PET 3008 - Strange screen

[Bacon2002]

cosam,

The 74LS244 has the PCB ref "U111" in front of it.

I have not looked at any schematics but I did notice that a few of the pins on this 74LS244 appear (on the underside) to be soldered together. Is this correct?

(I have ordered a 74LS244, hopefully the correct type, just in case I needed to replace it.)

Yes I have a digital ohm meter, should I be looking to check that each of the IC pins are connected to the circuit board (checking for dry/broken joints) or should I be checking for something more complicated?

Thanks,

David

 

[cosam]

The 74LS244 should be either UB3 or UC3.

If it's UB3, which buffers the top half of the address bus, that could be the problem right there.

I'll send David the ROMs and let's hope it will make any improvement. Since the computer boots up but never starts Basic, I would think either 901465-01 or -02 is bad, unless the error is elsewhere on the board.

Yep - going by the monitor output, I'd be guessing 901465-02 (as it's at 0xD000).

 

[cosam]

The 74LS244 has the PCB ref "U111" in front of it.

Ah, that you be UI11 which buffers the RAM to the data bus.

I have not looked at any schematics but I did notice that a few of the pins on this 74LS244 appear (on the underside) to be soldered together. Is this correct?

Yes, the inputs and outputs are "doubled up" as it were.

Yes I have a digital ohm meter, should I be looking to check that each of the IC pins are connected to the circuit board (checking for dry/broken joints) or should I be checking for something more complicated?

Seeing as this chip is on the data bus, which would appear to be working OK, I don't think it's the problem. If you still want to check, ideally you'd measure from the actual pin on the IC to a pin on another component it connects to. If you get no continuity or measure any significant resistance, you can zero in on the problem area by checking at other points along the path. Schematics are here.

 

[Bacon2002]

Hi,

The UB3 and UC3 74LS244's appear to be (visually) original and are at the other end to the UI11 74LS244.

It looks like when I was referring to the front I should have said something like at the keyboard end.

David

 

[cosam]

The UB3 and UC3 74LS244's appear to be (visually) original and are at the other end to the UI11 74LS244.

It looks like when I was referring to the front I should have said something like at the keyboard end.

Heh - no, my bad. I had a quick glance over my own board here and overlooked those two '244s at what I'd also call the front ;-)

 

[carlsson]

This is how my board looks on the bottom side:

 

[Bacon2002]

Carlsson,

It's difficult to tell from the photo which of the pins are connected together within the PCB.

Tomorrow morning I'll dig out my camera and take some pictures of the PET, it's PCB and this IC from the underside.

Thanks for all your help guys.

David

 

[carlsson]

Well, if they're pin compatible with 2532 perhaps I can use my EPROM programmer to read/dump them as 2532?

Great thinking. I configured the programmer for TI 2532 chips and successfully read all four ROMs. Then I set up WinVICE to use these ROM dumps instead of the default ones, and it works like a charm. I believe the dumps even are identical to the 3032 ROM set included with the emulator.

It means my ROM chips are fine, so if changing David's ones with this set doesn't fix his PET, something else is at fault. Perhaps a rusty ROM socket?

 

[Bacon2002]

Carlsson,

Thats great news that the chips function correctly.

Here are some pictures of the PET:-

Screen:-

74LS244 - Underside of UI11 and UI10

I'll post some more of the motherboard and the ROM chips etc.

Thanks,

David

 

[cosam]

74LS244 - Underside of UI11 and UI10

That looks correct. The pins connected to each other on UI11 should follow the same pattern as UI10, i.e. four pairs on each side, which they do. I suspect it's just more noticeable on UI11 as it is hand-soldered and maybe some solder mask has been scraped off between the pins. But anyway - if there was a problem here the symptoms would be a lot worse: blank or garbage screen and definitely no monitor running.

Hopefully carlsson's (now proven good) ROMs will get you running again.

 

[Bacon2002]

Here are some pictures of the Motherboard and ICs:-

Mainboard

ROMs

 

[Bacon2002]

Carlsson and Cosam,

I'm over the moon........

Many, many thanks for all your help, special thanks to Carlsson for the ROMs, without which my PET would be just a large and, of course, cool looking paper weight.

Cheers guys,

David

 

[cosam]

Good to see you're up and running again! Which ROM was it in the end?

 

[carlsson]

Ah, you do have 32K memory on that PET so it is upgraded from a 3008 to a 3032.

 

[Bacon2002]

Hi,

I think it was the 901465-02 ROM.

The good news keeps getting better, in all the excitement I didn't notice the 31743 bytes free.

Brilliant....


Ta,

David

 

[carlsson]

If you can identify your 901465-03 (Kernel) is fine, perhaps you can send it further to our German friend track18 who just figured his is bad? Both are 30xx machines so the ROMs should be identical.