My New Youtube Channel

I haven’t written in a while – have been busy with work and other projects. Speaking of the latter, one of those projects has been finally trying doing some proper-ish Youtube videos. I’ve had a Youtube channel for years (BradH – now defunct), but the videos I did there were all of the shaky handycam, no speaking type. The new videos I’m doing have much better lighting, sound and some are even mini documentary style.

Anyway, the new channel is Tech Time Traveller. Check it out if you love vintage gear!

The channel obviously will focus on vintage computers mostly, but also other tech. There will be documentaries, show’n’tells (titled 8BitBites), repairs and builds. Should be lots of fun.

And of course I’m not abandoning this blog, I see I need to do some… updates.. heh. 🙂 All in due time!

A 386-driven LED sign!

Sometimes you collect vintage gear without even realizing that it’s vintage gear.  Take this LED signboard for example:

Standard retail gear, right?  Nothing exciting from a vintage computer guy’s perspective.  But hey.. what’s this on the side?

A floppy drive?!  And an AT style keyboard connector?!  Hmmm… okay, now I’m really curious.  Let’s unscrew the back panel and see what’s going on here.

Gadzook!  Quite the array of electronics in here!  But look over the to left.. is that…

It is!  It’s a 386!  This signboard is driven by a very scaled down 386 PC!  So what’s the story?

In the early 1990s, a company called JVF was offering animated signboards for retail use.  According to a gentleman in the know some years back, the deal was you paid $5000 for the sign, and then had to pay JVF a subscription fee on an ongoing basis, with additional fees to change the message on the sign.   I was told that you never actually owned the sign – you were just paying for the rights to host and use it, and JVF could take it back.  The sign was capable of some pretty fancy animation.  Check out this video by one owner:

The sign was ‘booted’ up with an MSDOS disk that contained the drivers for the LED board as well as JVF’s custom software to produce messages.  The software was entirely proprietary and the end user could not make any changes without going through JVF first.

Sadly,  (or perhaps, understandably) the business model did not work.  Sometime in the late 1990s JVF met its demise, leaving signboard owners stranded and unable to change the signboard messages.  This was especially problematic if you were a retail outfit using the sign to display things like prices.  Over time, the signs were retired and dropped.  I got mine from a client – I was curious about the PC inside and thought perhaps someone might one day find a way to edit the sign contents.

Turned out I was right, a group of determined individuals came up with their own sign programming utilities.   I actually got my hands on a copy thanks to a guy who went by the handle Mr. Henderson, and managed to edit and use the sign for my business for a while.  The new tools didn’t quite enable you to do the kind of fancy animation JVF did, but it at least made the thing usable.  As I recall, you had to install the software on a PC running Windows 95 or less.  You would then edit the files in the ‘slideshow’ utility and save them to a 720K bootable disk.  As I mentioned, the disk was a standard MSDOS disk that called up the slideshow program via an AUTOEXEC.BAT file.

This is mine booting up:

I’m not sure what the purpose of the keyboard port was since DOS does not drive the LED directly.  However in the previous video I notice it appears there were some utilities you could use directly on the signboard.

I’d love to figure out how the driver works – I don’t know if it’s possible but it’d be cool to find a way to redirect text and graphics output from the 386 PC inside to the LED directly and actually use it as an oddware vintage PC.  I’m imagining my days playing Kings Quest on my father’s IBM Convertible, but in this case, writ large!

 

A Most Unusual Osborne 1

I’m a pretty diehard collector, so one would expect I’d already have a key milestone in computing, the ‘portable’ Osborne 1, in my collection.

Nope.

There’s a variety of reasons – one being that I kept getting lured off to more interesting machines. The other being that, I just had a sneaking suspicion vintage computer prices were too high and bound for a correction. Especially on something like the Osborne 1 (O1), which a decade ago was fetching hundreds of dollars. The O1 is not a rare machine by any stretch. It was famously successful, with Osborne ultimately producing tens of thousands of units before they ran into trouble with the successor model, the Executive. My strategy, such that I had one, was to just wait.

While I waited, I learned that in fact, there were a couple of variants of O1. The first 10 machines produced as prototypes were sold with metal cases. They looked like this:

Linked photo: oldcomputers.net

The prototypes appeared in much of the advertising. But the actual production design used plastic for the cases. The first O1s looked more like this:

Linked photo: oldcomputers.net

Later in production, the decision was made to revise the case design to incorporate a sturdier plastic. These machines are distinguishable from the first O1s by changed ‘bezel’ around the floppy drives. The drives behind the bezel and the guts of the machine are still the same – just the casing is different. The keyboard also received a ‘gully’ above the keys. Not sure what the purpose was.. maybe to hold pens? Anyway, this revised model was officially designated the Osborne 1a.

