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PET 2001-8 (Blue PET) needs HELP!

S

scouter3d

Member
Joined
Aug 21, 2011
Messages
49
Location
Wien, Österreich
Hi,

I have a sick blue PET (The one with 6540 ROMS and 6550 RAMS)

it started out with the famous random Character screen, and today after switching it on i got no display at all ...

I do have an oscilloscope, so i tested the Video out pin and it just stays High ... VSYNC and HSYNC are there and seem OK ...

I also tested the Powersupply (transformer) and i measured 14V AC, 7V AC and 14V DC, there is also 14V AC going to the Monitor
the 14V DC is going to all 4 7805 Chips and i have +5 on all the 4 Outputs ...

Is 14V DC OK? it seems very high ...????? and the 7805´s are getting pretty hot ...

Greetings TOM:) from Vienna
 
Too funny. I was just coming here to start my own thread about a Blue Pet 2001-8 I rescued from the recycling bin at the local dump last week :) Mine has the random character display, and seems to run fine in the 5-minute intervals I've left it on. I really should pull out the multimeter and test further voltages! My screen also jumps slightly up and down along the vertical axis (the whole display, not just a single/group of rows/columns). And one of the stems on the keyboard is broken (as well as missing about a dozen keycaps, sigh)

I'll refrain from starting my thread for now and see how your troubleshooting goes, making my own thread if need be :)
 
I had a random character screen problem on another PET, and it turned out to be bad capacitors in the reset circuitry.
You might want to check the reset pin on the 6502 - pin 40. It should be low for about a second, then go high and stay there. A reset pin stuck low would be a possible explanation for the random characters.
 
scouter3d said:
...I also tested the Powersupply (transformer) and i measured 14V AC, 7V AC and 14V DC, there is also 14V AC going to the Monitor
the 14V DC is going to all 4 7805 Chips and i have +5 on all the 4 Outputs ...

Is 14V DC OK? it seems very high ...????? and the 7805´s are getting pretty hot ...
Click to expand...
Hello Tom, and welcome!

No, assuming you're measuring correctly, 14VDC is definitely twice what it should be. Are you sure the power connector and/or the transformer is/are wired correctly?

That PET uses a centre-tapped full-wave circuit which should give you roughly 7.5 to 8V per leg; sounds like you've got them both in series instead of parallel somehow.

The monitor should connect to terminals 7 & 8.
The AC centre-tap (terminal 5, ground, black) should connect to pin 3 (the centre pin).
The two AC legs (terminals 4 and 6, brown) should connect to pins 1 and 5 (the two outside pins).
The DC to and from the capacitor (red) should connect to pins 2 and 4.
Orientation of the connector doesn't matter.


I think you should get video of some kind even if the CPU is halted; sounds like maybe the clock isn't running at all. Try removing the character generator IC and see if it makes any difference.

In case you didn't know, there are schematics for all 4 versions and the monitors here:

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html
 
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@ MikeS : Thank you for the voltage information! I think some preowner accidently switched 2 wires on the transformer!!!!!

I also think i should get Video even with the CPU haltet, or the video ram removed, ... removing/changing the character generator makes no difference

Thanks, i have allready loaded the shematics ...

Now i will switch back the wrong wires on the transformer ... and start checking signals again ...

PS: i have made a internet page with pictures of the repair process here: http://www.8bit-homecomputermuseum.at/repair/bluepet/bluepet.html

Greetings TOM:)
 
Yes! i have lots of fun and learning! with my little PET!!!

I have switched the wires on the transformer, now all the 7805´s run much cooler ... (on 8V instead of 14V)

Still no Video on PIN 1 of the Monitorconnector (only static High) ...


Question) would the PET generate video without the 2 PIAs and the VIA (or with a defect PIA/VIA) ????

Reset Signal on CPU seems OK (goes high after 1-2 seconds and stays there)
Clock on CPU is OK also Phi2 is OK
But there is no pulsing on the Address lines ... (maybe bad rams/ bad roms)

But the PET should generate Video even without the CPU running ...

So where to look next???

Greetings TOM:)
 
scouter3d said:
Hi,
I solved the "no video" problem: it was a defect 74LS74 (C1) !!!
Now i have Video
But the CPU seems to start only every 10th bootup ...
Click to expand...
Excellent! Have you tested the RESET circuit? How does it behave if you manually reset it at the 555 (DE9)?
 
