PET Deep Dive, Part 1, The Power Supply  

Addendum: As Bill pointed out below, I forgot to mention the transformer center tap. @05:53 The transformer secondary, or output side has a center-tap which is connected to ground. This is simply another wire connected to the center of the windings where the output would be half of the full winding. Because it's connected to ground, that means if you measure the AC voltage on one wire with respect to ground you would see half the voltage. @07:49 I mention the static PET has a half-wave rectifier, but that is incorrect. Because the center tap is grounded, CR1 and CR2 are rectifying in both directions, but only seeing 9 volts each with respect to ground. This is important because, if that center-tap wire is broken or disconnected, your rectifier output voltages will not be correct, so it's a good idea to check that the center tap is grounded by continuity or by verifying that the reference to ground is half the voltage.

Excellent video Chuck! It's a most excellent resource for the community! I thought your camera work, audio and especially the helpful diagrams were top notch.

Chuck...a terrific and clear tutorial. Thank you so much. I'm looking forward to number 2.

Awesome video Chuck. I've only been a PET 2001-N owner for 3 +/- years, but it's become one of my favorite vintage computers. You and your channel are a wealth of information for any Commodore PET computer user. Keep up the great work!

Deep dives are the best. Excellent series start Sir!

Great video. Looking forward to the rest of this series.

I've been wanting to find this information in an easy to understand way and you've done it here, Chuck. Thank you!

Excellent video! A must watch for anyone working on a Commodore PET. Thanks for sharing

This series is a great idea! Thanks!

Wow!. I will soon be diving into 2 PETs. This is perfect. I have one of each as well. Subscribed.

Very good video full of useful information.

As a kid I dreamt of owning a PET (with a decent keyboard)

More please.

Complimenti, video ben spiegato e ben fatto!! Bellissime macchine!!

Great video Hutch! I can’t wait to see part 2. I’m curious if you drew those schematics yourself, and what (if any) software you might have used.

Thanks Chuck! This is great! I’m looking forward to more. I’m restoring 2 4016s. Such slow work. Also curious how you go about cleaning the board (should I bother with ultrasonic cleaning?) - I’m assuming you’ll get to the much talked about (for a PET) keyboard and I’m always interested in that. I’ve gone down so many rabbit holes chasing red herrings. I’m always glad to see more on that subject haha.

I see a problem in the power supply description of the first PET. The transformer is shown with a centre tapped secondary, or at least two windings connected up that way. Typically the centre tap would be earthed for a simple supply arrangement. You have not shown the earth connection of the centre tap anywhere in your circuits. The two diodes you show are connected in the normal full wave rectifier arrangement, for when the secondary centre tap is earthed. Each diode on its own is a half wave rectifier, but their outputs are connected together and they are driven from the out of phase AC from either side of the earthed centre tap secondary. The load current in the computer flows between the +5 Volt supply and earth Without that earth return to the secondary winding, not only will no current flow in load (computer PCB) but you won't even have the +5 Volt supply. I don't know where the earthing is done, presumably around the transformer/filter capacitor. I haven't got that PET, or any PET. Now, assuming that the centre tap is earthed, what you have is full wave rectification of 9 Volts AC, not half wave rectification of 18 Volts AC. Still interesting Bill

Excellent video! A question: are the voltages at the transformer output the same for the 220 Volt European models?

Awesome video! It is really helpful. Please do more! I have a PET with dram that powers on to a black screen, two of the first batch of diods get very hot, do you think this could be the cause? I was not able to meassure the voltage, I get a -7 reading. Which diods should I replace with? Thank you very much!

Thank you for that great tutorial! I have a PAL 8032, it looks like the setup from the universal board shown here in this video. The gigantic Capacitor blew up while the pet was plugged in (but 5 minutes after I turned it off). Since this capacitor seems to be almost impossible to buy anywhere (I only found one 40 year old in the US for 70 Bucks + shipping around the world) is it possible to replace it with something different? Thank you an happy Commaydore!

Please do a video covering all differences between B series CBMs and N series PETs. Namely, give to their incompatibilities, what the heck is my B series machine good for?

This will help when I get into my PET 8032! Can you jumper a Toggle switch to switch between 80 and 40 Columns? And, I keep hearing that my PET 8032 has RIFA Cap(s) and sho9uld be changed out ASAP...that would be great if you covered that in Part 2. Thx. -Markj.

Canada runs on exactly the same electrical system; didn't you know that?

For long term reliability, is it worth swapping the linear regulators for their switch mode equivalents as this should reduce the amount of heat produced, quite significantly. Stephen G7VFY