Linked photo: oldcomputers.net

I had a several years’ think about it, and ultimately decided I wanted the original O1. I like the rougher look a bit better, and anyway, as a collector you tend to go as early as you can. I made no special effort to find one, prefering instead to wait until an opportunity presented itself. And sure enough it came up on ebay.

$99!! And a super low serial to boot – 337! Sure, the front bezel has been Swiss Cheesed by someone (rightly) concerned about heat dissipation. And okay, it doesn’t work. But when you are talking over 100,000 units produced, a number in the low hundreds is pretty unique. Plus the price was right! Who knows what I might find in there, this early in production? I know from experience with other makes and models that the earliest units sometimes have little differences from later, owing to tweaks during the production process. Anyway, I snapped it up.

When it arrived a couple weeks later, I naturally set to figuring out what was going on. From the description and pictures, I thought it was a simple case of bad RAM or similar. It seemed like it was being frozen at a garbage screen. However, that’s not quite what was going on. In fact, the machine behaved like it was possessed. Check it out yourself:

Taking apart the Osborne was a breeze – partly due to the fact that someone else already had and had not replaced about half the screws!

Anyway with the guts out, I tried the RAM piggybacking trick. You basically install a RAM chip of same type over an existing chip. If it’s bad, the good chip will complete the circuit correctly and the machine in theory will start working. If not, then that chip is likely working and you repeat with the next chip and so on until you find the culprit(s). Then you replace the bad chip(s). But no dice here. I even went full on and took a complete working set of 4116 RAMs from my Apple IIs and tried piggybacking the whole thing, but to no avail.

I then went into more rudimentary checks -I checked the voltages on the power supply, removed and checked the CPU in another machine (it was fine) and so forth. Nothing seemed to make any difference. I consulted photos and schematics, trying to decide where to hunt next. That’s when I discovered something odd: my O1’s motherboard didn’t look like the standard O1 board at all! Check it out. In the first pic is the standard O1 board. The second pic is mine.

The basic shape is the same, but everything is laid out differently! The external ports and brightness/contrast knobs are in their usual positions on mine, but the placement of the ICs – the CPU, floppy controller and such is totally differerent on my board! Same with the speaker and all the support logic. And check out that football field of nothingness on the far right side of my board! That’s a lot of unused space! The board in the topmost pic has just two EPROMs, mine has three! The date codes on most of the ICs, meanwhile, are almost all from 1980, a year before production got rolling. Only the EPROMs and one or two support chips are a little later.

I started digging around but every earlier O1 I found had the same motherboard as in the topmost photo. And I found machines considerably earlier than mine — down to serial number 55! — but they all seemed to have the same board as in the top photo. None had one like mine. Definitely it appeared that my board was an earlier revision than the others. I began to salivate as one word crossed my mind: prototype. You might wonder why a prototype board would appear in a machine several hundred units into production, but’s it’s not unheard of. Every dollar counts, and companies sometimes do strange or unusual things to move inventory. The Apple Lisa saga comes to mind – it’s believed a certain quantity of unsold Apple Lisa 1s were converted from 5.25″ Twiggy drives to 3.5″ Sony drives by Apple itself, and then sold as Lisa 2s. Some sold even still had their original Lisa 1 serial numbers intact. I have also seen examples of companies putting prototype or early-production components into current production machines, or even just putting a production serial number on a prototype/pilot run machine and selling it, just to clear them out. I was guessing that maybe my unit was a prototype/pilot machine that got issued a serial number and sold shortly after production started.

I decided to reach out to the O1’s designer on this one, Lee Felsenstein, via email. Lee emailed back said the board definitely appeared to be based on the prototype boards. The large empty space, he explained, would have been for linear voltage regulators installed on those units, which apparently had been revised out in mine. In particular, he found the handwritten revision number, in black marker at top left, to be interesting, as it wasn’t common practice to do that.