I will have a look at the RESET circuit ... but i think the RESET signal is fine because i can see a change on the Address and Datalines everytime i RESET ...

Does the PET need all ROM´s present to boot? or can i exclude one or another?

How many RAM chips need to be present?

Can i use Ruud Baltissens PET test rom with my 6540rom.com adaptor? and in which ROM slot? Alone (without other ROMs)? With howmuch RAM? Without RAM?

Greetings TOM:)
 
Hi,

I burned the PETTESTER ROM, used it in H4 and i works! (with 2 RAMs or even without! RAM)

BUT i get flickering lines in every Character ... (but not in the g´s or b´s the ramtest is outputing)

Also i had to remove one 6520 or else parts of the characterset were duplicated and the ramtest spills out b´s ...

greetings TOM:)

pictures here: http://www.8bit-homecomputermuseum.at/repair/bluepet/bluepet.html
 
scouter3d said:
Can i use Ruud Baltissens PET test rom
Click to expand...

Just as a minor note, it appears the pet tester ROM you're using isn't Ruud Baltissen's. The download you have on your page is a binary match for the pettester ROM I wrote and attached to this post:

http://www.vintage-computer.com/vcf...s.-(2001-8-4032-and-8032)&p=180148#post180148

There's actually a somewhat more sophisticated one (not written by me) in circulation, but I'm glad you're getting some use out of mine.

EDIT: On the same note, I do find it curious that you see flickering lines in the characters during the character set display but not during the RAM test display. I was about to say that flickering *may* be "normal" on such an old PET, because the delay loop as I coded it run through a loop reading the framebuffer RAM essentially as a time-consuming pile of NOPs in order to generate the roughly 2 second delay between the ramtest/charset cycles. (Really it's a bug, it should be doing something else. It does what it does simply because I basically reused another chunk of code for the delay.) On the really old PETs video RAM is access is contending, so those read cycles may well cause snow. But I'd think you should see it on both parts of the cycle.

Here's the offending code in assembly, in this case the second delay loop that runs after the character set display:

delayloop2:
lda $8000, x
lda $8100, x
lda $8200, x
lda $8300, x
lda $8080, x
lda $8180, x
lda $8280, x
lda $8380, x
inx
cpx #255
beq delay3
jmp delayloop2
delay3:
iny
cpy #255
beq loopexit
jmp delayloop2
loopexit:
jmp init

(This ROM is the first 6502 assembly I've ever written, so it does indeed suck.) The "after the memory test" loop is identical so, again, I'm not sure why you'd see flickering during one but not the other.
 
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It might be useful to know if the CPU starts consistently when you reset it with the rest of the system already powered up.

I'm pretty sure that the PET needs all ROMs present and working to boot properly, but I suppose removing certain ones would result in specific symptoms; that might be useful diagnostic information to compile...

You can remove the two 6520 PIAs however and it will still boot and display video.

You need a minimum of four RAM chips, the rightmost two pairs.

I'm not familiar with Ruud's test ROM; why not ask him directly or on the CBM hackers list where he's a regular.

Intermittent problems like yours are often caused by intermittent sockets on those old PETs; try sliding the chips sideways a bit, and you might even check the address lines at the chip pins with the NOP generator running.

Good luck!
 
scouter3d said:
I burned the PETTESTER ROM, used it in H4 and i works! (with 2 RAMs or even without! RAM)
Click to expand...

Thinking about this a little more I have to ask the obvious question: When you run the ROM with no system RAM installed do you get "g" or "b"s during the memory test cycle? If you get "g"s with no RAM installed then you have a serious problem. (A stuck address line A15 would explain that symptom, and that would certainly prevent you from getting to BASIC successfully.)

The test ROM does nothing with the 6520s or 6522 so their presence should have absolutely no effect. Having to pull one is possibly more evidence that there's something wrong with the address decoding circuitry.
 
Hi,

Clearly i used Eudimorphodons testrom and not Ruuds (thanks very much! and i updated the homepage ...)

@Eudimorphodon: If i remove all rams i get b´s ...

Greetings TOM:)
 
scouter3d said:
@Eudimorphodon: If i remove all rams i get b´s ...
Click to expand...

