Over the last few weeks I have been using the Mini VIC a lot for testing, and making a few minor changes along the way. Time to roll up all of those small updates. (this from Patreon November 2024)
Clocks
I identified the clock generation wasn't great in a previous post. I thought I should test out the theory, so make a clock generator that plugged into the 74HC02 socket on the Mini VIC.
I am a little worried that my bodges are getting neater.
I had already laid out the PCB, so I followed a similar arrangement.
This uses a more standard crystal oscillator approach. Two crystal oscillators in fact, so I am only switching the logic level clock signals rather than the connections to the crystal.
That works rather nicely and the system has been very stable throughout the Penultimate +3 testing.
I am pretty happy with that for "non-overlapping clocks".
I had an early NTSC VIC20 on the bench recently and took the opportunity to checked out the clocks generated on that board.
That looks a bit wobbly, but if I do the merge again, it seems clean enough (if the threshold is right).
I also captured the clock pulses on three other VIC20 boards I was using to test things on the Penultimate +3 but unfortunately I have lost all those traces.
When I get the Keysight scope that could do screenshots and save to USB, that was a game changer for showing scope traces in the blog posts. I use these a lot, so I have a script to automatically crop them to 4:3 these days.
When I looked for a USB stick to use, I found this 1GB USB stick that was otherwise not much use and thought it would be ideal. Turns out it wasn't. This USB stick has now spoiled three blog posts. It came from Keysight as part of some promo thing I guess in April 2017 based on the markings.
I lost a load of traces for the Sony cassette recorder post a few months ago, but reformatted it and it appeared to be working again. No more problems until today when it has failed again and lost most of the traces for this post and the VIC-1001 board repair post.
Power
I think the original 2 pin VIC 20 used a 9V AC power supply for convenience. That was the same way the PET had worked, AC in from a transformer and the rectification and regulation all handled on the board.
I am not 100% sure where the 7 pin DIN power supply was first used, the VIC20-CR, the Max or the C64?
Well, I say 7 pin, but many of them only had 4 pins connected. Two are 9V AC, and the rest are either 0V or 5V DC pins. All are wired in parallel on the socket, but the plugs often only have one 0V and one 5V.
It seems to suit the VIC20-CR best. The 5V DC is moved into the power supply itself, so that removes the need for the rectifier and regulator and the massive heatsink. The 9V AC is retained to be backwardly compatible with the userport of the previous model, and also used to generate an unregulated 9V DC supply for the datasette drive. It is regulated down to around 6V for that by the big TIP29 transistor near the port.
The Commodore 64 complicated things. Again the 9V AC is retained to be backwardly compatible with the userport of the previous model and for the datasette power. That unregulated 9V also provides power to a 7805 to generate a separate supply for the VIC-II and clock chip inside the tin can. But it also needs 12V. So the 9V AC feeds a voltage doubler to generate about 15V DC and then is regulated down to 12V for the VIC-II and SID.
Finally, a tap from the 9V AC is used to generate a line frequency reference, a 50Hz or 60Hz pulse that is fed to the TOD (Time of Day) input the 6526 VIA chips.
There are an awful lot of these things around. There is just one problem with them, well two.
Firstly, the 5V supply is flawed, and they quite often fail with the 5V rail rising upwards of 7V, taking out lots of the chips on the C64.
The second problem is they are potted, so you can't repair (or ideally replace) the 5V supply circuitry. Therefore, they should be avoided.
I had to hunt around to find this one for the photo. I have a box of them somewhere. I never use them. This one is showing about 5.31V on the 5V rail, so I wouldn't be happy using that.
I would be mad to design the Mini VIC to use one of those?
Well, I did, but with a very important caveat.
I do not connect to the 5V-7V DC supply at all. I ignore it and just connect to the 9V AC.
As such, it is quite a handy source of 9V AC from something which would otherwise be consigned to a box in a dark corner somewhere (or straight into e-waste).
I designed the board to accept either 9V AC or DC. The input is rectified to give 9V DC, and then regulated with a 7805 to give the 5V DC. The modern logic uses less power than the original board, so the approximately 1A rated supply will do just fine.
You can also power it form 9V DC if you prefer. There is no requirement for an AC dervied time of day signal (as on the C64), or any voltage doubler circuitry (as on the C64), or any other reason for AC on the board itself.
