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Commodore PET 2001-8 stuck on garbled screen
- Thread starter RetroGadgetMan
- Start date
Nivag Swerdna
Veteran Member
That second trace looks a bit anaemic. Do you have a nice 5V between pin 8 and pin 21 of the CPU (F3)?
Out of interest what is happening at pins 12 and 13 of C2?
Out of interest what is happening at pins 12 and 13 of C2?
RetroGadgetMan
Experienced Member
Just checked pin 7 again pic 1 after checking voltage as at first it was zero between 8 and 21. Now 5.8v
C2
Pic 2 pin 12
Pic 3 pin 13
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RetroGadgetMan
Experienced Member
daver2
10k Member
A bit...
Dave
Dave
RetroGadgetMan
Experienced Member
Just gone over them and some others with a fibre glass pen. Look abit better but you don't know how much corosion if any has gotten inside the package.
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daver2
10k Member
The corrosion shouldn't have got inside the package. I did have a few ICs on an Apple IIe disk card where the corrosion did actually eat completely through the pin close to the package - but you couldn't actually see it without a bright light and a magnifying glass. When I removed the ICs from their sockets (they were all in IC sockets) various pins got left behind!
What I would be more concerned about is contamination on the PCB and 'tin whiskers' forming between pins of the IC. These could be on the component or solder side.
I would also be concerned about the internal contact state of any IC sockets.
Dave
What I would be more concerned about is contamination on the PCB and 'tin whiskers' forming between pins of the IC. These could be on the component or solder side.
I would also be concerned about the internal contact state of any IC sockets.
Dave
RetroGadgetMan
Experienced Member
I planned on washing the board at the end of repairs in a ipa and giving it a good scrub.
Maybe I should do this now as itay help dislodge anything between pins etc.
I have seen people washing PCBs in water and then IPA to disperse any water left behind
But in this cool weather I am reluctant to use water. As it would.take ages to dry out completely.
daver2
10k Member
I wouldn't use tap water though. You would really need to use distilled water or deionised water.
I would just use a cotton bud, toothbrush, IPA and some quality time (and a Gin and Tonic - replace with a tipple of your choice) to clean up the board as far as you can. You can also use some dental Tepe brushes to get right in between the pins and under the IC. Dental stuff is great for cleaning PCBs!
The temptation is to try and do it 'quick and dirty'. Resist the temptation... It takes longer, but the result is better (in my opinion).
Dave
I would just use a cotton bud, toothbrush, IPA and some quality time (and a Gin and Tonic - replace with a tipple of your choice) to clean up the board as far as you can. You can also use some dental Tepe brushes to get right in between the pins and under the IC. Dental stuff is great for cleaning PCBs!
The temptation is to try and do it 'quick and dirty'. Resist the temptation... It takes longer, but the result is better (in my opinion).
Dave
Hugo Holden
Veteran Member
Actually those TI IC's are interesting.
If you look closely at them, you will see that their ends have saw marks where a type of grinding disc cutter was used to separate the IC's at the factory.
Also, if you check the resin body that the IC is made from, and try to melt it with the tip of your soldering iron, you will find that it has a much higher melting point than the resin case of a modern IC, and it is much more brittle.
These original TI logic IC's were very solid. Though, unfortunately TI used steel pins (if you check they are magnetic) and they Silver plated them. Good when new but as the years pass the silver plating fails and the steel rusts.
These IC's may well be ok, but it is good to clean off the rust with 1000 or 2000 grade paper and then a subtle brush with Fertan to deactivate the remaining rust and then apply a surface oil like Inox MX-3.
You may be able to save these unique early TI IC's.
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RetroGadgetMan
Experienced Member
Very interesting.
RetroGadgetMan
Experienced Member
Well I have just given the board an initial clean with ipa.
I still don't trust these old sockets as sometimes I have to apply pressure to get a stable character set. As they sometimes flicker on the screen. indicating a bad connection somewhere. Is there an easy way to remove the old sockets to replace them. Like snipping the legs.
Or is it case of removing them as a complete socket.
Looks like it's the characters set socket giving me trouble as when the board is out of the case and I apply pressure to that one IC it changes.
