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 I  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!

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)!!

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!