whoa, what's going on there?
CBM PET 2001 Restoration
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Thank you very much everyone. This discussion is most valuable.
In order to reanimate this PET there are still numerous issues to be solved:
- Kernal ROMs
- Missing 6522 VIA
- Missing two 6520 PIAs
- Glue logic still has some issues ( SN74100 ) -- probably more then I expect
- Running screen
- Some more 2114 ( this should be easy )
You can also use 6821 instead of 6520.
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whoa, what's going on there?
The board already has an inverted BA11 signal. So all you have to do is to cut the BA11 signal (only one cut) and use the existing inverted BA11 signal.
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whoa, what's going on there?
There board has already an inverted BA11 signal. So all you have to do is to cut the BA11 signal (only one cut) and use the exiting inverted BA11 signal.
Pure perfection.
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I'll wait for the latch to arrive, and try to stop the garbage screen from running as I am getting sea sick. Then I'll resolve the ROM issue. Thanks again.
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Just received a couple of 74100 latches - apparently from Poland...
Finally the image is not running anymore!
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I have modified the traces to match the mod detlef found in his PET:
Now [E5]-4 BA11 connects to [H1], [H2], [H3], [H4]
while [E5]-3 BA11 connects to [H5], [H6], [H7] Pin-18s.
No boot. I've double checked the ROM order, and that should be fine.
New symptoms:
- higher current intake (~3A) the last 7805 in the corner is trying to dissipate a lot of heat
- now i see some flickering in the characters so probably the RAM issue has appeared again
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- higher current intake (~3A) the last 7805 in the corner is trying to dissipate a lot of heat
What was the current intake before? Are all the voltage still ok?
I'm not shure if the PET will start with all the PIAs and VIA missing. I never checked this.
But you will need some RAM.
Next step could be a NOP-Generator:
If you don't have a spare CPU, you can use a socket. With this modification, the CPU constantly reads NOP instructions and counts all addresses on the address bus. This is how you can verify that the address lines are okay on any chip that uses them. And you can check the chip select line. -
Another way to spot the problem is a PET RAM/ROM board. With this board you can temporarily replace RAM and ROM to find out which causes the problem.
You can ask Bobbel if he can send you a complete kit.
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- higher current intake (~3A) the last 7805 in the corner is trying to dissipate a lot of heat
What was the current intake before? Are all the voltage still ok?
Voltages are still nominal, current was around ~2 Amps before, now 3-ish...
I think I've read somewhere that the PET should boot without peripheral controllers, but I didn't check the schematic either. ( lazyness kills time )
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Sorry, I did'n read this thread from the beginning: Did you check the CPU reset und clock pins?
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I think I've read somewhere that the PET should boot without peripheral controllers, but I didn't check the schematic either. ( lazyness kills time )
But it will not start without RAM.
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I think I've read somewhere that the PET should boot without peripheral controllers, but I didn't check the schematic either. ( lazyness kills time )
But it will not start without RAM.
Yes. Do I assume right that [I7],[I8],[J7],[J8] has to be populated as a minimum system RAM required for boot?
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Yes I think so. 2 KBytes should be sufficient. Maybe even 1 KByte will work (for boot only), but I never tried this.
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The "pile of the dead" is getting larger...
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OMG! What happend to this PET? I never had a PET with so many defects.
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OMG! What happend to this PET? I never had a PET with so many defects.
I started to feel at some point that this is not going to be an entry level fix...
I think a power surge might did this great job to the PET. I didn't tried it's PSU yet, because I am planning to replace it with a modern version anyway. -
I didn't tried it's PSU yet, because I am planning to replace it with a modern version anyway.
Please not!
What exactly do you mean with PSU?
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I didn't tried it's PSU yet, because I am planning to replace it with a modern version anyway.
Please not!
What exactly do you mean with PSU?
I meant the bulky transformer with the oversized capacitor and the full wave rectifier. I did not use it, and I am not planning to use it.
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But its a simple and reliable design. The most problematic things are the regulators themself. I don't see a benefit of ripping the transformer out of the system.
