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V2 Assembly Notes
When assembling a V2 card, the first you will have to do is make a few decisions:
The board is compatible with both the 6581 and 8580 SID models but just like in V1, this must be decided at build time since some components depend upon the SID model:
- C1/C2 = 470pF
- R3 = 1k
- R9 = See below - CAREFUL!
- SW1/RV2/R6 = Do not mount
- C1/C2 = 22nF
- R3 = Do not mount
- R6 = 180k
- R9 = See below - CAREFUL!
- SW1 = Mount normally
- RV2 = 1M
Be VERY CAREFUL with R9: it is this component that sets the auxiliary supply voltage of the SID, and if you provide your SID with the wrong voltage you are likely to destroy it.
There are three ways to create the SID auxiliary voltage in V2:
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The same TL497-based circuit used in V1: well, with R8 = 3.3 Ω, C15 = 100pF and L1 = 100uH it worked pretty well after all, didn't it? If you decide to go on this way, just close JP3 on the back of the board and do NOT mount U6. Be careful with R9 as usual: R9 = 11k for MOS 6581 and R9 = 7.87k for MOS 8580.
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One of those MT3608-based DC-DC step-up modules that can be bought on Chinese portals: this is a good choice because the switching frequency of said modules is way higher than the TL497's, they are cheeeeap and they will save you from soldering quite a few components. In fact if you go this way, you must avoid installing the following components: U5, U6, C15, C16, C17, C18, C19, L1, R8, R9, R10. Just solder your module inside the dashed box on the right area of the board, making sure to match the orientation (see the back of the board) and close JP3 on the back of the board.
Note that you must calibrate the output voltage before plugging in your SID or you will break it! The procedure is as follows:
- Mount all the components on your board EXCEPT the SID.
- Plug the board into your computer and turn it on.
- Take a multimeter, put it into voltage mode and put the black probe on the CENTRAL hole of U6 and the red one on its TOP hole.
- Turn the screw on the step-up module until you read either 12V for MOS 6581 or 9V for MOS 8580 (Be careful!). Try to get as close as possible.
- Power off the computer, wait 30 seconds, power it back on and measure again, correcting in case.
- If you have the possibility, drop a drop (!) of some threadlocking liquid on the screw so that it will never move again. If it moves in the wrong direction, it can kill your SID!
- Plug your SID, turn on the machine and measure again.
- Enjoy!
I know it's cumbersome but it only needs to be done once.
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Option 3 is actually complementary to one of the other two: in fact, if you are really paranoid about noise, you might frown upon switching regulators, so we give you the option of making a second stage of regulation through a linear regulator, which should "eat" any switching noise and provide the SID with very stable power. To take advantage of this option, pick your favorite between options 1 and 2 and ignore the instructions to close JP3. Then:
- If you went with option 1, use 12.7k for R9, regardless of the SID model.
- If you went with option 2, set the screw so that you read about 14V for MOS 6581 or 11V for MOS 8580.
- Install an LM7812 regulator at U6 for MOS 6581 or LM7809 for MOS 8580 (Be careful!).
- Make sure JP3 is open and measure the voltage between pins 14 and 28 of the SID before plugging it in.
In all honesty, I feel this option is a bit overkill, as the switching frequency is outside the audible range in all cases, but it's your choice.
Q1 is a dual footprint that can either accept a BC547/8/9 transistor in the bottom footprint (same transistor used on V1) or a 2SC1815 transistor in the top footprint, which is the same transistor used in the original C64 circuit and all over the C16/+4.
You can also use your favorite NPN transistor, just make sure to install it correctly using the markings on the back of the board.
In V2 it is no longer required to use an old CMOS 4520 counter IC at U2. If you manage to find one that's fine, then:
- Do not install C20
- Use 0R for R11 or just bridge it
If you prefer to use a modern fast chip like the CD74HCT4520E:
- Install C20 (470 pF)
- Use 100R for R11
Regardless of what you choose, make sure to solder the center and RST pads of JP2 on the back of the board. This should always work, but if your SID sounds bad (especially after a reset), try the other setting.
Strictly speaking, you should install 1.8 nF caps at C3/4 for MOS 6581 or 2.2 nF for MOS 8580. This is something we had ignored in V1, just using 3.9 nF caps in all cases. I did a lot of testing but have not come up with a final answer: at first it looked like using the proper caps would yield a somewhat "extended" usable range of the paddles (they would max out towards the actual end of the rotation), but then I started having strange problems with the X and Y pots conflicting with one another. I later realized that the problems could have been caused by oxidized contacts, so I'm really not sure what to recommend, but 3.9 nF should work decently in all cases.
Well, this is more of a recommendation: unfortunately I had to place those two ferrites (FB1 and FB2) at the bottom of the board. Since they are tall, it is possible that they touch the metal part of the expansion port cover: this will cause a short circuit on the power rail, since they are uncovered and one is connected to +5V and the other one to GND. You are strongly encouraged to cover them with insulation tape in order to avoid that (or put a strip on the metal cover, like I did).
If this bothers you, and you are ready to sacrifice a bit of noise prevention, just do not install the ferrites and short them out with wire or a resistor leg (i.e.: put a wire through the two holes of FB1 and another one through the two holes of FB2, do NOT connect FB1 and FB2 together).
V2 comes with new equations for the GAL (U4, which must be programmed), make sure to program it with the new file, equations for V1 are not compatible.
CN3 shorts AUDIO_IN to GND in order to avoid noise, when no input jack is inserted. If you don't like that, cut JP1 open.