Darn, that would of been an easy catch. ;^)

Have you tried swapping the 6520s to see if that makes any difference? (IE, you said that you need to "pull one of them" to get a passing grade. Does it matter which *chip* you pull, which 6520 socket is empty, or both?) I would suggest checking the chip select lines on the 6520s to make sure that the troublesome chip is *not* enabled while the tester ROM is running. If the chip select is stuck on for some reason it may be outputting garbage to the bus during memory reads.

(According the to the data sheet CS0 and CS1 must be high and CS2 must be low for the 6520 to be enabled. If you had a logic analyzer handy I'd suggest clamping it on those three pins and sampling for a while to make sure that combination never comes up. Without one I'd probably start by checking continuity from those pins on the 6520s/their sockets to the appropriate parts of the decoding logic and make sure one isn't being left floating.)
 
Eudimorphodon said:
Have you tried swapping the 6520s to see if that makes any difference?
Click to expand...
I have swapped the 6520s => one of them works in both sockets, the other one seems "dead"

I have tested all ramchips using the testrom, 4 displayed "b",6 prevent the PET from running the ROM, 6 of them displayed "g" but only 2 of them worked in the videoram sockets, the other 4 displayed wrong characters or only 1 stuck character
So i figure i only have 2 realy working rams ...

Greetings TOM:)
 
scouter3d said:
I have swapped the 6520s => one of them works in both sockets, the other one seems "dead"
Click to expand...

Sounds like a good bet it actually is dead. I seem to recall that one of the PIA's is mostly dedicated to the IEEE 488 port so you shouldn't need that to get the machine mostly running. (The other is needed to read the keyboard.)

I have tested all ramchips using the testrom, 4 displayed "b",6 prevent the PET from running the ROM, 6 of them displayed "g" but only 2 of them worked in the videoram sockets, the other 4 displayed wrong characters or only 1 stuck character
So i figure i only have 2 realy working rams ...
Click to expand...

The RAM test isn't very thorough so it's not surprising you might get a false "g" from a RAM that really does have a stuck bit. (it only does slightly more than checking to see if "it's there".) It's impressive that you have six chips that prevent the ROM from running *at all*, however. The ROM doesn't require any working RAM to operate so they must be fouling up the bus somehow when they're plugged in.

Sort of sounds like you could really use one of those RAM replacement boards (some of the 6550-> 6116 adapters featured here:

http://vic-20.de/x1541/hardware/adaptors.html

), unless someone has a stash of 6550s they can part with.
 
Eudimorphodon said:
...Sort of sounds like you could really use one of those RAM replacement boards (some of the 6550-> 6116 adapters featured here:

http://vic-20.de/x1541/hardware/adaptors.html
Click to expand...
Ummm, just for information, since I believe the OP already has one of each in his PET: Those are not system ROM replacements; that RAM adapter specifically replaces the two video RAM chips and the ROM adapter is for the character generator ROM, although I think you'd also be able to replace lower-half main ROMs with it.

If you don't mind running some jumpers to the mainboard it wouldn't be too hard to replace the RAM with a 6264 or even a 62256, but yeah, time for that all-in-one RAM/ROM replacement board we're discussing in another thread ;-)
 
MikeS said:
Ummm, just for information, since I believe the OP already has one of each in his PET: Those are not system ROM replacements; that RAM adapter specifically replaces the two video RAM chips and the ROM adapter is for the character generator ROM, although I think you'd also be able to replace lower-half main ROMs with it.
Click to expand...

Doh. I just skimmed the page after getting it a Google hit for for a 6550 adapter and totally missed the part where it said "just for the video RAM". I imagine the *schematic* might work for replacing a pair of the system RAMs but the board layout wouldn't fit the sockets. Sorry, my bad.

I'm too lazy to go through the hand-drawn schematics on Zimmers in much detail, but it looks to me as if the original PET has a connector (U4) that has buffered data and address lines available on it? It seems to me that the *very* simplest way to replace the memory in a PET would be to make a simple circuit board that hangs off of that connector. Seems at the very very minimum all you'd really need is a 62256 and a single gate that provides chip select based on the state of A15. You'd have to pull all the RAM from the motherboard sockets, of course, but otherwise...?

(Obviously this doesn't replace the potentially bad original ROMs.)
 
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