Lee couldn’t account for why this unusual and apparently very rare board ended up in SN 337, but his thoughts were close to mine – simply that someone, either by accident or intention, decided to install it. It could have been as simple as the board inadvertently being slipped in amongst the regular production boards at the factory. Or maybe on purpose, if someone in inventory wanted it gone. It could have been an early warranty replacement scenario and this prototype board just happened to be available as the replacement. Or maybe an employee owned this unit and built it this way themselves. It’s likely impossible to know for sure, although I will try to reach out to other Osborne people as Lee suggested and see if anyone recollects anything. All the same, it appears to be a one off situation. I’ve been researching madly since and still cannot find another unit that has one following this design. What a lucky break!

As a precaution, I had the EPROMs dumped and looked at by friends with much better skills at handling code than I am. One of the EPROMs is the standard character generator, same as all the other units. The BIOS version appears to be 1.3, which also was common on the earlier units. The only difference is mine has the BIOS split across two 2716 EPROMs, whereas in the main production board this is all on a single 2732.

In terms of operation, therefore, when (if?) the board is working, it should basically behave just like a regular O1. It’s quite possible it may have had an earlier BIOS installed previously – despite most ICs being from 1980, the two EPROMs both date to 1982. It seems to me like the board was built and used in some capacity before being configured to operate like the other production boards.

Unfortunately this rarity comes at a price – diagnosing why this machine won’t operate properly is going to be tricky since no schematic appears to be available for this board revision. I’ll keep plugging away at it.. but yeah… you learn something new every day, huh?

Connecting the Clock Lines

So with trepidation I install one of my precious 2518 line register ICs.  I have several of them, all dated to 1973.  I got very lucky finding these with acpsurplus on ebay.  I grabbed all they had, figuring there would be some attrition due to age and some mistakes on my part.  I just need *one* of the eight I’ve bought to work.  Can’t be that hard, right?

Anyway, I put it in there , and follow the directions in the Construction guide to connect my 330ohm ‘jumper’ to each of the bus ‘ports’ (pins) on the memory board to see if I can produce the same characters I was producing earlier by connecting the jumper to the resistors.   The difference here is that where before I was simply forcing certain bits high or low on the 2513 and thus it was spitting out whatever character the resulting ASCII code produced, this time I’m actually invoking the line register and getting the TVT to ‘store’ these characters for eight lines at a time.

I confirm with my jumper wire that I am getting… nothing.

None of the bus lines I probe work.  Something is wrong.  I try some elementary probing around trying to understand what is (or isn’t) happening.  The best I can come up with is that either my 2518 is bad, or we have perhaps a short or something not connected.

I decide to remove the Memory board and do some more cleaning between IC pins and such, to make doubly sure we have no accidental bridges.  When I plug everything back in and power up again, this time I have a screen full of < symbols instead of @.    If I remove the 2518 line register, it goes back to @.    As I fiddle around with the board, testing continuity, making sure the sockets are ok, each time I power up things change a little.  Sometimes I’ll get columns full of letters.

After scratching my head, I turned to Chuck on vcfed.   Since we know the line counter is derived from the outputs of IC5 on the timing board (a Signetics 8288), we should be seeing pulses on pins 2, 9 and 12.  We should also see signals on IC7, pins 3 and 8.

I decide to do a general checkover of the Timing board and discover that the socket at IC9 is bad.  I was supposed to have a pulse at pin 17 but did not; with the socket replaced we’re good.   But we’re still not getting any joy – pin 4 on the 2518, which should have a pulse, is solidly high according to my logic probe.  Why?

It turns out that the problem is pretty elementary – bus pin 17 on the memory board was not making direct contact with the attached trace.  Pin 17 is the line 1, 13, 25 transfer pin, so without a connection there we’re not going to see much.  I’m flummoxed as to how this could be, since there was no obvious break – it looked like the pin was solidly in there.    Anyway, I correct this and solder the crap out of it, and now we’ve got a connection, and now we’ve got some action when I connect my jumper.  Sure enough, the characters displayed onscreen are changing into different letters as they should be!  Wahoo!

So that’s a good result.  Now comes the bear: I need to create some jumpers that connect to various clocks on the Timing board for the next test.  Gingerly I take the unit apart and jumper in a few of the clocks recommended.  When connected to B1, they should produce different ‘stripes’ of characters on the screen.  For example, connecting clock ‘R’ to B1 should give me a screen that is half @s and half blanks.  The Q clock should give me two vertical columns, or ‘stripes’ as the guide calls them, with blanks in between.  That seems to be what I get, except I’m getting A’s instead of @s in some places.

