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PET 4016 Upgrade options?

WimWalther

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No key combination, but there is a POKE to switch in (or out) the other 'half' of the character generator.

It just eludes me at the moment, but a bit of searching should identify it.

Dave
If you scroll back a few posts, I identify it in my question about the odd behavior of ?CHR$(14).

It's 59468,14 / 12
 

daver2

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Missed those...

Your CHR$(14) may be reprogramming the CRTC to give you more scanlines per character. The EDIT ROM has both CRTC configuration options. If I am correct, this is not necessarily changing the character set, but (maybe) giving you better lowercase descenders for business use. It may mess up your graphics screens of course.

Dave
 

WimWalther

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When you use the control code the CRTC is being reprogrammed to add an extra blank line between character rows in lowercase mode. (Which as you've noticed can mess things up a little if your monitor's overscan is set up to fully fill the screen in graphics mode.)

The reason for this is Commodore decided that the extra line for descenders made lowercase more readable, and with the CRTC it was "free", or nearly so, to implement.
Dave,

Here's what Eudimorphodon said about it.
 

WimWalther

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Ran into another one..

Im trying to remove the keyboard for cleaning, but it's not coming easily. First off, it doesnt look like any CBM keyboard that I've ever seen, nor does it match the examples in any video on the subject. This kb is in a sheetmetal case with 12 black screws visible. It lacks the mounting tabs typical of CBM kb's, as seen on a VIC or 64 - and apparently most PETs.

When I try to remove screws, some come out relatively easily. But most seem to bind hard, only increasing in torque as I attempt to turn them. The removal torque is excessive, as if they are Loctited.. And I can't even tell if they are retaining the metal kb case.

Thoughts? I don't want to damage anything.

IMG_20220710_030253441~2.jpg
 
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WimWalther

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Those tabs you mention - they don't have any screws in them, at least not right now.

However, the (2) short #10 screws that hold the hinged case closed do pass through the holes in the tabs to reach the threaded inserts in the upper case half. So maybe the tabs provide a little extra security to mount the kb, but beyond that I don't see their purpose.

And that keyboard is VERY securely mounted. Even with several of the black screws removed, it shows no tendency to flex or move.
 

Eudimorphodon

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And that keyboard is VERY securely mounted. Even with several of the black screws removed, it shows no tendency to flex or move.

That’s kind of the point with that many screws. ;)

(again, see my edit about disregarding the thing about the side tabs being the main support, it had been a while since I’d opened my PET and apparently got it crossed with how Apple II keyboards are supported on the short sides.)

It should come off when you get them all. The only thing to worry about here is if somehow the screws are so corroded and sticking so hard that you manage to damage the plastic posts they’re going into. If you hear cracking noises and turning the screw no longer backs it out you have problems.
 

WimWalther

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Yeah.. when I applied force and turned one of the tight screws, I heard & felt something that may well have been a cast-in threaded insert slipping in its case boss.

Yuck. If those screws are actually seized in their inserts, this just became a big, ugly problem.
 

Eudimorphodon

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Yuck. That does indeed sound not good.

I’m hesitant to recommend hitting them with WD-40 or whatever because I’m not at all sure what would be safe for the plastic. Maybe other folks have thoughts.
 

Hutch

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There aren't any threaded inserts in there, just self tapping screws.
The "plastic" is structural foam which is brittle so it will sound rough.
On one PET I had to replace the keyboard screws with a slightly larger size since they were stripped out.
I usually re-assemble WITHOUT the metal tray. I believe that's only there as shielding for FCC compliance. It's heavy and cumbersome to re-install.
 

WimWalther

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There aren't any threaded inserts in there, just self tapping screws.

The screws that are coming out look like #6-32 machine screws, not the sort of wide-pitch, pointed 'wood' screws I'd expect to see if they were threading directly into plain plastic bosses as used in 64 / VIC etc,

That, and I've never seen screws stuck this hard in plastic..

But hey, we'll see! This is uncharted territory for me. Good news is that I got several more of them out.. only three really stubborn ones remain.
 

WimWalther

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And the keyboard function is restored..

All it really needed was a cleaning of the switch contacts, though to look at it you'd never have thought there was anything to clean off. I used medical swabs (the ones with long wooden shafts, not Q-Tips) & Energine fluid to clean both the PCB contacts and the dark gray conductive rubber tips of the key plungers. Isopropyl alcohol 91% might have worked, but I tend to go straight for the chlorinated solvents - they are the source of true power.

(Energine is just a brand name for 1,1,1 trichloroethane cleaning fluid. My can is 30+ years old, and I'm not sure if it's still sold - but most chlorinated automotive brake / electrical cleaners are the same or similar stuff.)

Now it's just a matter of reinstalling the kb..
 
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dave_m

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And the keyboard function is restored..
Good work. Yes, it is the conductive pads that need to be restored to conductivity. The circuit uses a 10 K-Ohm pull-up resistor. The pad can get up to several K-Ohms which screws up the voltage ratio and looks like a logic 1 instead of 0 when contact is made. I thought of a change of the resistor network chip to a 100K Ohm package would do the trick, but that might change the time constant enough to cause double characters.
 

WimWalther

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Dave,

After cleaning, I was getting key circuit resistance between roughly 50-100R. I only tested the Left Shift circuit as it was easy to identify..

But every key now works reliably, which is a major improvement!

And I also learned that Right & Left shift are not the same key scancode.. they're totally independent.
 

MikeS

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Something is escaping me here. I'm looking over this mod, but I cant find anything about the need to increase the video RAM on a 40 col. machine. I suppose that it's not a requirement if you're modifying an 80 col. machine - but in the case of a 40 col. it's a different story.

I'd initially assumed that the additional RAM would be on the daughterboard, but that only holds logic chips. Could someone please supply additional details on this?
Those 5 empty IC locations in front of the keyboard connector are for 2x2114 video RAM chips and three associated TTL chips.`

Schematics:
 

Eudimorphodon

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Those 5 empty IC locations in front of the keyboard connector are for 2x2114 video RAM chips and three associated TTL chips.`

FWIW, it’s mildly amusing how that memory is implemented on the 80 column PETs, although it’s definitively not anything you need to worry about unless you’re planning to do custom programming of the CRTC chip.

(Long and short of it is an 80 column PET’s CRTC still thinks it’s programmed for a 40 column display, but the hardware fetches two bytes/characters at once; the now 2k of VRAM is organized into even/odd bytes. This was necessary for the 80 column display’s effective 2mhz character line rate to continue to sync up with the 1Mhz CPU.)
 
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