitachi 74LS04 hex inverter used for various tasks such as the CPU reset circuit, a Hitachi 74LS244 octal driver, another Hitachi 6116 2K SRAM (the system RAM this time), and the two 2364 8K system and BASIC ROMs with date codes of 27th week and 32nd week 1983 respectively. On the Dick Smith schematic using the same order, these chips are numbered U15 (the 6847), then U7, U4, U3, U2 and U1 (74LS174), then U14 (74LS245), U13, U12 (assumed), no designation for the single 2K SRAM, and U10 and U9 for the ROMs. The ROMs are unsurprisingly the newest chips in the system and mean the computer could not have been assembled earlier than then, likely making it part of the last production runs before VTech abandoned the line in North America.

Obnoxiously, everything is soldered down; there are no sockets (cheap!). However, there are a number of unpopulated pads. Some extra pads near the ROMs were clearly intended to accommodate larger-capacity chips, and later machines indeed use a single 16K ROM with a change in board jumpers nearby. (Braver folks than I have extracted VZ-200 boards with 2364s and found bodge wires underneath. Apparently underpaid labour and smaller ROMs were less expensive at the time. Cheap!) There is also another set of pads near the VRAM chip between it and a big block of through-hole resistors. It's not clear what these pads were meant for, though the VZ-200 schematics show another resistor bank there serving as pull-ups. This bank is drawn on the schematic with dotted lines unlike the other set, so perhaps they were eliminated for cost reasons (cheap!).

Now let's talk about what we don't see. One thing we don't see here is a 3.58MHz master crystal. That's because ... we already saw it. The entire system runs from the 3.58MHz crystal on the colour encoder, so everything is precisely synchronized to the same clock source, including the TBA520, the MC6847 and the Z80. It is therefore impossible to merely switch the colour encoder boards and turn an NTSC VZ200 into a PAL VZ-200 or vice versa because of the added line padding circuit in the PAL unit and the absence of a second system crystal in the NTSC unit (cheap!). What about the VZ-300, where there's no 3.58MHz crystal either? In that system, the VDG is entirely clocked by one of the gate array chips, providing the same line padding logic, but also using the same 3.54MHz clock as the CPU which is apparently "close enough."

Another thing we don't see is something like a Motorola 6883 synchronous address multiplexer. Recall that the 6847 VDG has no externally exposed registers of its own (compare to, say, the VIC-II in a Commodore 64). Things like video modes and character attributes are twiddled by chip lines; for character attributes these lines are often wired to certain data bits from the video RAM, but to dynamically set a video mode under software control requires external hardware. Also, the VDG makes little attempt to cooperate with the CPU as to when it accesses video memory, other than indicating when it finishes a frame which is likewise asserted on one of its pins. In the Tandy Color Computers (prior to the CoCo 3, which uses a GIME), the MC6883 SAM sits between the 6809 CPU and the 6847 VDG and arbitrates all of this, managing the display mode and system timing, and servicing the VDG while the CPU is on the bus. On the other hand, this approach was judged too expensive for the cut-down Tandy MC-10, which instead uses a series of flip-flops to interleave bus access between its 6803 CPU and the 6847. A single 74LS245 bus transceiver allows the CPU unrestricted access to the video RAM when the processor is accessing memory.

Neither approach is suitable in this case, however. An MC6883 would be too expensive for the VZ200 also (cheap!), and the Z80 sits on the bus longer during a CPU cycle than a 6502 or 6800-family chip would, so the MC-10's interleaved approach won't work either. As it happens, the VZ200 does nearly exactly what the Scriptovision Super Micro Script does: during CPU video RAM access, the VDG's memory fetch is immediately suppressed using its MS pin and the 74LS245 bus transceiver temporarily kicks it off the bus. The contention problem is solved in both cases with software (cheap!) by simply not doing anything with VRAM until the VDG indicates it's between frames and not reading screen memory. The only difference is how they find that out; the SMS busy-waits on the VDG's FS signal before doing a screen update, while the VZ200 just wires FS to its IRQ line - screen updates using ROM routines are batched and when the interrupt is triggered, the ROM then blits the deferred changes to the screen all at once. Of course, if you write directly to the video RAM when the VDG is accessing it you'll get intermittent artifacts, and I'll show you what that looks like, but you can just watch for the IRQ yourself if you really care about it (many programs didn't). Otherwise, the VDG's mode pins for bitmapped graphics and alternate colour selection are handled through bits in the 74LS174 latch within the memory-mapped range, which also handles the piezo and cassette output. Overall this is a good demonstration of how the SMS was almost a home computer, because here's a home computer whose video architecture was almost the same.