I’m not sure what the deal is there, but I decide to persist a bit further and try connecting the 0, I, H, G, F and E clocks to B6 through B1 respectively.  This should produce a full screen of all the characters the 2513 can produce.  I connect, power up and:

Cool!  I mean, not quite what I should have, but we’re definitely getting there.  Still gotta address those vertical bars in my character output, but I can see we’re a good chunk of the way there.

Unable to find a problem with the IC sockets or anything amiss with bridged IC pins or broken board pins, I decide to swap ICs around to see what happens.  The TVT uses 4 8288s, so swapping them around in theory shouldn’t produce any change if they’re all working.  However, when I swap two, I do get a slightly different pattern.  I also notice that some characters in the middle of the screen are changing on the fly.  After subbing in another 8288 and swapping around with no change, I decide to look at the 2518 on the memory board.  On a lark, I swap it with another that i have.  Sure enough, the screen changes.. but it’s worse.  So I grab yet another.  I’m getting ABC as I should but then it all goes bad from there.

I try subbing in another 8288 and do another 8288 shuffle, and now I’m very close:

I really want that vertical line gone though.  Chuck suggests checking IC10 on the memory board – a 74165.  Chuck explains that since the characters are being formed on a 5×7 matrix, the 74165 is programmed to add two blank dots on one side of the character and one on the other.  Since the vertical lines appear to be happening on the 4th column of dots, we trace that to output pin 2 of the 2513, which feeds into pin 14 of the 74165.  He suggests something here might be amiss, maybe a short, and it’s causing the 74165 to pass on an extra vertical column of dots that shouldn’t be there.   Sure enough, with a bit of hunting I realize that a jumper that connects pin 14 of the ‘165 to the 2513 output hadn’t been properly soldered!  With that fixed, voila:

That’s almost perfect.  Granted, we still have an @ where there should be an H.  Why is that?  Everything looks good.  Chuck suggests that without a logic analyser or ‘scope, it’s going to be a real challenge to sort out.

I spend a full week trying to hunt it down, but to no avail.  Despite checking and rechecking the boards, the ICs, the sockets, jumpers… everything… I just can’t find it.  I speculate on causes and try my best to interpret what is going on.  I even consult the guru himself, Don Lancaster.  He gives me a few cryptic suggestions but nothing seems to change matters.   I finally decide, on Don’s advice, to leave it aside for now and move onto building the Cursor board.  We’ll have to hope whatever this is is just a problem with this particular test, and not a real problem with the unit.

Time for some characters!

Alright!

In my last post, I finally got my TV Typewriter to actually display something onscreen!  It wasn’t much, but seeing 16 rows of 32 well formed boxes on my screen made me feel like I’d reached low earth orbit, and was now on my way to the moon.

According to the TVT construction guide, the next step in our journey is to add the fabled 2513 character generator.  This is exciting because this is the point where the TV Typewriter actually starts putting recognizable stuff onscreen!  The instructions advise strongly to check all the power pins.  These ICs were rather exotic in their day (and pricey), and frying one in 1973 did not mean a quick call to Mouser.

Applying what I’ve learned so far, I check all the voltages and they’re all good.  I also apply extra caution and check all around for shorts caused by bad soldering or solder flux.  Where I see shiny flux where it shouldn’t be, I use a tiny jeweller’s flatblade screwdriver to scrape it out.  Not the ideal way but effective.

The construction guide advises the next step is to plug the board into the other two TVT boards, connect TV or Monitor, and fire it all up.  If all is ok, I should have a whole screen of @ symbols (this is the default binary code the address lines produce if nothing is trying to change them).  And we do!

Woohoo!  Now, I do see one little problem – there are some pixels activated that shouldn’t be.  I’m reassured by more experienced hands that this is likely just a short somewhere.

Still, the amazement isn’t wearing off.   This is a fairly complicated electronic device and I’ve got it working!  So many TVT builders before me did not even get past the planning stage, let alone get (mostly) working video output!

Ignoring the extra pixels for now, I now craft a ‘330 ohm jumper wire’ by attaching a 330 jumper to one end of a solid core #22 wire.  The goal here is to attach one end of the jumper to one of the +5V ‘ports’ on the bus, and then briefly touch the other end to the ‘signal end’ of each resistor tied to the 2513’s address lines.  The result should change the screen full of @s to As, Ds, Hs, Ps or blanks.  At least, according to the manual.  I try this out, and it sort f goes as I’m expecting, except when I connect R49, I get a B instead of P.   About 30 min of fooling around ensues.  I’m sending off what I’m getting to my vcfed friend Chuck, and he’s suggesting I’ve got something backwards, or that possibly the documentation is in error.  Eventually we figure out that B is what should be produced, not P, and that I’m suffering from a bit of PCB dyslexia and mixing up which pins are which on the 2513.  Once we account for that, we’re all good!