It is fed to the userport for use by external devices. I am not aware of any that specifically need AC. If any userport device requires AC, then feed the board AC. Otherwise, AC or DC is fine.
(and also note the caveat about 9V DC with a bridge rectifier and be aware of any ground loops or shared supplies - http://blog.tynemouthsoftware.co.uk/2024/03/why-dont-you-use-a-bridge-rectifier.html)
There are two sets of pads on the board, one for a 2.1mm DC power jack, as used on all my other kits, as well as one for the 7 pin DIN socket.
Testing
I didn't want to go directly to a C64 power supply, as I wanted to be able to monitor the current during testing, but I wanted to use the 7 pin DIN as that looked better for photos of the board.
I ended up making a 9V DC barrel jack to 7 pin DIN adapter so I could test both versions.
(actually, I made two. The first one I lost and I couldn't find it, so made a better one for these photos. I then almost immediately found the first one. It's been one of those weeks and it's only Monday)
(edit, as I come to post this, I have now lost both of those adapters)
However, the first time I tried it, it didn't work, totally dead. When I checked over, I found the fuse had blown. Not ideal.
I was impatient, so I tried clipping directly to the power switch pins and it worked fine, so I have just been using that throughout the testing.
Once I had finished testing, I had a look at fixing that.
I assumed I must have messed up the layout somehow to cause a short, or there was a solder bridge somewhere. I couldn't spot anything in the design that was wrong, so I took the board out of the VIC20 case and investigated further.
Well, there's ya problem.
Looks like I put the fuse holder too close to the edge of the board. I was trying to keep it away from the metalwork of the cartridge shield, and inadvertently moved it too close to the sideplate.
For the next revision, I have shuffled things around to move the fuse a bit lower, away from all the metalwork.
As a temporary fix, I desoldered that end of the fuse and cut the leg off. I soldered it to the trace on the top of the board and put a bit of tape over the bottom side.
One new fuse later and away we go.
The DC jack is connected to my bench supply so I can monitor the current use and try different voltages to check the limits.
The Mini VIC alone with a vintage VIC chip and MOS 6502 uses about 500mA. Add a Penultimate +3, and that rises to about 750mA. Still within the limits of the 1A supply.
If you need more power for accessories, use a more powerful supply.
I am impressed with how well the 7805 is going just rived to the board with no heatsink just a bit of tinned copper trace on the PCB. It gets warm, but no where near as hot as say the VIC chip.
I have had that running pretty much all day for several days when testing the Penultimate +3. (there are 250+ games on the menu, plus utilities, programming tools, file browser and different memory options, all of which have to be tested).
I will probably suggest a switching regulator for the final version, depends how things work out. Switching to a W65C02S should reduce that a bit. And VIC chip replacement current consumption is yet to be seen.
I did try it with the AC supply again, and it sort of works, but it seems a messed up with the reservoir capacitor. It was meant to be 2200uF, but somehow ended up as 220uF. With an extra capacitor clipped on, it worked fine.
Two problems remain.
- The chrominance signal is not very good.
- There occasional corruptions to the colour RAM only noticed during the Dead Test or similar soak testing.
(I don't have any of the video signal captures, I am going to have to do all those again, but not today. Frustratingly I have lost the before and after for the mods I did, so I will keep this bit short.)
Here you can see both problems. The R of colour has been corrupted and gone yellow.
This shows why composite video is never going to be as clear as RGB. Composite is just what it says, a composite of a sharp black and white image with some colour information overlaid on top of it.
And yes, I still need to fix the chroma timing on PAL.
Adverts
The Mini VIC kits are not available yet, but there are still Minstrel 2 and Minstrel 3 kits available.
I have also turned back on EU shipping, with a caveat that there will be extra delays and charges at customs, sorry, nothing I can do about that.
You can now get a Minstrel 2 kit for $200. 1980s pricing.
Or you can get a Minstrel 3 kit for $200
Patreon
You can support me via Patreon, and get access to advance previews of development logs on new projects and behind the scenes updates. These are often in more detail than I can fit in here, and this post contains bits from several Patreon posts. This also includes access to my Patreon only Discord server for even more regular updates.
