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Digital Group Z80 - almost working

Getafix

Experienced Member
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Apr 28, 2022
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54
Location
Vancouver, BC
Hi,

I fairly recently bought a digital group Z80 system (16K). It's in pretty decent shape, but not quite working. When powering up I get the message to Load Z80 INITIALIZE Cassette. No matter what I load, the load never completes - when the audio stops nothing more happens. If I then start another tape, the new program just keeps going into the bank where the first load stopped.

I did what Falter did in a thread by more or less the same name as this one - I "looked at" the RAM chips by putting them on the video board. I found (so far) only 1 faulty chip which I replaced with a chip from the last bank of RAM. Now, I don't get the periods anymore when I load through bank 6 RAM.

I have tried from the cassettes I have, as well as from the digital files I found on the bytecollector site. So far no luck, but I'll keep at it.

From one of the YouTube videos I found that shows a program loading, I see another difference with what I have going on. On my system, the display "loses sync" during some portions of the load. This is repeatable - I will make 100% sure - I believe it always happens during loading into specific banks. Maybe this is an indication of an issue to chase down?

I cannot DM anyone yet - this is my 1st post and only after 10 posts do I earn the trust to DM :)

Any ideas or help appreciated!

-Stefan
 

NeXT

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By losing sync, is it acting like the system is resetting and then immediately trying to keep loading?
What is pin 26 on the Z80 doing with a scope/logic probe?
 

falter

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Sounds to me like you have issues in the audio side of the TVC card. Is it the 64 char or 32 char card? I had a lot of challenges with the latter.
 

daver2

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Let us know exactly what cards you have and we can take it from there.

The first step is to make sure the power supply is working correctly. If you have lost some of the power supply voltages (or they are not smoothed because the capacitors have dried out) you will get nowhere.

What test equipment do you have access to?

Get a few posts under your belt. Ten posts do actually go pretty quickly!

Dave
 

Getafix

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Location
Vancouver, BC
Hi,

I have a TVC-32. For the "out of sync" I'll attach two photos to show "normal" and "out of sync" screen pictures. I have a Sanyo VM-4209 but I found that the little //c monitor is more stable than the Sanyo for knowing what's going on during these bad sync moments (happens on both so not the monitor).

Pin 26 on the Z80 stays high (unless I hit the reset toggle on the front plate).

The voltages I measure are near perfect on the motherboard: +12.87, -11.85, +5.0 and -5.0. I have a scope (and logic probe) and some other gear but very limited electronics knowledge - I am learning but it would be a big mistake to assume I know, well anything, really :oops:

As for cards:
DG-2002B - Motherboard
DG-1003A - TVC-32
DG-1010A - Z80 CPU
DG-1002A - Parallel I/O
DG-0011A - RAM cards (x2 fully populated with 2102s - so far only 1 chip known bad so the last bank on the 8-16K is blank now)

The keyboard itself is quite interesting - no idea if it works yet - picture included.

Stefan
 

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falter

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Something weird is going on with one of your ram banks. It should be going 1s, then 2s, then 3s onto 7s as it fills up memory. But you have 0s at the start and 0s where the 3s should be.

If you know for certain all RAM is good (at least per my video card test), the next thing I would do is get the jumper settings to run just the CPU card (with RAM installed on it) by itself. This should be sufficient to load the basic operating system. Once you have that down, then figure out the jumpers to add in the bigger memory boards. Either something isn't jumpered right, you have bad ram, or something maybe is going wrong with the addressing would be my guess.
 

Getafix

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It starts at 1s (after the text to load the Z80 initialize) and then does go up 2..7,0..7,... etc. I think it looks weird on the monitor just because of when the photo was taken - it had rolled over (0s at bottom and top from roll over - 2's is where it's currently loading).

Your point is still good and I'll try to get the machine to go with just the CPU card with CPU RAM (which passed the video test - thank you for that idea btw).

From the bytecollector site - is this the OS file I should be trying to load? z80_opsys-32char.mp3 , i.e. in
bytecollector.com\archive\digital_group\audio_images\z80_software_digitalcopy\32char

Thanks - Stefan
 

Getafix

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Well, the machine works the same with just the CPU 2K. The z80_opsys-32char.mp3 appears to be more than 2K as it loads banks 1 through 7 and then I start getting the .'s - no surprise because there's no RAM.

What I have noticed is some loads appear very fast while others load way slower - and by that I mean the octal numbers appear on the screen much slower. For example, from bytecollector.com referenced above "z80_loadtape_prog-1 hamdemo-32char.mp3" plays for 28 seconds and loads only part-way through bank 3 whereas "z80_loadtape_prog-2 flag-32char.mp3" is the same length/runtime but runs over the end of the 2K (past bank 7). Was this machine capable of selecting a baud rate based on the tape header, maybe? Maybe that's also an indication that there are problems with my TVC?