The next step according to the manual is to add the 2518 shift register.  This another rare IC, especially one dated to 1973, and I’m a bit nervous about plugging it in.  But after checking and rechecking, I go for it.  Now comes the real test – jumpering in several ‘clock’ wires from marked points on the timing board to bus ‘ports’ B1-B7, to try and get the TVT to produce a full screen of all available characters.  This should be interesting!

 

Timing & Memory Board Test

Having successfully gotten (I think) a tune into the TV using the TVT’s sketchy RF modulator, the next stage is to put some actual integrated circuits to work and see if we can get this this puppy to do something interesting.

The construction guide advises that I need to completely build up the Timing board and part of Memory Board A (Page A), just enough on the latter to try and get something onscreen. Here’s the Timing board all dolled up and ready for action!

At this stage of construction, we are not yet adding the character generator, just IC 10 (a PISO generator) and IC 11 (an open collector NAND gate). First, the TVT should establish 32 columns and 16 rows of boxes as the field where text would appear. Then, with everything sort of floating open, it should set each of those boxes to be fully on, meaning we’ll have 512 of them on screen together if it all works.

After a few very careful checks for solder bridges, I stack the two boards into the motherboard, and go for it. And….

Nada.

No boxes, nothing onscreen. I check pin 20 on the system bus (the composite video out pin) for voltage, and it has it. I then decide to connect a composite monitor directly and bypass the TV for now, just to be a bit more certain that what I’m not seeing isn’t due to the RF modulator playing games. But there’s nothing there.

On the advice of Chuck from VCFED, I start digging into the timing board circuitry. First, we want to make sure there’s an actual clock signal happening, otherwise nothing at all will function. We’re looking at the MC4024 ‘dual astable’ chip used in conjunction with the crystal to create our system clock. Based on pinouts and the TVT schematic, I should be seeing something on the XTAL (crystal) pins of that chip, pins 3 and 4. But all is silent. My logic probe indicates no pulse activity at all.

Mystified, I start probing around the chip with the logic probe. I’m really scratching my head. Is it a bad chip? Maybe.. these things are 40+ years old and failure is always an option. I swap one of my precious spares in. Same thing. I swap the other spare in. Nope. So I’m really scratching my head here trying to figure out what the heck is going on. Chuck’s concerned I might have gotten the wrong 4024 – there is a CD4024 made by TI and Fairchild – similar part number, but very different chip. I verify mine are Motorola MC4024s. Did I get a bad bunch? Certainly possible. But not likely. I get back to basics and look for voltage at pins 1 and 14. Without adequate power (+5V), nothing much is gonna happen. I find 5V at pin 14 but *not* at pin 1. Hrmm.

It turns out after an hour of puzzlement that part of the answer is at hand. As I look more closely, I realize that I have goofed! I failed to solder in one end of a jumper (dang blasted jumper wires!) that should have connected 5V to the trace leading to that pin. Soldering it in, I now have 5V.. but still nothing on screen when I power up. Argh! Thought that would do it.

More headscratching and investigation ensues, before it dawns on me that my IC socket might be a problem. Thus far, I have been conducting my voltage test from the socket pins on the backside of the board. Now it dawns on me to check from topside. Sure enough, although voltage is getting to the socket pin, it is *not* getting to the actual pin 1 of the MC4024. Aha! After testing things a bit, I realize something is broken in the socket. This was a hazard of using vintage sockets of dubious provenance. To test things out, I connect pin 1 and 14 via a jumper, and now I have something on both my monitor and TV:

Yay! Sort of. What’s onscreen *kind of* looks like boxes. But not nice, bright white boxes. It’s the same on composite… so it’s not an RF modulator issue. I’m going to get rid of this socket and replace it obviously since something’s borked on pin 1. I’m advancing, an inch at a time.

I keep probing around and find more problems. Pin 1 of IC9 (a 7402) on the timing board isn’t connected properly. I resolder that and now I have a signal there I didn’t, but still no change on screen.

