TRS-80 reborn

Hi, long time since the last post.

Today, covid containment push me up to put out of my closets my TRS-80. It’s a piece of computer history! It was given to me by skywodd for shipping cost only! He tell me it was out of order, don’t know what the problem is and have no time to repair! But thank again to him for the “present” 😅

First things first, I did not have the power supply unit for this beauty! And what the hell append in the head of people who designed this? It’s a dual supply, one part is DC and other one is AC (because of a small needs for negative voltage). Look at the mess :

At the left you have the power connector (we gonna talk about this connector later, trust me!). Pin 4 is ground. It’s our global reference voltage all around the schematics. Pin 2 is power (about 18.5V positive voltage becoming 13V when filtering them). But, pin 1 and 3 are AC voltage. Alternatively positive and negative. It’s absolutely not clear on this schematics, but Pin 5 (ground) is also the center tap of the transformer creating the AC of pin 1 and 3. So when AC on pin 1,3 is negative this value is 16 volt UNDER the global GND. This how you can produce +5V and -5V.

You can see that the -5 volt only is regulated with capacitor filter and one zener diode. This is because -5V is only for only a few things.

You can also see that +12V is produced (on top of the schematic) but it is used in the +5v regulation stage too. This is because the regulator (MC1723C), kind of old fashioned regulator only provide “reference voltage” and need to be powered from outside, with at least 9V. Even if it providing regulation for 5V. All those thing plus diode bridge constrain me to some modification. Because I didn’t want to run the street to find any special transformer with center tape… I removed the rectifier bridge and soldered directly in his hole +13v and -7V from my power supply. +13V goes to 12V regulation too.

See the original power supply unit :

I have a TTi dual power supply.  2x30v 2A each. But this is only DC power. So the be able to power this mess with this, I’ll need 13V to GND and GND to -7V in row. So you have an “center point” GND and positive 13V and negative 7V. But i preferred to remove the bridge rectifier to prevent any bad surprise.

Ok now let’s talk about the disgusting flat flex keyboard connector :

Even before trying to power this thing I removed it. Because it’s oxidized (rusty ?) and some connection are already broken. I don’t want to take any chance with it! Cut between lines, hot air and remove. Clean up and empty the holes.

Next step (you got it?), the cpu’s pins s are curiously oxidized too. But only cpu, no one else. So remove and clean up with very tiny tiny sand paper. Try to take care of static discharge with this!

One pin (/halt) on the CPU was left unconnected (first picture). Bent, out of the support. I preferred to reconnect them and cut the track on the PCB. this way I can reconnect them easily and without fear of breaking the pin.

So now, let’s talk about the problem. If it exists? I powered this carefully one stage at the time. First, the +12V. Then, shut it down, after the -5V then shut down. Between each test, checks if output regulation is OK. then after all power at the same time. OK, now it’s time to see if this mess can boot up.

First, I checked if the CPU booting up, but I don’t have any display to test at this moment… 😑 I don’t need them to know if this boot. The answer is yes because I can see signal changing state on ADDRESS pin and data moving from the ROM output with an oscilloscope. So the CPU fetch data and seem to execute them. I can show the video signal on an oscilloscope and see if it relevant or not. However, the video signal seems to be the problem because I don’t have any either 😫

I suspect that some one plug the power inside the video connector… What? Yes you read right. 🙃 You can plug the power into the video connector out. Because radio shark wanted to save few dollars on different connector. You don’t have any key and connectors are the same. I can’t imagine how many of those computer died with these mistakes (understand me right, designer make the mistake, not finals users). On the picture below you can see the 3 identical connectors.

Here you can see the schematics of the conditioning video signal. You can recognize connector. Same as Power. You have +5 volt populated out and reference GND. Video signal is on pin 4.

Technical data give use information about this “Simplified Video Mixing” (you kidding…😝)

On the schematics you can see a small mistake. Q2 and Q1 are labeled as the same reference. MPS3904 (NPN). But you can also see that the two symbols are different. One is NPN (Q1) and the other one is PNP (Q2). After some research, the symbol are true and the reference are false. So Q2 is a PNP. You can put here anything fast enough for the composite video signal.

AND… on the pcb board someone changed it for the schematics reference :

I removed it and… tada it dead.. So you have a wrong reference and it is dead. Replaced and also his capacitor friend.

Here you can view the video signal I get without display plug in. On the first picture you can see the 16 “burst” of each lines. This is because the video consol is only constituted of 16 lines of 64 characters.

After some research and quite few try i finally discover (another ? 🙄) miss understanding or error in schematic. Because it’s seems that pin out of the video display was inverted… quite strange. After that I was able to see something on the display :

You can see the mess on the display. This seems to append quite often with this computer. After replacing dip connections in the right side (? 🙃) and cleaned them. I arrived to this :

Error on loading memory rom. Time to reconnect the keyboard to see if it is part of the issue. So clean up the other side, create a connector to save time. If you want to solder all those small cable properly and save a lot of time, I advise you to warm the plastic of the ribbon cable with hot air. It gonna shrink a bit. Just enough to tin it. You have to tin it, trust me ! If not all small wire composing each cable gonna spread in any direction and you will lose a bunch of time. After that cut between them. Use a rule if you want to have them all the same length. But it’s not necessary the right way to do it. You can cut the cable in circle to offset when you’ll solder them on board. I put connector on both side of the board. This way it’s easy to plug it in enclosure case or out for debugging.

And finallllllly after all this : I get this :

Job done. Changing all caps at the same time…

Leave a Reply

Your email address will not be published. Required fields are marked *