AAAARGH! VIC-20!

[Phu]

!*"*£&!$&!!!

This one really has me stumped.

I've just spent 2.5 hours checking every single logic connection on the whole board. No broken connections anywhere!

I've replaced every single chip - including the VIC chip with the one provided by AndyT (cheers) - still just a blank screen!

This board is taking the smeg....

-- Richard

[billdooruk]

Mmmmm...you've done more than I would be able to....

Didn't know you were a Red Dwarf fan too Rich

[Phu]

I know nothing about the Red Dwarf VHS' on the shelf of which series 7 is currently in the VCR....

This board is weird. Seems Commodore did an interesting thing with their version of the 6502 (MOS Tech.).

Their 6502 has a slightly higher tolerance of voltage for a logic 0. And a suspicious R14 which has purpose other than to raise the clock voltage level slightly. A casual observer would say the circuit is engineered to only work with MOS (owned by Commodore at the time) 6502 CPUs!

When a Rockwell 6502 was used, the phase-shift clock outputs were non-existent, even though the clock input was healthy and the CPUs were known good. Desoldering the otherwise pointless R14 makes the clocks output again, though the CPU still locks up immediately its powered on.

More interesting is why with everything unplugged, the BLK7 line is floating at 2.5V......

I know you love reading this techy crap

-- Richard

[AndyRCM]

Im actually amazing myself by understanding MOST of what you are saying.
Richard, when you say blank screen - you mean blank black screen or something else?

Andy

[Panther]

Anyone know about Vic-20 cartridges ?

I've got 3 plus a 16k expansion cart that when plugged in, the Vic doesn't seem to recognise and reverts to the standard blue boot screen showing 3.5k free.

[AndyRCM]

Yeah I do . . . I know quite a bit about most of the Commodore machines . . .

Usually when a cart doesn't work it is because it is dirty! Basically you need to clean the edge connectors with something non-abrasive. I tend to use nail varnish remover on a baby wipe or something like that, this worked with all of my Vic carts as none of them were working either!

Andy

[Akastaff]

Cartridge will also fail to recognise if the I/O segment of the machine has been damaged, usually by a cart being pulled out whilst machine is on.

[Panther]

Will try cleaning the edge connectors, I've got one cart 'GORF' which loads OK, all the others don't want to play ! 

[Womble]

I always open the carts up, that way you can see what you are dealing with, eg any ROMs in sockets that need a reseat, or leaky batteries, or just oxidised contacts, far easier to deal with when its opened up.

[Phu]

Im actually amazing myself by understanding MOST of what you are saying.
Richard, when you say blank screen - you mean blank black screen or something else?

Meant to answer this sooner...

What I'm getting is a completely black, perfectly sync'd blank screen.

I've since done some more digging and found that the 74 133 13-input NAND gate* appears to be damaged. 12 out of 13 inputs are are +5V and the 13th is oscillating at CLK2. The output therefore *should* be oscillating too, but its stuck at 0.

74 133s are rare these days, and since I don't want to pay huge sums for them, I'm sourcing a few from eBay, which I should have my hands on in a few days.

* When I said I'd replaced every chip, this is one that I couldn't replace due to its rarity. I had built a replacement device out of 13 diodes, two resistors and a transistor, but it doesn't work quite the same with respect to values at 1.5v being treated as 0.

-- Richard

[Phu]

Right.. 74 133 replaced with working IC.

And the bloody thing still won't display!!

This board is really starting to <sound of birds twittering> me off!

-- Richard

[Womble]

Have met similar boards, they give the impression they don't want to be fixed.

[Phu]

Well, I made a little progress later on.

I grabbed hold of a kernal ROM disassembly for the startup routines. One of the first things the ROM does is call the cartridge ROM detect routine. At this point, the only RAM-based operation is the JSR instruction, but this only puts an address on the stack, RAM contents have yet to be made use of.

The cartridge detect routine then scans the cartridge area at $A000. If this code is being executed then the BLK5 line (select area $A000 - $BFFF) should go low for a number of times on startup. It doesn't.

This shows that the ROM is not being executed correctly. As an extension to this, the 6502 gets the start-of-execution address from $FFFC - $FFFD. Its quite likely that this isn't being read properly at all, and instead the CPU jumps to some random memory location and begins executing garbage, never touching the startup routine.

My new multimeter should be arriving today (my old one was becoming unreliable), so I can do some further checking of the connections between the kernal ROM and the CPU.

-- Richard

[AndyT]

Let me know if you need a replacement ROM chip. I think I have the one that went with the VIC chip I sent you.

[Phu]

AHA!

Tricksy CPU socketses.... not making contact with pin A10 !!

Still not got even so much as a border, but we have considerable progress through the init sequence.

I can see BLK5 going low during the cartridge scan. VA13 is being brought low multiple times, indicating the VIC is being written to. This also indicates that the RAM is working well enough for the stack to function correctly.

I can see *something* happening on screen, but its only subtle interference patterns.

I don't have any IRQ or NMI pulses. According to the disassembly, part of the kernal init is to turn on IRQs from one of the VIAs. Coupling this with the still blank screen, despite VIC being accessed suggests that the wrong values are being pushed around.

Investigation continues....

-- Richard