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Original TV Typewriter Rehab

falter

falter

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Jan 22, 2011
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I'm going to create a thread here to separate my repair efforts with both this TV Typewriter and my Mark-8. As always assistance is deeply appreciated. I recognize I am just a temporary caretaker, so I want to see these things go on to their future homes better than I got them.

I'm not looking to fix this overnight but just sort of getting organized and figuring out what I can do to safely test things.

As we know the label on the front of the case says built by Grant Runyan 1976. I'm not sure if that refers to the actual TVT itself or the box. I see evidence that this TVT may have been built to the basic spec earlier than 1976.. I haven't found a single chip so far that is later than 1974. The keyboard is a first generation SWTPC KBD1, which was designed by Don himself.

I see the remnants of the twinlead that would have been for TV use - it looks like it was clipped at one end. I suspect if it had never been used it would never have been installed, so that's why I kind of think this TVT arrived where it is in stages. There are two 'bonus' boards - one is Roger Smith's serial comm board, which I understand the hobbyist could etch themselves or order from techniques. I believe this board is the latter since it has professional quality silkscreening. There is another board composed mainly of 7400s and 7474s that seems to be inline with the video out. Not really sure what that does.. maybe enhanced cursor control or more function keys?

I guess the first place to start would be the 'mainframe' or motherboard, which is where power comes in. The power cable is chewed or has been rubbed in a way that the copper is exposed. I think I can save it as it's quite long so I'll just cut the bad bits off and resolder.

The PSU seems to mostly conform to the original design - it has a Signal 24-1A transformer. However, it looks like Grant deviated a bit - there is a 309 regulator with big heatsink installed - no 7805. Inside the box on the back wall is a warning not to power up the mainframe without other boards plugged in, otherwise it could damage the zeners. I don't know if that is applicable with this mod in place.

Also, the motherboard is missing one molex connector for the bus. I am not sure if this is on purpose or not.. I see a molex connector stuck to the underside of one board.

I do not have a variac.. but I'm willing to get one.. just have to understand how to hook it up properly.
 

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Consider that my TVT used 6 2102 SRAMs for screen memory. Ran hot as pistols. Each SRAM is rated at a dissipation of 1 Watt. Consider that 6 of the things means 6W at 5V. The LM7805 is rated at a maximum supply current of 1A. See the problem--and we haven't talked about the rest of the ICs on the board?
 
There is more text on that label which may give us more clues as to the power supply issue. It seems to refer in particular to the negative voltage supply.

Dave
 
falter said:
I do not have a variac.. but I'm willing to get one.. just have to understand how to hook it up properly.
Click to expand...

"Variac" was (is?) the original trademark name but search for "variable autotransformer"-- they are super simple devices. 110VAC (in your case) from the wall goes in, and adjustable 0~130VAC goes out the socket to your device under test. The idea is that you just slowly dial the voltage up to catch anything going sideways in the PSU instead of starting off with 100% mains voltage, possibly avoiding maximum damage.

I think the vintage General Radio ones look incredibly cool but I advise you to avoid ancient used ones for bench work (I have an old GenRad variac whose insulator had broken down and it immediately went hot chassis when I turned it on, oops). Note just for context that a variac set to 110V is != an isolation transformer so you still shouldn't poke your fingers into the PSU.
 
This is the full (mouse-nibbled) note. Pity how these things often end up in places mice like to be.
 

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I have done a very rough calculation on the unloaded negative power supply.

I have made a few assumptions though...

The power dissipation in the two negative Zenner diodes look OK.

0.543W for the 1N4734 (against a maximum of 1W).

0.723W for the 1N4736 (against a maximum of 1W).

The 47 Ohm resistor is a tad high (0.532W verses a rated 0.5W).

This is on the assumption that we have 12V AC from the transformer and this rectifies to approximately 17V DC.

Dave
 
falter said:
Also, the motherboard is missing one molex connector for the bus. I am not sure if this is on purpose or not.. I see a molex connector stuck to the underside of one board.
Click to expand...

That middle connector isn't used/connected to anything on the motherboard. It is used to pass signals between the stacked boards however.
 
labomb said:
That middle connector isn't used/connected to anything on the motherboard. It is used to pass signals between the stacked boards however.
Click to expand...
It's been a while since I've looked at it but that rings a bell now. Many thanks!
 
daver2 said:
I have done a very rough calculation on the unloaded negative power supply.

I have made a few assumptions though...

The power dissipation in the two negative Zenner diodes look OK.

0.543W for the 1N4734 (against a maximum of 1W).

0.723W for the 1N4736 (against a maximum of 1W).

The 47 Ohm resistor is a tad high (0.532W verses a rated 0.5W).

This is on the assumption that we have 12V AC from the transformer and this rectifies to approximately 17V DC.

Dave
Click to expand...
Yes I wouldn't think Don would be instructing people to test the PSU unloaded if it were going to blow something up, right? I'm thinking that note came from hard experience though..
 
Very possibly.

Hence the reason Variacs are very useful to test linear power supplies like these!

It could be that they had a high mains voltage at some point. That would increase the power dissipation - but I wouldn't have thought it sufficient to damage the Zener diodes (if they are of the correct 1W rating).

You can also add some light bulbs in place of your delicate electronics cards. You can use 6V and 12V car/motorcycle light bulbs. Select an appropriate wattage for the desired load you want. That should prevent damage to both your cards (if the PSU misbehaves) and the PSU from no load.

You can also use a resistor (or two) of the correct value and power to act as a dummy load.

Dave
 
Yep...

You plug the cable into a wall socket. Turn the knob anticlockwise (to set minimum voltage to the equipment under test). Plug the equipment to test into the socket on the Variac. Turn the power switch ON. Turn the knob clockwise slowly to increase the mains voltage to the equipment to test.

The voltage you are providing to the equipment to test is indicated on the display.

Dave
 
daver2 said:
I have done a very rough calculation on the unloaded negative power supply.

I have made a few assumptions though...

The power dissipation in the two negative Zenner diodes look OK.

0.543W for the 1N4734 (against a maximum of 1W).

0.723W for the 1N4736 (against a maximum of 1W).

The 47 Ohm resistor is a tad high (0.532W verses a rated 0.5W).

This is on the assumption that we have 12V AC from the transformer and this rectifies to approximately 17V DC.

Dave
Click to expand...
That's pretty reasonable, Dave, but the owner must have had issues with the zeners being borked; otherwise, the note would not have been written.
In any case, it's not a particularly good design, especially if the -12 is used to supply an RS232C setup.
 
I don't disagree. Hence the subsequent suggestion of the Variac or a dummy load.

Clearly the owner had an issue - but I still would like to know why.

Dave
 
falter said:
An ebay cheapie like this would work?

www.ebay.com

5Amp Variable Transformer 500VA 0-130V AC Voltage Regulator Metered LCD Display | eBay

(Capacity 500VA 0.5KVA 1KVA 1000VA 3000VA 3KVA. Just Different Color) Other Color 500VA / / / / /. A grounding port designed onto the device for much more stable and secure protections. Built-in Copper Coil.
www.ebay.com
Click to expand...
With your collection that seems like something you should definitely have, even if you didn't need it for this particular project. They allow you to power up old electronic devices in that haven't been turned on in decades in a safer way, as well as helping to debug issues. Of course you'll need to read up on how to use them safely, like knowing just how slowly you need to bring the voltage up for a particular piece of equipment.
 
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