This is a bit of an unusual one. It looks like a Commodore 64, but with bits missing, some extra wires and an extra board.
The following photo (thanks to the owner) shows the board in it's usual location.
This is one of a number of machines Commodore produced that were basically a Commodore 64 board in a PET style case.
It fits quite neatly, with the userport, datasette, IEC and AV jacks on the side, along with the cartridge slot.
The two joystick ports are on the front edge. There is no power DIN socket or power switch.
The circuit board on the left converts the composite video output from the C64 into something suitable to drive a standard PET 12" green screen monitor.
A Bit of History
You generally see these described as a way of using up spare PET cases and warranty return C64 boards, but I am not sure that is right.
All the boards I have seen have been factory produced with several parts not fitted.
I could understand if they removed the SIDs, but I see no reason to remove a bunch of passive components etc. and the board is clearly been flowed with those holes empty, rather than parts removed.
I think this is the oldest revision of the Commodore 64, board 326298 Rev A. This version of the has the 6510 CPU right next the DIN sockets, with the PLA below that and the SID to the side of the PLA.
Later boards swapped the PLA and the CPU, then they further confused things by swapping the SID and the PLA.
I think the choice of this older board is deliberate, and not just using up stock.
The way the video is generated is different on this revision, and allows them to remove the modulator.
In this version, the modulator just has audio and video inputs and RF out, so can easily be omitted.
On later boards, all of the circuitry amplifying the video signals and generating the composite video takes place inside the modulator, meaning they would not be able to remove it.
I don't think this is the collection of spare parts it is sometimes described as.
The case lid is also custom. It has a cutout for a C64 keyboard, which is a different shape.
The keyboard also had custom keycaps, with no colours on the number keys.
I am sure people will point out the the early VIC20 "had a PET keyboard with some keys taped over".
Whilst that is partly true, the housing was indeed from the PET, but the PCB was different, a different key matrix, and different printing on the key caps.
The metal chassis here is different, with cutouts for the C64 joystick ports, cartridge slots, DIN connectors etc. That is a custom piece.
The power supply is not a PET one, it is more like the C128D. It has 5V, 9V AC and 15V AC for the monitor.
The only thing that not custom is the PET 12" green screen monitor.
The 4064 has a small board to convert the composite video from the 4064 to the horizontal / vertical and video signals required for the PET. (more on that later)
It also has a custom KERNAL ROM the 901246-01.
This has a special routine which is called on every clock tick to resets the colours on one of two values, on or off.
Versions
There were various versions, and information varies about which is which and what features they had.
4064 is the name on the boot screen, following PET naming conventions, 40 column, 64K RAM.
These are also called PET 64, Educator 64 and Teacher's PET (although I think that last one was a program from the early 80s of donating 2001s to schools badged up as "Teacher's PET").
I have seen three types of displays, I know the first is factory, I don't know if the other two were as supplied or may have been user modified.
12" green screen digital
This is a standard PET monitor, with three digital input signals, two non standard sync and video. The video signal is digital, on or off, so there is no grey scale.
12" green screen analogue
I have seen some photos of these machines with a green screen showing levels of greyscale (greenscale?) The hardware I have here cannot do that, so someone may have swapped in a more standard 12" green screen monitor and a more standard C64 board?
12" colour screen
I was not aware of any 12" colour tubes, did they ever make them? or were they all 14". Someone may have been able to cram a 14" colour monitor in there, it would have needed the case back to be extended. Was that a factory thing, or again a customer mod?
This board would suit the first of those, a standard 12" green screen digital PET monitor.
There is no sound output and no SID chip. I have seen photos of one of the models with a speaker and amplifier board, again, not sure if that was factory.
The Fault
There is nothing displayed on the internal monitor.
Being a Commodore 64 board, there is a 5 pin DIN AV socket on the board, so the owner connected an external monitor, and this is there photo of the result.
Hmm, that's not right.
It is all white, which is not right for a 4064, and the text just appears to be small boxes.
Time to test it out.
The first step was to sort out the power supply.
It takes the same 9V AC and 5V DC as a normal C64, but is missing the power socket and power switch, so I made an adapter cable.
I had been told there was no output on the PET monitor. I checked the connector and there were no sync signals coming from the adapter board, so I unplugged that for the moment.
Connecting up an AV cable, I saw the same thing on the monitor, a collection of small squares, and on one occasion, just lines.
The squares were very difficult to see, everything was bright white, almost as if the black level wasn't being set. The video signal is meant to have a "front potch" to set the black level. This was missing on the ZX80 and early ZX81s, and although vintage TVs could cope with it, modern ones would give you the same sort of washed out greys.
Well, there is nothing much going on there other than the sync pulse. No front or back porch to set the black level as expected.
There characters dip slightly, but without the reference, it is understandable why the monitors display greys.
I don't think this was what the PET 64 video output was meant to look like.
At this point, I stopped and reverse engineered the conversion board.
The conclusion was no, that is not what it was meant to be generating. The full reverse engineering and theory of operation will follow in part 2, available from your newsagent next week, along with a card you can send off to get a free binder to collect and store all 52 issues of this repair series.
It is quite impressive what is going on, on that little board.
Is the VIC bad? time to open the lid on the rusty tin.
Within, more missing parts.
