Another PET 2001 with no cursor...

[Nama]

This is my Pet in a suitcase project that I repaired and built many years ago now. At the time it was working well, but as since decided to be a pain.

The system is basically a 2001 motherboard and transformer/capacitor in a suitcase with a small board mounted to the video out header to produce a composite signal. (see photos). The system has a RAM/ROM board installed.

The symptoms are as follows:
- Can boot to Basic 4 screen but no cursor present.
- No characters appear on screen when keyboard keys are pressed.
- Screen rolls like crazy (didn't used to do this and I have tested the composite board on another PET and it works fine)
- When I select Basic 1 or Basic 2 on the RAM/ROM board the screen is just blank. Only Basic 4 seems to display anything.
- Often boots to junk on the screen. Usually a power cycle fixes this.

What I have tried:
- Swapped both PIAs for W65C21s
- Installed another VIA
- Probed pin 7 of the CPU and get some solid pulses
- Probed pin 4 (IRQ) of the CPU and it is high.
- Tried other CPUs

I have a scope, a logic probe, a logic analyser and a multimeter for testing.
Any help would be greatly appreciate.

 

[Hugo Holden]

Maybe the screen rolling part is that the main board is producing the wrong frequency V sync or the wrong level. So the composite board could well be fine.

You could check the Vertical input signal to the composite board in the PET that it works in, and compare that with this one on the scope and that should show what the problem is for that issue.

So it is a case when you put it into storage some time ago it worked and now it doesn't ?

The typical part that fails on its own when not in use, is the Tant capacitor, often the old blue ones that go full short.

I think I can see one on your board. If it is part of the 555 power up reset timer it would stop that , if it was shorted out, you wouldn't have a proper reset pulse and all sorts of odd things could happen. Check that the 555 is producing the reset pulse , its about 1 second, after power up, and maybe check that Tant cap if it is not. The reset pulse should always be checked anyway as part of the very basic diagnostics.

 

[daver2]

I agree with Hugo's assessment.

The lack of an interrupt accounts for the lack of keyboard scanning. The keyboard is scanned as part of the interrupt handler.

This interrupt - in turn - is generated by one of the PIAs as a result of the VBLANK signal. So this further points to the video section circuitry if you have replaced the PIAs and VIA.

Dave

 

[Nama]

Thanks for the replies...I solved it.
I traced the vsync back, and back and finally found E8 (74ls20) pin 6 stuck high. , cutting that out, socketing and replacing E8 fixed the sync issue, and also I now got a cursor!
Seems this board is really fussy about IC's, and I had to do a bunch of 6520 and 6502 swapping before I could find a chip combo that works. Either it just wouldn't boot, to it would boot but the keyboard was all screwed up. In the end the only CPU I had that actually works in this board with the original two 6520 ICs is actually a 65C02...go figure!!!! Weird!
Anyway, hard to say it's 100% fixed yet, but I am able to load programs from my PETDISK, so thats something.
Thanks again for the pointers.

Philip

 

[daver2]

Well done!

Dave

 

[dave_m]

Hi Philip,
Nice to hear from you. I'm impressed you have keep that old Pet working for so long. I remember when you, MikeS and I (plus others) fixed that PET so many many years ago. You took many great scope photos that really helped us find the usual problem (flaky NAND gate?). Looks like you added a RAM/ROM board somewhere along the way...

You should use only NMOS 6520 or 6521 devices if possible. If you use the Western Digital PIA chip, make sure it is a 'W65C21N' version which is more NMOS compatible. The 'W65C21S' version has some differences in pull-up resisters and other things. It is great for higher speed CMOS systems but has a hard time driving a system with a lot of NMOS devices like the PET as I recall.
-dave_m