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Weird characters in kaypro II boot

Hi durgadas twins 😅, thank you very much for your support, I've changed the original cpu that was a little corrosion on the pins with one from a spectrum, but both work ok, it shows weird characters on the boot screen but boot up well. I'm going to triple check all the connections in the processor socket and the processor itself.

Other chips that can be responsible of??
 
Maybe time to reset, get a summary of all the current evidence. Can you post of picture of what the screen looks like now, and specifically which ROM image you have installed?
 
Ok I'll try to start from the beginning. I have two kaypros, one 2x and one II. The 2x is a ASSY 81-581 motherboard, and a 81-478A rom. The II has a 81-110 B motherboard, 81-149C monitor rom and 81-146 character rom. The 81-149C didn't have a label, but I compare with the 81-149c on Internet and are identical.


My problem right now is with the II. At the beginning didn't work, but after changed the rifa capacitors and another one in the video card, the computer booted up and show the boot screen with wrong characters on screen (first photos in the post). Trying to fix the issue, I observed corrosion in the pins of a bunch of chips, and changed the Z80. The computer show the same behaviour. I'll try then, to test all the socketed chips ( not many in this motherboard) and all were ok. I've read somewhere a method to test the video circuitry putting a empty rom and, test the machine. In fact, at this moment, the screen fills with 9's and apostrophe's, a behaviour expected.

My next step was try to change the place in the screen where the bios put the characters looking for the possible memory chip that can be damaged. Well I tried to burned the 2716 eprom with no success. Tried even burn with an external power supply, putting 25 volts on pin 21 but no success. In fact I think that I damaged the chip. I don't know if putting the damaged chip into the kaypro can harm the computer.. I don't think so, but my knowledge is limited.

In the meanwhile, the 28c16 that I've bought to replace the 2716 arrived, and to my surprise, when I put in the kaypro, the screen was full of garbage. At the beginning I thought in a possible incompatibility with the 28c16 but there was no sense. By the way I've ordered a bunch of 2716.

In the meanwhile I've tested with the diagrams the continuity of almost all the components on the motherboard but there were ok.

I've managed to burn the 2716 when arrived and no change at all.

Now the situation is, screen full of garbage at boot up, the same thing with the 2716, 28C16, with the original rom 81-149 C, the ones on the Internet, with the monitor rom by durgadas (bless you 😅) and the memory rom too.

It seems no change in the screen with any change in the previous things.

The voltages in the motherboard are fine. I've tested the motherboard of the 2x in the II (fingers crossed because I didn't now it something bad can happen...) but the lights on the disk drives was ok, but no video at all. I've changed the z80 and no news. The 2x boot fine with the two, and the II garbage with the two.

So, now I'm a little bit tired, so I'm try to dig in the boot process to identify the component, or what can be causing the issue.

I don't know too, what kind of cross tests could I do with the other kaypro without risks. So I'm going to send to you photos of the start screen of the II, the 2x motherboard. The picture of the II motherboard is already uploaded.

So thank you all for your support, especially you, durgadas,

By the way, I'm from Spain, and I'm using the 2x with the 220 mode of the motherboard, but the II can't have this possibility, so I'm using it with a transformer.

That's all, be free to ask please

In the pictures with garbage, one is with the monitor rom and the other without rom. With an empty rom, the screen is the same, no 9 or apostrophe at all
 

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OK, so it seems the Kaypro II has regressed, as originally there was evidence that the CPU was running instructions and even the ROM. (BTW, the photo of the mainboard is for your 2X, not the II). At this point we need to look for something that was altered or damaged while you were testing the chips. Perhaps the most likely thing is that the CPU or ROM sockets were damaged (or solder joins failed due to the mechanical pressure of removing/inserting chips), or that a IC pin got bent over and is not properly making contact in the socket. It is not always easy to see a bent pin without removing the chip.

One problem with working on Kaypros is that the mainboard is largely unsupported inside the case, and inserting chips - especially the larger ones - causes a great deal of flexing of the PCB (unless you take care to support the PCB from the back during insertion). If there were any marginal solder joints, this flexing could easily break them. It may require (careful) removal of the mainboard and close inspection of the back.

On the Kaypro II, the screen shows garbage until the CPU runs the ROM instructions to clear it. My memory test and monitor ROM images never clear the screen - you have to run a "memory fill" command on the video RAM to clear it. For the 81-146c ROM, it seems the CPU is not able to run the instructions in the ROM (either ROM does not work or the CPU does not work, or some path between those two is broken).
 
