MB-SID PSU

[cd_cd]

Hello,

I'm just working in a MB-SID just to get rid of some parts that have been lying around for ages. And one of these parts is a 12V AC power supply. And now I'm wondering if I can use it to get my MB-SID running. Since I dont' have much of a clue about electronics I think I need your help. Have a look a the schematic below. Is this a solution that could work? The 12V AC (a little more than 12V) should be enough to get the 6581 Sid-module running, the 5V supply comes from the core-module. I thought the 7809 whould take away too much power for the 7805 on the Core Module. Do you think this s a solution that could work? Does this solution make sense or does it make noise? If not, how whould the perfect solution look like? What about the capacitors and/or Elkos: Where do I have to place them and what specifications should they have (this is why there are questionmarks in the schematic)?

Sorry for beeing a complete noob in those things, and I hope my english is good enough to understand my intetion.

Here is what I thought could be OK:

[Wilba]

When I did something similar a long time ago, before I started using a C64 PSU, I recall I had a really wierd problem caused by the ground of the SID module not being the same ground as the Core. You have to pay attention that there is a bridge rectifier in the Core and also in the SID module (so each of those could take AC or DC power) and keep in mind that if you pass DC through another bridge rectifier, the ground is going to be 0.6v higher because it has to go through a diode.

I would suggest you do the following (and I'm no expert, so do what you feel is right):

* connect the SID module to the output of the bridge rectifier, and connect it to the input of the 7812 on the SID module (i.e. bypass the bridge rectifier on the SID module).

* connect the Core module to the output of the 7809, and connect it to the input of the 7805 on the Core module (i.e. bypass the bridge rectifier on the Core module).

This should mean the Core and SID will both have the same ground, and there's no need to connect ground between Core and SID module (and better you didn't), but you still need to connect 5v to the SID so connect it from the Core.

The SID module's current draw on the 12v supply is very low, so there's plenty left over to power the Core... but check the current rating on the 12 AC PSU... if it's really big (i.e. 1 amp) then you'll have something like 16v DC going into the 7809 and may need to insert another 7812 before the 7809 to drop the voltage... you will probaby end up having some really hot regulators so just be aware of this and buy some heatsinks!!!

[cd_cd]

Thanx – sounds very logic to me – seems to be the better alternative.

EDIT: But it doesn`t work after the rectivier there are just 10.4V DC left - not enough for the Sid. I think I'll get an original C64 PSU. ....

[Wilba]

You can't measure DC voltage out of the rectifier, it's a series of humps! The rectifer just turns the sine waveform of AC current into "positive only"... imagine you just inverted the negative parts of the waveform. You can't measure it as AC voltage either.

You need to put a capacitor after it to store the current between the peaks, and then measure that as DC (it will have a little ripple but that's OK).

A 12v AC power supply should be able to supply a 7812 (after rectifier) with enough current so it outputs 12v at the current rating of the power supply (i.e. 500mA, 1000mA, etc).

[cd_cd]

You can't measure DC voltage out of the rectifier, it's a series of humps! The rectifer just turns the sine waveform of AC current into "positive only"... imagine you just inverted the negative parts of the waveform. You can't measure it as AC voltage either.

You need to put a capacitor after it to store the current between the peaks, and then measure that as DC (it will have a little ripple but that's OK).

A 12v AC power supply should be able to supply a 7812 (after rectifier) with enough current so it outputs 12v at the current rating of the power supply (i.e. 500mA, 1000mA, etc).

Hello Wilba, thanx for your answer. Although I do not know much about all that stuff, I did know about the facts that you describe above. And I don't know why, but there where just 10.4 V left – including the usage of a capacitor.

...Anyway...

I think we can close this thread now. Yesterday I could buy three C64 incl 2 Original-PSU-Units for REALLY cheap money on eBay. That will solve my problems an there will be enough SID-Chips and PSUs left for a good friend who definitely wants to built a MB-SID as well. That sounds like the best solution, doesn't it?

Thank you for your support.

[Flying Panther]

I was thinking: If you connect all grounds right after the bridge rectifier of the Core and the Sid module, then the Core and Sid will share the same ground. Is this possible, or would that also involve ground loops?

[Wilba]

Yeah that should work...

What I was trying to warn about was, for example, using a 12v AC supply and putting that through a bridge rectifier (so you have 12v DC) and then supplying the SID module's 7812 with that, and then ALSO supplying the Core's bridge rectifier with the 12v DC. This would cause the Core's ground to be different (+0.6v) to the SID's ground and can cause logic errors (very wierd ones like what I had once).... and joining the ground between the Core and SID modules won't help... and is a bad idea.

If you follow something like this:

http://www.ucapps.de/mbhp/mbhp_4xsid_c64_psu_optimized.pdf

then you'll see where the (1) label is, all the grounds are connected at this point... you can connect the SID and Core modules in a chain OR use separate +5/ground wires to each module, it shouldn't matter too much... and that's essentially what you're thinking... you're bypassing the bridge rectifiers on all the modules.