Next I notice pins 3 and 4 on IC9 aren’t showing anything. The soldering looks ok and socket itself seems ok, so I swap to another 7402. Now i’ve got activity on those pins, but still no change onscreen. Agh! So frustrating!

Chuck suggests that I need to check pin 45, the character clock pin. I check it, but there’s no pulse there, so that indicates no dots are getting shifted out to the screen.

His next suggestion is to really check out those Signetics N8288s. These are very old divide by 12 counters that were a Signetics-only part and went extinct not long after their introduction. Chuck warns that the manufacturing processes used on these were not quite dialed in back in the early 70s, and it’s quite possible these have just degraded and died over the course of 40 something years.

I probe these ICs carefully following clock signals around. The probe’s speaker changes tone and the pulse speeds up or slows down depending on where I’m probing, as the clock is being divided progressively further and further. I’m scratching my head again – it looks like the 8288s are working just fine. However that proves to be incorrect. One of the 8288s, in IC2, is getting an inconsistent, flaky signal at pin 5 and 6. I notice if I press on the pin itself with my logic probe with a little bit of pressure, I get signal. As soon as I let the pressure off, nothing. I decide to (again) replace a socket, hoping the replacement works, as my stock of vintage sockets is dwindling fast.

That doesn’t solve the problem although now those pins are getting a much better signal. I keep digging around and eventually find IC9 is still not working correctly. I pull the board out and shine a bright light from on side so I can see the traces clearly on the other. It turns out I’ve accidentally bridged pins 2 and 3!

I make the correction, inspect the board one more time, and then do a literal Plug’n’Pray hoping I’ve found it. To my amazement and delight, this happens!

The RF modulator has dialed itself out a bit again but it’s clear from what I can see that we’ve finally got something to light up on screen. Connecting to the much clearer composite, it’s confirmed! I now have my boxes!

Wow. That was epic!

So I’ve learned a few things. One, if you’ve built it yourself, assume ‘user error’ is at fault before assuming your ICs or components are the issue. With traces, IC pads and the like packed in so tightly together, it is very easy, especially for a novice like me, to accidentally connect things that aren’t meant to be connected with solder, or not even solder them at all! Double, triple and quadruple check your work. Employ some form of illumination under your boards to really verify that you’ve got everything in order.

Further, be wary of sockets – especially vintage sockets. Even if unused, the quality of these varies and they do degrade over time, depending on how they are stored. Never assume because everything looks okay that it is. Check each and every pin on your IC with the pins on your socket and make sure they are actually connected!

And of course if you’re lost, it’s good to have the ‘phone a friend’ option. The vintage computing forums and email lists are fantastic for this. Everyone is so helpful and kind (and understanding!). Chuck’s expertise was invaluable here as it told me what to look for. It’s all a learning process.

I’m so impressed this thing is actually doing something! I honestly didn’t think I’d get this far. Okay, on to the next step!

RF modulator build and test

After successfully testing the power supply on my TVT’s mainboard, I now need to move on to actually making the TVT do something.  The next instructions from the guide have me building the RF modulator, which is the component that gives the TV Typewriter its name.  As mentioned before, I have had advice not to bother with this and just go with straight composite video.  However, I felt doing this, while easier, would sidestep the main achievement of the device, which was that it used something everyone had in their home (a TV) already.

Anyway, building the RF modulator is straightforward enough – the trickiest bit is winding the 14ga tinned bus wire into a coil that helps generate a signal for the TV to pick up.  You can see the result behind the trimmer capacitor.  The wire is very thick and does not want to twist into a coil easily.  It requires the use of a 3/8″ drill bit and very strong hands.  There is a point on the second coil from the right where a tiny piece of wire ‘taps’ (is soldered) into it to draw the signal.

After that, I install all the required pieces, including my 300ohm twinlead with ‘gimmick attenuator’.  If you’re of a certain age, you’ll remember that twinlead is basically a flat cable with two wires embedded in either side.  In the days before TVs had coax inputs, you had screw terminals on the back for VHF and UHF reception.  These twinlead cables went between there and the antenna attached to your house.   The ‘gimmick’ attenuator is nothing more than a short piece of twinlead that is cut and electrical taped a few inches up from where the twinlead is soldered into the mainframe board.  This provides a kind of capacitor that further hones the signal going to the TV, so as not to overpower it.

