Retrochallenge 2016 – End of the line. All change.

Well, another RC comes to an end an if there’s any truth in the saying “It’s not the winning, it’s the taking apart” then all is well. If, however, it is the winning then we have another “success deficit” on our hands.

So far I have the VT-180 in pieces. I looked at a few signals on the video board and compared them to those marked on the schematic but they all looked fine (see the photos).

160116-IMG_20160116_181840 160116-IMG_20160116_181754 160116-IMG_20160116_181725So next up I wondered if the 2114 static RAM chips have failed. I’ve seen them go on my DEC Rainbow and also on a Commodore PET a friend of mine has, so it’s quite common really. It would explain the garbled output.

One thing I like about DEC kit is it’s well made. The downside is that they used a lot of solder and getting chips out of any DEC board I’ve ever worked on is very difficult. So, as the last few hours of RC2016-1 tick away I have3 chips out, new ones on order and crud to clean out of the holes in order to add sockets.

The nice thing about Retrochallenge is there’ll be another one passing by any minute now.

Retrochallenge 2016 – Week 1 done. Some progress.

So here we are, one week in to RC2016-01 and what do we have to show for it?

As you saw in my previous post, I have taken the lid off the VT180 and it sits as a bare metal cage while I try to figure out what’s wrong.

151231-IMG_20151231_174248The initial display (shown on the left) has changed and is less verbose. 160106-IMG_20160106_194827 I’m not sure what happened to trigger the change.

The screen display is no longer in sync and rolls very quickly.

In order to help with the repair I have taken the VT100 board out of the card cage and I have it plugged in to the cable that was powering the card edge connector on the card cage. It’s clearly straight through so should be fine.

160106-IMG_20160106_194705That means that I can get to the board with my meter and ‘scope etc. I have also removed the AVO (advanced Video Output) board and the small inter-connect board. This hasn’t changed the symptoms at all.

The first real diagnostic work was to measure the voltages from the PSU. They all seem fine.

I have been able to use a logic analyser (borrowed – I really want one of these) and I’ve been taking a look on the bus.

have attached three screen shots taken of the logic analyser screens. I have 18 channels and so I have the data bus, the lower 8 bits of the address bus and the synch pin. All signals are from clips on the 8080.

I have the triggers set to stable high on sync (start of new instruction) and 0 on the address bus. I put the analyser to wait for trigger and turned on the VT180.
The three shots are from the same run and I’ve just stepped through the time a little to show what happens at start up.

vt180-digiview-3vt180-digiview-2vt180-digiview-1I get this with or without the keyboard. With the keyboard, all of the lights are lit. When I first started on this repair, a few, normal looking lights came on first and then, after a while, they all came on. It’s a funny old world.

 

Retrochallenge 2016 – First look.

151231-IMG_20151231_174248Let’s start at the very beginning. It’s a very good place to start – Apparently.

According to the documentation, the VT180 is a VT100 serial terminal with a VT18X add on board. This add on is a Z80 single board computer that talks to the VT100 though one of its communications ports. this make for quite a nice combination for the day.

You can use the VT100 as just a terminal to your corporate mainframe or mini-computer and when required, fire up the Z80 into CP/M-80 and compute like there’s no tomorrow.

Mine is computing like Yesterday was a little rough.

Onward.

At this point I am working without the VT-18X board present. I don’t think I need it yet. My understanding is that the VT100 side should come up cleanly without it. I could of course be wrong but I’m going down this road for a while.

The photo above shows what I’m dealing with. The keyboard has sensible lights on but I don’t know yet if that means anything.

My first job as always is to try and re-seat all of the socketed chips on the board. I’ve don’t that and it made no difference. Oh well.

Going in.

Next step, are the voltages correct? The VT100 has an Intel 8080A processor on board and that needs care and feeding.

Off with its head.

One thing I’ve always liked about DEC equipment is the way they built it. Their kit always feels like it was ment to br built and stay together but they also kept in mind that you will need to service it.

160102-IMG_20160102_140931The photo on the left shows one of four plastic “poppers” that hold the case halves together. There are also four bolts to hold the steel frame in but that’s it. Dead easy.

160102-IMG_20160102_141849

Check the power supply.

Using the diagrams in MP00633_VT100_Schematic_Feb82.pdf I checked the voltages on all of the pins. Everything is OK with the exception of the 12V line. I can see that there should be 12V on the orange wire but there is hardly anything there at all 0.4V and just ain’t enough.

Is that my problem?

Hmmm. The CRT uses the same 12V supply and you can see from the top photo that the CRT is working. When I tested the voltages I didn’t have the CPU board in and the CRT didn’t come on. When I plug the CPU board in, the 12V comes back.

Doh! Red herring.