Hi, yes the pic is for the 2x, the picture of the II I've already uploaded. Ok, I'll check again the continuity for all the main chips. I know that your roms don't clear the screen, but I think that with the monitor rom, there's is an "a" and a "b" switching on the first position, and a count between 1 to 9 in the memory rom, it's ok??
 
jmellidg said:
Hi, yes the pic is for the 2x, the picture of the II I've already uploaded. Ok, I'll check again the continuity for all the main chips. I know that your roms don't clear the screen, but I think that with the monitor rom, there's is an "a" and a "b" switching on the first position, and a count between 1 to 9 in the memory rom, it's ok??
Click to expand...
Correct. If you're not seeing those characters in the first position of the screen, that indicates the ROM is not being executed.
 
Hi people,

I've checked all the schematics with the motherboard and ALL the connections are fine.

So I guess that must be a faulty chip, what do you think?? I guess that the eprom is never loaded because the emergency ROM by durgadas don't show that first characters in the screen expected. I've tested the Z80 in another machine and it works, so it seems that isn't a problem with the processor.

So my knowledge ends here.. I'm afraid that there's no idea about what chips do things before the EPROM is loaded. Please can you help me identifying what chip can be the wrong one??

I have a working 2X so I can try cross tests, but I've already done all the trivial ones with no result.

Am I wrong in my guess??

Thank you in advance
 
jmellidg said:
Hi people,

I've checked all the schematics with the motherboard and ALL the connections are fine.
Click to expand...

If you've checked every wire to every chip across the PCB, and they are all working, then a chip would be the only logical conclusion. They are all mostly common chips IIRC. How are your desoldering skills?

Replacing chips one by one is going to be a difficult proposition. Also the chips you will need will be quite expensive. You can focus on the basics such as getting the ROM working, but if you aren't even writing video memory, that's going to be hard to do and I suspect you either missed a wire, or you have an intermittent connection or partial fault which is going to be difficult to locate.

At this point, you're pretty much flying blind. You really do need some suitable instruments to see what is going wrong, and some deep experience with how a z80 operates, at least to troubleshoot enough to get the board executing some instructions and writing to video memory.

A Hantek 32ch logic analyzer is probably the cheapest way to get some of that visibility since you need to confirm a large number of wires are doing what you expect. Also, as always, make sure your 4116 RAM voltages are correct ( 12v -5 and 5v ) and be careful as you go.

You next goal would be to ensure the z80 is operating with the ROM and is picking up and executing the ROM codes correctly. You'll want to make sure all the address lines to the ROM are working as expected. If you can read binary natively, it helps, but you can set up a logic analyzer to translate addresses and the like.

Otherwise, it's a game of guessing which chip and replacing it. A thermal camera ( if you have one ) can detect which chips are warmer, but some chips just run warm - it's not always indicative of a fault.

If you have an oscilloscope, you can check the decode lines are operating and at least appear to be running to the chips.

There is a fair amount of knowledge required to troubleshoot at that level, but you can pick it up as you go.
 
So, between the CPU and the ROM/RAM are a few fairly-common chips. You've got the high-address latch at U59 (74LS373), low address buffer at U49 (74LS241), data bus buffers at U64/U65 (74LS243), the memory decoder at U60 (74LS138), control bus buffer at U62 (74LS241), misc control logic at U56 and U37 (74LS02), and so on. But, you really should be doing some probing with an oscilloscope in order to find the likely cause(s) first. Just blindly desoldering chips and replacing them is going to be time-consuming and risks more damage to the board.

The first place to look is probably the /CE (18) and /OE (20) signals on the ROM (U47). That will tell you a lot about what is going wrong and where to look next. You should be seeing both /CE and /OE go low during the same time (although their leading/trailing edges may not line up). If you don't have a dual-trace 'scope, at least confirm that both of these signals are continually going low for about 400-800 nanoseconds (but also returning to high level in between).
 
Hi, thank you both. I have a pocket oscilloscope, and I'll try to work with it at Christmas!!. I'm not very good at this level of detail, but always is a good time to learn!!

Thank you very much
 
As long as you can get 20Msps or higher, it's enough to get a good idea of what is going on, on the main board. :)
And you can see what chips are being accessed through the chip select and output enable lines... And the direction lines to buffers.
 