With everything installed, we are ready to do a test!  The goal of this part of construction is to find a channel between 2 and 5 we can tune the device to.  At this stage, we are trying to tune it so that the screen goes completely blank, and any static noise is minimized or eliminated.   Ultimately I found the best channel to tune to was 5.  Here is a video of my dialing it in (please ignore my messy shop!):

You can see how the screen goes to a solid blue.  Offscreen, I am using a small screwdriver to achieve that screen by adjusting the 33pf trimmer capacitor next to the coil.  Anyway, there you have it!  Another exciting first step towards a working TVT!

Firing up the mainframe!

Okay so now comes the scary part.  The TV Typewriter has its own built in DC power supply, connected to the wall by a line cord soldered into the board.  That’s a little intimidating for someone who, until recently, had never built anything electronic in his life!  We’re talking live, exposed AC connections.  One wrong move under power and.. ouch!

The goal for this first major step in construction is to install the transformer, protection diodes, and any other circuitry related to the power supply.  We then want to plug in and switch on and do some voltage tests.

Now, I’m going to confess, I am no electronics engineer.  I did not understand the transformer wiring at all.  I got some help from friends on vcfed.org with this, as it is not as simple as just connecting the post marked 12V to the PCB point marked 12V.  You have to tie certain posts together and then connect them, per the schematic.  And prior to that, they had me do some quick voltage tests, to make sure Signal had labelled things correctly in the first place.

Anyway, this is what the completed wiring looks like:

Another problem that tripped me up was the orientation for diode D6.  Orientation of diodes here is critical – the diodes provide protection for the system and make sure different voltages aren’t crossed.  A mistake here could cause damage to the whole unit!

Complicating matters further, this is one of the (many) examples where the schematic is at odds with the PCB layouts themselves.  More experienced eyes than mine looked at the schematics and were convinced the orientation of several diodes were wrong.  However, the errata provided after the construction article came out suggested that only D6’s orientation was shown wrong on the PCB silkscreens.  Indeed, the scanned copy of the construction guide that I was relying on initially had a correction from the original owner:

After much debate on the forums, I decide to go with the errata and only change the orientation of D6.   Although I usually defer to the experts, everything I have read indicates that the PCBs are correct, minus this particular error.  So I install all the pieces.

One other note: I took this opportunity to correct a situation I wasn’t comfortable with.  In trying to keep the look of the ‘new’ TVT vintage, I had chosen some Mallory 5000uf caps.  These are big silver can caps rather than the coloured ‘sausage’ caps common today.  The caps I had picked had little metal tabs on them, and to make it fit on the mainframe I had been forced to rig the tabs up with wire and then solder the wires into the board.  They looked kinda sketchy.  Anyway, I later found some screw top GE 5000uf caps, which I was able to install much more securely.  They are silver also and look perfect for the role.  So that’s what you see installed in the photo.

With everything connected, I decide to go for a test.  I’m more than a little nervous here and go a bit crazy, setting up the unit on the concrete porch outside my (steel) door.  I’m going to set the power switch to on and connect the cord behind the door.  That way if it goes snap, crackle, pop, I’ve got more than adequate protection!  I realize this is of course a little paranoid, but this is my first time messing with something that uses live AC power that  built.

Anyway, after checking and rechecking, I plugged the AC line cord in, braced for explosion.  But none comes!  I wait a good 5 minutes.  Nada!  No smoke, no fire!  No scary crackling sounds!  Woohoo!

Getting more brave, I pull the unit into the house and onto my old tile floor.  I plug in again, still standing a few feet away.  No problem!  Now I man up, and grab my DMM to check voltages.  According to the manual, I should have 5V on pins 58 and 59 of the ‘bus’, -5V on pin 57, -12V on pin 56, and +12V at a spot on the mainframe board set aside to provide keyboard power.  My results are very close!  +5V on 58 and 59 exactly (thank you 7805 voltage regulator!), then -5.45V on pin 57, -12.4V on pin 56, and +13V on the keyboard +12V point.  A little out of spec, but given the machine is not under load and that the variance is so small, the power supply is given a clean bill of health!  Yahoo!

I know this is kind of a minor thing – after all this is just a power supply at this point, but the fact that I built A POWER SUPPLY THAT DIDN’T BLOW UP is kind of awesome nonetheless!  Thank you to all those that helped along the way!

 

Building a TVT sandwich (again)!