A missed opportunity with the more upmarket VZ-300 was the potential for lowercase or at least an alternative character set, especially because it even got a word processing cartridge released for it later (we'll play with it, it works with the VZ-200 also). An external font ROM can be lashed to the 6847 with a bit of additional circuitry and the Super Micro Script has one to generate higher-quality character glyphs. It seems like VTech could have done something like that controllable by another latch bit, and with a six-bit latch there are a couple more data bits there that could be used, but I guess that was either judged too risky or not even thought about. On both systems the 6847 INT/EXT pin that would have controlled this is merely hardwired to ground.

One note about the metal RF shield: if the cage is not pulled back down into position, it may distort and contact some of the pins on the 74LS174. This will cause weird graphical artifacts and knock out the piezo (no keybeep). It doesn't appear to harm the computer, but I was very careful to ensure it was bent back to as similar a position as before after this happened a couple times. Obviously this is no problem if you just completely take it off.

The situation is slightly more complicated on the VZ-200 with the 6K RAM daughterboard. Here you can see it sitting on standoffs with the lines from the three 2K SRAMs wired into where the single 2K SRAM would go, next to its NEC D780C CPU, another Z80 clone, with a date code of 21st week 1983. The 74LS244 in the front is not identified on the Dick Smith schematic, but the presence of six 4.7K resistors near it rats it out as the U12 chip in our keyboard matrix. These resistors have continuity with the +5V plane, so they are the column pull-ups. Yes, the solder is supposed to be bridged between some of the resistors and the 74LS244 like that; some checks with the continuity probe indicate it is wired exactly as indicated. The eight diodes for the rows are located near the cable. Probing these I got continuity between them and the 5 key as well. I was now starting to wonder about the 74LS244, because it had some weird bronzing like it had gotten burned or something. We know that the key is sensed if it goes logic low during scanning, so I ran a jumper between the metal braid (grounded) and pin 8 on the 74LS244 where that column should be connected, and turned the computer on. On my portable composite display you can see it acted like the G key was stuck down, which seemed plausible depending on the order the rows get scanned, so I concluded that chip line was probably fine. I went back to the keyboard PCB and started testing the other keys that weren't working. The T key and G key pads appeared to have good continuity also. Testing the bottom row, however, I abruptly lost continuity. Probing the individual connections between traces revealed a break in the conductive paint above the N key. This line gets propagated to all the other keys above it, and those keys' apparent connectivity was thus actually only on one side (and I was testing that), which is why they could never complete a junction and be sensed. On the other hand, the 6, Y and H keys on that column are wired separately into the ribbon cable, so they were unaffected.

I'm not sure how such a fault would have happened. The keys move, but they don't sweep or scour, and ordinarily they shouldn't be contacting the painted portions anyway. It also doesn't seem likely to have been a factory defect because that would have made the computer very difficult to use, and this computer was clearly used.

I pondered the best way to fix it, since any repair would have to be flat or it would distort the key sheet on top (e.g., no solder blobs, no top bodge wires). I have a circuit pen I could use to draw a new trace, but it's temperamental, and I didn't want to do something I couldn't undo later in case it wasn't actually the problem.

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Original: http://oldvcr.blogspot.com/2026/09/a-dick-smith-vz200-without-dick-smith.html