9VAC/5VDC Simple Power Supply!

[MrAureliusR]

Hey everybody.

So I recently acquired a Commodore 64, for those of you that don't know a while back I was looking for SID chips because I'm interested in building a MidiBox SID V2... So the C64 I got came with the power supply but no video cable. I brought it home, plugged it in, turned in on, power LED came on, I think to myself 'Well that's at least a good sign.' It sits for about a month till I get around to getting a video cable together and ... no output, whatsoever.

So, as Dave says, golden rule of troubleshooting is Thou Shalt Check Voltages. So I checked the voltages directly out of the brick. There's supposed to be 2 pins that have 9VAC and one pin with 5VDC, plus a ground pin.

Get this. I'm told that usually it's the 5VDC regulator (which is literally coated in a block of epoxy, making replacement impossible) that starts to go. The voltage creeps up towards 6, 7 V and it fries your C64, as there's no internal regulation. However, my 5VDC checks in at about 5.12V. Not bad, really.

One 9VAC pin is giving me about 2-3VAC. The other --- 20VAC!!! TWENTY volts for chrissake!! I double and triple checked this and was quite surprised as I can't even imagine a scenario where the brick is that #%@&ed up as to give out such wonky AC.

So -- now I need to build a simple 9VAC/5VDC power supply. Obviously I need to buy a 120VAC--> 9VAC transformer, a bridge rectifier, and a 5V regulator. I was thinking, to give the best performance, get a double-tapped 9V transformer so each pin has it's own tap (I'm not exactly a power supply aficionado, so I'm just guessing here). However, clearly I need some line filtering/regulation of some point. This is where I need help/guidance. I found one video on YouTube where this guy has re-built his own C64 power supply but I want to do this as simply as possible.

5V rail needs to handle ~1.5-2A max. 9V @ 1A (or so I've been told).

I was thinking a) common mode choke (or are they for after DC rectification?), and b) filter caps for both before and after transformer/regulation stages. Plus fuses, and any other obvious safety related stuff. Anything else obvious I'm missing? I want to put 2 analog meters on the front of the box, for voltage and current output. (On that note, would it be more important to have the voltage meter reading the 9VAC or the 5VDC? I was thinking the 5V, because the regulator seems more likely to fail than the transformer -- but then again, my screwed-up PSU seems to shoot that in the foot. Or I suppose I could find a meter that does AC/DC and install a switch).

Any advice for a first-time PSU builder is much appreciated! I've already looked at transformers on element14/Newark and I'm leaning towards this one:
http://canada.newark.com/block/fl30-9/transformer-2-x-9v-30va/dp/23M6267
Actually, that's basically the only one they have that meets the VA requirements, and is available without shipping from the UK. Plus it's on sale which is great.

All help and advice is MUCH appreciated! Thanks a million guys.

[SeanB]

Did you check between the 2 pins, as this is a floating 9VAC supply. As a simple test just use an external transformer to supply 9VAC to the unit and try it. There is no connection between the AC pins and the 5V rail other than capacitive coupling through the transformer. 20V is about right for the coupling into a 10Meg meter input resistance from the mains side.

[MrAureliusR]

I wasn't sure (for some stupid reason) whether to check them against ground or each other. Against each other yields about 7.2VAC. Against ground is the other measurements above. Either way something is off, no?

[SeanB]

About right, but a little low voltage wise. Try checking the inside for the usual loose chips, corroded traces from the leaky battery and the other usual things like that. Chips in sockets extract and reinsert to clean the sockets and pins again so they contact well.

[MrAureliusR]

Yeah, before I even powered it up I did a thorough visual inspection. Also cleaned off all the heat sink paste and used fresh stuff, reseated some of the chips, even popped open the modulation can to see if it was okay. It's actually in great shape as far as I can tell. Put the heat shield back on, sealed it up and then it sat there for a couple weeks before I tested it again.

7.2V doesn't seem too low? I know something might be pulling it down, you don't think it's worth getting another transformer to test it with? Maybe I'll just pull the SID chip when I get the MidiBox kit and say screw it to the C64. At that point I have to build a new power supply anyway.

[Shock]

The original C64 had a simple RCA out so an RF cable will work.

[MrAureliusR]

The original C64 had a simple RCA out so an RF cable will work.

Do you mean like the sort of cable the SNES used? It comes out as RCA then demodulates it to a straight coax input?

[Shock]

Straight cable carries both the audio and video signal.  This was before people were plugging into monitors you could connect it straight into the TV and find the channel it displays on or switch your TV to AV.  This is about $1 worth of parts to make

[MrAureliusR]

So it literally goes straight from the RCA to the 'cable' input? That's awesome, I'm going to try that out and see if that works. I hope so or else there's an even bigger problem.

However I ordered the MidiBox SID kit so in a few weeks it might get harvested anyway!