I still don't trust these old sockets as sometimes I have to apply pressure to get a stable character set. As they sometimes flicker on the screen. indicating a bad connection somewhere. Is there an easy way to remove the old sockets to replace them. Like snipping the legs.
Or is it case of removing them as a complete socket.
Looks like it's the characters set socket giving me trouble as when the board is out of the case and I apply pressure to that one IC it changes.
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Nivag Swerdna
Veteran Member
Absence of life on C2 pin 13 suggests that the video RAM is never being addressed.
Try pins 8 and 9 of E5
Maybe the program isn't running... maybe the decoding isn't working...
As for the crusty ICs... if they are rusted through then you have a problem... rusty on the outside.. par for the course.
But... you really need something that is writing to the screen to exercise this... not sure what your tester does... hopefully it loops and tries to display something.
Try pins 8 and 9 of E5
Maybe the program isn't running... maybe the decoding isn't working...
As for the crusty ICs... if they are rusted through then you have a problem... rusty on the outside.. par for the course.
But... you really need something that is writing to the screen to exercise this... not sure what your tester does... hopefully it loops and tries to display something.
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Hugo Holden
Veteran Member
It depends on the design of the particular socket.
A lot of the early IC sockets you can lever off the plastic shroud leaving the socket claws on the board. (If you do it make sure to protect the pcb surface with a thin sheet of card or the tool/screwdriver will mark/damage it). Then you can remove the pins one by one and then that makes it easier, after applying fresh solder, to clear the holes with the solder sucker.
Most modern dual wipe or machine pin IC sockets, this is not possible, so you have to solder suck each pin, make sure each is free in the hole, and remove the socket as one piece. Don't attempt to remove it unless every pin is free in the hole.
For new sockets, I generally (for the old computer pcb's) fit new dual wipe sockets. The reason being that they have very thin pins, and are easy to solder suck and remove if they get damaged later. Although the machine pins sockets in many ways are better, especially if you have an adapter with thin round pins, but the socket's pin has a larger cross sectional area and these sockets are more difficult to remove again later from the pcb, if that is required, as even with good solder sucking, the round pin can still remain attached to the side wall of the plated through hole on the pcb. The aim of the game is to do what is better for the pcb. Most IC's and IC sockets are expendable, the pcb is not. So everything must be done to protect the pcb.
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RetroGadgetMan
Experienced Member
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Nivag Swerdna
Veteran Member
This is the point at which I would whip out my trusty NOP adapter and Digital Signature gadget...
However, you could logic probe all the pins of G2 and see if it is decoding ANY addresses.
PS
I'll sell you a deluxe NivagSwerdna NOP adapter... if you interested... PM me
However, you could logic probe all the pins of G2 and see if it is decoding ANY addresses.
PS
I'll sell you a deluxe NivagSwerdna NOP adapter... if you interested... PM me
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RetroGadgetMan
Experienced Member
G2
Pin 1 hi pulse
Pin 2 hi pulse
Pin 3 hi pulse
Pin 4 hi
Pin 5 hi
Pin 6 hi
Pin 7 hi pulse
Pin 8 hi
Pin 9 hi
Pin 10 hi
Pin 11 hi
Pin 12 lo
Pin 13 hi
Pin 14 hi
Pin 15 hi
Pin 16 hi lo pulse
Pin 17 hi pulse
Pin 18 lo
Pin 19 lo
Pin 20 hi pulse
Pin 21 hi pulse
Pin 22 hi pulse
Pin 23 lo
Pin 24hi
I always do a pass twice and this is the 2nd pass.
The first pass pin 1 through 11 read as hi pulsing. Ghosts in the machine
RetroGadgetMan
Experienced Member
Nivag Swerdna
Veteran Member
The activity will reflect the program that is running... either reading ROM or reading/writing RAM. The fact you have life on some of the outputs (1,2,3,4,5,6,7,8,9,10,11,13,14,15,16,17) suggests that it is doing some decoding... the lack of activity on pin 9 suggests it is not accessing the screen. $8000...
Do you have the technology to flash a 6540 ROM replacement? I would suggest using daver2's tester.
Do you have the technology to flash a 6540 ROM replacement? I would suggest using daver2's tester.
RetroGadgetMan
Experienced Member
Unfortunately not. I would like to going forward but not found a device yet