I received 10 official and 3 home-made tapes with the machine. Most of the tape have versions of Maxi Basic on it (with some notes about patching some bytes), or Tiny Basic, or programs for/in basic. I have one tape labeled "Z80 Mem Chkr/Cursor O/S SUPER CLOCK - GGSS,Inc.". The Mem Ch(ec)k(e)r? appears to be a small program that does fit into 2K, but as with everything else, once the data on the tape has run past, nothing happens on the machine. Whatever is supposed to pickup/indicate an end of load is not triggering.

Are any of the files I have mentioned, or that are on bytecollector.com, the Z80 INITIALIZE cassette the Z80 CPU BOARD documentation speaks of? If not, does anyone have a .wav or other audio version of this cassette that they could share with me? I'd even be happy to pay for a cassette version (copy of with shipping) if the copy is verified to load on someone else's system. Probably easier to grab an audio copy and verify that, though :)

Thank you
Stefan
 

falter

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I'm pretty sure OPSYS should fit in 2k. It was released before the memory board expansions were available. I think Bryan's copy there is just the base one, although I have seen some images with extra junk included that bloats up the size.

Definitely the TVC could be at issue. I know the 32 char board had a reputation for being finicky. Mine was heavily modified and only required a couple of mc1458s to get fully working. I also had to replace a trimmer pot. If it had gone deeper than I that I would have been stuck as I didn't have a scope or anything at the time.

If I get some time later I'll pull my machine down to 2k and try that opsys tape.
 

Getafix

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Thank you falter, I would really appreciate that. Just had a look through the IMSAI boards and found some more 2012's (some are white ceramic). I should be able to get this machine back to 16K if I can get the loading working.

As soon as I know I am trying to load something that works I'll tackle the TVC in more earnest. Mostly that means I'll try to learn more so I might identify non-working components.

Do do you happen to have a picture of your TVC card? Curious about the mods.
 

Getafix

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Success!

My digital group Z80 system now reads tapes and works. I have only read the opsys tape, but this at least looks great! :)

I didn't have to replace a trimmer pot, but I did have to mess with R42, the 50K, to get this working. This was after cleaning all of the chips and using a bit of deoxit on all the sockets. Thank you falter for mentioning the pot, not sure I would have looked there next.
I am so happy! :)

Stefan
 

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Getafix

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First I have to get the keyboard to work. There’s some life there but it’s not usable at the moment. Not just because I have disassembled it completely ;)

Hopefully when I put it back together it works as well.
 

Getafix

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Keyboard also works. Everything works now. Just had a quick game of Sargon Chess. Unbelievable what people were doing way back in the mid-70s. I guess technology rocketed forward after that - Atari in '79 had a home computer with pretty descent graphics in color.

I sure am happy with this machine though!
 

NeXT

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First I have to get the keyboard to work. There’s some life there but it’s not usable at the moment. Not just because I have disassembled it completely ;)

Hopefully when I put it back together it works as well.
That keyboard screams this was a hobbyist build. That edge connector soldering was all kinds of jacked up.
 

Getafix

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The boards are beautifully done and was done by a computer shop in Toronto, Canada (Computer Mart on Bayview Ave - there's a sticker inside). It was picked up by the first owner, Mr Schwarz, on Jan 28/77. The receipt is still with the machine - unfortunately no price. There are several other receipts for cassettes and magazines (Byte and Interface) that are dated through '77. Came with some documentation, flyers (from the digital group and computer shops in Toronto). It's a nice little package.

The keyboard is a different matter. I think that was a hobby build and the soldering is "all kinds of jacked up". I had to fix some of the traces/edge connectors too. But, it all works now. Once in a while, after running some program, the keyboard will stop working and a power cycle is necessary to get it going again, not just a reset. Still, I'll take it from where I was just a couple of days ago :)

The TVC is still a little odd - still "sync" issues during a load and the last line is half off-screen. I will mess with it more later. For now I am just enjoying the fact that it works. Wrote some basic programs this afternoon. All pretty good fun!
 

Getafix

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In my "Introductions" thread @daver2 asked for a video of the computer working, so I made a video and here it is, cross posted from the Introductions thread.

Stefan
 

NeXT

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Hammond Manufacturing operates out of my old home town Guelph, ON which is just down the road from Toronto. I remember whent ehre was still farmland between the two. ;)
That enclosure was very likely catalog ordered (their catalog is STILL amazing) for carrying around shallow depth rackmount equipment but was repurposed accordingly.
 

Marty

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Jul 26, 2009
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Hi All;
Getafix, there is another Operating System for the Digital Group, but it takes about twice as much memory as You have..
It loads and works in about 30K of memory..
I gave it to Bryan Blackburn (I don't know if Falter has it), it is called ICOSE, it has an Assembler , an Editor as well as Reading and Writing to Cassette and a few other Commands..
I don't know if I have a copy of it on Cassette any more, All I might have is the Listing for it.. Which is quite long and many pages of hand dis-assembly..

THANK YOU Marty
 
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