It looks like R21 is the only part which has actually been desoldered, you can see the holes. C71 looks to have been cut out. I am not sure why as they are only connected to other parts which are not fitted?
Everything else is factory not fit.
The 14.31818MHz crystal confirms this is an NTSC board, at least it means 60Hz timing, there is no actual colour.
The VIC chip has a bit of heatsink compound on, using the lid as a heatsink.
With that removed, you can see this is a standard NTSC VIC II chip 6567 R8. This one is dated later 1984, but the rest of the board is early 1983, so this may have been previously replaced.
I don't have a spare NTSC VIC II chip, but since we're not doing colour, I think I can get away with a PAL one, at least to test to see if it makes a difference.
With a PAL VIC II fitted, the signal on the scope was now much better
Trying that on the monitor, the black level was now being set, so the background was black and the boxes could clearly be seen.
Hmm, I wonder.
This is a C64 board, right? so I can use C64 cartridges.
One of the things the Dead Test does is bypass the onboard character ROM.
Ah, we have life. So, the font ROM is bad.
Seems to pass the initial RAM test, but fails the colour RAM test.
Yes, well, there's a good reason for that, the colour RAM U6 is one of the chips that is not fitted on the 4064.
It does point at some bad RAM chips though.
I replaced the bad character ROM with one from a C64 board and we have a boot screen!
Driving the PET Monitor
It seems to be working OK, so I connected up the video board again, and it was now generating a proper sync signals now.
The PET monitors require an unusual version of horizontal sync which continues into the visible area.
Time to wire this up to a PET monitor.
Looking good (certainly better than my photography skills, maybe if I wait until it gets dark?).
For some reason, they removed the number of bytes free message when they change the boot screen?
Testing the 4064
Dead Test seemed to suggest there was some bad RAM. Time for a second opinion.
Des Test also seemed to pass the RAM but failed the colour RAM so wouldn't progress further.
It was the same for the other diagnostics I tried, the colour RAM fail was blocking out any other results.
All seemed to be passing, but still indicating some faults, potentially false results due to the absence of the colour RAM.
Further Testing
This is a C64, right? so I can run C64 programs.
Well, I didn't have a C64 program, but I did have a PET program, written in BASIC, which could be loaded in a C64 and used to test RAM.
I use this when testing Mini PETs and PET ROM/RAM boards, it is a good stability check as it takes half an hour to run and is constantly switching between ROM and RAM.
But how to load it?
Of course, an SD2IEC, in this case the TFW8b SD2IEC / Fastload hybrid.
I wonder how many 4064s have fastloaders?
I modified it to test RAM from 4K to 40K, and the extra 4K at $Cxxx. I couldn't test the RAM behind the ROMs since I was running from BASIC, but that seemed a pretty good test.
That took about an hour and all test passed and still seems to be running fine.
Testing the ROMs
The ROMs all seem to be working, but I have not been able to get the diagnostic cartridges to test them as the all give up when the hit the bad (i.e. entirely missing) colour RAM.
My plan was to read the ROMs and dump them out to files on disk. I could do this from BASIC, but I would need to write a program to do it, and I am a bit rusty on Commodore BASIC file handling.
The SD2IEC Fastload Combo has a built in machine code monitor, which is handy for this sort of thing.
I went into full details on this in a bonus post on my Patreon -
But the short version is I saved them to disk, and compared them to reference versions, and they were identical.
Can it run Crysis?
Let's try some more things.
I started with something simple, my C64 version of Perilous Swamp.
It was all going so well
Then I got eaten.
Oh well.
At least the 4064 seems to be working well.
What about something a bit more challenging?
That seems to work, but that is via the AV jack and is monochrome greyscale.
The PET monitor can only do black or green, so the result is different, but still pretty good.
The game is still very playable.
All good so far.
PETSCII Robots anyone?
SD2IEC seems to be working nicely.
The robot looks good, but of course there is no audio, so you don't get the nifty title music.
The only issue I had here is the player character is not particularly visible.
That all seems to be working nicely, ready to go back.
It should have an NTSC VIC II chip. They are not that easy to find here in the UK, but the owner is in the US, so has a spare one.
Reality Check
N.B. just in case there is any confusion, this was a real product, unlike the SX-20 featured last week, which is a one off TFW8b project.
I don't know, VIC20 in an SX-64, C64 in a PET, what next? PET in a C64 case?
Already done that.
Adverts
Tindie seems to be back from it's unplanned downtime. My store contains all sort of kits, test gear and upgrades for the ZX80, ZX81, Jupiter ACE, and Commodore PET.
Something else you could put into a PET case for example.
If there are any further problems, you can always contact me using the link at the top of the page and let me know what you want and which country you are in, and I will get you a price if you want to buy direct.
SD2IEC Combo
The SD2IEC Fastload Hybrid is available from TFW8b.com
Patreon
If you enjoy posts like these, you can support me via Patreon, and get access to advance previews of blog posts (e.g. the second part with the reverse engineering of the converter board), and exclusive posts (e.g. part 1.5 of this series with the look at using the debugger to dump the ROMs).
You also get progress updates on new projects (like the one shown at the end of the post) and other behind the scenes updates, as well as access to my Patreon only Discord server for even more regular updates, and to discuss your own projects.
There is now a free trial on the Patreon, if you want to find out what goes on in there, and I hope you will want to stick around.
