Can I use this Voltage Regulator?

[opg]

So, it's very possible I have TWO bad power supplies for the Commodore 64. I read online that the transistor and a 300 ohm resistor can be replaced with a +5v 750mA voltage regulator.

The first thing I do, of course, is look on Radio Shack's site to get a part fast. The closest thing they have is a +5v Fixed-Voltage Regulator (7805) "with capabilities up to +5v at 1 Amp." But it also states the max input voltage is 35V DC.

The power supply for the C64 has an input of 116V, 60Hz, 40W.

Am I just out of luck on this one? I don't think Mouser had anything closer, just the same as the 7805 model.

[DrJustice]

Given the tip of using a standard 7805 regulator, I think it's safe to assume that you can use just about any 3 terminal positive 5V regulator with a high enough current rating. It doesn't matter if the current (Ampere) rating is 'too high', in fact, the more the merrier; the device will run better and cooler and will last longer.

A 7805 rated at 1A should be just fine, if 0.75A is the requirement - you can get beefier regulators too:

Here is a catalogue page from Mouser with some regulators that may be suitable. Perhaps something like the L78S05CV rated at 2A, or something more beefy like a LM323K rated at 3A. These choices were just random picks, most well stocked component shops will have a range of suitable regulators.

DJ
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[saul2600]

First, what repair plan are you referencing? can you provide us with a link or two?

If you are trying to replace a bipolar transistor and resistor pair with one 7805, then, yes, many 7805's will work... under some/many circumstances. But a schematic will really help with identifying wether it's good advice or not. There are many variables to weigh, however, and it should not be done without some initial steps being done first.

From the general descriptions that i've seen, the external power supply has a DC rail at +5 V, and supplies 9V AC as well.

*If* you are experiencing problems only on the DC rail, then repairing it or replacing it with a proper 5V rail makes sense. Perhaps the schematics you have seen do this... but perhaps a simple repair to the originals may be easier. Complicating the issue, though, is that the C64 itself may be having problems independant of the power supply rails. Depending on some specifics, of course.

However, i think it wise to do this in somewhat proper steps.

1. What does your supply read unloaded? Both the AC and DC voltages should be measured. You should also try to read any AC voltages on your DC supply... Make sure you read the pins correctly... you may get some pretty screwy readings otherwise.

2. Any idea how much current actually runs through either the DC or AC supply? This would be helpfull info, or may just be some needless data.

3. Can you see a failed transistor in the supply? This would be a convenient failure mode, as replacing it will surely help, if not eliminate the problem. Often, one failed transistor will take out another, but it gives us a great start.

4. If your power supply is potted in epoxy, and you cannot open it to find a burned transistor, then test the other supply next.

5. I take it from your original post that you have two supplies which fail to start the machine. If both measure reasonably close to the original 5V DC and 9V AC, then likely your C64 has some problems, and not the supplies themselves. Where did you measure the anomolous voltages you found? Which pins of which IC's?

Finally, there is always the option of using a 5V DC supply and a seperate 9V AC supply solderred and shrink-wrapped to the original connector going into the C64. This may very well provide you with a vastly superior power supply section (or inferior, depending on which are chosen).

Again, I'll need more information in order to proceed with more specific advice...

[jksuperstar]

Oh, and don't think the input to the 5v regulation is 116v AC. There should be a transformer that takes that 110-120v down to 24v AC (or so), which then goes through a diode bridge(4 hefty diodes). The output of that diode bridge should be about 24v DC, which gets filtered through a cap or two, before hitting the 5v regulator (your transistor combo).

[DrJustice]

Below is the 7805 pinout and the schematic of the original C64 power supply which I believe opg has.

The transistor and 330ohm resistor can be replaced by the 7805 by connecting pin 1 to the left side diodes/capacitor, pin 2 to ground where the resistor was and pin 3 (which is the output) to the capacitor at the right.

[opg]

Thanks, DrJustice! That's the same schematic I'm looking at. I've been reading up on the repairs and troubleshooting at Ray Carlsen's site, since that pops up EVERYTIME I look up C64 maintenance.

http://personalpages.tds.net/~rcarlsen/cbm/c64blank.txt

Here, he mentions that one of many, many, possible causes of a blank screen is "A missing 9VAC (power supply problem)."

He also has a great page on how to test the power supply and what the results mean. There is too much good info to quote from it:

http://personalpages.tds.net/~rcarlsen/cbm/pstester.txt

Since I have two power supplies and two C64s, I can afford to lose one of each. The first power supply I purchased did absolutely nothing, no LED light in particular. According to Carlsen, if it is a power supply problem, it is the +5v line that has gone bad.

The second power supply I have will cause the LED to light up (and it is not a dim light - full strength) on BOTH C64s. In this case, Carlsen believes it is the 9V AC that is bad. I like this, because it may mean that I did not fry the first C64 I bought, and that if there are other problems with any of the chips, it is possible that I could have some spares. And because the 2nd power supply lit up the LED on both, I am for now assuming that both computers are okay.

Anyway, my goal was to start with the first power supply that doesn't do a damn thing, crack it open with the "handle of a screwdriver" as Carlsen suggests, check for any loose components, then try out this voltage regulator modification.