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.
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!
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 did a video a while back on testing and repairing a keyboard. Look for "PET Business Keyboard Repair" on my channel from Oct 2018. Also touch on it in "Commodore "Super PET" Restoration (Part 2)"
Re: cleaning the board. I blow the dust off with compressed air and/or brush it off with an unused paintbrush reserved for that task. I have rarely washed a board with tap water or a bit of soap and tap water but I always do a final rinse with distilled water to be sure it doesn't leave behind any mineral residue. I know someone else who repairs a lot of C64 boards and he always uses an ultrasonic cleaner, but I don't have one that's big enough for PET boards.
This is an excellent question. The PCB is the same, so the input voltages should be roughly the same, but might vary a volt or two on the AC and unregulated side. The unregulated voltages are not critical as long as they are at least 4 volts more than the regulated voltage, but the higher the voltage the hotter the linear regulators will get, so you don't want them to be much higher.
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!
If the diodes are hot, that means they are working. Their temperature will depend on how much current is being drawn through them. It's usually obvious when one burns up. Check them with your meters diode tester is it has one. If your unregulated voltages are too low, a bad diode seems likely.
The schematics are from Commodore. I just annotated them using Windows Paint. I did draw the power cord diagram using paint and some graphics I found online.
The Commodore schematics from the 70s were hand drawn with stencils and straight edge. They've been scanned and put online. link is in the description.
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
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!
That kind of cap with the screw terminals might be hard to find but any sufficiently large capacitor should work in its place. The old one should be 23000-27000 uF but I'm sure it would be OK with less, or more.
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.
See my earlier video about RIFA caps. There's a pop-up card @19:19 Link: ruclips.net/video/5V_bgdxlUjs/видео.html I'll make a mention of that in my next video.
Changing 40/80 columns requires changing several jumpers and changing one ROM chip. There's an add-on card that does this, the Comspec 40/80-S2 Board. Link: www.6502.org/users/sjgray/projects/cbm4080/index.html
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
It depends on your philosophy. Some prefer to keep the parts as original as possible, but I've seen users pull the linear regulators and wire in a Meanwell switching power supply. It would cut down the heat, but in my experience the linear regulators are fairly reliable. I have yet to replace one and I've only heard about a couple of failures in other PETs
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?
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.
Excellent video! A must watch for anyone working on a Commodore PET. Thanks for sharing
This series is a great idea! Thanks!
Complimenti, video ben spiegato e ben fatto!! Bellissime macchine!!
I've been wanting to find this information in an easy to understand way and you've done it here, Chuck. Thank you!
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.
More please.
As a kid I dreamt of owning a PET (with a decent keyboard)
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 did a video a while back on testing and repairing a keyboard. Look for "PET Business Keyboard Repair" on my channel from Oct 2018. Also touch on it in "Commodore "Super PET" Restoration (Part 2)"
Re: cleaning the board. I blow the dust off with compressed air and/or brush it off with an unused paintbrush reserved for that task.
I have rarely washed a board with tap water or a bit of soap and tap water but I always do a final rinse with distilled water to be sure it doesn't leave behind any mineral residue.
I know someone else who repairs a lot of C64 boards and he always uses an ultrasonic cleaner, but I don't have one that's big enough for PET boards.
Excellent video! A question: are the voltages at the transformer output the same for the 220 Volt European models?
This is an excellent question. The PCB is the same, so the input voltages should be roughly the same, but might vary a volt or two on the AC and unregulated side.
The unregulated voltages are not critical as long as they are at least 4 volts more than the regulated voltage, but the higher the voltage the hotter the linear regulators will get, so you don't want them to be much higher.
@@HutchCA Thanks for the useful information! 👍
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!
If the diodes are hot, that means they are working. Their temperature will depend on how much current is being drawn through them. It's usually obvious when one burns up.
Check them with your meters diode tester is it has one. If your unregulated voltages are too low, a bad diode seems likely.
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.
The schematics are from Commodore. I just annotated them using Windows Paint. I did draw the power cord diagram using paint and some graphics I found online.
@@HutchCA Wow. All my schematics look like 4th generation copies of library books that were scanned upside down by a 5 year old at 60dpi.
The Commodore schematics from the 70s were hand drawn with stencils and straight edge. They've been scanned and put online. link is in the description.
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
Thanks. Yes, the transformer does have a center tap that is grounded on both versions of the PET.
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!
That kind of cap with the screw terminals might be hard to find but any sufficiently large capacitor should work in its place. The old one should be 23000-27000 uF but I'm sure it would be OK with less, or more.
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.
See my earlier video about RIFA caps. There's a pop-up card @19:19
Link: ruclips.net/video/5V_bgdxlUjs/видео.html
I'll make a mention of that in my next video.
Changing 40/80 columns requires changing several jumpers and changing one ROM chip.
There's an add-on card that does this, the Comspec 40/80-S2 Board.
Link: www.6502.org/users/sjgray/projects/cbm4080/index.html
I posted a couple of comments with links in them, so let me know if you don't see my previous replies. YT sometimes blocks comments with links.
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
It depends on your philosophy. Some prefer to keep the parts as original as possible, but I've seen users pull the linear regulators and wire in a Meanwell switching power supply.
It would cut down the heat, but in my experience the linear regulators are fairly reliable. I have yet to replace one and I've only heard about a couple of failures in other PETs
@@HutchCA My thought would be to keep the transformer and swap out the linear voltage regulators with Traco voltage regulators.
@@MattKasdorf That'd work too, as long as those switching regulators can handle the same amount of current.
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?
Unfortunately, I don't own any B series PETs. They aren't as common in the US.
Canada runs on exactly the same electrical system; didn't you know that?