Following the instructions of the TV Typewriter construction guide, the first thing to do after recreating my TVT boards (again and again) is to drill them out and work on aligning the stack connectors.  I knew this was going to be a problem from the get go — to test alignment as well as make sure scale wasn’t being lost during scanning or printing, I actually printed the artwork onto transparencies and then compared to the originals in the guide.  The issue I ran into was the stack connectors – the ‘bus’ that the four TVT boards connect via.  A quick study of the alignment of these using the transparencies revealed that between boards, they didn’t quite line up.  I supposed this could have been due to 40 years of humidity acting upon the paper of the guide, although I also have two copies of the Mark-8 construction guide and there is no difference between the artwork in those dimensionally or otherwise.  It could also simply be errors – I had read some contemporaneous accounts of TVT builders that complained of issues with alignment.  Anyway, after carefully considering the issue, I decided I could just ‘make it work’ by carefully adjusting the pins as I installed them.  Altering the artwork seemed like a fools errand – it might fix the connector problem but risked distorting everything else.

Anyway, after making the boards, I’ve drilled them and began removing the Molex connectors from my first, erroneous board set, and installing them on the ‘new’.  I also installed my new transformer, as well as the control switches (bottom) for test fitting.  So far so good, although the transformer sits a little too close to the board stack.

In terms of alignment vertically.. yeah.. it’s not pretty.  But I’ve confirmed it’ll work.  Now to solder in all 24 molex connectors and 240 pins (again.. ugh)!!

 

I redid the boards. And then redid them again. And again…

Okay, so when we last left off, I had discovered an unfortunate mistake in the original PCBs I had recreated for my TV Typewriter project: they were too small, a victim of the distortions that happen when ‘line art’ is scanned.

It’s all good.  As I mentioned also, I had managed to run into some actual vintage 1973 board stock.  Now re-doing my work was no longer a matter of desire – it was necessity, just to get that inch or so closer to a full replica.

If you read my post about my 2-5-2-2 toner transfer process you’ll see the first board I worked on – a redo of the TVT mainframe board.  But the TVT gremlins weren’t going to let me off that easy — it turned out, after etching, that it was still the wrong size!  Apparently printing onto magazine paper causes some kind of weird dimensional loss.  I suspect it may be because the inks used to print on magazine paper are water based and perhaps the heat from the laser printer causes a bit of that to boil off and thus shrink?  Not sure.  Anyway, after some trial and error I figured out the loss was about 1% all around.  Adjusting for this and reprinting gave me the right dimensions, and finally the correct size board.

Now, I mentioned before that these vintage copper clad sheets, which I was lucky to find, were only 0.03″ thick.  That’s a problem, because most PCBs commonly are around 0.06″ thick.  And we want that thickness, because the TVT has some heavy components, like the transformer, and we don’t want any flex.  So what I decided to do was etch my PCB pattern on one board, and then completely etch off the copper on another, and then cut to size and sandwich them together with epoxy.  This seemed straightforward enough, and so I trundled ahead — made my first mainframe board and then epoxied together.  And it looked good until  I noticed that bubbles were forming internally.  And no matter what I did, I couldn’t make them go away.  The result was a splotch effect that made the board look obviously glued together.

Nope.  Don’t want that.  It was suggested to me that I buy a book press.  Apparently you want to apply a huge amount of pressure to prevent bubbles from forming.   I couldn’t find one though – not one that wouldn’t incur a fortune in shipping fees from the U.S., which was the only place I could find them.

So I played around.  On the mainframe board, I actually ripped it apart (the epoxy is suprisingly easy to remove once exposed), cleaned and retried, this time waiting a little while before bonding the two boards.  This helped, but there’s still some discoloration.  I must have tried four different ways on five different boards before realizing there wasn’t much I could do.  My final trick was to switch to contact cement.  This made a world of difference – on my cursor board, it left it looking mostly like an original piece of 0.06″, save for a couple of splotches or ‘birthmarks’, as I like to call them.  I decided to accept that and move on.  A couple of boards I did – my Memory and Timing boards, I actually started drilling right after bonding, and this produced a kind of ‘craquelure’.   Here’s a picture of the ‘new’ timing board, next to the old one as I’m stripping the part.  Notice the ‘cracks’.

Oh well — over time they faded and now, frankly, I don’t notice them.  On my final board, the Cursor, I waited a full 15 minutes after applying the cement to both boards and then attached them.  This made for a much cleaner look, albeit with some ‘birthmarks’.  I decided to live with those.  This TVT will live inside a case anyway.  I doubt anyone but the most discerning will bother to note the splotches.   Alright, now to actually follow the directions this time and start building.  Here comes the mainframe!