Hi everybody again, I've been trying to fix the kaypro again..
But no success 😭
I've socketed all the memory and memory video chips. One 4164 was bad , but it doesn't fix the issue.
The screen is full of garbage when the computer start.
I've seen in the tech manual that garbage on screen can be a problem with the reset switch but it seems that there's no short in the switch, but it doesn't work at all.

The lights of the drives still on all time, I don't know it it could be a clue.

So, please, be patient with me, I have a basic knowledge about electronics. I have a pocket oscilloscope, so please can you tell me in a very basic way where can I start to search for the problem??

As you earlier say perhaps the /CE and /OE of the rom could be a good starting point??

Thank you all!!
 
jmellidg said:
Hi everybody again, I've been trying to fix the kaypro again..
But no success 😭
I've socketed all the memory and memory video chips. One 4164 was bad , but it doesn't fix the issue.
The screen is full of garbage when the computer start.
I've seen in the tech manual that garbage on screen can be a problem with the reset switch but it seems that there's no short in the switch, but it doesn't work at all.

The lights of the drives still on all time, I don't know it it could be a clue.

So, please, be patient with me, I have a basic knowledge about electronics. I have a pocket oscilloscope, so please can you tell me in a very basic way where can I start to search for the problem??

As you earlier say perhaps the /CE and /OE of the rom could be a good starting point??

Thank you all!!
Click to expand...

Start with the z80. Check stuff like the clock, and that it's reading memory, and the reset is working, and it's reading and writing and doing both IO and Memory requests.

If you want to go further, you could try a logic analyzer, or writing your own debug software to rewrite the video memory on startup. Things like that are good milestone tests.

Debugging something like a complete computer is hard work. Some members probably have test roms for it if you ask around?
 
Hi, thank you!! I'm sure that the Z80 work in other computers, but I'll test it again. And yes, durgadas has several monitor roms but I'm afraid that the computer even doesn't run the rom.
 
jmellidg said:
Hi, thank you!! I'm sure that the Z80 work in other computers, but I'll test it again. And yes, durgadas has several monitor roms but I'm afraid that the computer even doesn't run the rom.
Click to expand...
Sorry, you've misunderstood me - I'm not suggesting that the z80 is faulty ( though it might be ) - I'm suggesting that the first point in troubleshooting a z80 system that is not working is to check that the z80 is doing what it is supposed to be doing - eg, reading and writing data, and that all the signals to it look correct. Then you track this back to memory to make sure what it is seeing is correct - is it reading the rom and executing? Is it getting a good clock? At the right frequency? Is it clean? Are you seeing the right pins strobing? Are any held low or look weird compared to other similar pins?

If the z80 is working, and the ROM is working, then even if other things are failed, you can write your own ROM to do stuff like rewriting the video memory with information. Once you have a z80 and ROM and screen, you can write diagnostics. Check stuff piece by piece. Narrow down what is faulty.

I had a system that had been given up on by several repairers. I used an oscilloscope to slowly read the addresses where it got stuck, and it was in a waiting loop while it expected something from an AY-3-8910 chip - So I guessed it was getting a bad signal from the AY chip, replaced it, and it started working again.

So when I say start with the z80, I mean begin at the place that needs to work the most, and start making sure what it needs the most is working, and follow this path until you find where it's not working - :)

If the z80 is reading the ROM, you can begin just by writing a simple ROM that does something, eg, Write patterns to memory, and see what it does next. It writes I/O operations to make something happen so that a chip is activated. If the z80 isn't reading the ROM, then that's the first thing to track down as it should be relatively simply to figure out - after all, all a z80 needs is a working address and data bus to a ROM chip, and some ROM decode logic to be working so that the ROM is selected. Trigger the scope with the address decode and make sure your address lines to the ROM are all working.... Even a ROM of NOPs with a RST0 at the end is useful as a test ROM, because it will produce recognizeable patterns on an oscilloscope. As long as you have around 10MHz bandwidth on your scope you should be fine with a z80.

There are cheap $10 logic analyzers too that can decode multiple lines and show you what some address lines and data look like when certain signals exist.

You can even make up a board that extends the z80 out of the socket, and provides pin headers to each pin to attach probes.

Never underestimate custom designing simple hardware to assist in the debug process.
 
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