MB-6582 PSU Option A modification

MIDIbox SID
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I’ve built the MB-6582 base board up to a certain point and have stopped at the PSU stage in order to test sections as they are built. I’ve opted for the PSU Option A, since I  have four C64 PSU’s in possession. Here’s the rub:

All four PSU’s have issues, which I didn’t discover until after I got to this point.

    PSU 1: completely dead, the unrepairable type

    PSU 2: outputs 5v only on the correct pin. No 9v AC output. unrepairable type.

    PSU 3: outputs 9V AC only on correct pins. No 5v output. unrepairable type.

    PSU 4: third party C64 PSU that is repairable! I can open this puppy and see all the goodies inside. However, it only outputs 5 V on the correct pin. No 9V AC output.

Instead of buying another C64 PSU on eBay and taking the risk of getting yet another unusable brick, I’ve opted to work with PSU 4. I opened up the PSU and discovered a trim-pot resistor on the circuit board inside. I turned it all the way to the right, and the voltage dropped from 9v to 8v. hmm..

So, I decided to pick up a 2.7 ohm resistor to bring the 9V DC down to 5V DC. Here’s my decision: Should I put the resistor in the PSU or on the MB-6582 base PCB?

Option One: put in the psu. Instead of taking the thing apart and risking damage, I thought I would locate the 5V line as it comes off the PCB inside the PSU and cut the wire, then solder the resistor between the pcb and the wire that goes out to the DIN connector.

Option Two: Reviewing the MB-6582 base PCB, it looks like I could remove the bridge I soldered at J73, and replace it with the 2.7 ohm resistor. From tracking the 5V line on the PCB from the power input/power switch area to the rest of the component areas, I think that will work.

Option Three: Take apart the third party PSU and figure out where on the PCB inside of it that I can replace a resistor with one of higher value to bring the voltage down to 5.

Option Four: Replace the trim pot inside the old PSU that will allow me to crank the voltage down from 9V to 5V.

Any thoughts or suggestions? I’m leaning towards Option Two, since taking apart the innards of the third-party PSU may be risky. I don’t want to damage any components in this old thing. Nor do I want to zap myself since I don’t know how to properly discharge the PSU before working on it.

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  1. Do not mess with the PCB. You don’t “fix” something that’s not broken so that it will only work with a faulty PSU…

  2. Resistors are not really a linear voltage regulator…

  3. I have bought and sold about 15 of the bricks and had ONE that really really didn’t work. Seems odd to me that _all_ the ones you have are garbled.

My suggestion:

Option 5. Get a new one, see if it works, if not repeat. From what you’re saying I think it would be better if you were NOT to poke around in a PSU.

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  1. Do not mess with the PCB. You don’t “fix” something that’s not broken so that it will only work with a faulty PSU…

Good point. This had crossed my mind, and my original plan was to get another PSU from eBay or Craig’s List. But, the frustration of going through four PSU’s was getting to me! I just need to bite the bullet and get another one…

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  1. Resistors are not really a linear voltage regulator…

Another good point. As you can see, I’m learning. I figured the best way to learn was to do a lot of reading and work on a project as well. I should have remembered this from when I built a Core + 1 Sid with the optimized PSU. But, it’s been a lot of months since I did that project and I’m having to brush up on my electronics knowledge.

Just for my learning:

So, why would using a voltage regulator be better than using a resistor? Don’t they both drop the voltage?

I’m assuming in this case (dropping 9V DC down to 5V DC) I would use a 7805 voltage regulator. But, would it need to be used in conjunction with electrolytic capacitors, just like in the optimized PSU?

I just purchased another C64 PSU on eBay, so I don’t have any plans on modifying my PCB or that old PSU. Just curious so I’ll know for future projects.

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Regulators and resistors both “drop the voltage”. Correct. But a resistor does that depending on the load (Ohm’s Law, voltage = current * resistance) so when the current changes the voltage does as well. A voltage regulator always puts out the correct voltage.

The voltage regulator itself does not _need_ the electrolytic (and/or ceramic/polyfilm/…) caps, they are used to reduce noise. Which is something you really wanna do in a synth.

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Thanks! I’m going to the library tonight…

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Received a used C64 PSU yesterday (purchased on eBay). And…it works! Testing the used C64 PSU’s was a pain for me since my meter has probes instead of alligator clamps. Trying to hold the probes on pins 7 and 9 at the same time without them slipping off before I could get a read was near impossible. So, I purchased a 7-pin din female chassis mount plug in my last Mouser order. I’ve found it’s much easier to test the output of the PSU by plugging the PSU into this part and applying the meter probes to the backside of the female chassis plug.

Now on to better things…finishing this MB-6582 base PCB. I’ve finished all four cores and they pass test! I’ve also finished the first two SID modules. Now, I’m struggling with getting MIOS Studio to output MIDI properly on crappy Dell-version of the Creative LiveSB sound card living in my Debian box…but that’s another story for another time…once I figure that out, I’ll post a thread with the solution for the benefit of anyone that may be using budget hardware scrounged from dead PC’s like I am.