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I think a power surge might did this great job to the PET. I didn't tried it's PSU yet, because I am planning to replace it with a modern version anyway.
Killing 80% of all ICs seems not a great sign that the modern replacement is actually better.
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But its a simple and reliable design. The most problematic things are the regulators themself. I don't see a benefit of ripping the transformer out of the system.
Okay. Either that or the 7805 regulators. Looking the pile of the bad chips I somehow not convinced of the reliability of this design.
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Please not!
What exactly do you mean with PSU?
I meant the bulky transformer with the oversized capacitor and the full wave rectifier. I did not use it, and I am not planning to use it.
The transformater is an essential part of the original PET. If you remove it please keep it for later reinstallation.
Or sell it here as original PET spare parts so it does not end up in the dumpster.
Okay. Either that or the 7805 regulators. Looking the pile of the bad chips I somehow not convinced of the reliability of this design.
There is no problem with the power supply of the PET. Most of the PETs work fine with it after more than 40 years. Even the big condensator does not cause any trouble. It's more reliable than modern chinese regulators.
But it was designed for 220 Volt and now we have up to 240 Volt. So the regulators produce more heat. If I use a PET for several hours I use an old DDR voltage regulator to reduce the voltage to 220 Volt.
But you should check the 5 and 12 Volt for to much ripple and replace the condensators around the regulators.
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Please not!
What exactly do you mean with PSU?
I meant the bulky transformer with the oversized capacitor and the full wave rectifier. I did not use it, and I am not planning to use it.
The transformater is an essential part of the original PET. If you remove it please keep it for later reinstallation.
Or sell it here as original PET spare parts so it does not end up in the dumpster.
I am not throwing operational original parts out, I can assure you. Also I have not decided yet what to do with the power regulation. I just would like to make sure that this type of failure won't happen again.
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So.. the voltage regulators on the board have not been checked nor replaced yet?
Then please do, asap. Together with the diode over the 7905. These are killing chips, not the transformer.
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Then please do, asap. Together with the diode over the 7905. These are killing chips, not the transformer.
This is a PET 2001 board. There is no -5V volt and no 7905 regulator.
I never replaced a regulator on a PET/CBM board but I already had 2 defective diodes at the 7905 regulator that killed all dynamic RAM chips. But this is a problem of the later CBM board with 4116 dynamic RAM.
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Then please do, asap. Together with the diode over the 7905. These are killing chips, not the transformer.
This is a PET 2001 board. There is no 7905 regulator.
I never replaced a regulator on a PET/CBM board but I already had 2 defective diodes at the 7905 regulator that killed all dynamic RAM chips. But this is a problem of the later board with 4116 dynamic RAM.
I am currently running the motherboard from a laboratory power supply and I am skipping the diodes. I've replaced the 7805, but I am thinking of having DCDC converters in their place once we place back the machine to the exhibition. Not sure yet. First it needs to boot, then we'll see.
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I am currently running the motherboard from a laboratory power supply and I am skipping the diodes. I've replaced the 7805, but I am thinking of having DCDC converters in their place once we place back the machine to the exhibition. Not sure yet. First it needs to boot, then we'll see.
I really like opening up the PET and showing the inner workings. And I don't want to look at modern DC-DC converters, where every layman can see that they are not original. And every expert will have tears in his eyes.
But it's your PET and your decision.
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I am currently running the motherboard from a laboratory power supply and I am skipping the diodes. I've replaced the 7805, but I am thinking of having DCDC converters in their place once we place back the machine to the exhibition. Not sure yet. First it needs to boot, then we'll see.
I really like opening up the PET and showing the inner workings. And I don't want to look at modern DC-DC converters, where every layman can see that they are not original. And every expert will have tears in his eyes.
But it's your PET and your decision.
You are right. I'll keep it as it is.
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As I mentioned I use such a cheap old DDR voltage regulator to reduce the voltage to 220 volt to keep the PET and other old equipment cooler.
It's a Feutron 1223.